rab proteins and endocytic trafficking: potential targets for therapeutic intervention
TRANSCRIPT
www.elsevier.com/locate/addr
Advanced Drug Delivery Reviews 55 (2003) 1421–1437
Rab proteins and endocytic trafficking:
potential targets for therapeutic intervention
Mary-Pat Stein, Jianbo Dong, Angela Wandinger-Ness*
Molecular Trafficking Laboratory, Department of Pathology, University of New Mexico School of Medicine, Albuquerque, NM 87131, USA
Received 5 July 2003; accepted 30 July 2003
Abstract
Rab GTPases serve as master regulators of vesicular membrane transport on both the exo- and endocytic pathways. In their
active forms, rab proteins serve in cargo selection and as scaffolds for the sequential assembly of effectors requisite for vesicle
budding, cytoskeletal transport, and target membrane fusion. Rab protein function is in turn tightly regulated at the level of
protein expression, localization, membrane association, and activation. Alterations in the rab GTPases and associated regulatory
proteins or effectors have increasingly been implicated in causing human disease. Some diseases such as those resulting in
bleeding and pigmentation disorders (Griscelli syndrome), mental retardation, neuropathy (Charcot–Marie–Tooth), kidney
disease (tuberous sclerosis), and blindness (choroideremia) arise from direct loss of function mutations of rab GTPases or
associated regulatory molecules. In contrast, in a number of cancers (prostate, liver, breast) as well as vascular, lung, and thyroid
diseases, the overexpression of select rab GTPases have been tightly correlated with disease pathogenesis. Unique therapeutic
opportunities lie ahead in developing strategies that target rab proteins and modulate the endocytic pathway.
D 2003 Elsevier B.V. All rights reserved.
Keywords: Small GTPases; Pigmentation and bleeding disorders; Neuropathy; Thyroid disease; Cancer
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1422
1.1. Constitutive endocytosis and rab proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1422
1.2. Maintenance of normal cell physiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1424
2. Regulation of rab protein function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1424
2.1. Regulated expression levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1424
2.2. Regulated membrane association and localization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1425
2.3. Regulated activation through nucleotide binding and hydrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1425
2.4. Regulated function through specific effectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1427
3. Altered rab proteins in disease. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1429
3.1. Loss of function mutations in rab proteins, rab regulatory molecules, or rab effectors . . . . . . . . . . . . . . . . . . 1429
3.2. Altered rab expression or activation in disease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1431
0169-409X/$ - see front matter D 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.addr.2003.07.009
* Corresponding author. University of New Mexico HSC, 2325 Camino de Salud NE, CRF 225, Albuquerque, NM 87131, USA. Tel.: +1-
505-272-1459; fax: +1-505-272-4193.
E-mail address: [email protected] (A. Wandinger-Ness).
M.-P. Stein et al. / Advanced Drug Delivery Reviews 55 (2003) 1421–14371422
4. Therapeutic targets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1432
4.1. Modalities for stimulating rab protein expression and/or function . . . . . . . . . . . . . . . . . . . . . . . . . . 1432
4.2. Modalities inhibiting rab protein expression and/or function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1433
5. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1433
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1433
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1433
1. Introduction endocytic transport, providing the spatial and temporal
Table 1
Endocytic rab proteins
Rab Rab function References
Rab4 Localized on EEa and RE; regulates
sorting/recycling
[84,85]
Rab5 Formation of CCV from PM; CCV–EE
and EE–EE homotypic fusion;
endosome motility
[41,42,86–88]
Rab7 Required for EE–LE transport and
LE–Lys fusion
[10,89,90]
Rab9 Endosome to TGN transport [12,91]
Rab11 Recycling through perinuclear RE;
exocytosis from TGN to PM; implicated
in polarization of the Drosophila oocyte
[92–95]
Rab14 Implicated in endocytosis, lysosome
fusion, and phagocytosis
[96,97]
Rab15 Implicated in EE sorting and perinuclear
recycling
[6]
Rab17 Required for transcytosis in epithelial
cells
[98,99]
Rab20 Implicated in endocytosis and recycling
in epithelial cells
[100]
Rab22 Localized on EE and LE; implicated in
endocytosis
[101,102]
Rab25 Implicated in ARE recycling [93]
Rab34 Implicated in macropinosome formation
and lysosome distribution
[103,104]
Rab39 Implicated in endocytosis [105]
a ARE, apical recycling endosome; CCV, clathrin-coated vesicle;
EE, early endosome; LE, late endosome; RE, recycling endosome;
TGN, trans-Golgi network.
Numerous human diseases can be attributed to
alterations in endocytic trafficking. The rab GTPases
and associated proteins are critical regulators of endo-
cytic transport. Increasingly, rab proteins and their
effectors are found overexpressed or subject to loss of
function mutations in human disease. The alterations
impact cellular physiology by perturbing the homeo-
stasis of key cell surface receptors, lipid metabolism,
hormone processing, and specialized secretory path-
ways. Consequently, diverse diseases ranging from
mental retardation to cancer may be attributed to the
derangement of the regulatory machinery governing
endocytic membrane trafficking. This review begins
with an overview of rab protein function and regulation
and highlights how these processes are disrupted in
disease with the aim of identifying potential avenues
for therapeutic intervention.
1.1. Constitutive endocytosis and rab proteins
Endocytosis is a fundamental cellular process re-
quired for the regulated uptake and intracellular trans-
port of macromolecules. There are numerous routes
whereby molecules may be internalized including re-
ceptor-mediated endocytosis via clathrin-coated vesi-
cles, caveolar uptake, macropinocytosis, and phagocy-
tosis (reviewed in Ref. [1]). The discussions in this
review will center on receptor-mediated endocytosis.
Beginning with the initial steps of receptor seques-
tration and culminating with the delivery of cargo to
specific intracellular destinations, endocytosis is a
highly regulated process. Following internalization,
molecules in vesicular carriers derived from the plasma
membrane are transported to specific intracellular des-
tinations allowing for coordinate regulation of hor-
mone and growth factor receptor signaling, sampling
and presentation of antigens for immune recognition,
and general cellular homeostasis. The small ras-related
rab GTPases have emerged as important regulators of
control required for endocytic transport fidelity. Based
on our current understanding, the rab proteins regulate
individual transport steps by controlling cargo selec-
tion [2] and modulating vesicle budding, directed
targeting, and fusion [3]. As detailed further below,
rab protein activity depends on their selective activa-
tion and ability to act as scaffolds for the recruitment of
specific effectors in a spatially and temporally con-
trolled manner. Approximately 60 rab proteins are
encoded by the human genome, with additional rab
proteins possibly generated by alternative splicing [4].
