IQGAP1 Integrates Ca2+/Calmodulin and Cdc42 Signaling (1999)

Calmodulin regulates diverse Ca2+-dependent cellular processes, including cell cycle progression and cytoskeletal rearrangement. A recently identified calmodulin-binding protein, IQGAP1, interacts with both actin and Cdc42. In this study, evidence is presented that, in the absence of Ca2+, IQGAP1 bound to Cdc42, which maintained Cdc42 in the active GTP-bound state. Addition of Ca2+ both directly abrogated the effect of IQGAP1 on the intrinsic GTPase activity of Cdc42 and, in the presence of calmodulin, dissociated Cdc42 from IQGAP1. In addition, in vitro binding assays revealed that calmodulin associated with both the calponin homology domain and the IQ motifs of IQGAP1. Moreover, F-actin competed with Ca2+/calmodulin for binding to the calponin homology domain, but not the IQ motifs, of IQGAP1. Analysis of cell lysates revealed that calmodulin bound to IQGAP1 in a ternary complex with Cdc42. Increasing the Ca2+ concentration enhanced the interaction between calmodulin and IQGAP1, with a concomitant decrease in the association of IQGAP1 with Cdc42. Our data suggest that IQGAP1 functions as a scaffolding protein, providing a molecular link between Ca2+/calmodulin and Cdc42 signaling. Calmodulin regulates diverse Ca2+-dependent cellular processes, including cell cycle progression and cytoskeletal rearrangement. A recently identified calmodulin-binding protein, IQGAP1, interacts with both actin and Cdc42. In this study, evidence is presented that, in the absence of Ca2+, IQGAP1 bound to Cdc42, which maintained Cdc42 in the active GTP-bound state. Addition of Ca2+ both directly abrogated the effect of IQGAP1 on the intrinsic GTPase activity of Cdc42 and, in the presence of calmodulin, dissociated Cdc42 from IQGAP1. In addition, in vitro binding assays revealed that calmodulin associated with both the calponin homology domain and the IQ motifs of IQGAP1. Moreover, F-actin competed with Ca2+/calmodulin for binding to the calponin homology domain, but not the IQ motifs, of IQGAP1. Analysis of cell lysates revealed that calmodulin bound to IQGAP1 in a ternary complex with Cdc42. Increasing the Ca2+ concentration enhanced the interaction between calmodulin and IQGAP1, with a concomitant decrease in the association of IQGAP1 with Cdc42. Our data suggest that IQGAP1 functions as a scaffolding protein, providing a molecular link between Ca2+/calmodulin and Cdc42 signaling. GTPase-activating protein glutathione S-transferase guanosine 5′-O-(3-thiotriphosphate) polyacrylamide gel electrophoresis polyvinylidene difluoride enhanced chemiluminescence phosphate-buffered saline calponin homology domain. The Ras superfamily comprises a group of small GTPases that function in intracellular signaling cascades, affecting cell growth and differentiation as well as cytoskeletal organization, vesicle trafficking, and nuclear transport (1Boguski M.S. McCormick F. Nature. 1993; 366: 643-654Crossref PubMed Scopus (1782) Google Scholar). These proteins act as molecular switches, alternating between an active GTP bound form and an inactive GDP bound form. Regulators include GTPase-activating proteins (GAPs)1 that facilitate conversion from active to inactive states, guanine nucleotide exchange factors that catalyze release of GDP from the GTPase, and GDP-dissociation inhibitory factors that inhibit both GAP-mediated and intrinsic GTP hydrolysis. Deregulation of both GTPases and their regulators may be involved in cell cycle derangement and oncogenesis (2Barbacid M. Eur. J. Clin. Invest. 