the stability of the nuclear lamina polymer changes with the … · 2004-07-28 · summary the...

The Stability of the Nuclear Lamina Polymer Changes With the Composition of

Lamin Subtypes According to Their Individual Binding Strengths

Eric C. Schirmer and Larry Gerace

Department of Cell Biology, The Scripps Research Institute, La Jolla, CA 92037

Running title: Lamin Subtype Composition and Lamina Stability

To whom correspondence should be addressed:

Larry Gerace

10550 N. Torrey Pines Rd.

IMM10, R209

La Jolla, CA 92037

JBC Papers in Press. Published on July 27, 2004 as Manuscript M407705200

Copyright 2004 by The American Society for Biochemistry and Molecular Biology, Inc.

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Phone: (858) 784-8514

Fax: (858) 784-9132

e-mail: [email protected]

keywords: Lamin; intermediate filament; nuclear lamina; nuclear architecture; mechanical

stability.

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SUMMARY

The nuclear lamina, which provides a structural scaffolding for the nuclear envelope,

consists largely of a polymer of the intermediate filament lamin proteins. Although different cell

types contain distinctive relative amounts of the major lamin subtypes (A, C, B1 and B2), the

functions of this variation are not understood. We have investigated the possibility that subtype

variation affects lamina stability. We find that homotypic and heterotypic binding interactions of

lamin B2 are substantially less resistant to chemical dissociation in vitro than those between the

other lamin subtypes, while lamin A interactions are the most stable. Surprisingly, removal of the

central 4/5 of the rod domain did not substantially weaken the interactions of lamins A and B2,

suggesting that other regions also strongly contribute to their binding interactions. In contrast,

this rod deletion strongly destabilizes the binding interactions of lamins B1 and C. Consistent

with the binding studies, lamins are more readily solubilized by chemical extraction from cells

enriched for lamin B2 than from cells enriched for lamin A. This suggests that the distinctive

ensemble of heterotypic lamin interactions in a particular cell type affects the stability of the

lamin polymer, and, correspondingly, could be relevant to tissue-specific properties of the

lamina including its involvement in disease.

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INTRODUCTION

The nuclear lamina is a filamentous protein meshwork that lines the inner nuclear

membrane. Its core consists of a polymer of the intermediate filament (IF)1 lamin proteins which

binds to chromatin and connects to the inner nuclear membrane via integral membrane proteins

(reviewed in 1,2). Mutations in lamins and associated membrane proteins cause a variety of

debilitating human diseases (laminopathies) including muscular dystrophy, neuropathy, and

developmental disorders (reviewed in 3,4).

Differences are observed in the relative amounts of the major products of the three lamin genes

(encoding lamins A/C, B1, and B2) in different cell types and at different stages of development.

Lamins B1 and B2 are expressed throughout development, whereas lamins A and C, which are

splice variants differing in their C-termini, usually appear only near the time of, or following

differentiation in specific tissues of chicken and mammals (5-7). These different subtypes are

presumed to interact in vivo, since binding of A/C lamins to lamin B1 is observed in blot

overlays (8), column binding assays (9), and 2-hybrid analysis (10). However, the binding

between other pairs of lamin subtypes has not yet been examined. The observation that different

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1 The abbreviations used are: IF, intermediate filament; NE, nuclear envelope; WT, wild-type.

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nuclear envelope (NE) diseases preferentially affect certain tissues (3,4) may relate to the

distinctive expression patterns of lamins in different tissues and the specific biochemical

properties of each subtype.

Similar to other IF proteins, lamins contain a central α-helical rod domain composed of

~48 heptad repeats, flanked by N- and C-terminal head and tail domains. The rod domain

assembles into a parallel, unstaggered coiled-coil homodimer that is stable in 8 M urea. In vitro,

dimers of each major lamin subtype can assemble laterally into homotypic filaments by

staggered anti-parallel interactions (11-13). The five heptads at each end of the rod, particularly

those at the tail end, have been suggested to be critical for initiating the coiled-coil (14,15).

Fragments containing these terminal heptads together with either the head or tail can assemble

into dimers and tetramers in vitro, but not into filaments (14). Both charged and hydrophobic

residues on the outer surface of the dimer rod are thought to contribute to stabilizing filaments.