A subset of 13 rab proteins is utilized on the endocytic
M.-P. Stein et al. / Advanced Drug Delivery Reviews 55 (2003) 1421–1437 1423
pathway (Table 1) and serves to determine the fate of
endocytosed cargo.
Althoughmany rab proteins have been implicated in
endocytic transport (Table 1), only a small subset has
been extensively characterized (Fig. 1). Early endo-
cytic events are primarily regulated by rab5 and rab15.
Rab5 facilitates segregation of cargo into clathrin-
coated pits and together with a number of identified
effector molecules, promotes cytoskeletal motility and
homotypic early endosome fusion (reviewed in Ref.
[5]). In contrast, rab15 inhibits transport to early endo-
somes, directly opposing the activity of rab5 [6,7].
Molecules that reach early endosomes are subsequently
sorted for recycling back to the plasma membrane or
Fig. 1. Schematic diagram of the rab-regulated endocytic pathway. Mole
endosomal compartments that are characterized by the presence of rab5
within early endosomes results in immediate transport back to the plasma m
transport either to perinuclear localized recycling compartments or tow
endosomes is subsequently followed by rab11-mediated recycling to the pl
is facilitated by rab7, and cycling of molecules from late endosomes to the T
mediated by rab7 and RILP. Finally, secretory lysosomes, such as those fou
calcium-regulated process. Abbreviations: CCP, clathrin-coated pits; CCV
LYS, lysosome; RE, recycling endosome; TGN, trans-Golgi network.
transport to lysosomes for degradation. Recycled mol-
ecules can either be sorted into rab4 containing micro-
domains within early endosomes that permit fast
recycling to the plasma membrane [8] or be transported
to perinuclear recycling endosomes where rab11 regu-
lates transport back to the plasma membrane [9].
Molecules destined for degradation are delivered in a
rab7-dependent transport step from early to late endo-
somes [10]. Rab7may also function downstream of late
endosomes facilitating transport to lysosomes in asso-
ciation with its one identified effector protein, RILP
[11]. An additional arm of the endocytic pathway
provides for rab9-mediated recycling of molecules
such as the mannose 6V-phosphate receptor (M6PR)
cules internalized by endocytosis are initially transported to early
and rab4. Sorting of molecules into rab4-containing microdomains
embrane while sorting into rab5-containing microdomains leads to
ard lysosomes for degradation. Transport to perinuclear recycling
asma membrane. Transport from early endosomes to late endosomes
GN is mediated by rab9. The transport of molecules to lysosomes is
nd in melanosomes, are delivered back to the plasma membrane in a
, clathrin-coated vesicle; EE, early endosome; LE, late endosome;
M.-P. Stein et al. / Advanced Drug Delivery Reviews 55 (2003) 1421–14371424
from late endosomes to the TGN [12]. These rab
proteins are the most highly understood of the consti-
tutively expressed endocytic rabs. Several additional
endocytic rab proteins, primarily localized to special-
ized cells, have also been characterized: rab27 (trans-
port of melanosomes), rab25 (apical recycling
endosome to TGN), and rab17 (endosomal recycling
in epithelia) [3]. Each endocytic rab protein character-
ized to date functions in concert with a relatively
unique set of effector molecules. Therefore, continued
characterization of the less well-characterized rab pro-
teins is imperative to achieve a more complete under-
standing of the molecular mechanisms governing
endocytosis.
1.2. Maintenance of normal cell physiology
Functional endocytic trafficking is central to normal
cellular physiology. For this reason, the manipulation
of constitutive or cell-type-specific rab proteins could
provide a means for treating diseases resulting from
aberrant intracellular transport. For example, overex-
pression and delayed degradation of epidermal growth
factor receptors, in particular ErbB2, has been associ-
ated with tumor formation and poor prognosis in breast
cancer patients [13]. Targeting ErbB2 receptors to
lysosomes might aid in reducing intracellular pools of
ErbB2 receptors resulting in decreased cell signaling
and reduced cell proliferation. In thyrocytes, increased
overall levels and increased membrane-bound fractions
of rab5a and rab7 have been linked to formation of
benign thyroid autonomous adenomas [14]. Altering
the amount and rate of thyroglobulin transport and
processing, and the subsequent basolateral secretion
of T3 and T4 by reducing rab5a and rab7 expression,
might lead to a reduction in tumor formation. In
Alzheimer’s patients, aberrant processing and transport
of amyloid precursor protein (APP) in axons leads to
the generation of Ah plaques and the progression of
neurodegenerative disease (reviewed in Ref. [15]).
Manipulating the transport of Ah-containing vesicles
may facilitate clearance of accumulated APP. Finally,
roles for rab proteins in G-protein-coupled receptor
endocytosis, desensitization, and downregulation
(reviewed in Ref. [16]) and in cholesterol and lipid
metabolism and trafficking [17,18] demonstrate that
rab proteins are critically important for a wide variety
of normal cellular functions.
2. Regulation of rab protein function
Rab proteins direct vesicular transport by means of
their localization to select intracellular compartments
and through specific interactions with multiple effec-
tor proteins. Downstream regulation of rab effector
proteins is in turn dependent on the tight regulation of
the rab proteins themselves. Regulation occurs at
multiple levels and in conjunction with numerous
regulatory proteins. Transcriptional and translational
mechanisms control rab protein expression. Mem-
brane localization is controlled through specific post-
translational modification and membrane recruitment.
Selective rab protein activation depends on nucleotide
exchange and hydrolysis, and localized function
occurs through interaction with unique effector pro-
teins. Thus, the rab proteins govern intracellular
vesicular trafficking by acting as localized scaffolding
platforms to exert temporal and spatial control of
transport. Below, the well-characterized regulatory
events associated with rab protein expression and
functional control are presented. As the identification
of factors that modulate rab function in vivo contin-
ues, additional regulatory mechanisms will surely be
revealed.