1990; 20: 225-235Crossref PubMed Scopus (262) Google Scholar). The Rho family GTPase Cdc42 is involved in cytoskeletal rearrangement and cell cycle progression (3Symons M. Trends Biochem. Sci. 1996; 21: 178-181Abstract Full Text PDF PubMed Scopus (260) Google Scholar, 4Olson M.F. Ashworth A. Hall A. Science. 1995; 269: 1270-1272Crossref PubMed Scopus (1063) Google Scholar). Specifically, active Cdc42 stimulates filopodium and microspike formation in fibroblasts (5Nobes C.D. Hall A. Cell. 1995; 81: 53-62Abstract Full Text PDF PubMed Scopus (3783) Google Scholar, 6Kozma R. Ahmed S. Best A. Lim L. Mol. Cell. Biol. 1995; 15: 1942-1952Crossref PubMed Scopus (889) Google Scholar) and polarization of actin and microtubules in T cells (7Stowers L. Yelon D. Berg L.J. Chant J. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 5027-5031Crossref PubMed Scopus (347) Google Scholar). In addition, microinjection of Cdc42 into Swiss 3T3 fibroblasts stimulates DNA synthesis (4Olson M.F. Ashworth A. Hall A. Science. 1995; 269: 1270-1272Crossref PubMed Scopus (1063) Google Scholar). It is therefore not surprising that Cdc42 has been implicated in carcinogenesis. Transforming potential has been confirmed by overexpression studies in which dominant negative mutants of Cdc42 inhibit Ras-mediated transformation, whereas constitutively active mutants promote anchorage-independent growth in Rat1 fibroblasts (8Qiu R.G. Abo A. McCormick F. Symons M. Mol. Cell. Biol. 1997; 17: 3449-3458Crossref PubMed Scopus (269) Google Scholar). Downstream effectors of Cdc42 are being elucidated and may include phosphatidylinositol 3-kinase (9Zheng Y. Bagrodia S. Cerione R.A. J. Biol. Chem. 1994; 269: 18727-18730Abstract Full Text PDF PubMed Google Scholar) and pp70S6K (10Chow M.M. Blenis J. Cell. 1996; 85: 573-583Abstract Full Text Full Text PDF PubMed Scopus (269) Google Scholar), which are involved in cell cycle progression, and the Wiskott-Aldrich syndrome protein (11Symons M. Derry J.M.J. Karlak B. Jiang S. Lemahieu V. McCormick F. Francke U. Abo A. Cell. 1996; 84: 723-734Abstract Full Text Full Text PDF PubMed Scopus (749) Google Scholar) and n-chimaerin (12Kozma R. Ahmed S. Best A. Lim L. Mol. Cell. Biol. 1996; 16: 5069-5080Crossref PubMed Scopus (131) Google Scholar), which may regulate actin polymerization. Cdc42 also stimulates p21-activated Ser/Thr kinases, which in turn regulate the activation of the nuclear mitogenic protein kinases, c-Jun kinase and p38 (13Coso O.A. Chiariello M. Yu J.C. Teramoto H. Crespo P. Xu N. Miki T. Gutkind J.S. Cell. 1995; 81: 1137-1146Abstract Full Text PDF PubMed Scopus (1574) Google Scholar, 14Minden A. Lin A. Claret F.X. Abo A. Karin M. Cell. 1995; 81: 1147-1157Abstract Full Text PDF PubMed Scopus (1450) Google Scholar). Interestingly, another potential downstream effector of Cdc42, IQGAP1, displays significant sequence similarity to Sar1 and the tumor suppressor neurofibromin (15Weissbach L. Settleman J. Kalady M.F. Snijders A.J. Murthy A.E. Yan Y.-X. Bernards A. J. Biol. Chem. 1994; 269: 20517-20521Abstract Full Text PDF PubMed Google Scholar), and is the major calmodulin-binding protein in Ca2+-free breast cell lysates (16Joyal J.L. Annan R.S. Ho Y.D. Huddleston M.E. Carr S.A. Hart M.J. Sacks D.B. J. Biol. Chem. 1997; 272: 15419-15425Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar). Calmodulin is a highly conserved, ubiquitous protein involved in diverse Ca2+-dependent cellular processes, including cell cycle progression and proliferation, cyclic nucleotide metabolism, glycogen metabolism, cytoskeletal arrangement, and smooth muscle contraction (17Cohen P. Klee C.B. Calmodulin. Elsevier Science Publishing Co., Inc., New York1988Google Scholar). It possesses four Ca2+-binding sites, occupation of which effects a conformational change that facilitates association with multiple target proteins. Binding to calmodulin occurs via either basic amphiphilic α-helices or IQ motifs, 23 amino acid sequences with the consensus IQXXXRGXXXR (18Houdusse A. Cohen C. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 10644-10647Crossref PubMed Scopus (101) Google Scholar). A “complete” IQ motif contains a C-terminal arginine in its consensus sequence, dictating no Ca2+ requirement for calmodulin binding. Alternatively, an “incomplete” IQ motif without arginine requires Ca2+for binding (18Houdusse A. Cohen C. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 10644-10647Crossref PubMed Scopus (101) Google Scholar). A substantial body of evidence implicates calmodulin in carcinogenesis. For example, the level of calmodulin is significantly increased in malignant tissue (19Van Eldik L.J. Burgess W.H. J. Biol. Chem. 1983; 258: 4539-4547Abstract Full Text PDF PubMed Google Scholar), including breast carcinoma (20Singer A.L. Sherwin R.P. Dunn A.S. Appleman M.M. Cancer Res. 1976; 36: 60-66PubMed Google Scholar), and overexpression of calmodulin alters cell morphology and shortens the cell cycle (21Rasmussen C.D. Means A.R. Cell. Motil. Cytoskel. 1992; 21: 45-57Crossref PubMed Scopus (12) Google Scholar). Although a causal relationship between calmodulin concentration and malignancy has not been demonstrated, it is hypothesized that increased concentrations of calmodulin may contribute to neoplastic transformation. The downstream effectors of such transformation are unknown. One candidate is the recently isolated 189-kDa protein, IQGAP1. IQGAP1 contains three complete IQ motifs, one incomplete IQ, and a N-terminal region homologous to the actin and calmodulin-binding domain of calponin (22Hart M.F. Callow M.G. Souza B. Polakis P. EMBO J. 1996; 15: 2887-3005Crossref Scopus (335) Google Scholar, 23Mezgueldi M. Mendre C. Calas B. Kassab R. Fattoum A. J. Biol. Chem. 1995; 270: 8867-8876Abstract Full Text Full Text PDF PubMed Scopus (61) Google Scholar) (Fig. 1). In addition, IQGAP1 contains a region with significant sequence similarity to the catalytic domain of Ras-GAPs (15Weissbach L. Settleman J. Kalady M.F. Snijders A.J. Murthy A.E. Yan Y.-X. Bernards A. J. Biol. Chem. 1994; 269: 20517-20521Abstract Full Text PDF PubMed Google Scholar). IQGAP1 binds Rac and Cdc42 (22Hart M.F. Callow M.G. Souza B. Polakis P. EMBO J. 1996; 15: 2887-3005Crossref Scopus (335) Google Scholar) and also cross-links microfilaments (24Bashour A.M. Fullerton A.T. Hart M.J. Bloom G.S. J. Cell Biol. 1997; 137: 1555-1566Crossref PubMed Scopus (216) Google Scholar). Recently, it was demonstrated that calmodulin binds the N-terminal region of IQGAP1 (22Hart M.F. Callow M.G. Souza B. Polakis P. EMBO J. 1996; 15: 2887-3005Crossref Scopus (335) Google Scholar), which contains both the calponin homology domain (CHD) and the IQ motifs. Ca2+/calmodulin attenuates the association of IQGAP1 with Cdc42 (16Joyal J.L. Annan R.S. Ho Y.D. Huddleston M.E. Carr S.A. Hart M.J. Sacks D.B. J. Biol. Chem. 1997; 272: 15419-15425Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar) and F-actin (24Bashour A.M. Fullerton A.T. Hart M.J. Bloom G.S. J. Cell Biol. 1997; 137: 1555-1566Crossref PubMed Scopus (216) Google Scholar). IQGAP1 thus appears to be an actin-associated protein that can transduce Ca2+/calmodulin signals to Cdc42 at the cytoskeleton. To further explore the functional sequelae of the interaction between Ca2+/calmodulin and IQGAP1, we isolated both full-length endogenous human IQGAP1 and glutathione S-transferase (GST) fusion constructs containing selected regions of IQGAP1. We show here that Ca2+ binds directly to IQGAP1 and modulates the IQGAP1-mediated inhibition of Cdc42-catalyzed GTP hydrolysis. We