Although there are few details on how different subtypes assemble into the lamina in vivo, their

resistance to chemical extraction from cells and the low rates of exchange for lamins A and B1

fused to GFP (16) argue that they are stably integrated into a polymeric structure.

In this study we have used biochemical approaches to compare the properties of all four

principal mammalian lamin subtypes. We found that each pairing yielded a different binding

‘strength’ as defined by its resistance to chemical dissociation. Interestingly, we found that lamin

B2 had much weaker homotypic and heterotypic interactions than the interactions seen among

the other lamin subtypes and, correspondingly, all lamin subtypes were more readily extracted

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from cultured cells that were induced to express relatively high levels of lamin B2. Paralleling

this at the opposite extreme, lamin A interactions were the strongest among the different lamin

subtypes and lamins were less readily extracted from cells expressing high levels of lamin A.

Analysis of lamin mutants lacking the middle 4/5 of the rod domain suggested that this region

contributes significantly to the homotypic binding interactions of lamins B1, A and C, and that

lamin A contains additional binding sites unique to its C-terminus that are important for its

unusually strong interactions. These results suggest that changes in the ratio of lamins A and B2

during development or in different cell types could directly influence the dynamic properties of

the lamina and its functions as a structural scaffolding for the NE and chromatin.

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EXPERIMENTAL PROCEDURES

Plasmid Construction — Primers that added 5’ Bam HI/ Nde 1 and 3’ Not 1 sites were

used to amplify by PCR the coding sequences of human lamins A, C, and B1 and mouse lamin

B2. To produce deletion mutants lacking the middle 4/5 of the rod domain (referred to as in

text), these primers were used in conjunction with internal primers containing Hind III sites that

fused nucleotides 207 and 1017 for lamin B1 and the corresponding residues of the other lamins

via an added alanine codon. All genes were moved to pET28a (Novagen) for protein expression

and pHHS10B (which bears an HA epitope tag) for mammalian transfection.

Protein Purification — WT lamins and deletion mutants were purified from inclusion

bodies. The proteins were expressed in BL21-(DE3) cells by induction with 0.3 mM isopropyl-

1-thio-ß-D-galactopyranoside at A595 0.7 for 3 h at 37°C, collected by centrifugation, and

lysed by sonication in 25 mM HEPES pH 8.0, 0.1 mM MgCl2, 3 mM ß-mercaptoethanol

containing 1 mM phenylmethylsulfonyl fluoride (PMSF), 1 µg/ml aprotinin, 1 µM leupeptin, and

1 µM pepstatin. The pellets from a 20 min centrifugation at 27,000xg were washed with 1%

Triton X-100 and resuspended in 20 mM HEPES pH 8.0, 8 M urea, 3 mM ß-mercaptoethanol.

For further purification, this was incubated with nickel resin (Qiagen) for interaction of the 6x-

His tag from the pET28a vector and proteins were eluted with the same buffer containing 200

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mM imidazole. Proteins were then dialyzed into 20 mM Tris-HCl pH 8.0, 8 M urea, 2 mM DTT,

1 mM EDTA with protease inhibitors for storage.

Lamin Binding Assays — Purified WT lamins and deletion mutants were covalently

coupled individually to Affi-gel 10 or 15 (based on their PIs; BioRad) for a solid support as in

(9,17), except that ~3 mg was bound per ml of matrix. Matrices were pre-washed with the

buffers used in subsequent binding assays to avoid confusion of coupled versus bound protein

when testing homotypic interactions. Uncoupled lamins were diluted to 20 µg/ml and dialyzed

out of urea into binding buffer: 25 mM HEPES pH 7.4, 300 mM NaCl, 5 mM ß-

mercaptoethanol with protease inhibitors. This was centrifuged (16,000xg, 15 min) to remove

significantly polymerized lamins and incubated with the matrix overnight at 4°C. The columns

were washed in the same buffer and eluted with increasing concentrations of urea in 25 mM

HEPES pH 8.0, 500 mM NaCl, 5 mM ß-mercaptoethanol (4 column volumes each fraction).

