The IQGAP1-Rac1 and IQGAP1-Cdc42 interactions: interfaces differ between the complexes

IQGAP1 contains a domain related to the catalytic portion of the GTPase-activating proteins (GAPs) for the Ras small G proteins, yet it has no RasGAP activity and binds to the Rho family small G proteins Cdc42 and Rac1. It is thought that IQGAP1 is an effector of Rac1 and Cdc42, regulating cell-cell adhesion through the E-cadherin-catenin complex, which controls formation and maintenance of adherens junctions. This study investigates the binding interfaces of the Rac1-IQGAP1 and Cdc42-IQGAP1 complexes. We mutated Rac1 and Cdc42 and measured the effects of mutations on their affinity for IQGAP1. We have identified similarities and differences in the relative importance of residues used by Rac1 and Cdc42 to bind IQGAP1. Furthermore, the residues involved in the complexes formed with IQGAP1 differ from those formed with other effector proteins and GAPs. Relatively few mutations in switch I of Cdc42 or Rac1 affect IQGAP1 binding; only mutations in residues 32 and 36 significantly decrease affinity for IQGAP1. Switch II mutations also affect binding to IQGAP1 although the effects differ between Rac1 and Cdc42; mutation of either Asp-63, Arg-68, or Leu-70 abrogate Rac1 binding, whereas no switch II mutations affect Cdc42 binding to IQGAP1. The Rho family "insert loop" does not contribute to the binding affinity of Rac1/Cdc42 for IQGAP1. We also present thermodynamic data pertaining to the Rac1/Cdc42-RhoGAP complexes. Switch II contributes a large portion of the total binding energy to these complexes, whereas switch I mutations also affect binding. In addition we identify "cold spots" in the Rac1/Cdc42-RhoGAP/IQGAP1 interfaces. Competition data reveal that the binding sites for IQGAP1 and RhoGAP on the small G proteins overlap only partially. Overall, the data presented here suggest that, despite their 71% identity, Cdc42 and Rac1 appear to have only partially overlapping binding sites on IQGAP1, and each uses different determinants to achieve high affinity binding.     

Localization of the PAK1-, WASP-, and IQGAP1-specifying regions of Cdc42.

The Rho family small GTPase Cdc42 transmits divergent intracellular signals through multiple effector proteins to elicit cellular responses such as cytoskeletal reorganization. Potential effectors of Cdc42 implicated in mediating its cytoskeletal effect in mammalian cells include PAK1, WASP, and IQGAP1. To investigate the determinants of Cdc42-effector specificity, we utilized recombinant Cdc42 mutants and chimeras made between Cdc42 and RhoA to map the regions of Cdc42 contributing to specific effector p21-binding domain (PBD) interaction. Site-directed mutants of the switch I domain and neighboring regions of Cdc42 demonstrated differential binding patterns toward the PBDs of PAK1, WASP, and IQGAP1, suggesting that switch I provides essential determinants for the effector binding, but recognition of each effector by Cdc42 involves a distinct mechanism. Differing from Rac1, the switch I domain and the surrounding region (amino acids 29 to 55) of Cdc42 appeared to be sufficient for specific binding to PAK1, whereas determinants outside the switch I domain, residues 157-191 and 84-120 in particular, were necessary and sufficient to confer specificity to WASP and IQGAP1, respectively. In addition, IQGAP1, but not PAK1 nor WASP, required the unique "insert region," residues 122-134, of Cdc42 to achieve high affinity binding. Microinjection of the constitutively active Cdc42/RhoA chimeras into serum-starved Swiss 3T3 cells showed that although preserving PAK1- and WASP-binding activity could retain the peripheral actin microspike (PAM)-inducing activity of Cdc42, interaction with PAK1 or WASP was not required for this activity. Moreover, IQGAP1-binding alone by Cdc42 was insufficient for PAM-induction. Thus, Cdc42 utilizes multiple distinct structural determinants to specify different effector recognition and to elicit PAM-inducing effect.     

The Ras GTPase-activating-protein-related human protein IQGAP2 harbors a potential actin binding domain and interacts with calmodulin and Rho family GTPases.