also present evidence that Ca2+/calmodulin competes with F-actin for binding to the CHD of IQGAP1. Finally, we demonstrate that Ca2+ the binding of calmodulin to IQGAP1 the release of Cdc42 from IQGAP1. We that IQGAP1 may a molecular link between Ca2+/calmodulin signaling and from was from of the fusion protein has been (22Hart M.F. Callow M.G. Souza B. Polakis P. EMBO J. 1996; 15: 2887-3005Crossref Scopus (335) Google Scholar). and DNA from New for from from was from was from of and from and was by of of has been D.B. Biochem. PubMed Scopus Google Scholar). To amino was with and was by glutathione was by with the N-terminal of IQGAP1. For was by protein human breast carcinoma cells in with in a three in phosphate-buffered saline and in A and with or and in at IQGAP1 was isolated from cells by as (16Joyal J.L. Annan R.S. Ho Y.D. Huddleston M.E. Carr S.A. Hart M.J. Sacks D.B. J. Biol. Chem. 1997; 272: 15419-15425Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar). The of and amino (Fig. has been (22Hart M.F. Callow M.G. Souza B. Polakis P. EMBO J. 1996; 15: 2887-3005Crossref Scopus (335) Google Scholar). The for homology domain, amino was by of gel of and To the for motifs, amino on containing full-length of IQGAP1 was and of IQGAP1, with the containing The was gel and with with DNA The was and into the of fusion proteins in by in and was to and by on and with containing fusion proteins with glutathione and fusion proteins to the on of was with in a containing and for Cdc42 was with GTP by with in for of full-length IQGAP1, calmodulin, and to of GTP was by at without in The was by containing on and with a with containing and GTP by Cdc42 was by GTP was the between in the presence and absence of and IQGAP1 The was with at in containing and for at in for and to To that protein bound to the the was with in and acid for at by in and acid for of and with in A containing or for at on a of F-actin or four in A. by and to with with and by was also by fusion protein and concentrations of F-actin in and for at for at proteins isolated with and with by to and with with and by IQGAP1 was isolated from cell lysates with the four with A containing F-actin was and in A with for at four with by to and with cells in A containing or and to at for at to The for protein or was with for at to bound guanine and with in for to lysates with of or for at and isolated with for at on a In of protein from cells with in A containing either or for at on a with by and to with or with and by cells in A containing and to at for at into one of which was to with protein for at or was to the for at by for at with protein by with and for at in by to and with or with the or and by cells in A containing or and to at for at The for protein of with in A with or for at on a by the of either or to for an at by and in the by at for in by and to of was in with with and by assays the from of signals was in and with was with and for to was by We demonstrated that Ca2+/calmodulin modulates the association of IQGAP1 with Cdc42 (16Joyal J.L. Annan R.S. Ho Y.D. Huddleston M.E. Carr S.A. Hart M.J. Sacks D.B. J. Biol. Chem. 1997; 272: 15419-15425Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar), but no of function was To the of both Ca2+/calmodulin and IQGAP1 on Cdc42 GTPase activity was by binding assays (Fig. Ca2+ not significantly GTP in the Ca2+/calmodulin a significant of the intrinsic GTPase activity of Cdc42 In IQGAP1 GTPase activity of Cdc42 by in the absence of inhibition was not by the of calmodulin (Fig. of Ca2+ abrogated the effect of IQGAP1 on Cdc42-catalyzed GTP hydrolysis. of which the association of Cdc42 with IQGAP1 (16Joyal J.L. Annan R.S. Ho Y.D. Huddleston M.E. Carr S.A. Hart