Alternatively, matrices were eluted with 5 M NaCl followed by 8 M urea. Fractions were

analyzed by Western blotting with antibodies specific to each lamin subtype (17). Lamins in each

fraction were quantified using a PhosphorImage densitometer with ImageQuant software and

graphed as the percentage of the total eluted material. Although lamins were eluted in steps with

increasing urea concentrations, the median urea concentration (urea50) was calculated as if a

gradient had been used (i.e. if the median occurred 3/4 of the way between 6 and 8 M, a urea50

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value of 7.5 M was calculated).

Solubility Assays — The solubility of lamins was tested by similarly diluting and

dialyzing purified lamins into binding buffer, and then incubating overnight at 4°C. Samples

were then subjected to centrifugation at 16,000xg for 15 min to pellet significantly polymerized

material. Supernatants and pellets were equivalently loaded onto SDS-PAGE, resolved, and

analyzed by Western blotting and densitometry. Alternatively, lamins in the supernatants were

quantified using the Bradford protein assay compared to freshly diluted controls.

Salt and Detergent Extraction of Lamins — 293T cells were transfected with either lamin

A or lamin B2 constructs in the pHHS10B vector using polyfect (Qiagen) according to the

manufacturer’s instructions. Transfection efficiencies were measured at 30-40%. At 54 h post-

transfection, cells were recovered by pipetting, washed in PBS and divided into separate tubes

for extractions. Cells were resuspended and incubated on ice for 5 min in 25 mM HEPES pH 7.4,

450 mM NaCl, 3 mM MgCl2, 0.5 mM EDTA, 1 mM DTT, 0.4% Triton X-100. Insoluble

material was then separated by centrifugation at 9000xg in a microcentrifuge for 2 min and both

fractions resolved by Western blotting and densitometry.

Alternatively HL-60 cells were treated with 1 µM trans-retinoic acid (Calbiochem) from

a 1 mM stock dissolved in ethanol or 16 nM TPA (phorbol 13 mysistate acetate; Calbiochem)

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from a 160 µM stock dissolved in acetone. After 4 days, attached differentiating cells were

treated with trypsin-EDTA and all cells were counted and collected by centrifugation. Equal

numbers of cells lysed in SDS-sample buffer were analyzed by SDS-polyacrylamide gel

electrophoresis, western blotting with lamin subtype-specific antibodies, and densitometry to

determine the effects of drug treatments on relative lamin levels. Treated cells were also

extracted and analyzed as above to determine changes in the relative solubility of each subtype.

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RESULTS

Assay for Measuring Lamin Dimer-Dimer Interactions — To directly compare the

binding properties of the four major lamin subtypes, recombinant WT lamins and deletion

mutants that lacked the middle ~4/5 of the rod (Fig. 1A, ) were prepared from bacteria. Each of

the rod deletion mutants retained 5 heptads from both ends of the rod (indicated by /) that were

fused in register. These were predicted by computer algorithm to form a single coiled-coil of 10

heptads, and experimental support for this comes from biophysical analysis of the lamin B1

mutant that retained a strong α-helical signal in solution by circular dichroism and assembled

into filaments which had a similar Fourier transform infrared spectroscopy (FTIR) pattern as WT

filaments (17). In contrast to the short coiled-coil, the WT lamin dimers are predicted to have

four distinct coiled-coil helices formed by sets of 6 to 20 heptads each, separated by short

linkers. The ends of the rod are the most conserved regions among lamins (~77-100%

homologous), but the central portions of the rod, as well as the heads and tails are also highly

conserved (~53-71% homologous; Fig. 1B).

We directly compared the homotypic and heterotypic binding of recombinant lamins A,

C, B1, B2, and the homotypic binding of their respective mutants (Fig. 1C) using an in vitro

assay that preferentially measured the strength of dimer-dimer interactions. For this assay

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(modified from a previous method; References 9,17), each purified recombinant lamin was

covalently coupled to an agarose bead support in 7 M urea (a condition favoring homotypic

dimers) at a relatively high lamin concentration (~3 mg/ml). Separately, each lamin was diluted

and dialyzed out of urea into a “binding buffer” at a protein concentration that was too low to

support self-assembly into polymers (20 µg/ml), and then incubated with the immobilized lamin

beads. Thus, the assay would strongly favor the interaction of the soluble lamins (present at a

low concentration) with the immobilized lamins (present in excess). A large amount (~1 ml) of

matrix was used for each assay to reduce potential sampling errors associated with small sample

sizes. Matrices were washed and then non-covalently bound lamins were eluted stepwise in the

binding buffer containing increasing concentrations of urea. Fractions were analyzed by Western

blotting with subtype-specific lamin antibodies followed by densitometry. Each column assay

was repeated two or more times, yielding similar results. To facilitate comparison of the different

lamin subtypes, a calculation of the median urea molarity at which 50% of the total bound lamin

had been eluted (urea50) was used as an index of the binding ‘strength’.

