be expected of a mouse model of Hunting-ton’s disease.

But the unexpected happened when thegene was switched off. Yamamoto et al. didthis by administering an antibiotic in the ani-mals’ drinking water; this antibiotic inhibit-ed an activator of the HD gene that was alsoinserted into the mice. In a group of the mice,the HD gene was kept switched on until themice were 18 weeks old. By this point, theanimals showed clear symptoms and neuro-logical features of the disease. Then, half ofthe group were continually given the anti-biotic (‘gene-off ’ group), while the other halfwere maintained with the gene switched on(‘gene-on’ group). After another 16 weeks, inthe gene-off group the nuclear staining andintranuclear and extranuclear polygluta-mine aggregates had disappeared from thestriatum, and were much reduced in the cor-tex (Fig. 1). These mice showed no furtherreduction in striatal size and the decrease in binding of dopamine to its receptors was slightly reversed, compared with the 18-week time point. But an equally remark-able finding was that the progression of themotor disorder had been reversed to levelsapproaching those of control animals.

This reversal of aggregate accumulationand neurological symptoms would havebeen difficult to predict. Polyglutamineaggregates are highly insoluble, and are dif-ficult to dissolve or denature biochemi-cally13,14. But Yamamoto et al. have shownthat the neuron is capable of dispensing withsuch structures. It seems likely that aggre-gates can be dismantled and made accessibleto the proteasome — the cell’s main protein-degrading machinery — and so can bedestroyed by common cellular mechanisms.

One open question in this area of researchis whether or not the protein aggregates areresponsible for the onset and progression ofthe disease. Although Yamamoto et al.1 haveshown that aggregation and symptoms arereversed in parallel in their mice, they cannotestablish a definitive, causal link between theaggregation pathway and the disease. Butdisease progression clearly depends on thecontinuous expression of the mutant HDgene.

What does all this mean for our ability toapply the brakes to Huntington’s disease inhumans? Turning off the mutant HD gene(and hence production of the mutant hunt-ingtin protein) reverses the disease symp-toms in the mice. So it seems that neither the soluble nor the aggregated form of theprotein irreversibly commits a neuron to die.At least the early symptoms are probablycaused not by neuronal death, but rather by a neuronal malfunction that can be reversed.So, therapeutic approaches to reduce theexpression of the mutant protein or to blockany of the steps that lead to the formation ofaggregates might not only prevent or delaythe onset of the disease. They may also offer a

cure — if used when neuronal malfunctioncan still be turned around. ■

Gillian Bates is in the Division of Medical andMolecular Genetics, GKT School of Medicine, King’sCollege, 8th Floor Guy’s Tower, Guy’s Hospital,London SE1 9RT, UK.e-mail: gillian.bates@kcl.ac.uk1. Yamamoto, A., Lucas, J. J. & Hen, R. Cell 101, 57–66 (2000).

2. Harper, P. S. Huntington’s Disease (Saunders, London, 1996).

3. Huntington’s Disease Collaborative Research Group Cell 72,971–983 (1993).

4. Vonsattel, J.-P. et al. J. Neuropathol. Exp. Neurol. 44, 559–577 (1985).

5. DiFiglia, M. et al. Science 277, 1990–1993 (1997).

6. Gutekunst, C.-A. et al. J. Neurosci. 19, 2522–2534 (1999).

7. Mangiarini, L. et al. Cell 87, 493–506 (1996).

8. Davies, S. W. et al. Cell 90, 537–548 (1997).

9. Li, H. et al. Hum. Mol. Genet. 8, 1227–1236 (1999).

10.Cha, J.-H. et al. Phil. Trans. R. Soc. Lond. B 354, 981–989

(1999).

11.Augood, S. J., Faull, R. L. & Emson P. C. Ann. Neurol. 42,215–221 (1997).

12.Gossen, M. & Bujard, H. Proc. Natl Acad. Sci. USA 89,5547–5551 (1992).

13.Scherzinger, E. et al. Cell 90, 549–558 (1997).

14.Kasantsev, A. et al. Proc. Natl Acad. Sci. USA 96, 11404–11409

(1999).

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NATURE | VOL 404 | 27 APRIL 2000 | www.nature.com 945

Cells do not exist in isolation from theirexternal environment, and need to beable to modify their activity according

to what is happening outside them. Molec-ular switches act to relay information from acell’s exterior to its interior. One set of molec-ular triggers are the Src-family kinases(SFKs), the best known of which, Src, lendsits name to the family. The SFKs regulate aplethora of events, including cell growth,division, differentiation, survival and death,as well as specialized functions such asimmune responses, the attachment of cells toextracellular matrix, cell movement, anduptake of molecules from the cell surface1,2.Unrestricted SFK activity can lead to cancer,but ordinarily SFKs are held in a stable, inactive state until needed.

