Bustelo (2012) Rac-ing the plasma membrane

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Rac-ing to the plasma membraneThe long and complex work commute of Rac1 during cell signaling

Xosé R. Bustelo,* Virginia Ojeda, María Barreira, Vincent Sauzeau† and Antonio Castro-CastroCentro de Investigación del Cáncer; Instituto de Biología Molecular y Celular del Cáncer; CSIC—University of Salamanca; Campus Unamuno; Salamanca, Spain

†Current address: L’Institut du Thorax-UMR 915; Nantes, France

Keywords: coronin, Rac1, F-actin,cytoskeleton, lipid rafts, β-Pix, ArhGEF7,Pak, RhoGDI

Submitted: 11/05/11

Revised: 12/16/11

Accepted: 12/19/11

http://dx.doi.org/10.4161/sgtp.19111*Correspondence to: Xosé R. Bustelo;Email: [email protected]

Commentary to: Castro-Castro A, Ojeda V,Barreira M, Sauzeau V, Navarro-Lérida I, Muriel O,et al. Coronin 1A promotes a cytoskeletal-basedfeedback loop that facilitates Rac1 translocationand activation. EMBO J 2011; 30:3913–27;PMID:21873980; http://dx.doi.org/10.1038/emboj.2011.310

The functional cycle of the Rac1GTPase involves a large number

of steps, including post-translational pro-cessing, cytosolic sequestration byRhoGDIs, translocation to specific sub-cellular localizations, activation by GDP/GTP exchange, inactivation by GTPhydrolysis, and re-formation of cytosolicRac1/RhoGDI inhibitory complexes.Here, we summarize the current know-ledge about the regulation of those steps.In addition, we discuss a recently descri-bed, cytoskeletal-dependent feed-backloop that favors the efficient translocationand activation of Rac subfamily proteinsduring cell signaling. This route ismediated by a heteromolecular proteincomplex composed of the cytoskeletalprotein coronin1A, the Dbl familymember ArhGEF7, the serine/threoninekinase Pak1, and the Rac1/RhoGDIdimer. This route promotes the trans-location of Rac1/RhoGDI to F-actin-richjuxtamembrane areas, the Pak1-dependentrelease of Rac1 from the Rac1/RhoGDIcomplex, and Rac1 activation. Thispathway is important for optimal Rac1activation during the signaling of theEGF receptor, integrins, and the anti-genic T-cell receptor.

Rac1, one of the best characterizedmembers of the Rho/Rac GTPase subfam-ily, regulates ubiquitous processes such asthe formation of membrane ruffles andlamellipodia, cell adhesion, proliferation,intercellular attraction/repulsion, and trans-criptomal dynamics. In addition, it modu-lates cell-type-specific processes such as

axon migration/guidance, phagocytosis,or the formation of the immunologicalsynapse.1 To trigger most of those func-tions, Rac1 has to fulfill two basic con-ditions. One of them is to be anchored atthe plasma membrane to make it possiblethe subsequent activation of its primaryeffectors in the correct subcellular locali-zation. The second condition is that ithas to be bound to GTP, since this isthe only conformational state compatiblewith the interaction of most downstreameffectors.1 These requirements onlychange in few signaling scenarios, suchas the cell cycle-regulated transfer ofRac1 to the nucleus2 or the indistinctivebinding of GDP-Rac and GTP-Rac1 tomTOR.3

Like the mythical Greek Odysseus, thetravel of Rac1 from the cytosol to theplasma membrane is a stepwise mechanismsubjected to multiple regulatory challenges(Fig. 1). The first step required to reachits particular Ithaca is the attachment ofa geranyl-geranyl group to the most C-terminal cysteine residue of the translatedRac1 protein, a modification catalyzed inthe cytosol by type I geranyl-geranyl trans-ferase (GGT1)1,4 (Fig. 1, step 1a). Rac1then moves to the endoplasmic reticulum,where it is subjected to further processingby the protease Rce1 and the methyl-transferase Icmt.1,5 Rcc1 cleaves off theC-terminal Rae1 LLL tripeptide (Fig. 1,step 1b). Icmt incorporates a methyl groupat the a-carboxyl group of the C-terminalcysteine that becomes exposed upon thecompletion of the Rcc1-catalyzed reaction(Fig. 1, step 1c). Based on data gatheredusing Ras subfamily proteins, it has been