Comparison of the Binding Strength Between Various Lamin Dimers — We first used

this approach to examine homotypic dimer-dimer interactions (Fig. 2). We found that the

majority of the lamin B2 homotypic linkages were disrupted at relatively low concentrations of

urea (urea50 2.9 M) (Fig. 2A). In contrast, ~2.5-fold higher urea concentrations were required to

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separate the lamin A (urea50 8.0 M), lamin C (urea50 6.9 M), and lamin B1 (urea50 7.6 M)

homotypic interactions (Fig. 2A). If 5 M NaCl was used to elute the matrices instead of urea,

only a small fraction of homotypically-bound lamins was released. However, nearly 10-fold

more lamin B2 was eluted (~10% of the total bound) as compared to lamin B1 (Fig. 2B). Thus,

in relation to lamin B1, lamin B2 homotypic interactions are substantially more susceptible to

disruption by chemical conditions that perturb hydrophobic associations (urea) as well as

electrostatic interactions (high NaCl). Since the coiled-coil dimer formed by the lamin rod is not

disrupted by high urea or salt, we speculate that the greater sensitivity of lamin B2 to elution by

these conditions results from disruption of binding interfaces between dimers, although we

cannot rule out the possibility that the tail domain of recombinant lamin B2 is misfolded, or that

the folding of the tail domain of lamin B2, rather than its binding interfaces per se, is selectively

disrupted by the urea elution. However, we note in this regard that the predicted

immunoglobulin fold domain of the lamin B2 tail (18,19) is as similar in sequence to the

homologous regions of lamin B1 and lamin A as the homologous region of lamin B1 is similar to

lamin A (residues 436-552, % identity/ % homology; A:B1 49/ 64, A:B2 54/ 66, B1:B2 47/ 63).

To confirm that the soluble lamins were not themselves significantly polymerizing or

aggregating during the course of the assays, the lamins were tested for their solubility under

assay conditions. Proteins were diluted into the assay buffer and incubated overnight as in the

column binding studies. Samples were then subjected to centrifugation and the percent of

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pelletable material determined. We found that the vast majority of all the lamin constructs

analyzed remained soluble (Fig. 3A), thus validating the binding assay by demonstrating that it

was measuring neither polymerization nor aggregation and arguing that the proteins are properly

folded.

Heterotypic interactions of lamin B1 with a mixture of A/C lamins were previously

shown to be stronger than the lamin B1 homotypic interactions (9), an observation we confirmed

in our assay (Fig. 3B). However, by analyzing lamins A and C separately, we further learned that

the lamin B1 heterotypic interaction with lamin A (urea50 8.0 M) is stronger than its interaction

with lamin C (urea50 6.8 M). When we examined the binding interactions of lamin B2 by this

method, we found that the heterotypic interactions of lamin B2/A (urea50 3.4 M), lamin B2/C

(urea50 3.3 M), and lamin B2/B1 (urea50 3.8 M) were substantially weaker than the homotypic

lamin A and lamin B1 associations (Fig. 3C). Nonetheless, they were slightly stronger than the

homotypic lamin B2 interactions (urea50 2.9 M).

Contributions of the Peripheral and Central Regions to Lamin Interactions The lamin

mutants are expected to maintain the same orientation of the heads and tails that occurs in the

WT dimers and thus could be used to analyze the relative contributions of the central 4/5 of the

rod versus more peripheral lamin regions for lamin-lamin interactions e.g. central rod:central rod

and head/tail:central rod interactions compared to head:tail and rod end interactions. We found

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that the A construct retained a strong homotypic interaction (Fig. 4; A, urea50 7.1 M) that was

quite similar to the homotypic interaction of WT lamin A (urea50 8.0 M; Fig. 2A). Even

interactions between WT lamin A and lamin A, which might be expected to be unstable due to

the differences in the lengths of the rod, were relatively strong (data not shown; urea50 6.5 M).