2.1. Regulated expression levels
Most of the endocytic rab proteins are constitutively
expressed in all mammalian cells. Nevertheless, indi-
vidual rab protein levels vary quantitatively between
cell types (reviewed in Ref. [19]). In addition, tissue-
specific expression of rab proteins in cells such as
epithelia and neurons fulfill the need to regulate
specialized transport processes in these polarized cells.
The data imply that individual cells express a partic-
ular repertoire of rab proteins to fulfill their cellular
vesicular transport needs.
Altered rab gene expression in response to various
inflammatory stimuli and diseases has been reported
and suggests additional plasticity is exerted through
the control of rab gene expression. Modulation of
expression offers another level of control that might
be manipulated for therapeutic purposes. Increased
expression of rab5a and rab7 occurs in response to
cAMP [14], while rab5a can also be increased by
interferon gamma in macrophages [20]. Conversely,
intestinal epithelia treated with transforming growth
M.-P. Stein et al. / Advanced Drug Delivery Reviews 55 (2003) 1421–1437 1425
factor h, exhibited decreased rab11 expression [21].
The possibility of regulating membrane trafficking
through the control of rab protein expression is clearly
illustrated through these studies on inflammatory
responses and may serve as a paradigm for other
disease situations with altered rab expression. Indeed,
altered expression of rab proteins in diseases such as
low-grade dysplasia associated with Barret’s esopha-
gus (rab11) [22], cardiomyopathy (rab1, rab4, and
rab6) [23], lung tumor progression (rab2) [24], pros-
tate and liver cancer (rab25) [25,26], and possibly
atherogenesis (rab7) [27], point to the importance of
understanding RAB gene control and the identification
of factors affecting RAB expression. Such pathways
may then be targeted to selectively increase or decrease
rab protein expression and achieve the desired modu-
lation of endocytic transport.
2.2. Regulated membrane association and localization
Following protein synthesis, rab proteins interact
with several common rab regulatory molecules, which
result in posttranslational modification and membrane
association of rab proteins. One or two cysteine
residues in the C-termini of all rab proteins are
modified with isoprenoid moieties to allow for mem-
brane localization. Modification occurs when newly
synthesized rab proteins first interact with rab escort
proteins (REP) (Fig. 2, steps 1–4). Next, the rab:REP
complex is specifically recognized by a rab geranyl-
geranyl transferase (RabGGTase), which catalyzes the
addition of geranylgeranyl groups to C-terminal
cycteine residues in the motifs CC, CXC, CXXX,
CCXX, or CCXXX (where X is any amino acid). The
geranylgeranylated rab proteins are subsequently de-
livered to intracellular membranes in a complex with
REP, which maintains the modified rab protein in a
soluble form. Targeting of REP proteins to specific
intracellular membranes may involve membrane
receptors, which have not yet been identified. Once
the geranylgeranylated rab protein is presented to the
specified target membrane, REP is released and
recycled to the cytosol for additional rounds of escort.
REP proteins share sequence homology with rab GDP
dissociation inhibitors (rabGDI), another rab regula-
tory protein.
Similar to REP, rabGDI binds GDP-bound rab
proteins, producing soluble rabGDI:GDP-bound rab
(GDP-rab) complexes in the cytosol. Importantly,
rabGDI extracts GDP-rab from intracellular mem-
branes and recycles GDP-rab back to donor mem-
branes (Fig. 2, steps 11–13). Major conformational
changes in rabGDI occur upon binding to membrane-
associated GDP-rab, resulting in the dissociation of the
rabGDI:GDP-rab complex from acceptor membranes
[28]. Recycling of GDP-rab proteins to specific mem-
brane compartments is mediated by compartment-
specific receptors that recognize the soluble rabG-
DI:GDP-rab complex. The membrane-bound Hsp90
chaperone complex has been identified as a putative
rabGDI:rab receptor [29]. Alternatively, the rab GDI
displacement factor (GDF, discussed below) may also
serve as a rabGDI:GDP-rab receptor. Upon binding to
donor membrane receptors, rabGDI is released from
GDP-rab, allowing recycling of rabGDI for additional
rounds GDP-rab extraction and transport.
Highly conserved sequences within REP and
rabGDI form a general binding platform that facilitates
rab protein recognition. Complementary conserved
residues within all mammalian rab proteins enable
rab protein binding to these and other common regu-
latory molecules [30]. Since the conserved binding
motifs within the regulatory molecules bind to a variety
of rab proteins, the disruption of multiple cellular
functions by mutation would be predicted to have dire
consequences. However, due to the expression of
multiple isoforms of rab regulatory molecules with
redundant function, mutation of REP1 results specifi-
cally in retinal degeneration whereas loss of GDI
function results in severe mental retardation. Modulat-
ing endocytosis through the manipulation of common
regulatory molecules is therefore unlikely to be a
tractable therapeutic strategy. Deficits in these regula-
tory molecules should be considered candidates for
gene therapy.
2.3. Regulated activation through nucleotide binding
and hydrolysis
In addition to cycling between membrane compart-
ments, all rab proteins also cycle between GDP-bound
(‘‘inactive’’) and GTP-bound (‘‘active’’) states (Fig. 2,
steps 5–10). After extraction from membranes by
rabGDI, rab proteins either remain in a cytosolic
complex associated with rabGDI or are delivered back
to donor membranes. Dissociation of rabGDI at the
Fig. 2. Regulation of rab membrane localization and nucleotide cycling. Newly synthesized GDP-bound rab proteins are bound by REP (1),
which then presents the new rab proteins to rabGGTase (2). Following geranylgeranylation of rab protein C-termini, REP delivers the newly
modified rab proteins to their appropriate donor membranes (3), dissociates and recycles for additional rounds of rab escort (4). Once released,
rab proteins quickly insert into the donor membrane and undergo exchange of GTP for GDP with the aid of GEFs (5–7). Rab proteins may exist
in a nonnucleotide-bound transition state in vivo (6), stabilized by guanine nucleotide-free chaperones (GNFC) such as Mss4-like TCTP
proteins. GTP-bound rab proteins interact with effector (E) molecules on ECV (8) and at their acceptor membranes, facilitating transport,
tethering, and fusion of vesicles at their appropriate destinations. Hydrolysis of GTP is accelerated by GAPs, leading to dissociation of
associated effector molecules (9,10). Removal of GDP-bound rab proteins from target membranes is mediated by GDI, which solubilizes rab
proteins and targets rab proteins back to donor membranes (11). Interaction of GDI with GDF releases the rab protein allowing reinsertion into
the donor membrane (12), and GDI recycles into the cytosol for additional rounds of extraction and transport (13,14). Abbreviations: REP, rab
escort protein; rabGGTase, rab geranylgeranyl transferase; GEF, guanine exchange factor; TCTP, translationally controlled tumor-associated
proteins; E1, E2, E3, effector molecules; ECV, endocytic carrier vesicle; GAP, GTPase activating protein; GDI, guanine dissociation inhibitor;
GDF, GDI dissociation factor.