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assumed that the prenylation, cleavage andmethylation steps were conditio sine quanon for the biological activity of Rac1proteins.1,4,5 Such model is consistent withcurrent experimental evidence,6,7 althoughsome recent reports have unexpectedlyshown that Rac1 can be functional inprimary GTT1 deficient macrophages8

and in embryonic fibroblasts obtainedfrom Rce1–/– and Icmt–/– mice.9

After its transit through the endoplas-mic reticulum, Rac1 moves back to thecytosol where it stays in an inactivereservoir until the reception of extracellularsignals by the cell. This pool is stabilizedby the formation of stoichiometric com-plexes with RhoGDIs10 (Fig. 1, step 2).RhoGDIs perform both negative andpositive actions in this complex. On thenegative side, they inhibit the usuallyhigh intrinsic GDP/GTP exchange ofRac1, thus favoring the maintenance ofthe bound GTPase in the inactive con-formation in non-stimulated cells. Thisfunction is mediated by the direct inter-action of the RhoGDI molecule with theRac1 switch regions.10 In addition, theyuse a deep hydrophobic cavity to trap theRac1 prenyl group, a mechanism thatkeeps the GTPase away from the mem-brane in a cytosolic, fully soluble pool10

(Fig. 1). On the positive side, RhoGDIsprotect the bound Rac1 molecules fromproteolytic degradation11 and, in addition,are likely involved in the final transitof Rho/Rac GTPases to the plasmamembrane.10

The cytosolic reservoir of Rac1 israpidly mobilized to the plasma membraneupon cell stimulation1 (Fig. 1, step 3a) or,alternatively, to the nucleus in the G2

phase of the cell cycle2 (Fig. 1, step 3b).The main factors involved in the formermobilization route are the Rac1 GEFs, agroup of enzymes containing either Dbl-homology or Dock-homology domainsthat promote the rapid transition of Rac1from the inactive (GDP-bound) to theactive (GTP-bound) state12 (Fig. 1, step 4).However, alternative physiological activationsteps exist, such as the transglutaminase-dependent serotonylation of Rac1 down-stream of G-coupled receptors.13 Thismobilization step is also facilitated by theC-terminal Rac1 polybasic and proline-rich regions. The former region favors the

formation of hydrophilic interactions withthe negatively-charged heads of lipidsonly present in the plasma membrane,because Rac-related proteins lacking thishydrophilic signal are localized in intra-cellular vesicles14,15 (Fig. 1, step 3a). TheRac1 proline-rich region interacts withthe SH3 region of ArhGEF7 (also knownas β-Pix), a Dbl-homology family proteinthat can associate with F-actin rich areasof the cell16 (Fig. 1, step 3a). Recent dataindicate that other ancillary signals col-laborate in this process, including thepost-translational modification of Rac1by phosphorylation,17 sumoylation18 andubiquitination.19,20 Rac1 localization canbe also regulated positively by extrinsicsignals such as the presence of specificphospholipids (PtdIns(4,5)P2, PtdIns(3,4,5)P3)21,22 and lipid rafts23-25 at theplasma membrane (Fig. 1, step 3a). Atthe end of the effector phase (Fig. 1, step5), the standard regulatory model holdsthat Rac1 goes back to the GDP boundstate via the action of Rho/Rac GTPaseactivating proteins12 (Fig. 1, step 6a),re-associates with RhoGDI molecules(Fig. 1, step 6b) and moves back to theinactive cytosolic reservoir until a newstimulation cycle starts.1 In addition tothis standard model, recent data demon-strated that Rac1 can undergo activation/inactivation cycles by shuttling betweenthe plasma membrane and endocyticcompartments.23,26 The shuttling of Rac1in and out of the nucleus requires theRac1 polybasic region and karyopherina2,27 an importin that transfers cargomolecules inside the nucleus (Fig. 1, step3b). Whether this step requires GEFs,carrier proteins, or intracellular dockingproteins is unknown as yet. The mechanismby which Rac1 is returned to the cytosolremains ill defined (Fig. 1, step 6c).