The switch between the inactive andactive states of an SFK involves both confor-mational changes and modification by phos-phorylation — the addition of phosphategroups to particular amino-acid residues of the protein. But it is still unclear how thecell coordinates these events in response toextracellular stimuli1. A significant step for-ward has now been taken by Kawabuchi andcolleagues3. Writing on page 999 of this issue,they identify a new protein, called Cbp, thatlinks SFKs to a negative regulator.

The SFKs are a subset of enzymes knownas tyrosine kinases, which transfer phosphategroups onto the amino-acid residue tyrosinein proteins. A characteristic of a phosphory-lated tyrosine residue is its ability to bind aprotein region known as an SH2 domain.Non-phosphorylated tyrosine residues, incontrast, cannot bind SH2 domains. So, byphosphorylating specific residues, tyrosinekinases can mediate the binding of one pro-tein to another, or induce conformationalchanges within proteins that modify theiractivity.

Many tyrosine kinases span the mem-brane encapsulating a cell, and are activatedonly by binding of one specific extracellularmolecule. In contrast, SFKs associate with

the cell membrane but reside totally insidethe cell, and are activated in response to avariety of extracellular stimuli. An SFK is in an inactive state when it is itself phospho-rylated at an inhibitory carboxy-terminaltyrosine; the protein then folds up into aninactive conformation in which its inhibi-tory tyrosine residue binds to the protein’sown SH2 domain (Fig. 1a, page 947). TheSFK is activated when this phosphate isremoved by a phosphatase, or when anotherprotein binds and displaces these intramol-ecular interactions. These events result in adifferent tyrosine residue becoming exposedand phosphorylated, thus activating the pro-tein. Once active, the SFK can phosphorylateother proteins and then bind to themthrough its now exposed SH2 domain.

Phosphorylation of the inhibitory tyro-sine is done by another kinase, known as Csk(for carboxy-terminal Src kinase). And, atleast in lymphocytes (immune cells), Csk hasa second way of inactivating SFKs. In thesecells, Csk is bound to a phosphatase, calledPEP, that seems to specifically dephosphory-late the second (activating) phosphorylationsite in the SFK4. So, the Csk–PEP complexboth adds an inhibitory phosphate to SFKsand removes an activating phosphate.

Csk (and a closely related kinase of simi-lar function, Chk) is thus critical for keepingSFKs inactive. Indeed, removal of the Cskgene deregulates SFK activity5–7. But is Cskitself regulated in some way? Or is it always‘on’, ready to inactivate SFKs soon after theyhave been activated? Unlike SFKs, Csk is nottyrosine-phosphorylated at either inhibito-ry or stimulatory sites. However, it does con-tain an SH2 domain, and Kawabuchi et al.3

now provide compelling evidence that Cskuses this domain to relocate from the cytosolto the vicinity of active SFKs (that is, to themembrane). So, Csk might be regulated spatially more than catalytically.

Kawabuchi et al. find that Csk uses itsSH2 domain to bind to a new protein, Csk-binding protein (Cbp), when the latter is

Signal transduction

Molecular switches in lipid raftsLeslie A. Cary and Jonathan A. Cooper

© 2000 Macmillan Magazines Ltd

phosphorylated at a specific tyrosineresidue. Although Csk floats free in thecytosol, Cbp is firmly anchored in the mem-brane. So, binding to phosphorylated Cbpbrings Csk to the membrane, where the SFKsare found. Cbp is phosphorylated not by Cskbut apparently by another tyrosine kinase —quite possibly an SFK. From these results, weget to the model shown in Fig. 1b. A cell-surface receptor binds to its particular extra-cellular partner and activates a specific SFK.The active SFK phosphorylates many pro-teins of varying cellular functions, and possi-bly Cbp as well. Cbp can now recruit Csk tothe membrane, where it can phosphorylate

and inactivate the SFK, turning off the sig-nalling events at the appropriate time.

The discovery of Cbp explains severalobservations. First, we knew previously thatCsk’s SH2 domain is important for SFK reg-ulation, but we did not know why8,9. Second,we knew that Csk relocates to the membrane— specifically to regions at which SFKs areactive — but we did not know how8,9. Third,genetic analysis indicates that SFKs are acti-vated by certain phosphatases10,11. However,these phosphatases do not show the expectedspecificity towards the inhibitory tyrosineon SFKs. Kawabuchi et al.3 suggest that thesephosphatases might instead act mainly on

Cbp, so promoting the activation of SFKsindirectly.