M.J. Sacks D.B. J. Biol. Chem. 1997; 272: 15419-15425Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar), with IQGAP1 of the GTPase activity of Cdc42 by IQGAP1 (Fig. The effect of Ca2+ on the of Cdc42 activity by IQGAP1 that IQGAP1 may To this fusion proteins containing the or C-terminal regions of IQGAP1, and IQGAP1 on confirmed that proteins bound to the with not bound directly to full-length IQGAP1 and to the but not the C-terminal region of IQGAP1 (Fig. To further the Ca2+-binding fusion proteins of the IQ and CHD regions of IQGAP1 demonstrated Ca2+ binding to the CHD of IQGAP1, but not to the IQ region (Fig. not Calmodulin binds to the N-terminal region of IQGAP1 (22Hart M.F. Callow M.G. Souza B. Polakis P. EMBO J. 1996; 15: 2887-3005Crossref Scopus (335) Google Scholar), but the of the calmodulin-binding on IQGAP1 not been regions include the four IQ motifs and the which displays sequence similarity to the calmodulin-binding region of calponin (22Hart M.F. Callow M.G. Souza B. Polakis P. EMBO J. 1996; 15: 2887-3005Crossref Scopus (335) Google Scholar). we the binding of fusion proteins containing the or IQ motifs of IQGAP1 to calmodulin bound to both the N-terminal domain and IQ motifs of IQGAP1 (Fig. was with full-length IQGAP1 (16Joyal J.L. Annan R.S. Ho Y.D. Huddleston M.E. Carr S.A. Hart M.J. Sacks D.B. J. Biol. Chem. 1997; 272: 15419-15425Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar), binding of calmodulin to both of fusion proteins was in the presence in the absence of Ca2+/calmodulin also bound to the CHD of IQGAP1, but not (Fig. on it is that the binding of Ca2+/calmodulin for the CHD was for the IQ motifs. with data (22Hart M.F. Callow M.G. Souza B. Polakis P. EMBO J. 1996; 15: 2887-3005Crossref Scopus (335) Google Scholar), bound to calmodulin The molecular present in the and of the fusion proteins The CHD displays sequence similarity to the region of calponin (22Hart M.F. Callow M.G. Souza B. Polakis P. EMBO J. 1996; 15: 2887-3005Crossref Scopus (335) Google Scholar, 23Mezgueldi M. Mendre C. Calas B. Kassab R. Fattoum A. J. Biol. Chem. 1995; 270: 8867-8876Abstract Full Text Full Text PDF PubMed Scopus (61) Google Scholar). actin with IQGAP1 (24Bashour A.M. Fullerton A.T. Hart M.J. Bloom G.S. J. Cell Biol. 1997; 137: 1555-1566Crossref PubMed Scopus (216) Google Scholar), the of actin to with calmodulin for binding to the CHD of IQGAP1 was Addition of F-actin the binding of the to calmodulin of a binding by F-actin was also with calmodulin in and isolated with the with confirmed that F-actin calmodulin binding to the CHD of IQGAP1. not decrease calmodulin binding to the CHD not by actin revealed that F-actin bound to the CHD of IQGAP1 not calmodulin binding to the IQ region (Fig. and to full-length IQGAP1 not by the F-actin concentration by to no effect not is calmodulin in the in the in of the significantly of calmodulin for the IQ region Ca2+/calmodulin vitro binding of IQGAP1 to Cdc42 (16Joyal J.L. Annan R.S. Ho Y.D. Huddleston M.E. Carr S.A. Hart M.J. Sacks D.B. J. Biol. Chem. 1997; 272: 15419-15425Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar). To this interaction in a cellular we the effect of calmodulin on the binding of to endogenous IQGAP1 in human breast cell IQGAP1 bound to (Fig. Binding was in the absence in the presence of In addition, calmodulin was isolated from cell lysates by (Fig. calmodulin not directly with Cdc42 (16Joyal J.L. Annan R.S. Ho Y.D. Huddleston M.E. Carr S.A. Hart M.J. Sacks D.B. J. Biol. Chem. 1997; 272: 15419-15425Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar, M.F. Callow M.G. Souza B. Polakis P. EMBO J. 1996; 15: 2887-3005Crossref Scopus (335) Google Scholar), the calmodulin was