Whereas the GTPase/RhoGDI complexand the catalytic steps involved in Rac1GDP/GTP exchange and GTP hydrolysisare well understood in structural terms,10,12

the dynamic aspects that modulate therelease of Rac1 from RhoGDIs during theactivation process are not well understood.For instance, we do not know whetherRac1 dissociates from the RhoGDI beforecontacting the upstream GEFs or, alter-natively, whether the GEFs promote boththe dissociation and activation of the

GTPase. In either case, it is unclear theintracellular cues used by either Rac1 orthe Rac1/RhoGDI complex to reach thesubcellular regions where the stimulatedGEFs are localized. Finally, we do notknow whether these Rac1 mobilization-related steps are general and/or cell type-specific. Recent inroads in this subjectsuggest that the release of Rac1 is mediatedby the direct phosphorylation of RhoGDIby either upstream (Src, protein kinase C)or downstream (Pak1) kinases.28-31 In anycase, it is still difficult to explain how thecytosolic RhoGDI/Rac1 complexes man-age to get close to those kinases duringcell stimulation.

To shed light on this process, wedecided to search for proteins involved inthe regulation of the translocation ofRac1 to the plasma membrane using agenome-wide functional screen (Fig. 2).This approach led to the isolation ofcoronin1A (Coro1A) as a protein capableof inducing the translocation and activa-tion of Rac1 during cell signaling (Fig. 2,bottom).32 Coro1A belongs to a largefamily of cytoskeletal regulators that showa phylogenetic distribution from unicel-lular eukaryotes to humans.33 Theseproteins control the bundling of F-actinfilaments, the growth and orientationangle of new F-actin branches, and thedisassembly of old actin cables.33 Suchactivities suggested to us that Coro1Acould be possibly involved in a cross-talkbetween the F-actin cytoskeleton and theprocess of Rac1 translocation and/oractivation. Consistent with this idea, wecould demonstrate that the overexpres-sion of Coro1A induced the F-actin-dependent translocation and activationof Rac1 in the plasma membrane. Con-versely, its inactivation led to ineffectiveactivation of Rac1 and Rac1-downstreamroutes. This pathway was active in anumber of cell types (COS1, 293T,Jurkat cells) and cellular stimulation con-ditions (EGF signaling, integrin-mediatedadhesion, T-cell receptor stimulation).

Mechanistic studies revealed thatCoro1A promotes the translocation andactivation of Rac1 via the formation of aF-actin-dependent, heteromolecular com-plex with ArhGEF7, Pak1, RhoGDI andRac1. According to our proposed model(Fig. 3), the formation of this Rac1

COMMENTARY

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translocation complex requires upstreamsignals such as Rac1 activity, aggregatedlipid rafts in the plasma membrane and

the presence of F-actin in cells (Fig. 3,stage 1). In the absence of those upstreamsignals, Coro1A can only associate in a

stable manner with ArhGEF7 in the cyto-plasm (Fig. 3, stage 2). Upon conditionstriggering F-actin polymerization, this

Figure 1. Depiction of the main steps of the functional cycle of Rac1. Enzymes and other cellular factors collaborating in each of those steps are shown ingreen. Other symbols are indicated in the figure. Please note that the sequence of the C-terminal LLL tripeptide is different in some Rac1 orthologs, suchas those present in horses (TVF), chimpanzees (LQL) or Drosophila (ALL). The status of Rac1 inside the nucleus is still poorly characterized so we have notincorporated the activation/inactivation cycle of Rac1 in that compartment. It is also unclear whether the insertion of Rac1 in membranes is achievedwhen in the GDP- or GTP-bound state. The latter case has not been contemplated in the scheme for the sake of simplicity. Abbreviations used are:ER, endoplasmic reticulum; GAP, GTPase activating protein; Me, methyl group; Pi, inorganic phosphate; PKC, protein kinase C; PM, plasma membrane;PRR BP, proline-rich region binding protein. Other abbreviations have been described in the main text.

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inert complex is tethered to membraneruffles and lamellipodia, a process made itpossible by the intrinsic F-actin bindingproperties of Coro1A (Fig. 3, stage 3). TheF-actin bound complex binds then Pak1and Rac1/RhoGDI (Fig. 3, stage 4), lead-ing to the phosphorylation of RhoGDIby Pak1, the release of Rac1 from thephosphorylated RhoGDI and, finally, toRac1 activation (Fig. 3, stage 5). Severalproperties of this new translocation process

are important from a signaling point ofview. First, the F-actin binding propertiesof Coro1A offer a rather simple explana-tion for understanding how the cytosolicRac1/RhoGDI pool is shuttled toward theplasma membrane during cell stimulationconditions. Second, its dependency onupstream Rac1 signals suggest that theCoro1A route is involved in the generationof secondary waves of Rac1 activationrather than being directly involved in the

initial burst of Rac1 activity that takesplace upon the reception of the extra-cellular signal. Thus, according to ourproposed regulatory model, cells will haveto trigger Rac1 or RhoG activation inorder to engage the downstream Coro1A-dependent relay mechanism (Fig. 3).Finally, the need of pre-formed F-actincytoskeletal structures to engage theCoro1A-dependent translocation routesuggests that F-actin can induce a positive