In contrast, the stability of lamin C homotypic interactions to extraction was drastically

diminished by the deletion of the central 4/5 of the rod (C), dropping from a urea50 of 6.9 M to

3.6 M (2.6 M for WT lamin C:C). Because lamins A and C are identical except in their

carboxyl-termini which are encoded by different exons (residues 567-572 of lamin C and

residues 567-664 of lamin A), the greater binding stability of the A versus C mutants indicates

that the unique C-terminal region of lamin A (residues 567-664) makes a significant

contribution to the relatively strong lamin A homotypic interactions.

Deletion of the central 4/5 of the rod had a significant impact on the binding stability of

lamin B1. The binding strength of the B1 construct was greatly reduced compared to the strong

WT homotypic interaction (Fig. 4; B1, urea50 4.5 M versus B1, urea50 7.6 M). The difference of

3.1 M between the WT lamin B1 and B1 urea50 values was similar to the 3.3 M difference

observed for lamin C. B2:B2 association was somewhat greater than that of the WT B2:B2

association (urea50 4.3 M versus 2.9 M; Fig. 4). This result raises the possibility that the central

4/5 of the rod domain present in WT lamin B2 has a modest destabilizing influence on the lamin

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B2 homotypic binding interactions. Alternatively, we cannot rule out the possibility that our

results arise from peculiarities with lamin B2 not seen with the other lamin subtypes, i.e., that the

column-immobilized lamin B2 is more available for binding interactions than WT lamin B2.

Effects of Lamin Subtype Composition on the Solubility of the Lamin Polymer in Cells

— The in vitro binding assays predict that a lamina enriched in lamin B2 in situ would be less

stable than one enriched in lamin A. Furthermore, if different lamin subtypes co-assemble into

the same filaments, then increasing the relative level of lamin B2 in the nuclear lamina of cells

would increase the sensitivity to chemical extraction of all endogenous lamin subtypes, whereas

increasing the relative amount of lamin A would reduce their extraction. To test this model,

lamin B2 or lamin A was transfected into 293T mammalian cultured cells, and the cells were

then analyzed by Western blotting for changes in the percent of each lamin subtype extracted

using a buffer containing high salt and nonionic detergent, which disrupts both hydrophobic and

electrostatic interactions. The percent of all lamin subtypes that were extracted was decreased in

cultures of cells transfected with lamin A as compared to controls, whereas the percent of

extracted lamins B1 and B2 was increased in cells transfected with lamin B2, as compared to

control cells (Fig. 5A). Our values significantly underestimate the effects of exogenously

expressed lamins on the solubility of the endogenous lamins, since only ~30% of the cells in the

populations analyzed were transfected. As cells were lysed at times when all transfected lamins

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were concentrated at the nuclear rim as seen by immunofluorescence staining (Fig. 5B), the

changes in the solubility of the transfected lamin is not due to its improper targeting or

aggregation. Similarly, all endogenous lamins retained their normal distribution, yet their

solubility was also altered. Untransfected cells yielded somewhat variable levels of solubilized

lamins from experiment to experiment, so after quantification by densitometry, the percent of

solubilized lamins from the transfected samples was normalized to the percent solubilized in a

non-transfected control sample of the same experiment. The averaged results from four

experiments (Fig. 5C) confirmed that overexpressing lamin B2 destabilizes the lamins to

chemical extraction, while lamin A stabilizes them.

As an alternative to changing the relative levels of lamins in the NE by overexpression,

we used a cell system where endogenous lamin levels are normally altered when the cells are

pharmacologically induced to differentiate (20). Treatment of HL-60 cells with retinoic acid

(RA) for granulocyte induction elevated the relative level of lamin B2, whereas treatment with

phorbol esters (TPA) for monocyte induction dramatically increased A/C lamins (Fig. 6A;

Reference 21). After 4 days, cells were analyzed for changes in the percentage of each

extractable lamin subtype by Western blotting. A notable increase in soluble lamins was

observed in the RA-treated cells (enriched in lamin B2) as compared to those treated with TPA

(enriched in A/C lamins). The chemical extractability of lamins in uninduced HL-60 cells was

intermediate between that of the RA- and TPA-treated cells (Fig. 6B and C). Changes in the

expression pattern of other lamina proteins or post-translational lamin modifications induced by

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the drug treatments could contribute to the lamin solubility differences in this experiment.