bound to IQGAP1 in a ternary complex with Cdc42. The of calmodulin of Ca2+ (Fig. can be for by Ca2+/calmodulin may a decrease in the of IQGAP1 for Cdc42 (16Joyal J.L. Annan R.S. Ho Y.D. Huddleston M.E. Carr S.A. Hart M.J. Sacks D.B. J. Biol. Chem. 1997; 272: 15419-15425Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar). that has to be is a decrease in the of IQGAP1 for To between was Ca2+ increased the binding of endogenous IQGAP1 to calmodulin by with a concomitant decrease in the of endogenous Cdc42 that was by the These data suggest that Ca2+ the binding of calmodulin to IQGAP1 and that this binding effects the release of Cdc42 from IQGAP1. The of the was it in IQGAP1 was the protein in cell lysates by the (Fig. The which was the N-terminal of IQGAP1, bound to but not not The effect of Ca2+ on the interaction calmodulin, Cdc42, and IQGAP1 was further by Ca2+ not the binding of IQGAP1 to its (Fig. Ca2+ by the of Cdc42 that with IQGAP1 (Fig. with the data in calmodulin with IQGAP1 in the presence of in the absence of Ca2+ (Fig. of the revealed that a small of calmodulin with IQGAP1 in the absence of Ca2+ not The of the is revealed by the absence of proteins in the with (Fig. in vitro assays that Ca2+ not the binding of IQGAP1 to Cdc42 (16Joyal J.L. Annan R.S. Ho Y.D. Huddleston M.E. Carr S.A. Hart M.J. Sacks D.B. J. Biol. Chem. 1997; 272: 15419-15425Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar). IQGAP1 from lysates bound to in the presence in the absence of Ca2+ (Fig. Moreover, Ca2+ the of Cdc42 that with IQGAP1 (Fig. for may be that in the cellular Ca2+/calmodulin binds a substantial of endogenous IQGAP1, its association with Cdc42. To this cell lysates to with in the presence of Ca2+, but not with by the of IQGAP1 in lysates (Fig. with an not the of IQGAP1 in the These data that a significant of endogenous IQGAP1 is bound to endogenous with the data in suggest that Ca2+ the binding of calmodulin to IQGAP1 and thus the of Cdc42 from IQGAP1. Cdc42 facilitates cell cycle progression and cytoskeletal rearrangement (3Symons M. Trends Biochem. Sci. 1996; 21: 178-181Abstract Full Text PDF PubMed Scopus (260) Google Scholar, 4Olson M.F. Ashworth A. Hall A. Science. 1995; 269: 1270-1272Crossref PubMed Scopus (1063) Google Scholar). the regulators that Cdc42 function and the downstream effectors that link Cdc42 to actin In this we the Ca2+, calmodulin, IQGAP1, and Cdc42. in vitro data a in which Ca2+/calmodulin regulates GTPase activity the protein IQGAP1 (16Joyal J.L. Annan R.S. Ho Y.D. Huddleston M.E. Carr S.A. Hart M.J. Sacks D.B. J. Biol. Chem. 1997; 272: 15419-15425Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar, M.F. Callow M.G. Souza B. Polakis P. EMBO J. 1996; 15: 2887-3005Crossref Scopus (335) Google Scholar). The C-terminal region of IQGAP1 has significant sequence similarity to the catalytic domain of Ras-GAPs and has been hypothesized to act as a (15Weissbach L. Settleman J. Kalady M.F. Snijders A.J. Murthy A.E. Yan Y.-X. Bernards A. J. Biol. Chem. 1994; 269: 20517-20521Abstract Full Text PDF PubMed Google Scholar). data from Hart (22Hart M.F. Callow M.G. Souza B. Polakis P. EMBO J. 1996; 15: 2887-3005Crossref Scopus (335) Google Scholar) that IQGAP1 the GTP-bound of Cdc42. To this the effect of endogenous IQGAP1 on Cdc42 activity was We demonstrate that Ca2+, of calmodulin, modulates the of Cdc42 activity by IQGAP1. Specifically, IQGAP1 Cdc42 in its active GTP-bound to inhibit the intrinsic GTPase activity of Cdc42. not the binding of IQGAP1 to Cdc42 (16Joyal J.L. Annan R.S. Ho Y.D. Huddleston M.E. Carr S.A. Hart M.J. Sacks D.B. J. Biol. Chem. 1997; 272: 15419-15425Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar), this Ca2+ effect is not to of IQGAP1 from Cdc42. We that the effect of Ca2+ on IQGAP1 may be to association of Ca2+ with IQGAP1. amino in the N-terminal region of IQGAP1 significant sequence similarity to a Ca2+-binding muscle protein (15Weissbach L. Settleman J. Kalady M.F. Snijders A.J. Murthy A.E. Yan Y.