Figure 2. Scheme of the cellomic screen used in the work reviewed here. The first screening was conducted using an expression library of 135,000independent cDNA clones obtained from human T cells. To this end, we used a reporter HEK293T stably expressing a cytoplasmic EGFP-Rac1 protein(step 1). To increase the efficiency of the screening, this cell line also expressed anti-apoptotic proteins to avoid the loss of clones due to the presence ofpro-apoptotic molecules in the transfected cDNA pools. The screening was conducted by transfecting separate pools of 90 cDNAs in the reporter cell lineusing the calcium phosphate precipitation method (step 2) and the subsequent score of cells showing plasma membrane localized EGFP-Rac1 usingepifluorescence microscopy (step 3). Positive pools were progressively subdivided and transfected in the reporter cell line (steps 4a and 4b) until theisolation of the cDNA clones responsible for the Rac1 translocating activity (step 5). The crystal structure of Coro1A, one of the clones identified in thisscreening, is shown at the bottom. The structure is composed of WD40 domains arranged in a prototypical b-propeller conformation. See inset at thebottom for the shape and color code use for the indicated proteins used in these experiments.

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feed-back mechanism that will allow thegeneration of additional waves of Rac1activation and F-actin polymerization atthe plasma membrane (Fig. 3).

Despite these advances, the role ofCoro1A in the translocation and activationof Rac1 is far from being an open and shutcase. Thus, the mechanism that regulatesthe assembly of Pak1 and the RhoGDI/Rac1 pair onto the Coro1A/ArhGEF7complex has not been fully elucidatedyet. Our data indicate that such assembly

requires the presence of ArhGEF7 in theCoro1A complex, a result consistent withthe known physical interaction betweenArhGEF7 and Pak1.34 However, therequirement of F-actin for the formationof the entire Coro1A/Pak1/RhoGDI com-plex also indicates that additional ancillarypartners and/or signals must also contrib-ute to this process. Whether these extraelements are proteins, membrane lipids,and/or other upstream signals remains tobe determined. It is also unclear the Rac1

GEF in charge of activating Rac1 upon itsrelease from the Coro1A-nucleated proteincomplex (Fig. 3). One option is ArhGEF7itself, since it is obvious that its presencein the Coro1A complex will ensure itsclose proximity with the released Rac1molecules (Fig. 3). However, the catalyticactivity of ArhGEF7 is controversial,suggesting that other GEFs could beinvolved (Fig. 3, protein labeled as X). Itis possible therefore that such activationstep is at the hands of other GEFs that,

Figure 3. Schematic representation of the regulatory model for the proposed Coro1A-mediated translocation and activation of Rac1 reviewed here. Thefirst stimulus triggering the first burst of Rac1 activity via a Coro1A-independent route is shown on the left in gray color and thin lanes. The Coro1A-basedrelay mechanism involved in the subsequent amplification of Rac1 signals is shown with thicker lanes. L, ligand; X, a putative Rac1 GEF factor associatedto ArhGEF7. Phosphorylated residues are shown as yellow lollipops. See further details in the main text of this work.

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due to their localization in F-actin cablesor the plasma membrane, could be in closeproximity to the Coro1A-nucleated com-plex. The resolution of all those lingeringissues will need further experimental workin the near future. It is also worth notingthat recent data obtained in our laboratorysuggest that Coro1A can use alternativemechanisms to induce the translocationof Rac1 to the plasma membrane. Hence,we have observed that Coro1A can triggerthe membrane localization of Rac1R66E, amutant protein that cannot bind RhoGDI.This alternative route is mechanisticallydistinct from that characterized in thisreviewed publication, because it cannotbe inhibited by blocking Pak1 function(Castro-Castro A, Bustelo XR, unpub-lished observations). Although theseresults are probably not physiologicallyrelevant given the lack of significantamounts of RhoGDI-free Rac1 proteinsin the cytosol, they are interesting becausethey reveal a pathway that can furtherenhance the membrane anchoring ofRac1 upon its liberation from theRhoGDI complexes by the Coro1A/ArhGEF7/Pak1 complex. Although thisalternative translocation mechanism

