关键词： Biochemistry  

摘要： Calmodulin regulates the function of numerous proteins by binding to short regions on the target molecule. IQ motifs, which are found in over 100 human proteins, appear in tandem repeats and bind calmodulin in the absence of Ca2+. One of these IQ-containing proteins, IQGAP1, interacts with several targets, including Cdc42, beta-catenin, E-cadherin, and actin, in a calmodulin-regulated manner. To elucidate the molecular mechanism by which apocalmodulin and Ca2+/calmodulin- differentially regulate IQGAP1, a series of constructs of IQGAP1 with selected point mutations of the four tandem IQ motifs were generated. Mutating the basic charged arginine residues in all four IQ motifs abrogated binding of IQGAP1 to apocalmodulin, but had no effect on its interaction with Ca2+/calmodulin. Analysis of IQGAP1 constructs with point mutations in single, double, or triple IQ motifs revealed that apocalmodulin bound only to IQ3 and IQ4. By contrast to the arginine mutant constructs, mutation of selected hydrophobic residues in the IQ motifs produced an IQGAP1 protein incapable of binding either apocalmodulin or Ca2+/calmodulin. These results, which differ from the conventional model of Ca2+-independent binding of calmodulin to IQ motifs, provide insight into the complexity of the molecular interactions between calmodulin and IQ motifs.

关键词： Proteins  

摘要： The Zn2+- and Ca2+-binding S100B protein is implicated in multiple intracellular and extracellular regulatory events. In glial cells, a relationship exists between cytoplasmic S100B accumulation and cell morphological changes. We have identified the IQGAP1 protein as the major cytoplasmic S100B target protein in different rat and human glial cell lines in the presence of Zn2+ and Ca2+. Zn2+ binding to S100B is sufficient to promote interaction with IQGAP1. IQ motifs on IQGAP1 represent the minimal interaction sites for S100B. We also provide evidence that, in human astrocytoma cell lines, S100B co-localizes with IQGAP1 at the polarized leading edge and areas of membrane ruffling and that both proteins relocate in a Ca2+-dependent manner within newly formed vesicle-like structures. Our data identify IQGAP1 as a potential target protein of S100B during processes of dynamic rearrangement of cell membrane morphology. They also reveal an additional cellular function for IQGAP1 associated with Zn2+/Ca2+-dependent relocation of S100B.

关键词： Proteins  

摘要： The abilities of the M-3 muscarinic acetylcholine receptor (mAChR) and Rac1 to regulate similar cellular responses, including cadherin-mediated adhesion, prompted us to investigate Rac1 regulation by M-3 mAChR. We characterized changes in Rac1 induced by stimulating transfected M-3 mAChR in Chinese hamster ovary cells stably expressing hemagglutinin (HA)tagged wild-type or mutant Rac1. mAChR activation converts endogenous Rac1 to the GTP-bound form in cells expressing HA-Rac1 but not in cells expressing dominant negative HA-Rac1(Asn-17) or constitutively active HA-Rac1(Val-12). The competitive binding of endogenous IQGAP1 by HA-Rac1(Val-12) may diminish the mAChR-mediated activation of endogenous Rac1. HA-Rac1 and HA-Rac1(Val-12), but not HA-Rac1(Asn-17), accumulate with IQGAP1 at cell junctions during mAChR-induced cell-cell compaction. Co-localization studies suggest that Rac1 can accumulate at junctions without IQGAP1, but IQGAP1 cannot accumulate at junctions without Rac1. mAChR activation also induces GTP-independent changes in Rac1 because mAChR activation redistributes HA-Rac1(Asn-17), which does not bind GTP. Actin associates with complexes containing HA-Rac1 or HA-Rac1(Val-12) after prolonged mAChR activation. We also demonstrate that Rac1 participates in mAChR-induced cell-cell compaction and c-Jun phosphorylation. These results indicate that M-3 mAChR activation converts Rac1 to the GTP-bound form, alters interactions between Rac1, IQGAP1, and actin, and causes the junctional accumulation of Rac1 and IQGAP1.