We previously described IQGAP1 as a human protein related to a putative Ras GTPase-activating protein (RasGAP) from the fission yeast Schizosaccharomyces pombe. Here we report the identification of a liver-specific human protein that is 62% identical to IQGAP1. Like IQGAP1, the novel IQGAP2 protein harbors an N-terminal calponin homology motif which functions as an F-actin binding domain in members of the spectrin, filamin, and fimbrin families. Both IQGAPs also harbor several copies of a novel 50- to 55-amino-acid repeat, a single WW domain, and four IQ motifs and have 25% sequence identity with almost the entire S. pombe sar1 RasGAP homolog. As predicted by the presence of IQ motifs, IQGAP2 binds calmodulin. However, neither full-length nor truncated IQGAP2 stimulated the GTPase activity of Ras or its close relatives. Instead, IQGAP2 binds Cdc42 and Racl but not RhoA. This interaction involves the C-terminal half of IQGAP2 and appears to be independent of the nucleotide binding status of the GTPases. Although IQGAP2 shows no GAP activity towards Cdc42 and Rac1, the protein did inhibit both the intrinsic and RhoGAP-stimulated GTP hydrolysis rates of Cdc42 and Rac1, suggesting an alternative mechanism via which IQGAPs might modulate signaling by these GTPases. Since IQGAPs harbor a potential actin binding domain, they could play roles in the Cdc42 and Rac1 controlled generation of specific actin structures.     

Salmonella enterica serotype Typhimurium usurps the scaffold protein IQGAP1 to manipulate Rac1 and MAPK signalling

Salmonella enterica serotype Typhimurium invades eukaryotic cells by re-arranging the host-cell cytoskeleton. However, the precise mechanisms by which Salmonella induces cytoskeletal changes remain undefined. IQGAP1 (IQ motif-containing GTPase-activating protein 1) is a scaffold protein that binds multiple proteins including actin, the Rho GTPases Rac1 and Cdc42 (cell division cycle 42), and components of the MAPK (mitogen-activated protein kinase) pathway. We have shown previously that optimal invasion of Salmonella into HeLa cells requires IQGAP1. In the present paper, we use IQGAP1-null MEFs (mouse embryonic fibroblasts) and selected well-characterized IQGAP1 mutant constructs to dissect the molecular determinants of Salmonella invasion. Knockout of IQGAP1 expression reduced Salmonella invasion into MEFs by 75%. Reconstituting IQGAP1-null MEFs with wild-type IQGAP1 completely rescued invasion. By contrast, reconstituting IQGAP1-null cells with mutant IQGAP1 constructs that specifically lack binding to either Cdc42 and Rac1 (termed IQGAP1ΔMK24), actin, MEK [MAPK/ERK (extracellular-signal-regulated kinase) kinase] or ERK only partially restored Salmonella entry. Cell-permeant inhibitors of Rac1 activation or MAPK signalling reduced Salmonella invasion into control cells by 50%, but had no effect on bacterial entry into IQGAP1-null MEFs. Importantly, the ability of IQGAP1ΔMK24 to promote Salmonella invasion into IQGAP1-null cells was abrogated by chemical inhibition of MAPK signalling. Collectively, these results imply that the scaffolding function of IQGAP1, which integrates Rac1 and MAPK signalling, is usurped by Salmonella to invade fibroblasts and suggest that IQGAP1 may be a potential therapeutic target for Salmonella pathogenesis.     