remains to be elucidated, we surmise thatit could involve the generation of F-actin-dependent “permissive” conditions at theplasma membrane. Consistent with thisview, we have observed that the over-expression of Coro1A induces lipid raftaggregation in the plasma membrane.Furthermore, we found that the aggrega-tion of lipid rafts promotes the transloca-tion of the Rac1R66E mutant to the plasmamembrane (Castro-Castro A, BusteloXR, unpublished observations). Finally, itshould be pointed out that Coro1A andArhGEF7 are not ubiquitously expressed,so it is unlikely that the Rac1 transloca-tion/activation mechanism reviewed herewill be utilized by all cell types. In thiscontext, it will be interesting to investigatewhether other cytoskeletal proteins fulfillCoro1A-like functions in cells that donot express Coro1A. This is a feasiblescenario, because our genome-wide func-tional screen has resulted in the identifi-cation of a second WD40 family protein(WDR26) that can also induce thetranslocation of Rac1 to the plasmamembrane (Castro-Castro A, Bustelo XR,unpublished data). It is also possible thatthe increase in the membrane-localized

pool of Rac1 can be achieved by routesalternative to those reported here. Forexample, our cellomic screen has alsoidentified transmembrane proteins thatfavor the membrane localization ofRac1 by blocking the internalization oflipid rafts. Unlike the case of Coro1A-dependent route, this alternative pathwaycannot be inhibited by either Rac1dominant negative mutants or Pak1 inter-ference strategies. Taken together, theseresults suggest that cells can resort to manypathways to regulate the translocation ofRac1 during cell signaling.

Acknowledgments

X.R.B.’s work is supported by grantsfrom the NIH (5R01-CA73735-13), theSpanish Ministry of Science andInnovation (SAF2009-07172, GEN2003-20239-C06-01), the Red Temática deInvestigación Cooperativa en Cáncer(RD06/0020/0001), the Castilla y LeónAutonomous Government (GR97), andthe 7th Framework European UnionProgram (FP7-HEALTH-2007-A-201862).X.R.B.’s lab is a Consolidated CancerResearch Group of the AsociaciónEspañola contra el Cáncer.

References1. Bustelo XR, Sauzeau V, Berenjeno IM. GTP-binding

proteins of the Rho/Rac family: regulation, effectorsand functions in vivo. Bioessays 2007; 29:356-70;PMID:17373658; http://dx.doi.org/10.1002/bies.20558

2. Michaelson D, Abidi W, Guardavaccaro D, Zhou M,Ahearn I, Pagano M, et al. Rac1 accumulates in thenucleus during the G2 phase of the cell cycle andpromotes cell division. J Cell Biol 2008; 181:485-96; PMID:18443222; http://dx.doi.org/10.1083/jcb.200801047

3. Saci A, Cantley LC, Carpenter CL. Rac1 regulates theactivity of mTORC1 and mTORC2 and controlscellular size. Mol Cell 2011; 42:50-61; PMID:21474067; http://dx.doi.org/10.1016/j.molcel.2011.03.017

4. Seabra MC. Membrane association and targeting ofprenylated Ras-like GTPases. Cell Signal 1998; 10:167-72; PMID:9607139; http://dx.doi.org/10.1016/S0898-6568(97)00120-4

5. Winter-Vann AM, Casey PJ. Post-prenylation-proces-sing enzymes as new targets in oncogenesis. Nat RevCancer 2005; 5:405-12; PMID:15864282; http://dx.doi.org/10.1038/nrc1612

6. Roberts PJ, Mitin N, Keller PJ, Chenette EJ, MadiganJP, Currin RO, et al. Rho Family GTPase modificationand dependence on CAAX motif-signaled posttransla-tional modification. J Biol Chem 2008; 283:25150-63; PMID:18614539; http://dx.doi.org/10.1074/jbc.M800882200

7. Cushman I, Casey PJ. RHO methylation matters: arole for isoprenylcysteine carboxylmethyltransferase incell migration and adhesion. Cell Adh Migr 2011;5:11-5; PMID:20798596; http://dx.doi.org/10.4161/cam.5.1.13196