关键词： tumor invasion   cell motility   cohort migration   IQGAP1   HGF/SF   cross-talk  

摘要： In histopathological sections, it is frequently observed that carcinoma cells invade the stroma as coherent cell nests rather than single cells. We have called this type of movement "cohort migration (CM)" and developed an in vitro model, in which human colon carcinoma cells move as coherent cell sheets when stimulated with hepatocyte growth factor/scatter factor (HGF/SF). In this CM model, localized release from cell-cell adhesion at the lower portion of cells is essential for cell movement. Its mechanism was investigated in this study with special reference to the E-cadherin/catenin complex (Ecc) and IQGAP1. IQGAP1 is a target molecule of Cdc42 and Rac1 and negatively regulates the Ecc-based cell-cell adhesion by dissociating alpha-catenin, a key molecule that links Ecc to actin cytoskeleton, from Ecc. In our study, the amount of IQGAP1 bound to Ecc increased in migrating cells in association with a decrease in the alpha-catenin level in Ecc. In accordance with this, IQGAP1 showed a shift from the cytosol to the membrane fraction. Moreover, confocal laser microscopic study demonstrated the localization of IQGAP1 at the membranes of the lower portion of migrating cells, where cell-cell adhesion was specifically disrupted during CM. Furthermore, when HGF/SF-induced CM was enhanced with pre-coated extracellular matrix (ECM) components, the level of IQGAP I in Ecc increased more than that caused by HGF/SF alone. On the contrary, when CM was inhibited by interrupting cell-ECM interaction, the level of IQGAP1 in Ecc did not increase despite HGF/SF stimulation. Taken together, these results indicate close association of IQGAP1 with localized disruption of cell-cell adhesion during CM and that modulation of CM by cross-talk between signals induced by HGF/SF and cell-ECM interactions also involves IQGAP1-related mechanisms.

关键词： Biochemistry  

摘要： 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 (G) under bar AP-(r) under bar elated (d) under bar omain (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.

摘要： 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 1Q-GAP1, 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.

关键词： Biochemistry  

摘要： IQGAP1 colocalizes with actin filaments in the cell cortex and binds in vitro to F-actin and several signaling proteins, including calmodulin, Cdc42, Rac1, and beta-catenin. It is thought that the F-actin binding activity of IQGAP1 is regulated by its reversible association with these signaling molecules, but the mechanisms have remained obscure. Here we describe the regulatory mechanism for calmodulin. Purified adrenal IQGAP1 was found to consist of two distinct protein pools, one of which bound F-actin and lacked calmodulin, and the other of which did not bind F-actin but was tightly associated with calmodulin. Based on this finding we hypothesized that calmodulin negatively regulates binding of IQGAP1 to F-actin. This hypothesis was tested in vitro using recombinant wild type and mutated IQGAP1s and in live cells that transiently expressed IQGAP1-YFP. In vitro, the affinity of wild type IQGAP1 for F-actin decreased with increasing concentrations of calmodulin, and this effect was dramatically enhanced by Ca2+ and required the IQ domains of IQGAP1. In addition, we found that calmodulin bound wild type IQGAPI much more efficiently in the presence of Ca2+ than EGTA, and all 8 IQ motifs in each IQGAP1 dimer could bind calmodulin simultaneously. In live cells, IQGAP1-YFP localized to the cell cortex, but elevation of intracellular Ca2+ reversibly induced the fluorescent fusion protein to become diffusely distributed. Taken together, these results support a model in which a rise in f ree intracellular Ca2+ promotes binding of calmodulin to IQGAP1, which in turn inhibits IQGAP1 from binding to cortical actin filaments.