M.-P. Stein et al. / Advanced Drug Delivery Reviews 55 (2003) 1421–14371426
donor membrane is facilitated by membrane-bound
GDI-displacement factor (GDF) [31]. Upon release of
rabGDI, the geranylgeranyl moieties of GDP-rab rein-
sert into the donor membrane, readying the rab for
another cycle of activation. Membrane association of
GDP-rab promotes nucleotide exchange from GDP to
GTP; however, the intrinsic rate of nucleotide exchange
is very slow. The rate of nucleotide exchange is greatly
enhanced by specific guanine nucleotide exchange
factors (GEFs). GEFs catalyze the exchange of GTP
for GDP; however, the molecular mechanisms govern-
ing nucleotide exchange are not yet fully appreciated.
A class of proteins sharing sequence similarity with
the guanine nucleotide-free chaperones Mss4/Dss4
[32] has recently been identified. Translationally con-
trolled tumor-associated proteins (TCTP) are highly
expressed in a wide range of mammalian cells and may
function as guanine nucleotide-free chaperones [33].
M.-P. Stein et al. / Advanced Drug Delivery Reviews 55 (2003) 1421–1437 1427
Evidence for a functional role of nonnucleotide bound
rab proteins in endocytosis derives from recent evi-
dence demonstrating that rab15 function requires Mss4
binding and that Mss4 is a rab15 effector [34]. Func-
tional roles for other rab proteins bound to nucleotide-
free chaperones will undoubtedly be uncovered. Fur-
thermore, understanding how cycling from GDP- to
nonnucleotide- to GTP-bound states is regulated will
be critical for determining how and when conforma-
tional changes in the rab proteins allow for functional
interactions with downstream effector molecules.
GTP hydrolysis leads to the cycling of rab proteins
from their active GTP-bound to inactive GDP-bound
state. Although all rab proteins contain some intrinsic
GTPase activity, the rate of phosphate hydrolysis is
greatly enhanced by the presence of GTPase activating
proteins (GAPs). In mammalian cells, cytosolic GAP
activity has been demonstrated to exist for a number of
rab proteins. However, only two mammalian rab
GAPs have been definitively identified: GAPCenA,
which has sequence similarity to a yeast Ypt/rab GAP
[35]; and Rab3-GAP, which has no sequence similar-
ity to any known GAP proteins [36]. Acceleration of
GTP hydrolysis by GAPs results from the insertion of
a common arginine finger motif found in yeast Ypt/
rabs, ras, and rho, into their substrate [37]. Rates of
hydrolysis up to 1000-fold over baseline have been
observed.
The factors regulating nucleotide exchange and
hydrolysis appear to be highly specific for individual
rab proteins. The exchange of GTP for GDP by GEFs
provide the molecular ‘‘on-switch’’ for rab proteins
while the hydrolysis stimulated by GAPs provides the
‘‘off-switch’’. These opposing reactions therefore play
a crucial role in regulating the temporal dynamics of
vesicular membrane trafficking. Thus, these proteins
may be attractive therapeutic targets to enhance or
diminish rab protein activity and modulate select endo-
cytic trafficking pathways.
2.4. Regulated function through specific effectors
Rab proteins serve to promote trafficking by coor-
dinating the individual steps in vesicular transport. In
this context, the rab proteins can be viewed as molec-
ular scaffolds that promote temporal and spatial control
through their compartment-specific localization and
activation. Rab proteins also serve in cargo selection
and as interfaces to intracellular signaling cascades. In
this context, rab proteins may control the fate of
individual cargo molecules by interacting with cargo
directly or via specialized effectors that are induced in
response to signaling. Examples of rab proteins and
their effectors serving in each of these capacities are
detailed below.
Principally, the interaction of rab proteins with
effector molecules facilitates membrane transport
through a coordinate assembly process. The rab pro-
teins serve as molecular scaffolds for the sequential
recruitment of proteins and lipids that are required for
transport vesicle budding, cytoskeletal motility, and
finally vesicle tethering, docking, and fusion with the
target membrane. Effector molecules for a number of
rab proteins have been identified and characterized
(Table 2). The best-characterized set of rab effector
proteins are those that interact with rab5. Analysis of
the rab5 effectors suggest a general model by which
interactions between rab proteins and effector mole-
cules may lead to the formation of macromolecular
scaffolds for efficient and accurate membrane trans-
port [5].
Rab5 effector interactions are tightly coupled to rab
activation. GTP-rab5 initially recruits regulatory com-
plexes of rabaptin-5 and rabex-5 to early endosomes.
