IQGAP1 stimulates actin assembly through the N-WASP-Arp2/3 pathway.

IQGAP1 is a conserved modular protein overexpressed in cancer and involved in organizing actin and microtubules in motile processes such as adhesion, migration, and cytokinesis. A variety of proteins have been shown to interact with IQGAP1, including the small G proteins Rac1 and Cdc42, actin, calmodulin, beta-catenin, the microtubule plus end-binding proteins CLIP170 (cytoplasmic linker protein) and adenomatous polyposis coli. However, the molecular mechanism by which IQGAP1 controls actin dynamics in cell motility is not understood. Quantitative co-localization analysis and down-regulation of IQGAP1 revealed that IQGAP1 controls the co-localization of N-WASP with the Arp2/3 complex in lamellipodia. Co-immunoprecipitation supports an in vivo link between IQGAP1 and N-WASP. Pull-down experiments and kinetic assays of branched actin polymerization with N-WASP and Arp2/3 complex demonstrated that the C-terminal half of IQGAP1 activates N-WASP by interacting with its BR-CRIB domain in a Cdc42-like manner, whereas the N-terminal half of IQGAP1 antagonizes this activation by association with a C-terminal region of IQGAP1. We propose that signal-induced relief of the autoinhibited fold of IQGAP1 allows activation of N-WASP to stimulate Arp2/3-dependent actin assembly.

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.