关键词： Biochemistry  

摘要： beta-Catenin, an oncoprotein integral to cell-cell adhesion and proliferative signaling, is increased in several malignancies. The recently discovered calmodulin-binding protein IQGAP1 binds stoichiometrically to beta-catenin and regulates the association of beta-catenin with the cell-cell adhesion complex. In the present work, we investigated the role of IQGAP1 on the transcriptional co-activator function of beta-catenin, and whether calmodulin modulated the functional interaction between IQGAP1 and beta-catenin. In vitro competition assays revealed that both Ca2+/calmodulin and apo-calmodulin, when pre-bound to IQGAP1, prevented binding of beta-catenin to IQGAP1, and partially displaced beta-catenin pre-bound to IQGAP1 when added subsequently. Conversely, beta-catenin partially displaced apo-calmodulin, but not Ca2+/calmodulin, from IQGAP1. Overexpression of IQGAP1 in SW480 colon carcinoma cells enhanced beta-catenin-mediated transcriptional co-activation by 1.72-fold, and this stimulation was significantly attenuated upon antagonism of calmodulin using the cell-permeable antagonist CGS9343B. Moreover, an IQGAP1 mutant that does not bind calmodulin was unable to stimulate beta-catenin transcriptional function. Results of pulse-chase analyses suggested that IQGAP1 slowed the turnover of soluble, but not total, beta-catenin. Immunocytochemistry revealed that IQGAP1 overexpression increased the amount of beta-catenin located in the nucleus, whereas incubation of cells with CGS9343B blocked this accumulation. Together, our results imply that IQGAP1 enhances the function of beta-catenin in the nucleus and that calmodulin regulates this stimulation.

关键词： IQGAP1   colorectal cancer   cell migration   invasion   cell-cell adhesion   immunohistochemistry  

摘要： In histopathological sections, it is frequently observed that carcinoma cells move in and invade the stroma as coherent cell nests, rather than single cells. We have called this type of movement 'cohort migration (CM)' and developed an in vitro model, in which human colon adenocarcinoma cells move as coherent cell sheets when stimulated with naturally occurring motogenic factor, hepatocyte growth factor/scatter factor (HGF/SF). In this CM model, localized release from cell-cell adhesion is essential for cell movement. Recently, we have shown that IQGAP1, a target molecule of Cdc42 and Rac1 small GTPases, is involved in this localized release from the E-cadherin-based cell-cell adhesion during CM. In this study, we examined expression of IQGAP1 immunohistochemically in human colorectal tissues and found that IQGAP1 was overexpressed in carcinoma tissues compared with normal counterparts. Within the carcinoma tissue, IQGAP1 tended to be expressed more at the invasion front than at the upper portions, and higher levels of expression were observed in deeper two-thirds of carcinoma tissues than in the superficial one-third. This expression pattern showing stronger signals in deeper portions was most apparent in advanced carcinomas that invaded into the subserosa. These findings supported a role of IQGAP1 in colon carcinoma invasion. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.

关键词： Proteins  

摘要： IQGAP1 contains a number of protein recognition motifs through which it binds to targets. Several in vitro studies have documented that IQGAP1 interacts directly with calmodulin, actin, E-cadherin, beta -catenin, and the small GTPases Cdc42 and Rac. Nevertheless, direct demonstration of in vivo function of mammalian IQGAP1 is limited. Using a novel assay to evaluate in vivo function of IQGAP1, we document here that microinjection of IQGAP1 into early Xenopus embryos generates superficial ectoderm. lesions at late blastula stages. This activity was retained by the mutated variants of IQGAP1 in which the calponin homology domain or the WW domain was deleted. By contrast, deletion of the IQ (IQGAP1-Delta IQ), Ras-GAP-related (IQGAP1-Delta GRD), or C-terminal (IQGAP1-DeltaC) domains abrogated the effect of IQGAP1 on the embryos. None of the latter mutants bound Cdc42, suggesting that the binding of Cdc42 by IQGAP1 is critical for its function. Moreover, overexpression of IQGAP1, but not IQGAP1-Delta GRD, significantly increased the amount of active Cdc42 in embryonic cells. Co-injection of wild type IQGAP1 with dominant negative Cdc42, but not the dominant negative forms of Rac or Rho, blocked the effect of IQGAP1 on embryonic ectoderm. Together these data indicate that the activity of IQGAP1 in embryonic ectoderm. requires Cdc42 function.