IQGAP1 is a component of Cdc42 signaling to the cytoskeleton

The Ras-GAP related protein IQGAP1 binds several proteins, including actin, calmodulin, E-cadherin and the Rho family GTPase Cdc42. To gain insight into its in vivo function, IQGAP1 was overexpressed in mammalian cells. Transfection of IQGAP1 significantly increased the levels of active, GTP-bound Cdc42, resulting in the formation of peripheral actin microspikes. By contrast, transfection of an IQGAP1 mutant lacking part of the GAP-related domain (IQGAP1deltaGRD) substantially decreased the amount of GTP-bound Cdc42 in cell lysates. Consistent with these findings, IQGAP1DeltaGRD blocked Cdc42 function in cells that stably overexpress constitutively active Cdc42 and abrogated the effect of bradykinin on Cdc42. In cells transfected with IQGAP1deltaGRD, bradykinin was unable to activate Cdc42, translocate Cdc42 to the membrane fraction, or induce filopodia production. IQGAP1deltaGRD transfection altered cellular morphology, producing small, round cells that closely resemble Cdc42-/- cells. Some insight into the mechanism was provided by in vitro analysis, which revealed that IQGAP1deltaGRD increased the intrinsic GTPase activity of Cdc42, thereby increasing the amount of inactive, GDP-bound Cdc42. These data imply that IQGAP1 has a crucial role in transducing Cdc42 signaling to the cytoskeleton.     

IQGAP1 promotes cell motility and invasion

The dynamic processes of cell migration and invasion are largely coordinated by Rho family GTPases. The scaffolding protein IQGAP1 binds to Cdc42, increasing the amount of active Cdc42 both in vitro and in cells. Here we show that overexpression of IQGAP1 in mammalian cells enhances cell migration in a Cdc42- and Rac1-dependent manner. Importantly, cell motility was significantly decreased both by knock down of endogenous IQGAP1 using small interfering RNA and by transfection of a dominant negative IQGAP1 construct, IQGAP1DeltaGRD. Cell invasion was similarly altered by manipulating intracellular IQGAP1 concentrations. Moreover, invasion mediated by constitutively active Cdc42 was attenuated by IQGAP1DeltaGRD. Thus, IQGAP1 has a fundamental role in cell motility and invasion.     

RhoG signals in parallel with Rac1 and Cdc42

RhoG is a member of the Rho family of small GTPases and shares high sequence identity with Rac1 and Cdc42. Previous studies suggested that RhoG mediates its effects through activation of Rac1 and Cdc42. To further understand the mechanism of RhoG signaling, we studied its potential activation pathways, downstream signaling properties, and functional relationship to Rac1 and Cdc42 in vivo. First, we determined that RhoG was regulated by guanine nucleotide exchange factors that also activate Rac and/or Cdc42. Vav2 (which activates RhoA, Rac1, and Cdc42) and to a lesser degree Dbs (which activates RhoA and Cdc42) activated RhoG in vitro. Thus, RhoG may be activated concurrently with Rac1 and Cdc42. Second, some effectors of Rac/Cdc42 (IQGAP2, MLK-3, PLD1), but not others (e.g. PAKs, POSH, WASP, Par-6, IRSp53), interacted with RhoG in a GTP-dependent manner. Third, consistent with this differential interaction with effectors, activated RhoG stimulated some (JNK and Akt) but not other (SRF and NF-kappaB) downstream signaling targets of activated Rac1 and Cdc42. Finally, transient transduction of a tat-tagged Rac1(17N) dominant-negative fusion protein inhibited the induction of lamellipodia by the Rac-specific activator, Tiam1, but not by activated RhoG. Together, these data argue that RhoG function is mediated by signals independent of Rac1 and Cdc42 activation and instead by direct utilization of a subset of common effectors.     

Regulation of Spine Density and Morphology by IQGAP1 Protein Domains

IQGAP1 is a scaffolding protein that regulates spine number. We now show a differential role for IQGAP1 domains in spine morphogenesis, in which a region of the N-terminus that promotes Arp2/3-mediated actin polymerization and branching stimulates spine head formation while a region that binds to Cdc42 and Rac is required for stalk extension. Conversely, IQGAP1 rescues spine deficiency induced by expression of dominant negative Cdc42 by stimulating formation of stubby spines. Together, our observations place IQGAP1 as a crucial regulator of spine number and shape acting through the N-Wasp Arp2/3 complex, as well as upstream and downstream of Cdc42.     