However, we propose that changes in the relative lamin levels are likely to underlie the solubility

changes, since both systems that we used for altering lamin levels in vivo correlated lamin B2-

enriched polymers with decreased resistance to chemical extraction compared to lamin A-

enriched polymers.

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DISCUSSION

Many IF protein families have undergone amplification and diversification in higher

organisms. Nearly 50 keratin subtypes form specific heterodimer pairs in different epithelial cell

types, each with a different stability to disruption by urea in vitro (22,23). Neurofilament H, M,

and L subtypes combine in different ratios that are suggested to determine axon diameter (24).

Lamins have no particular obligate pairing and can occur in any ratio. However, the distinctive

patterns of lamina composition in differentiated cell types and at various stages of development

are largely conserved between mouse (5,7), cow (25), and chicken (6), suggesting that each cell

type gains a functional advantage from its particular lamin ratio. We are investigating the

hypothesis that the unique biochemical properties of each lamin subtype and its heterotypic

associations are responsible for functional changes in the lamina. We find that each of the

possible pairings of the four principal mammalian lamin subtypes has a different interaction

strength, as defined by our column binding/urea elution assay, and that different regions of

different proteins contribute to this variation (Fig. 7). In particular, the homotypic and

heterotypic interactions of lamin B2 (the least studied mammalian subtype) are much less stable

than those of other lamin subtypes, which may explain why distinctive conditions were required

for in vitro assembly of chicken lamin B2 into filaments (26). The relative ‘weakness’ of lamin

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B2 correlated with an increased sensitivity to extraction from cultured cells using salt and

detergent. The reports that lamin B2 is expressed in most or all cell types of adult mice (27,28)

and that lamin B2 is essential in cultured cells (29) supports the hypothesis that unique

biochemical properties of lamin B2 related to this ‘weakness’ may be important for the functions

of some cells.

By using recombinant proteins and being able to analyze lamin A and lamin C separately,

our results provide a potential explanation for why one earlier study found the lamin A/C:lamin

B1 interactions to be stronger than lamin A/C:lamin A/C interactions (9), while another (8) found

no difference. We observed differences between the binding interactions of lamin A and lamin C,

attributed to the unique carboxyl-terminal extensions of these two proteins. Therefore,

differences in the ratio of lamins A and C between the preparations analyzed in the earlier studies

could produce differences in the average measured strength of binding. Our findings add to other

studies showing differences between lamins A and C: lamin C appears to follow a different

pathway from lamin A for integrating into the lamina (30) and may differ in its binding affinity

for emerin from lamin A (31).

Post-translational modifications clearly also contribute to lamina stability as

phosporylation drives lamina disassembly in mitosis (32-34) and lamins are also phosphorylated

during interphase (35,36). The binding studies presented here using recombinant proteins

measured the basal interaction without phosphorylation. Once a greater understanding exists of

which residues are modified and the timing of that modification in the cell cycle, it will be

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possible to test this additional layer of complexity in these binding studies. However, as S-phase

phosphorylation of lamins should have occurred in our in vivo assays, we do not expect our

general conclusions to be altered.

In our in vitro binding assays, we used the disruption of binding by the chaotrope urea as

an operational definition of interaction ‘strength’. This is a commonly used approach for studies

of IF proteins that are inherently insoluble (22,23). The strong α-helical signal from the rod

domain, which even is seen in the mutants that contain only 10 heptad repeats in circular

dichroism studies (17), indicates that circular dichroism is unlikely to be useful for determining if

unfolding of protein domains by the chaotrope contributes to the disruption of binding.

