BMC Cell Biology BioMed Central Research article Open Access Arf6 recruits the Rac GEF Kalirin to the plasma membrane facilitating Rac activation Tae Hyeon Koo1, Betty A Eipper2 and Julie G Donaldson*1 Address: 1Laboratory of Cell Biology, NHLBI, NIH, Building 50, Rm 2503, Bethesda, MD 20892, USA and 2Department of Neuroscience, University of Connecticut Health Center, Farmington, CT 06030, USA Email: Tae Hyeon Koo - [email protected]; Betty A Eipper - [email protected]; Julie G Donaldson* - [email protected] * Corresponding author Published: 18 July 2007 Received: 2 March 2007 Accepted: 18 July 2007 BMC Cell Biology 2007, 8:29 doi:10.1186/1471-2121-8-29 This article is available from: http://www.biomedcentral.com/1471-2121/8/29 © 2007 Koo et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract Background: Many studies implicate Arf6 activity in Rac-mediated membrane ruffling and cytoskeletal reorganization. Although Arf6 facilitates the trafficking of Rac1 to the plasma membrane and in many cases Arf6 activation leads to the activation of Rac1, the details of how Arf6 influences Rac function remain to be elucidated. Results: We demonstrate in binding assays and by co-immunoprecipitation that GDP-bound Arf6 binds to Kalirin5, a Rho family guanine nucleotide exchange factor, through interaction with the spectrin repeat region. In cells, expression of wild type Arf6 recruits spectrin repeat 5 and Kalirin to the plasma membrane and leads to enhanced Kalirin5-induced ruffling. By contrast, expression of an Arf6 mutant that cannot become activated, Arf6 T27N, still recruits spectrin repeat 5 and Kalirin to membranes but inhibits Kalirin5-induced ruffling in HeLa cells. Kalirin5-induced Rac1 activation is increased by the expression of wild type Arf6 and decreased by Arf6T27N. Furthermore, expression of a catalytically-inactive mutant of Kalirin5 inhibits cytoskeletal changes observed in cells expressing EFA6, an Arf6 guanine nucleotide exchange factor that leads to activation of Rac. Conclusion: We show here with over-expressed proteins that the GDP-bound form of Arf6 can bind to the spectrin repeat regions in Kalirin Rho family GEFs thereby recruiting Kalirin to membranes. Although Kalirin is recruited onto membranes by Arf6-GDP, subsequent Rac activation and membrane ruffling requires Arf6 activation. From these results, we suggest that Arf6 can regulate through its GTPase cycle the activation of Rac. Background cytoskeleton. To date, at least 25 Rho family proteins have Arfs are small, ubiquitous, Ras-related GTPases that play been identified and of these, RhoA, Rac1 and Cdc42 have an essential role in membrane traffic and structure. been the most widely studied for their effects on actin Among the 6 mammalian Arf proteins, Arf6 has been organization. In particular, Rac1 controls the formation of implicated in the regulation of endocytic membrane traf- lamellipodia and membrane ruffles, the sites of protrusive fic and cortical actin structure at the plasma membrane actin polymerization and early adhesion formation initi- (PM) [1,2]. The small GTPases of the Rho subfamily are ated at the leading edge of migrating cells [3,4]. also known to be important regulators of the actin Page 1 of 10 (page number not for citation purposes) BMC Cell Biology 2007, 8:29 http://www.biomedcentral.com/1471-2121/8/29 Recent studies have uncovered crosstalk between these 2 the Dbl family of Rho GEFs. There are multiple splice families, in which both Arf6 and Rac proteins have forms of Kalirin family GEFs, the longest of which is emerged as key regulators in actin remodeling and mem- Kalirin12, a dual Rho GEF homologous to Trio [17,18]. In brane trafficking events [5-8]. We showed that Rac1 colo- both Kalirin and Trio, the first DH-PH motif activates Rac calizes with Arf6 at the plasma membrane and recycling and RhoG while the second activates Rho (Fig. 1A) endosomes, and that Rac1-stimulated ruffling requires [19,20]. Both Kalirin5 (also called ΔKalirin7) and Kalirin7 Arf6 activity [9]. Stimulation of the hepatocyte growth fac- (also called Duo) lack the second DH-PH motif, terminat- tor receptor in MDCK cells [10] or the angiotensin II ing with a PDZ motif; Kalirin5 lacks the Sec14 and first receptor in Hek cells [11]leads to the sequential activation four spectrin repeats of Kalirin7 [17]. We examined of Arf6 and then Rac. On the other hand, during platelet whether we could detect a specific interaction of Kalirin5 activation Arf6-GTP levels decrease, followed by activa- and its SR with Arf6 by a GST pull-down assay. COS7 cells tion of Rho GTPases [12]. Santy and Casanova showed were transfected with HA-tagged Arf1, Arf5 or Arf6 and the that activation of Arf6 by the guanine nucleotide exchange lysates were incubated