Cdc42 protein acts upstream of IQGAP1 and regulates cytokinesis in mouse oocytes and embryos

Cdc42 and Rac1 Rho family GTPases, and their interacting protein IQGAP1 are the key regulators of cell polarity. We examined the role of Cdc42 and IQGAP1 in establishing the polarity of mouse oocyte and regulation of meiotic and mitotic divisions. We showed that Cdc42 was localized on the microtubules of meiotic and mitotic spindle and in the cortex of mouse oocytes and cleaving embryos. IQGAP1 was present in the cytoplasm and cortex of growing and fully-grown oocytes. During maturation it disappeared from the cortex and during meiotic and mitotic cytokinesis it concentrated in the contractile ring. Toxin B inhibition of the binding activity of Cdc42 changed the localization of IQGAP1, inhibited emission of the first polar body, and caused disappearance of the cortical actin without affecting the migration of meiotic spindle. This indicates, that in maturing oocytes accumulation of cortical actin is not indispensable for spindle migration. In zygotes treated with toxin B actin cytoskeleton was rearranged and the first and/or subsequent cytokinesis were inhibited. Our results indicate that Cdc42 acts upstream of IQGAP1 and is involved in regulation of cytokinesis in mouse oocytes and cleaving embryos, rather than in establishing the polarity of the oocyte.     

Regulation of cadherin-mediated cell-cell adhesion by the Rho family GTPases.

Reports in the past two years have shown that Cdc42, Rac1, and Rho - belonging to the Rho small GTPase family - participate in the regulation of cadherin-mediated cell-cell adhesion. IQGAP1, an effector of Cdc42 and Rac1, interacts with cadherin and beta-catenin and induces the dissociation of alpha-catenin from the cadherin-catenins complex leading to disruption of cell-cell adhesion: activated Cdc42 and Rac1 counteract the effect of IQGAP1. Thus, Cdc42 and Rac1 appear to regulate cadherin-mediated cell-cell adhesion acting through IQGAP1.     

BNIP-2 induces cell elongation and membrane protrusions by interacting with Cdc42 via a unique Cdc42-binding motif within its BNIP-2 and Cdc42GAP homology domain

The Cdc42 small GTPase regulates cytoskeletal reorganization and cell morphological changes that result in cellular extensions, migration, or cytokinesis. We previously showed that BNIP-2 interacted with Cdc42 and its cognate inactivator, p50RhoGAP/Cdc42GAP via its BNIP-2 and Cdc42GAP homology (BCH) domain, but its cellular and physiological roles still remain unclear. We report here that following transient expression of BNIP-2 in various cells, the expressed protein was located in irregular spots throughout the cytoplasm and concentrated at the leading edge of cellular extensions. The induced cell elongation and membrane protrusions required an intact BCH domain and were variously inhibited by coexpression of dominant negative mutants of Cdc42 (completely inhibited), Rac1 (partially inhibited), and RhoA (least inhibited). Presence of the Cdc42/Rac1 interactive binding (CRIB) motif alone as the dominant negative mutant of p21-activated kinase also inhibited the BNIP-2 effect. Bioinformatic analyses together with progressive deletional mutagenesis and binding studies revealed that a distal part of the BNIP-2 BCH domain contained a sequence with low homology to CRIB motif. However, in contrary to most effectors, BNIP-2 binding to Cdc42 was mediated exclusively via the unique sequence motif 285VPMEYVGI292. Cells expressing the BNIP-2 mutants devoid of this motif or/and the 34-amino acids immediately upstream to this sequence failed to elicit cell elongation and membrane protrusions despite that the protein still remained in the cytoplasm and interacted with Cdc42GAP. Evidence is presented where BNIP-2 in vivo induces cell dynamics by recruiting Cdc42 via its BCH domain, thus providing a novel mechanism for regulating Cdc42 signaling pathway.     