Rabex-5 acts as a rab5 nucleotide exchange factor and
rabaptin serves as an accessory factor for coated
vesicle formation [38], as well as vesicle tethering
[39]. The GTP-rab5 bound to early endosomes also
recruits the p150/hVPS34 complex to early endo-
somal membranes by binding the p150 adapter pro-
tein [40]. Once membrane associated, hVPS34, a
phosphatidylinositol (PI) 3V-kinase, generates PI 3V-phosphate (PI3P) in the plane of the early endosomal
membrane. The newly formed PI3P microdomains
regulate endosome motility on microtubules [41] and
provide platforms for assembly of the fusion machin-
ery. Molecules containing the FYVE domain specif-
ically and preferentially bind to PI3P [42,43]. Two
such FYVE-domain containing proteins specifically
recognize PI3P on early endosomes and promote ve-
sicle docking and fusion [44–46]. EEA1, early endo-
somal autoantigen 1, is a tethering molecule that
interacts with syntaxins 6 and 13 to establish vesicle
contact with the target membrane [47]. Rabenosyn-5
is required for SNARE complex formation and thus
promotes fusion [48]. The assemblies of macromo-
Table 2
Endocytic rab effector proteins
Rab Rab effector Function References
Rab4-GTP Rabip4 Implicated in
retrograde transport
from REa to SE
[106]
Rabaptin-5 Coordinates
endocytosis and
recycling from EE
[107]
Rabaptin4 Implicated in
transport from EE
to RE
[108]
RCP Implicated in
recycling
[109]
Rab5-GTP EEA1 EE homotypic
tethering and fusion
[110]
Rabaptin-5 Forms a stable
complex with
Rabex-5; implicated
in membrane
docking and fusion;
recently shown to
participate in clathrin
adapter binding
[38,111]
Rabaptin-5h Implicated in fusion [112]
Rabenosyn-5 Required for CCV–
EE and EE–EE
fusion
[48]
p110h Class I PI3K
catalytic subunit
[113]
PI3K
(hVPS34)
Class III PI3K [113]
p150 Class III PI3K
adapter protein for
hVPS34; serine/
threonine protein
kinase
[40,114]
RIN2 Guanine nucleotide
exchange
[115]
Rab5-GDP RIN1 Guanine nucleotide
exchange
[52]
Rab7-GTP RILP Regulates LE to
lysosome transport
through interactions
with dynein
[11]
Rab9-GTP p40 Stimulates transport
of M6PR from LE to
TGN
[116]
TIP47 Bind to both M6PR
and Rab9-GTP;
cytosolic recognition
factor for M6PR
[117]
Rab11-GTP Rip11 Required for ARE to
apical PM transport
[118]
RCP Homologue of Rip11 [109]
Table 2 (continued )
Rab Rab effector Function References
Rab11-GTP Eferin EF-hand domain
containing protein,
implicated in rab11
localization
[119]
Rab11BP
(Rabphilin 11)
Implicated in vesicle
recycling
[120,121]
Rab11-GDP,
Rab11-GTP
Rab11-FIP Implicated in
recycling; homo- and
heterodimerization to
create protein
platforms
[122]
Rab34-GTP RILP Regulates
lysosomal
positioning
[104]
a ARE, apical recycling endosome; CCV, clathrin-coated vesicle;
EE, early endosome; Eferin, EF-hands-containing Rab11-interacting
protein; Rab11-FIP, rab11 family-interacting protein; LE, late
endosome; M6PR, mannose 6-phosphate receptors; PI3K, phosphoi-
nositol-3V-kinase; RCP, rab coupling protein; RE, recycling endo-
some; SE, sorting endosome; TGN, trans-Golgi network.
M.-P. Stein et al. / Advanced Drug Delivery Reviews 55 (2003) 1421–14371428
lecular complexes on clathrin-coated vesicles and
early endosomes are triggered by rab5 activation.
Together, rab5, phospholipids, and a variety of rab
effector molecules contribute to the coordinate control
of early endocytic trafficking and culminate in regu-
lated endosomal fusion. These events provide a par-
adigm for other intracellular membrane trafficking
and fusion events. The directionality and specificity
of transport relies on the specific interactions between
rab proteins and their effector molecules while the
temporal specificity of these interactions relies on the
nucleotide cycling of the rabGTPases.
New data document rab protein function in cargo
selection, offering unprecedented opportunities for
modulating the fate of specific molecules. Rab pro-
teins may directly interact with some cargo molecules,
as is the case for the interaction between rab3b and
polymeric IgA receptor [2,49]. Alternatively, rab
proteins may interact with specific cargo via a unique
intermediary, as illustrated by the requirement for
TIP47, which mediates the interaction between rab9
and the mannose 6-phosphate receptor [50,51]. Cargo
selection may also depend on interfaces with intracel-
lular signaling cascades that can serve to increase or
decrease rab protein activity. The regulation of EGFR
endocytosis illustrates the complexity of such stimuli.
Ras activation induced by EGFR may on the one hand
M.-P. Stein et al. / Advanced Drug Delivery Reviews 55 (2003) 1421–1437 1429
increase endocytosis and promote receptor downregu-
lation. Endocytosis, in this case, is stimulated by
Rin1, a protein with rab5 GEF activity that converts
rab5 to its active form [52]. On the other hand, ras
activation may also attenuate endocytosis. In this case,
rab5 is inactivated by the action of a specific rab5
GAP, RN-Tre, which is recruited by the activated ras
effector Eps8 [53]. Such opposing activities are most
probably temporally balanced during normal signal-
ing, but may be aberrant in some disease states where
EGFR is constitutively active.
In summary, selective manipulation of rab proteins
or their effectors offers the potential to disrupt specific
transport steps and preferentially shunt cargo for recy-
cling or degradation. In addition, as we learn more
about specialized cargo selection pathways, it is con-
ceivable that the trafficking of individual receptors may
be precisely controlled and modulated for therapeutic
benefit.
3. Altered rab proteins in disease
Increasingly, rab proteins and associated regulatory
molecules or effectors are shown to be altered in
human disease. The alterations may be associated
with a loss of function, in which case, an underlying
germline mutation is frequently responsible. Loss of
function mutations in the rab protein or associated
regulatory molecules and effectors frequently have
similar phenotypic outcomes. Rab protein overexpres-
sion or aberrant activation, triggered by somatic
mutation or altered signaling, also underlie a number
of disease states.
3.1. Loss of function mutations in rab proteins, rab
regulatory molecules, or rab effectors
Genetic mutations affecting rab proteins and asso-
ciated regulatory molecules have recently come to
light. To date, two diseases have been characterized
with causal mutations in rab genes, while others
impact accessory factors as detailed below.
The first human disease identified to result from a
mutation of a rab gene was Griscelli syndrome type 2
(GS2). GS2 is a rare autosomal recessive disorder
originally described in 1978 [54]. Patients exhibit
immune impairment and increased susceptibility to
infections due to defects in T cell cytotoxicity and
cytolytic granule release. Partial albinism results from
the accumulation of melanosomes in melanocytes.