with spectrin-like repeats 4–7 of rat factor (GEF) ARNO leads to the subsequent activation of Kalirin7 (KalSR4-7) fused to GST [21]. Arf6 bound to Rac [7]. Recently, these investigators found that expres- GST-KalSR4-7 but not to GST alone (Fig. 1B). Moreover, sion of a dominant negative form of Dock180/ELMO, a neither Arf1 nor Arf5 bound to GST-KalSR4-7. Rac GEF, inhibited ARNO activation of Rac suggesting that this Rac GEF may couple ARNO activities to Rac acti- We then examined the interaction between Kalirin5 and vation [13]. Arf6 in cells expressing FLAG-tagged Kalirin and various HA-tagged Arf proteins by immunoprecipitation of FLAG- Like other GTPases, both Arf and Rho proteins are regu- Kalirin. Wild type Arf6 and the GTP-binding defective lated by cycling between active, GTP-bound and inactive, mutant of Arf6 (Arf6T27N) co-immunoprecipitated with GDP-bound conformations. This cycle is regulated by Kalirin5 but a constitutively active mutant of Arf6 GEFs that catalyze exchange of GTP for GDP and GTPase- (Arf6Q67L) did not (Fig. 1C). Neither Arf1 nor Arf5 co- activating proteins (GAPs) that catalyze GTP hydrolysis. precipitated with Kalirin, consistent with the results in the These regulators control the activation of these GTPases GST pull-down assays. Similar results were obtained when spatially and temporally. Rho family GEFs have a Dbl Arf proteins were immunoprecipitated with antibody to homology (DH) domain, a conserved amino acid HA and the immunoprecipitates probed for Kalirin5 (data sequence that is the catalytic domain. Many Dbl family not shown). Since cells expressing Arf6 will have both GEFs also contain other protein-protein interaction GDP and GTP-bound forms, whereas cells expressing domains that allow GTPases to be activated in specific sig- Arf6T27N will have only the GDP-bound form of Arf6, nal transduction pathways and coordinate more elaborate these results suggest that Kalirin5 binds to the GDP- responses to specific demands at localized cellular sites bound, and not the GTP-bound, form of Arf6. Although [14,15]. we could see this interaction following over-expression of Arf6 and Kalirin, we were not able to detect it with endog- In the present study, we identified Kalirin, a Rho family enous proteins; this may be due to low levels of expres- GEF, as a specific Arf6 binding protein. Kalirin, a novel sion, difficulty extracting the proteins in a native state and member of the Dbl family, is a multi-domain protein with lack of sensitive immunological reagents. many isoforms in rat adult brain tissues as determined by Northern blot analysis [16,17]. Kalirin5 is the shortest To confirm this Arf6-GDP-specific interaction, we co- protein of the Kalirin isoforms, containing 5 spectrin-like expressed Arf6 and Kalirin5 with EFA6, an Arf6 GEF that repeats (SR), a Dbl homology (DH) domain, a pleckstrin will lead to an increase in Arf6-GTP [22,23] and hence a homology (PH) domain, and a PDZ binding motif. Kali- decrease in Arf6-GDP in cells. Co-expression of EFA6 rin is highly homologous to Trio, which is more widely decreased the amount of Arf6 recovered in the Kalirin5 expressed [18]. Here we provide evidence that Arf6-GDP immunoprecipitate (Fig. 1D). Secondly, lowering ATP can bind to Kalirin through interaction with these spectrin and GTP levels in cells by treatment with sodium deoxy- repeats, thereby recruiting Kalirin to the membrane lead- glucose and sodium azide, used previously by others ing to Rac activation. [24,25], led to a time-dependent increase in Arf6 and Kalirin5 binding (Fig. 1E). Enhanced Arf6 binding to SR5 Results in the GST-spectrin pull down assays was also observed Arf6-GDP binds to Kalirin via the spectrin repeats when cells were depleted of energy as above (data not To identify proteins that interact with Arf6, we used the shown). Similar co-immunoprecipitation results were yeast two-hybrid system to screen a human fetal brain obtained from transfected HeLa cells (data not shown). library. One clone was identified that contained the spec- We also attempted co-immunoprecipitation of larger Kali- trin repeat (SR) domain 5 of Kalirin, a novel member of rin-like isoforms (Kalirin7 and 12) but were unsuccessful Page 2 of 10 (page number not for citation purposes) BMC Cell Biology 2007, 8:29 http://www.biomedcentral.com/1471-2121/8/29 Kalirin5Figure 1interacts with Arf6-GDP Kalirin5 interacts with Arf6-GDP. A. Domain structure of Kalirin isoforms and Trio showing SR, common first and second DH-PH motifs. (Also depicted, Sec14 domain, oval and SH2 domain, hexagon). B. Arf6, but not Arf1 or Arf5, binds to SR region of Kalirin. COS7 cells were transfected with C-terminally HA-tagged Arf isoforms, and cell extracts were incubated with GST-KalSR4-7 or with GST.