Self-association of IQGAP1: characterization and functional sequelae

The scaffolding protein IQGAP1 participates in numerous cellular functions by binding to target proteins such as actin, calmodulin, E-cadherin, beta-catenin, Cdc42, Rac1, and CLIP-170. IQGAP1 regulates the cytoskeleton, promotes cell motility, and modulates E-cadherin-mediated cell-cell adhesion. However, how IQGAP1 exerts its functions in vivo is still unclear. In this study we investigate the self-association of IQGAP1 and its role in IQGAP1 function. Endogenous IQGAP1 co-immunoprecipitated from MCF-7 cells with IQGAP1 tagged with enhanced green fluorescent protein, indicating that IQGAP1 self-associates in cells. In vitro assays confirmed that IQGAP1 can self-associate and that this effect is mediated by the N-terminal half of the protein. Gel filtration analysis suggested that full-length IQGAP1 exists as a combination of monomers, dimers, and larger oligomers. Analysis performed with multiple fragments of IQGAP1 narrowed the self-association region to amino acids 763-863. In support of this observation, a peptide comprising residues 763-863 disrupted self-association of full-length IQGAP1 in a dose-dependent manner. Similarly, deleting this sequence from IQGAP1 abolished binding to full-length IQGAP1. In addition, the ability of IQGAP1 to increase the amount of active Cdc42 in cells is abrogated upon removal of this region. Consistent with these findings, transfection into cells of a peptide containing the self-association domain significantly reduced the amount of active Cdc42 in cell lysates. These observations define a sequence of IQGAP1 that is necessary for its oligomerization and demonstrate that self-association is required for the normal cellular function of IQGAP1.     

Gastric hyperplasia in mice lacking the putative Cdc42 effector IQGAP1

Human IQGAP1 is a widely expressed 190-kDa Cdc42-, Rac1-, and calmodulin-binding protein that interacts with F-actin in vivo and that can cross-link F-actin microfilaments in vitro. Recent results have implicated IQGAP1 as a component of pathways via which Cdc42 or Rac1 modulates cadherin-based cell adhesion (S. Kuroda et al., Science 281:832-835, 1998), whereas yeast IQGAP-related proteins have been found to play essential roles during cytokinesis. To identify critical in vivo functions of IQGAP1, we generated deficient mice by gene targeting. We demonstrate that IQGAP1 null mutants arise at normal frequency and show no obvious defects during development or for most of their adult life. Loss of IQGAP1 also does not affect tumor development or tumor progression, but mutant mice exhibit a significant (P < 0.0001) increase in late-onset gastric hyperplasia relative to wild-type animals of the same genetic background. While we cannot exclude that functional redundancy with IQGAP2 contributes to the lack of developmental phenotypes, the restricted expression pattern of IQGAP2 is not obviously altered in adult IQGAP1 mutant mice. Thus, IQGAP1 does not serve any essential nonredundant functions during murine development but may serve to maintain the integrity of the gastric mucosa in older animals.     

Interaction with IQGAP1 links APC to Rac1, Cdc42, and actin filaments during cell polarization and migration

Rho family GTPases, particularly Rac1 and Cdc42, are key regulators of cell polarization and directional migration. Adenomatous polyposis coli (APC) is also thought to play a pivotal role in polarized cell migration. We have found that IQGAP1, an effector of Rac1 and Cdc42, interacts directly with APC. IQGAP1 and APC localize interdependently to the leading edge in migrating Vero cells, and activated Rac1/Cdc42 form a ternary complex with IQGAP1 and APC. Depletion of either IQGAP1 or APC inhibits actin meshwork formation and polarized migration. Depletion of IQGAP1 or APC also disrupts localization of CLIP-170, a microtubule-stabilizing protein that interacts with IQGAP1. Taken together, these results suggest a model in which activation of Rac1 and Cdc42 in response to migration signals leads to recruitment of IQGAP1 and APC which, together with CLIP-170, form a complex that links the actin cytoskeleton and microtubule dynamics during cell polarization and directional migration.     