Hemophagocytic syndrome is caused by hyperstimu-
lated T cells and macrophages [55]. The genetic
defects responsible for GS2 include three distinct
missense mutations in highly conserved residues and
numerous microdeletions or larger deletions in
RAB27A on chromosome 15q21[55]. Rab27a has
been shown to be critically important for the transport
and release of melanosomes, a form of secretory
lysosome, from melanocytes [56]. Similar defects in
secretory lysosome release from immune cells and
platelets account for the bleeding disorders and im-
mune dysfunctions associated with the disease [57,58].
A related syndrome, Griscelli syndrome type 1
(GS1), is also characterized by partial albinism but
exhibits a primary severe neurological impairment
without deficits in immune function [59]. The primary
defect in GS1 was localized to MYO5A. MYO5A en-
codes an unconventional myosinVa motor protein [60].
Mouse coat-color variants equated with GS1 (dilute
[61]) and GS2 (ashen [58]) originally helped to define
the intracellular transport pathway ofmelanosomes. An
additional mouse model, the leaden mouse, identified
melanophilin [62] as a rab27a-interacting partner that
mediates the recruitment and binding of myosinVa to
actin [63,64]. Thus, human and mouse studies have
elucidated the components and fundamental mecha-
nisms requisite for secretory lysosome transport and
release, demonstrating that phenotypically similar dis-
eases may arise due to disruptions of multiple compo-
nent on the same intracellular pathway.
Recently, a second genetic disorder was pinpointed
to defects in a rab gene. Mutations in the RAB7 gene
cause Charcot–Marie–Tooth type 2B neuropathy
[65]. The disease is marked by sensory and motor
neuron impairment, distal muscle weakness and atro-
phy, and ulcerations often requiring amputation. Mis-
sense mutations in RAB7 were localized to exons 3
and 4, where either a C to T transition led to substi-
tution of Leu129 for Phe or a G to A transition resulted
in mutation of Val162 to Met [65]. The Val162 residue
is highly conserved in all species and Leu129 is
localized adjacent to the GTP-binding domain of
rab7. As such, both mutations are predicted to disrupt
rab7 function. A second form of Marie–Charcot–
Tooth disease (MCT4B1) results from a genetic defect
M.-P. Stein et al. / Advanced Drug Delivery Reviews 55 (2003) 1421–14371430
in myotubularin-related protein 2 [66], a dual speci-
ficity phosphatase required for PI3P and PI3,5P2metabolism [67]. A third form of Marie–Charcot–
Tooth disease (MCT2A) was previously characterized
based on missense mutations in KIF1B [68]. The
KIF1B gene encodes a kinesin motor protein thought
to play a crucial role in synaptic vesicle transport. By
analogy with Griscelli syndrome, it seems likely that
rab7, myotubularin, and KIF1B are also part of a
common molecular pathway such that disruption of
any one of the genes causes a similar disease pheno-
type that is manifested as peripheral neuropathy. In this
context, it is of interest to consider our observations
demonstrating that rab7 and a PI 3V-kinase, hVPS34,coordinately control late endocytic transport*. The
rab7-regulated formation of PI3P on late endosomes
may facilitate kinesin-regulated membrane recycling,
particularly in motor neurons. Turnover of PI3P may
subsequently be temporally regulated by myotubu-
larin-related protein phosphatase, providing a molec-
ular ‘‘off-switch’’ for the kinesin-mediated vesicular
transport. Further characterization of the intracellular
pathways governed by rab proteins and identification
of rab effector molecules will undoubtedly uncover
additional molecules that are mutated to cause a
variety of human diseases. Consequently, effective
gene therapy strategies for reconstitution of these
molecules will be increasingly important.
In the case of rab27, the disease etiology matches
the expression profile of the protein. Rab27 is highly
expressed in keratinocytes, platelets, and lymphocytes,
where it promotes secretory lysosome fusion. In the
absence of rab 27, secretory lysosome fusion is dis-
rupted, leading to bleeding and pigmentation disor-
ders, as well as immune dysfunction. In contrast, rab7
is a ubiquitous rab that controls transport between
early and late endosomes. Determining how mutations
in rab7 result in sensory and motor neuropathy and
identifying how the loss of rab7 function is compen-
sated for in other cells, tissues, and organs remain
critical unresolved issues for understanding the com-
plexities of rab function in vivo.
Genetic defects in rab regulatory molecules are
associated with retinal degeneration in choroideremia,
X-linked mental retardation, and kidney disease in
tuberous sclerosis [69–71]. Choroideremia and X-
linked mental retardation result from germline muta-
tions in general regulatory factors that impact the
membrane association of rab proteins. Altered regula-
tion of rab5 nucleotide hydrolysis promoted by dis-
crete cofactors has been implicated in both prostate
cancer and tuberous sclerosis. In the case of tuberous
sclerosis, this is precipitated by germline mutation of
the cofactor’s coding sequences, while in the case of
prostate cancer, the cofactor protein is highly overex-
pressed. The nature of a causal mutation in prostate
cancer has not been defined, but may arise from
somatic rearrangements.
Choroideremia, a form of retinal degeneration due
to loss of retinal epithelium, choroids, and retinal
photoreceptor cells, ultimately causes blindness in
affected individuals [30]. A number of point mutations
in rab escort protein1 (REP1), as well as translocations
that disrupt REP1 gene expression, have been de-
scribed [72]. Identification of two REP genes, REP1
and REP2, suggest that functional redundancies in rab
regulatory molecules may exist. However, tissue-spe-
cific expression of REP1 or the specificity of particular
rab proteins for REP1 could result in an absolute
requirement for REP1 in a given tissue. This appears
to be the case in retinal epithelia, where loss of REP1
activity results in retinal degeneration and loss of
eyesight in affected individuals. Functional redundan-
cy through the expression of REP2 may partially
compensate for loss of REP1 activity, except where
REP1 activity is absolutely required.
X-linked nonspecific mental retardation (MRX) has
also been associated with genetic defects in a rab
regulatory molecule, rabGDIa. Both a truncation mu-
tation in rabGDI (MRX48) and a T to C transition that
resulted in a nonconservative amino acid change
(Leu92 to Pro) in GDIa were described in two distinct
families with MRX [73]. RabGDIa is critical for
neurotransmitter release and rab3a recycling in the
brain, and the introduction of proline at position 92 in
rabGDIa decreased the affinity of rabGDIa for rab3a.