Nonetheless, we think the latter is unlikely because of the considerable homology between the

different subtypes in the core of the tail, which has been shown to form a stable all-ß

immunoglobulin-like fold in A/C lamins (18,19). Also, the validity of the in vitro binding data is

supported by its correlation to our observations on the chemical extractability of lamins from

cultured cells: in two different cell models, an increase in expression of the ‘weaker’ binding

lamin B2 increased the percentage of all lamin subtypes that are chemically extracted, whereas

an increase in levels of the ‘stronger’ binding lamin A led to a decrease in the extractability of all

lamin subtypes. Although it is possible that differences in integral membrane proteins that bind

lamins in the HL-60 model (21) also influence lamina solubility, this effect is likely minor due

to the much greater excess of lamins and the integral membrane proteins are not likely to be

altered in the second model system (transient transfections).

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We predict that the aggregate binding strengths for each unique ratio of lamin subtypes in

different cells will correlate to the mechanical stability of the lamina in each of those cell types.

Such a correlation has been made for the intermediate filament keratin proteins. Certain keratin

subtypes (K5/K14) that are the least soluble in urea are expressed in stratified epithelia while the

most readily dissociated (K8/K18) occur in the internal epithelia: the stratified epithelium is

believed to require greater mechanical stability than internal epithelia, a hypothesis borne out by

keratin mutations that cause blistering skin diseases (23).

The notion that lamin A promotes lamina stability is consistent with the observations that

lamin A disruption in cells results in altered nuclear morphology (37), nuclear herniations (38),

and greater susceptibility to mechanical stress (39). Loss of lamina stability is likely relevant to

laminopathies, as lamins exhibited increased solubility in fibroblasts from Emery-Dreifuss

muscular dystrophy patients (40). The functional advantage that lamin B2 confers could be a

relative reduction in the mechanical stability of the lamina that would be important for rapid

nuclear morphogenesis in proliferating cell types. Consistent with this suggestion, proliferating

basal epithelial cells and lymphocytes are rich in lamin B2 and poor in lamin A (27,28). Reduced

lamina stability may also be advantageous for tumorigenic cells that frequently undergo a

reduction in levels of lamin A (41,42). Though still providing structural support, such relative

‘instability’ compared to lamin A-enriched systems may additionally facilitate migration of

tumorigenic or immune cells by enabling nuclear shape changes. This is consistent with the

lamin B2-rich neutrophils that undergo extensive lobulation during terminal differentiation

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(which is blocked in the laminopathy, Pelger-Huet anomaly; 43). Decreased lamina “stability” in

neutrophils would also facilitate the "extrusion" of their nuclear contents that occurs during

pathogen invasion and is thought to be part of the immune defense mechanism (44).

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ACKNOWLEDGEMENTS

We thank K. Kanelakis and I.B. Chen for critical reading of the manuscript. This work was

supported by NIH postdoctoral fellowship F32 GM19085 to E.C.S. and NIH GM28521 to L.G.

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FIGURE LEGENDS

FIG. 1. Design and purification of mutants. A, The domain structure of lamins (drawn to scale

for lamin A). Lamins contain a short amino-terminal head domain (~33 amino acids) followed

by a coiled-coil rod domain (354 amino acids) and a carboxyl-terminal tail domain (~200-300

amino acids). The rod is predicted to contain four separate α-helices: coil 1A, 1B, 2A, and 2B of

6, 20, 6, and 16 heptads, respectively (/ delineates each heptad), that are separated by short,

flexible linker regions (filled boxes). The lamin mutants lack the central 4/5 of the rod, retaining

only 5 heptads at each end that are fused in register to form a single coiled-coil of 10 heptads. B,

The % identity/ % homology for the head, first five rod heptads, central rod, last five rod

heptads, and tail is indicated with AV, VLI, ST, DE, QN, RK, or FY counted as homologous

residues. Because lamins A and C are identical except for the last 6 residues of lamin C and the

last 98 residues of lamin A, the calculations for lamin C are given separately in parenthesis. C,

The quality of the protein preparations used in this study was examined by coomassie staining of

a 10% SDS-polyacrylamide gel. Molecular weights of protein standards are designated on the

right.