IQGAP1 regulates Salmonella invasion through interactions with actin, Rac1, and Cdc42

To infect host cells, Salmonella utilizes an intricate system to manipulate the actin cytoskeleton and promote bacterial uptake. Proteins injected into the host cell by Salmonella activate the Rho GTPases, Rac1 and Cdc42, to induce actin polymerization. Following uptake, a different set of proteins inactivates Rac1 and Cdc42, returning the cytoskeleton to normal. Although the signaling pathways allowing Salmonella to invade host cells are beginning to be understood, many of the contributing factors remain to be elucidated. IQGAP1 is a multidomain protein that influences numerous cellular functions, including modulation of Rac1/Cdc42 signaling and actin polymerization. Here, we report that IQGAP1 regulates Salmonella invasion. Through its interaction with actin, IQGAP1 co-localizes with Rac1, Cdc42, and actin at sites of bacterial uptake, whereas infection promotes the interaction of IQGAP1 with both Rac1 and Cdc42. Knockdown of IQGAP1 significantly reduces Salmonella invasion and abrogates activation of Cdc42 and Rac1 by Salmonella. Overexpression of IQGAP1 significantly increases the ability of Salmonella to enter host cells and required interaction with both actin and Cdc42/Rac1. Together, these data identify IQGAP1 as a novel regulator of Salmonella invasion.     

Involvement of IQGAP3, a regulator of Ras/ERK‐related cascade,in hepatocyte proliferation in mouse liver regeneration and development

The spatio‐temporal regulation of hepatocyte proliferation is a critical issue in liver regeneration. Here, in normal and regenerating liver as well as in developing liver, we examined its expression/localization of IQGAP3, which was most recently reported as a Ras/Rac/Cdc42‐binding proliferation factor associated with cell–cell contacts in epithelial‐type cells. In parallel, the expression/localization of Rac/Cdc42‐binding IQGAP1/2 was examined. IQGAP3 showed a specific expression in proliferating hepatocytes positive for the proliferating marker Ki‐67, the levels of expressions of mRNAs and proteins were significantly increased in hepatocytes in liver regeneration and development. In immunofluorescence, IQGAP3 was highly enriched at cell–cell contacts of hepatocytes. IQGAP1 and IQGAP2 were exclusively expressed in Kupffer and sinusoidal endothelial cells, respectively, in normal, regenerating, and developing liver. The expression of IQGAP1, but not of IQGAP2, was increased in CCl₄‐induced (but not in partial hepatectomy‐induced) liver regeneration. Exclusive expression/localization of IQGAP3 to hepatocytes in the liver likely reflects the specific involvement of the IQGAP3/Ras/ERK signaling cascade in hepatocyte proliferation in addition to the previously identified signaling pathways, possibly by integrating cell–cell contact‐related proliferating signaling events. On the other hand, the Rac/Cdc42‐binding properties of IQGAP1/2/3 may be related to the distinct modes of remodeling due to the different strategies which induced proliferation of liver cells; partial hepatectomy, CCl₄ injury, or embryonic development. Thus, the functional orchestration of Ras and the Ras homologous (Rho) family proteins Rac/Cdc42 likely plays a critical role in liver regeneration and development. J. Cell. Physiol. 220: 621–631, 2009. © 2009 Wiley‐Liss, Inc.     

Discoidin domain receptor 1 activation suppresses alpha2beta1 integrin-dependent cell spreading through inhibition of Cdc42 activity

Upregulation and overexpression of discoidin domain receptor 1 (DDR1) have been implied in the regulation of kidney development and progression of cancers. Our previous studies with Mardin-Darby canine kidney (MDCK) cells showed that overexpression of DDR1 inhibited cell spreading, whereas dominant negative DDR1 promoted cell spreading on collagen-coated dish. Cell spreading is an important characteristic for cell differentiation and survival. However, little is known about the molecular mechanisms underlying the role of DDR1 in cell spreading. We have found here a novel signaling pathway of DDR1 consisting of Cdc42 that regulates the assembly and disassembly of cytoskeleton and cell spreading in MDCK cells. Cell spreading involves the organization of cytoskeleton that is mainly regulated by Rho-family GTPases. We assessed the activity of Rho-family GTPases and transfected MDCK cells with constitutively active or dominant negative GTPases, and quantified the extent of cell spreading. These results showed that DDR1 decreased the filamentous actin ratio and Rac1/Cdc42 activities, but had no effects on RhoA activity. Neither constitutively active nor dominant negative Rac1 altered DDR1-inhibited cell spreading. Constitutively active Cdc42 could rescue the DDR1-inhibited cell spreading, whereas dominant negative Cdc42 inhibited cell spreading, indicating that DDR1-inhibited cell spreading is Cdc42 dependent. With the use of alpha(2)beta(1) integrin blocking antibody, we showed that collagen-induced Cdc42 activation was mediated by alpha(2)beta(1) integrin. Moreover, ectopic FAK expression enhanced the Cdc42 activity. Reducing FAK activity by dominant negative FAK (FRNK) markedly abolished the Cdc42 activity. These findings show that DDR1a/b activation inhibits cell spreading through suppressing alpha(2)beta(1) integrin-mediated Cdc42 activation.     