Consequently, this mutation might cause inefficient
recycling of rab3a from synaptosomes, possibly lead-
ing to mental defects. Furthermore, rabGDIa has been
implicated in neuronal development based on expres-
sion at embryonic day 9 and its requirement for axonal
extension [73]. Expression of additional rabGDI pro-
teins and promiscuous binding of rab proteins to
rabGDIs must allow for sufficient recycling of rab
proteins in tissues and organs outside the brain. Similar
to the case of REP2, expression of additional rabGDI
M.-P. Stein et al. / Advanced Drug Delivery Reviews 55 (2003) 1421–1437 1431
proteins does not compensate for loss of rabGDIa
expression in brain.
Genetic defects in less well-characterized, rab-re-
lated proteins may also result in severe disease. One
example, tuberous sclerosis (TSC), is an autosomal
dominant disease with a variety of manifestations
including the formation of benign tumors called
hamartomas, mental deficits that include behavioral
and learning difficulties, and renal complications in-
cluding but not limited to renal lesions. Genetic defects
in two distinct loci, TSC1 and TSC2, account for the
majority of cases. TSC1 encodes hamartin, a 130-kDa
protein of unknown function while TSC2 encodes
tuberin, a protein that stimulates GTP hydrolysis on
rab5 and rap1 and thus behaves like a GAP [74].
Hamartin and tuberin interact with one another in vivo
and act as tumor suppressor genes [75,76]. Defects in
either TSC1 or TSC2 are predicted to disrupt the
intracellular complex formed by these proteins, there-
by inhibiting the unknown activity of hamartin. A role
for the tuberin/hamartin complex in intracellular trans-
port was suggested by recent findings that polycystin-
1, an integral membrane component of basolateral
adherens junctions, was mislocalized to the Golgi in
cells deficient for TSC2 expression [77]. Further detail
regarding the functions of tuberin and hamartin in
vivo, as well as their relationships to rab5 and other
components of the endocytic regulatory machinery, is
essential to gain clues about the molecules that are
most acutely affected by the defects in TSC.
A role for rab proteins in the transport and mainte-
nance of cellular cholesterol and lipids was recently
revealed. Niemann–Pick type C (NPC) disease is a
genetic disorder in which the accumulation of lipids in
late endosomes ultimately causes a severe neurode-
generative disorder resulting in premature death. The
vast majority of NPC cases result from genetic muta-
tions in NPC1, while a small percentage result from
mutations within a second gene, NPC2 [78]. These
genes encode proteins required for lipid trafficking,
although the precise molecular mechanisms have not
yet been elucidated. Overexpression of rab7 or rab9 in
NPC cells alleviated cholesterol accumulation and
restored transport of several glycospingolipids, lacto-
sylceramide, and GM1 to the Golgi [17]. Importantly,
the motility of rab7-containing late endosomes is
directly altered by cholesterol accumulation, possibly
through the inhibition of motor protein binding [18].
These results suggest that rab7, cholesterol, kinesin
motor proteins, and the NPC1 and NPC2 proteins may
interact on late endosomes, resulting in the proper
transport and maintenance of cellular lipids. A greater
understanding of these lipid transport processes will be
invaluable, providing information both about the NPC
disease process as well as identifying molecules re-
quired for lipid homeostasis. With increased under-
standing, manipulation of cholesterol and lipid
transport processes may provide therapeutic benefit
to those suffering from lipid storage diseases.
3.2. Altered rab expression or activation in disease
As discussed above, many proteins participate in
the regulation of rab protein expression and localiza-
tion and genetic defects in rabs or their regulatory
molecules can result in disease. In addition, control
over the levels of rab protein expression must exist.
Overexpression of requisite endocytic rab proteins or
regulatory proteins has been associated with human
thyroid, vascular, and lung diseases, as well as some
cancers [4,14,24,26,27]. Overexpression may be pre-
cipitated by somatic rearrangements as in the case of
some prostate cancers or may arise in response to
sustained stimuli from intracellular signaling.
Thyroid hormone production requires the uptake
and processing of thyroglobulin (Tg) from apical
extracellular colloidal stores, endocytosis and pro-
cessing of Tg in late endosomes and lysosomes,
and transport and release of mature thyroid hormone
at the basal surface. Thyroid autonomous adenomas
(AA) are benign tumors of the thyroid associated
with elevated levels of rab5a and rab7 and decreased
levels of follicular Tg [14]. Membrane association of
both rab5a and rab7 are also increased in AA,
suggesting that rab5 and rab7 are both activated in
AA. Increased expression of rab5a and rab7 increase
the rate of thyroglobulin endocytosis and processing
in response to elevated cAMP [14]; however, it
remains to be determined if the enhanced processing
of Tg results in tumor formation or if additional
factors are involved.
Alterations in endo- and exocytic rab protein
expression have also been revealed in several models
of human disease and in a variety of cancers. Upre-
gulation of rab1a, rab4, and rab6, and altered Golgi
morphology were observed in a h2-adrenergic recep-
M.-P. Stein et al. / Advanced Drug Delivery Reviews 55 (2003) 1421–14371432
tor model of cardiomyopathy [23], while increased
expression of rab7 was observed in a rabbit model of
atherogenesis [27]. Similarly, in a mouse model for
lung tumor progression, increased expression of rab2
was detected [24] whereas in prostate cancer cell
lines, alterations in rab25 expression were noted
[25]. Recent work identified six rab and three Arf/
Sar proteins that are upregulated in human liver
cancers, including hepatocellular carcinomas and
cholangiohepatomas [26]. Thus, alterations in expres-
sion of a variety of rab proteins, as determined by
gene expression profiling, suggest that rab proteins
play a multitude of roles in maintaining normal
cellular physiology.
