Tyrosine phosphorylation of the scaffold protein IQGAP1 by VEGFR2 modulates cell proliferation.

IQGAP1 (IQ motif-containing GTPase activating protein 1), a scaffold protein with >250 interacting partners, regulates several intracellular signaling cascades, thereby modulating cell functions. Post-translational modifications of IQGAP1 by ubiquitination or phosphorylation have been shown to alter its interactions with selected proteins. We previously demonstrated that phosphorylation of Tyr-1510 of IQGAP1 by the MET receptor tyrosine kinase creates a docking site for Src homology 2 (SH2) domains of Abl. The vascular endothelial growth factor receptor 2 (VEGFR2) orchestrates angiogenesis by promoting endothelial cell survival, proliferation, and migration. Here, we show that the VEGFR2 kinase stimulates tyrosine phosphorylation of IQGAP1 in vitro and in cells; the latter is independent of SRC. Analysis by LC-MS/MS identifies Tyr-855 as the predominant site of IQGAP1 phosphorylation. Using a microarray screen, we identify that the SH2 domains of 7 proteins bind to a peptide containing Tyr-855 of IQGAP1 in a phosphorylation-dependent manner. To assess the functional effects of Tyr-855 phosphorylation, we replaced Tyr-855 of IQGAP1 with alanine (IQGAP1 Y855A). The ability of VEGF to activate Akt and ERK and increase cell proliferation is abrogated when IQGAP1-null cells are reconstituted with IQGAP1 Y855A, indicating that phosphorylation of Tyr-855 of IQGAP1 modulates the ability of VEGF to activate at least some signaling pathways and promote cell proliferation. Taken together, our data suggest that IQGAP1 phosphorylation is a potential therapeutic target for neoplasms and other disorders characterized by increased VEGFR2 signaling.