FIG. 2. Binding interactions of different lamin subtypes. One lamin was covalently bound to a

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solid phase matrix at high concentration. This was incubated with a second lamin at low

concentration in a high salt buffer to favor binding while minimizing polymerization. After

washing, bound lamins were eluted with the increasing concentrations of urea indicated. The

percentage of the total material eluted in each fraction is plotted and the median indicated

(urea50 in text). A, Lamins A and C exhibited the strongest homotypic interactions, eluting between 6

and 8 M urea. Lamin B1 exhibited relatively strong interactions (4-8 M); however lamin B2

homotypic interactions were notably weak (2-4 M). B, Most homotypically bound lamins

resisted extraction by 5 M NaCl. Nonetheless, the percent eluted (black) was ~10x greater for

lamin B2 than for lamin B1.

FIG. 3. Lamin heterotypic interactions. A, To affirm that the soluble lamins in the column-

binding assay were not undergoing significant self-assembly, they were incubated in the same

binding buffer for the same amount of time and at the same concentration used in binding assays

(20 µg/ml). Higher and lower concentrations were also tested. The samples were then subjected

to centrifugation to pellet polymerized lamins and the supernatants were assayed for protein

concentration compared to the starting concentration. The percent of material remaining soluble

is presented. B, The assay system in Figure 2 was used to test heterotypic binding. Lamin B1

interactions with lamin A were stronger than its interactions with lamin C. C, Lamin B2

heterotypic interactions were similarly analyzed. All lamins tested were observed to slightly

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stabilize lamin B2 as indicated by the increase in the urea50 value for the heterotypic interaction

(line and number marked in black) versus the homotypic interaction (line and number marked in

gray).

FIG. 4. Contribution of the central 4/5 of the rod to binding. The urea50 value for lamins is

indicated by a line and number marked in black, and, for comparison, the value for WT lamins

from Figure 2 is indicated by a line and number marked in gray. Both WT lamins A and C

exhibited strong homotypic interactions indicated by their retention on the column until the

highest concentrations of urea. However, deletion of the rod domain specifically reduced the

strength of C interactions. Stabilization of lamin B1 interactions depended more on the central

rod than for lamin A while lamin B2 interactions appeared to be destabilized by the central rod.

FIG. 5. Effect of expressing specific lamins on the stability of the lamina in cells. A, Lamin B2

and lamin A overexpression affect the endogenous lamin polymer in opposite ways. Lamins

were overexpressed in 293T cells, and cells were extracted at 54 h with detergent and salt. A,

Solubilized (s) and pelletable (p) material was analyzed by Western blotting with antibodies

specific for each lamin subtype. B, The exogenously expressed lamin targeted to the NE as

determined by immunofluorescence staining for its epitope tag. This indicates that the increased

solubility is not merely due to excess lamin. C, The percentage of soluble material from 4

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experiments calculated after densitometry is presented as a ratio to that of the non-transfected

control for each experiment. Bars indicate the calculated standard deviation.

FIG. 6. Effect of altering lamin ratios on the stability of the lamina in cells. A, Manipulation of

the lamin composition in a model differentiation system alters the solubility properties of the

endogenous lamin polymer. A, HL-60 cells were treated with retinoic acid (RA) or phorbol

esters (TPA). The lamin protein levels from equal numbers of cells were analyzed by

densitometry, revealing a 50% increase in lamin B2 with RA and a ~3-fold increase in A/C

lamins with TPA. B, After 4 days of induction, cells were collected and extracted with 450 mM

NaCl and 0.4% Triton X-100. Solubilized (s) and pelletable (p) lamins were analyzed with

subtype-specific antibodies. The full range of immuno-reactive species is shown: multiple post-

translationally modified species or degradation products appeared which were analyzed together

for C. Pelletable A/C lamins were loaded at 1/4 the level for TPA induced cells because of their

high levels. C, Densitometric analysis of the results of 4 experiments. The percentage of soluble

material is presented as a ratio to that of the untreated HL-60 cells for each experiment. Bars

indicate the calculated standard deviation.

FIG. 7. Schematic summary of lamin binding interactions. Binding strength of each lamin

pairing as measured by stability to extraction with urea is presented to scale by the urea50 value.

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Lamin B2 interactions were clearly ‘weaker’ than those of all other lamin subtypes.

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Eric C. Schirmer and Larry Geracesubtypes according to their individual binding strengths

The stability of the nuclear lamina polymer changes with the composition of lamin

published online July 27, 2004J. Biol. Chem.

10.1074/jbc.M407705200Access the most updated version of this article at doi:

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