Phosphorylation of IQGAP1 modulates its binding to Cdc42, revealing a new type of rho-GTPase regulator

The Rho-GTPase Cdc42 is important for the establishment and maintenance of epithelial polarity. Signaling from Cdc42 is propagated via its effector molecules that specifically bind to Cdc42 in the GTP-bound form. The cell-cell contact regulator and actin-binding protein IQGAP1 is described as effector of Cdc42 and Rac. Unexpectedly, we show in this study that IQGAP1 bound also directly nucleotide-depleted Cdc42 (Cdc42-ND). This interaction was enhanced in the presence of phosphatase inhibitors and in epithelial cells without cell-cell contacts. Tandem mass spectrometry analysis and immunoprecipitation experiments revealed that IQGAP1 was Ser1443-phosphorylated in vivo, potentially by protein kinase Cepsilon and upon loss of cell-cell contacts. In addition, we identified two independent domains of the IQGAP1 C terminus that bound exclusively Cdc42-ND. These domains interacted with each other, favoring the binding to Cdc42-GTP. Moreover, phosphorylation on Ser1443 strongly inhibited this intramolecular interaction. Thus, we unraveled a molecular mechanism that reveals a novel type of Rho-GTPase regulator. We propose that, depending on its phosphorylation state, IQGAP1 might serve as an effector or sequester nucleotide-free Cdc42 to prevent signaling.     

Cdc42, Rac1, and their effector IQGAP1 as molecular switches for cadherin-mediated cell-cell adhesion.

Cell-cell adhesion is a dynamic process in various cellular and developmental situations. Cadherins, well-known Ca(2+)-dependent adhesion molecules, are thought to play a major role in the regulation of cell-cell adhesion. However, the molecular mechanism underlying the rearrangement of cadherin-mediated cell-cell adhesion is largely unknown. Cdc42 and Rac1, belonging to the Rho small GTPase family, have recently been shown to be involved in the regulation of cell-cell adhesion. In addition, IQGAP1, an effector for Cdc42 and Rac1, has been shown to regulate the cadherin function through interaction with beta-catenin, a molecule associated with cadherin. In this review, we will summarize the mode of action of Cdc42 and Rac1 as well as IQGAP1 as molecular switches for the cadherin function, and then discuss physiological processes in which the Cdc42/Rac1/IQGAP1 system may be involved.     

Rac1 and Cdc42 capture microtubules through IQGAP1 and CLIP-170

Linkage of microtubules to special cortical regions is essential for cell polarization. CLIP-170 binds to the growing ends of microtubules and plays pivotal roles in orientation. We have found that IQGAP1, an effector of Rac1 and Cdc42, interacts with CLIP-170. In Vero fibroblasts, IQGAP1 localizes at the polarized leading edge. Expression of carboxy-terminal fragment of IQGAP1, which includes the CLIP-170 binding region, delocalizes GFP-CLIP-170 from the tips of microtubules and alters the microtubule array. Activated Rac1/Cdc42, IQGAP1, and CLIP-170 form a tripartite complex. Furthermore, expression of an IQGAP1 mutant defective in Rac1/Cdc42 binding induces multiple leading edges. These results indicate that Rac1/Cdc42 marks special cortical spots where the IQGAP1 and CLIP-170 complex is targeted, leading to a polarized microtubule array and cell polarization.