-X. Bernards A. J. Biol. Chem. 1994; 269: 20517-20521Abstract Full Text PDF PubMed Google Scholar, A. P. C. J. Cell Biol. PubMed Scopus Google Scholar). the N-terminal region and CHD amino of IQGAP1 to Ca2+ directly the homology region that with Cdc42 is in the C-terminal of IQGAP1 Ca2+ binding a conformational change in IQGAP1. Our revealed that calmodulin in the presence of Ca2+ effects the of IQGAP1 from Cdc42 (16Joyal J.L. Annan R.S. Ho Y.D. Huddleston M.E. Carr S.A. Hart M.J. Sacks D.B. J. Biol. Chem. 1997; 272: 15419-15425Abstract Full Text Full Text PDF PubMed Scopus (126) Google Ca2+/calmodulin appears to be a negative of Cdc42 activity on In the absence of Ca2+, IQGAP1 binds Cdc42 and it in its active GTP bound state. in Ca2+ concentration both the inhibition of Cdc42 GTPase activity by IQGAP1 and, in the presence of calmodulin, IQGAP1 from Cdc42. Although we not demonstrated this in the data in for this in the cell The presented in further a for calmodulin in IQGAP1 as a substantial of endogenous IQGAP1 is bound to A for Ca2+/calmodulin of Cdc42 via IQGAP1 A.M. Fullerton A.T. Hart M.J. Bloom G.S. J. Cell Biol. 1997; 137: 1555-1566Crossref PubMed Scopus (216) Google Scholar) that IQGAP1 interacts with and cross-links Calmodulin this In addition, studies IQGAP1 with microfilaments in and (24Bashour A.M. Fullerton A.T. Hart M.J. Bloom G.S. J. Cell Biol. 1997; 137: 1555-1566Crossref PubMed Scopus (216) Google Scholar). we demonstrate that Ca2+/calmodulin binds to the CHD of IQGAP1 and F-actin this F-actin not directly to calmodulin (17Cohen P. Klee C.B. Calmodulin. Elsevier Science Publishing Co., Inc., New York1988Google Scholar), data that F-actin binds to the CHD of IQGAP1. Ca2+/calmodulin and F-actin for binding to the IQGAP1 may Cdc42 to microfilaments in the absence of GTP-bound Cdc42 with IQGAP1 and F-actin Cerione R.A. Hart M.J. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar) and enhanced F-actin by IQGAP1 M. S. T. Y. A. A. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar). In the presence of Ca2+, calmodulin may IQGAP1 from not Cdc42 but also from a of Cdc42 from the of in vitro GTPase are in cells and IQGAP1 inhibit the intrinsic GTPase activity of Cdc42 in the absence of Ca2+, an GTP-bound Cdc42 may act of IQGAP1 and may inhibit a function of IQGAP1 at the cytoskeleton. In the absence of Ca2+, IQGAP1 may Cdc42, its active GTP-bound and it to cytoskeletal active Cdc42 may inhibit the effect of IQGAP1 on cytoskeletal rearrangement. intracellular the inhibition of Cdc42 GTPase activity by IQGAP1 and may IQGAP1 from both Cdc42 and In this IQGAP1 is both a of Cdc42 and and a downstream target of Cdc42 The N-terminal of IQGAP1 binds Ca2+/calmodulin and the C-terminal interacts with Cdc42. IQGAP1 may as a for a actin providing a molecular link between the and Cdc42 and a by The functional sequelae of the interaction of Ca2+/calmodulin with IQGAP1 The interaction between calmodulin and IQGAP1 is as IQGAP1 is the calmodulin-binding protein in Ca2+-free breast carcinoma cell lysates (16Joyal J.L. Annan R.S. Ho Y.D. Huddleston M.E. Carr S.A. Hart M.J. Sacks D.B. J. Biol. Chem. 1997; 272: 15419-15425Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar) and a substantial of endogenous IQGAP1 is bound to Ca2+/calmodulin in the cellular Cdc42 in cell and of the of Ca2+ and calmodulin in such may IQGAP1 as an We Hart for for the and for providing the