8. Khan OM, Ibrahim MX, Jonsson IM, Karlsson C, LiuM, Sjogren AK, et al. Geranylgeranyltransferase type I(GGTase-I) deficiency hyperactivates macrophages andinduces erosive arthritis in mice. J Clin Invest 2011;121:628-39; PMID:21266780; http://dx.doi.org/10.1172/JCI43758

9. Michaelson D, Ali W, Chiu VK, Bergo M, Silletti J,Wright L, et al. Postprenylation CAAX processing isrequired for proper localization of Ras but not RhoGTPases. Mol Biol Cell 2005; 16:1606-16; PMID:15659645; http://dx.doi.org/10.1091/mbc.E04-11-0960

10. DerMardirossian C, Bokoch GM. GDIs: centralregulatory molecules in Rho GTPase activation.Trends Cell Biol 2005; 15:356-63; PMID:15921909;http://dx.doi.org/10.1016/j.tcb.2005.05.001

11. Boulter E, Garcia-Mata R, Guilluy C, Dubash A, RossiG, Brennwald PJ, et al. Regulation of Rho GTPasecrosstalk, degradation and activity by RhoGDI1. NatCell Biol 2010; 12:477-83; PMID:20400958; http://dx.doi.org/10.1038/ncb2049

12. Bos JL, Rehmann H, Wittinghofer A. GEFs and GAPs:critical elements in the control of small G proteins. Cell2007; 129:865-77; PMID:17540168; http://dx.doi.org/10.1016/j.cell.2007.05.018

13. Walther DJ, Peter JU, Winter S, Höltje M, PaulmannN, Grohmann M, et al. Serotonylation of smallGTPases is a signal transduction pathway that triggersplatelet alpha-granule release. Cell 2003; 115:851-62;PMID:14697203; http://dx.doi.org/10.1016/S0092-8674(03)01014-6

14. Filippi MD, Harris CE, Meller J, Gu Y, Zheng Y,Williams DA. Localization of Rac2 via the C terminusand aspartic acid 150 specifies superoxide generation,actin polarity and chemotaxis in neutrophils. NatImmunol 2004; 5:744-51; PMID:15170212; http://dx.doi.org/10.1038/ni1081

15. Prieto-Sánchez RM, Bustelo XR. Structural basis forthe signaling specificity of RhoG and Rac1 GTPases. JBiol Chem 2003; 278:37916-25; PMID:12805377;http://dx.doi.org/10.1074/jbc.M301437200

16. ten Klooster JP, Jaffer ZM, Chernoff J, Hordijk PL.Targeting and activation of Rac1 are mediated bythe exchange factor beta-Pix. J Cell Biol 2006; 172:759-69; PMID:16492808; http://dx.doi.org/10.1083/jcb.200509096

17. Kwon T, Kwon DY, Chun J, Kim JH, Kang SS. Aktprotein kinase inhibits Rac1-GTP binding throughphosphorylation at serine 71 of Rac1. J Biol Chem2000; 275:423-8; PMID:10617634; http://dx.doi.org/10.1074/jbc.275.1.423

www.landesbioscience.com Small GTPases 65

© 2012 Landes Bioscience.

Do not distribute.

18. Castillo-Lluva S, Tatham MH, Jones RC, Jaffray EG,Edmondson RD, Hay RT, et al. SUMOylation of theGTPase Rac1 is required for optimal cell migration.Nat Cell Biol 2010; 12:1078-85; PMID:20935639;http://dx.doi.org/10.1038/ncb2112

19. Nethe M, Anthony EC, Fernandez-Borja M, Dee R,Geerts D, Hensbergen PJ, et al. Focal-adhesion target-ing links caveolin-1 to a Rac1-degradation pathway. JCell Sci 2010; 123:1948-58; PMID:20460433; http://dx.doi.org/10.1242/jcs.062919

20. de la Vega M, Kelvin AA, Dunican DJ, McFarlane C,Burrows JF, Jaworski J, et al. The deubiquitinatingenzyme USP17 is essential for GTPase subcellularlocalization and cell motility. Nat Commun 2011;2:259; PMID:21448158; http://dx.doi.org/10.1038/ncomms1243