The novel prostate cancer gene 17 (PRC17) pro-
vides an example where overexpression of a rab
regulatory factor decreases rab activation and results
in disease. PRC17 was recently identified from a panel
of prostate tumors to contain a GTPase-activating
domain that can interact with rab5 [4]. PRC17 was
shown to be highly upregulated in metastatic prostate
tumors to transform mouse fibroblast 3T3 cells and
when mutated in the GAP domain, to lose its trans-
forming capability [4]. These results demonstrate that
the GAP activity of PRC17 is responsible for its
oncogenic activity and suggest that upregulation of
GAP activity might alter rab5 and/or rap1 activity in
human prostate disease. Thus, defects or alterations in
rab proteins or their effectors may account for a number
of diseases in which the molecular mechanisms have
yet to be identified.
4. Therapeutic targets
Based on the prevalence of altered rab protein
expression and/or regulation as an underlying cause
of human disease, it is of significant interest to consider
the therapeutic potential of modulating rab protein
function. There are several considerations of import
in this regard. The first issue pertains to the requirement
for cell- or tissue-specific targeting. This is an impor-
tant consideration for ubiquitous rab proteins, which
may exhibit altered function only in select tissues as
well as for rab proteins expressed in a tissue-specific
manner. The second issue pertains to transient versus a
more permanent modulation of expression. For exam-
ple, in the treatment of some cancers or thyroid ade-
nomas, it may be sufficient to transiently downregulate
rab protein expression, while in the case of genetic loss
of function diseases, it will be crucial to have sustained
and permanent reconstitution of protein function. Fi-
nally, it is important to consider therapeutic interven-
tions that can restore rab protein function, as well as
those that can block function. Such situations arise
when the trafficking of specific cargo may need to be
modulated.
4.1. Modalities for stimulating rab protein expression
and/or function
As discussed above, rab proteins may be regulat-
ed at the level of expression or activation. Changes
in expression or activation may be achieved either
by overexpressing specific genes or by modulating
signaling.
Rab proteins that function on constitutive endocytic
pathways are continuously cycling between active and
inactive states. Nevertheless, interfaces with intracel-
lular signaling cascades afford considerable plasticity
in membrane trafficking. Signaling may result in en-
hanced rab5 and rab7 protein expression and activation
with notable enhancements caused by cAMP [14],
interferon g [20], and ras activation [79]. Conversely,
signaling via the stress-induced MAP kinase p38
promotes rab5–GDI association and thus decreases
endocytosis [80]. Intracellular signaling may also af-
fect the fate of internalized cargo by altering transport
along specific rab5- or rab4-regulated pathways, as is
the case for EGF-regulated trafficking of its receptor
[52,53] and PDGF-regulated integrin recycling [81].
Therefore, treatments that stimulate or interfere with
these signaling cascades present one avenue for ma-
nipulating endocytic transport and rab function.
Rab protein and regulatory protein overexpression
may be achieved using available gene therapy strate-
gies. However, caution must prevail since overexpres-
sion can in some cases have deleterious effects and
result in disease. Rab5 might be modulated to affect
immune cell function and increase phagocytosis and
intracellular killing, while enhancing rab7 function
may have utility in treating lipid storage diseases such
as Niemann–Pick type C disease, stimulating bone
resorption by osteoclasts [82] and treating Charcot–
Marie–Tooth neuropathy. Upregulation of both rab5
and rab7 function might be used to enhance EGFR
M.-P. Stein et al. / Advanced Drug Delivery Reviews 55 (2003) 1421–1437 1433
downregulation, and slow tumor growth and upregu-
lation of rab4 function might enhance plasma mem-
brane aVh3 integrin recycling and promote cell
adhesion and differentiation. Reconstitution of rab27
or associated cofactors could restore secretory lyso-
some function in melanosomes, platelets, and immune
cells. Similarly, reconstitution of the rab regulatory
proteins REP1 and GDIa may ameliorate choroider-
emia and X-linked mental retardation. Thus, a variety
of possibilities exist for therapies that increase rab
expression or activity with the caveat that overexpres-
sion must be carefully controlled and/or transient
where possible.
4.2. Modalities inhibiting rab protein expression and/
or function
Diseases, such as certain cancers, lung, and vascular
diseases, where rab protein expression is elevated, will
require careful analysis to pinpoint which pathway or
rab protein is central to disease pathogenesis. Once
clarified, small interfering RNA (siRNA) approaches
may be combined with adenoviral vectors to reduce the
expression of the ‘‘offending’’ rab protein. In the case
of thyroid adenomas, the elevated expression of rab5
and rab7 might be reduced with specific antagonists of
cAMP-mediated signaling. In prostate cancer, reduc-
tion of PRC17 expression via siRNA or inhibition of
function with specific inhibitors of its GAP activity
may block metastatic spread.
In some cases, diseases result from the improper
sorting and trafficking of key cargo molecules along
rab-regulated pathways. For example, processing of
amyloid precursor protein to its plaque-producing
products by g secretase is enhanced by treatments that
inhibit cholesterol transport. Thus, ensuring normal
cellular cholesterol levels and enhancing transport to
rab7-positive late endosomes may positively reduce
aberrant APP processing [83]. A number of cancers
result from the increased recycling of growth factor
receptors. Therefore, implementing therapies that re-
sult in receptor downregulation in place of recycling,
effectively shifting the balance between interconnected
pathways, would be expected to positively impact
outcomes. Such therapies could be achieved either by
selectively blocking rab protein function or by stimu-
lating cofactors that shunt cargo transport along the
desired pathway.
5. Concluding remarks
In summary, the rab GTPases and associated regu-
latory factors are frequent targets of mutation and/or
altered expression in a variety of human disease states.
Although our understanding of the molecular mecha-
nisms of rab protein function have made significant
progress in the recent past, research to identify and
characterize additional rab-interacting proteins that are
required for endocytosis will undoubtedly provide
crucial insights into disease processes. Treatments
may vary from gene therapy to small molecule inter-
ventions, geared toward restoring normal function or
modulating pathways central to normal physiology.
*Note: Data concerning rab7 and hVPS34 interaction
now published:M.P. Stein, Y. Feng, K.C. Cooper, A.M.
Welford, A.Wandinger-Ness, Human VPS34 and p150
are rab7 interacting partners, Traffic 4 (2003) 1–18.
Acknowledgements
This work was supported by the National Science
Foundation under grant number MCB9982161 to
A.W.N. Partial salary support was provided to MPS
through a grant from the University of New Mexico
Cancer Center.
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