Most SFKs are modified with specificlipids that bring them to subdomains of thecell membrane that have high cholesteroland glycolipid content, called membrane‘rafts’1 (Fig. 1). A number of factors that acti-vate SFKs bind to stimulus receptors that donot penetrate through the membrane but areinstead concentrated in the rafts in the outermembrane, pointing outwards from the cellsurface12. Interestingly, Cbp is also found inrafts — probably by virtue of the same lipidmodification as the SFKs3. Even thoughKawabuchi et al. could not detect a complexbetween Src and Cbp, it is likely that mostSFKs would be near Cbp in rafts. So, we spec-ulate that one way in which outer-membranereceptors located in rafts could stimulateSFKs is if they were to alter the balancebetween phosphorylation and dephospho-rylation of Cbp. The fact that Cbp has extra-cellular and transmembrane regions mightallow it to interact with the receptor.

Coincidentally, Brdic̆ka et al.13 have inde-pendently identified Cbp (which they callPAG) as an SFK-associated protein in raftspurified from lymphocytes. They show thatCbp can be phosphorylated by SFKs andassociates with Csk, and also that Cbpbecomes dephosphorylated when lympho-cytes are stimulated, at the same time thatSFKs are activated. So, dephosphorylation of Cbp coincides with, and probably con-tributes to, SFK activation.

Thus, two groups3,13 have identified whatcould be a key regulator of SFKs. If they areright, ablation of the Cbp gene might havesimilar effects to the loss of Csk — that is,constitutive SFK activation. If so, turning offCbp expression would be one way for cancerto arise, and the Cbp gene might be lost dur-ing the evolution of human cancers. A lotmore should be learnt from experiments in which the Cbp gene is knocked out —experiments that are eagerly awaited. ■

Leslie A. Cary and Jonathan A. Cooper are at theFred Hutchinson Cancer Research Center,1100 Fairview Avenue North, PO Box 19024,Seattle, Washington 98109-1024, USA.e-mail: jcooper@fred.fhcrc.org1. Thomas, S. M. & Brugge, J. S. Annu. Rev. Cell Dev. Biol. 13,

513–609 (1997).

2. Wilde, A. et al. Cell 96, 677–687 (1999).

3. Kawabuchi, M. et al. Nature 404, 999–1003 (2000).

4. Cloutier, J. F. & Veillette, A. J. Exp. Med. 189, 111–121

(1999).

5. Imamoto, A. & Soriano, P. Cell 73, 1117–1124 (1993).

6. Nada, S. et al. Cell 73, 1125–1135 (1993).

7. Schmedt, C. et al. Nature 394, 901–904 (1998).

8. Sabe, H., Hata, A., Okada, M., Nakagawa, H. & Hanafusa, H.

Proc. Natl Acad. Sci. USA 91, 3984–3988 (1994).

9. Howell, B. W. & Cooper, J. A. Mol. Cell. Biol. 14, 5402–5411

(1994).

10.Ponniah, S., Wang, D. Z., Lim, K. L. & Pallen, C. J. Curr. Biol. 9,

535–538 (1999).

11.Thomas, M. L. & Brown, E. J. Immunol. Today 20, 406–411

(1999).

12.Stefanova, I., Horejsi, V., Ansotegui, I. J., Knapp, W. &

Stockinger, H. Science 254, 1016–1019 (1991).

13. Brdic̆ka, T. et al. J. Exp. Med. (in the press).

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NATURE | VOL 404 | 27 APRIL 2000 | www.nature.com 947

Figure 1 Turning a molecular switch on and off. Src-family kinases (SFKs) are molecular switches thatrelay extracellular information to the cell’s interior. a, Left, when phosphorylated (circled ‘P’) at itsinhibitory tyrosine residue (minus symbol), an SFK forms an inactive conformation as a result ofbinding of this tyrosine to the SFK’s SH2 domain. Right, dephosphorylation of this residue (emptyorange circle) induces an activated conformation, with accessible kinase and SH2 domains and aphosphorylated stimulatory tyrosine residue (plus symbol). b, As shown by Kawabuchi et al.3, an SFK might be localized with Cbp and cell-surface receptors in membrane regions called ‘rafts’ (darkyellow areas). Left, binding of an extracellular stimulus to its cell-surface receptor leads to theactivation of an SFK, which then phosphorylates other proteins to regulate cellular function andmight also phosphorylate a tyrosine residue of Cbp. Right, this phosphorylated tyrosine allows thebinding of Cbp to the SH2 domain of Csk, and so recruits Csk to the membrane. Csk can nowinactivate the SFK by phosphorylating the SFK’s inhibitory tyrosine residue.

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