21. Heo WD, Inoue T, Park WS, Kim ML, Park BO,Wandless TJ, et al. PI(3,4,5)P3 and PI(4,5)P2 lipidstarget proteins with polybasic clusters to the plasmamembrane. Science 2006; 314:1458-61; PMID:17095657; http://dx.doi.org/10.1126/science.1134389

22. Yeung T, Gilbert GE, Shi J, Silvius J, Kapus A,Grinstein S. Membrane phosphatidylserine regulatessurface charge and protein localization. Science 2008;319:210-3; PMID:18187657; http://dx.doi.org/10.1126/science.1152066

23. Balasubramanian N, Scott DW, Castle JD, CasanovaJE, Schwartz MA. Arf6 and microtubules in adhesion-dependent trafficking of lipid rafts. Nat Cell Biol 2007;9:1381-91; PMID:18026091; http://dx.doi.org/10.1038/ncb1657

24. del Pozo MA, Alderson NB, Kiosses WB, Chiang HH,Anderson RG, Schwartz MA. Integrins regulate Ractargeting by internalization of membrane domains.Science 2004; 303:839-42; PMID:14764880; http://dx.doi.org/10.1126/science.1092571

25. del Pozo MA, Balasubramanian N, Alderson NB,Kiosses WB, Grande-García A, Anderson RG, et al.Phospho-caveolin-1 mediates integrin-regulated mem-brane domain internalization. Nat Cell Biol 2005;7:901-8; PMID:16113676; http://dx.doi.org/10.1038/ncb1293

26. Palamidessi A, Frittoli E, Garré M, Faretta M, MioneM, Testa I, et al. Endocytic trafficking of Rac isrequired for the spatial restriction of signaling in cellmigration. Cell 2008; 134:135-47; PMID:18614017;http://dx.doi.org/10.1016/j.cell.2008.05.034

27. Sandrock K, Bielek H, Schradi K, Schmidt G,Klugbauer N. The nuclear import of the smallGTPase Rac1 is mediated by the direct interactionwith karyopherin alpha2. Traffic 2010; 11:198-209;PMID:19961560; http://dx.doi.org/10.1111/j.1600-0854.2009.01015.x

28. DerMardirossian C, Rocklin G, Seo JY, Bokoch GM.Phosphorylation of RhoGDI by Src regulates RhoGTPase binding and cytosol-membrane cycling. MolBiol Cell 2006; 17:4760-8; PMID:16943322; http://dx.doi.org/10.1091/mbc.E06-06-0533

29. DerMardirossian C, Schnelzer A, Bokoch GM.Phosphorylation of RhoGDI by Pak1 mediates dissoci-ation of Rac GTPase. Mol Cell 2004; 15:117-27;PMID:15225553; http://dx.doi.org/10.1016/j.molcel.2004.05.019

30. Price LS, Langeslag M, ten Klooster JP, Hordijk PL,Jalink K, Collard JG. Calcium signaling regulatestranslocation and activation of Rac. J Biol Chem 2003;278:39413-21; PMID:12888567; http://dx.doi.org/10.1074/jbc.M302083200

31. Abramovici H, Mojtabaie P, Parks RJ, Zhong XP,Koretzky GA, Topham MK, et al. Diacylglycerolkinase zeta regulates actin cytoskeleton reorganizationthrough dissociation of Rac1 from RhoGDI. Mol BiolCell 2009; 20:2049-59; PMID:19211846; http://dx.doi.org/10.1091/mbc.E07-12-1248

32. Castro-Castro A, Ojeda V, Barreira M, Sauzeau V,Navarro-Lérida I, Muriel O, et al. Coronin 1A pro-motes a cytoskeletal-based feedback loop that facilitatesRac1 translocation and activation. EMBO J 2011;30:3913-27; PMID:21873980; http://dx.doi.org/10.1038/emboj.2011.310

33. Rybakin V, Clemen CS. Coronin proteins as multi-functional regulators of the cytoskeleton and mem-brane trafficking. Bioessays 2005; 27:625-32; PMID:15892111; http://dx.doi.org/10.1002/bies.20235

34. Manser E, Loo TH, Koh CG, Zhao ZS, Chen XQ,Tan L, et al. PAK kinases are directly coupled to thePIX family of nucleotide exchange factors. Mol Cell1998; 1:183-92; PMID:9659915; http://dx.doi.org/10.1016/S1097-2765(00)80019-2

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