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Thesis Advisor: Robert Nagele, Ph.D.
Cell and Molecular Biology Program
Science Center, Room 290
Friday, August 14, 2009
IQGAP1 is a multifunctional scaffold protein that regulates many signaling pathways and cellular functions. Through binding and modulating a diverse set of known target proteins, IQGAP1 participates in actin dynamics and cytoskeletal organization, microtubule dynamics, and cell-cell adhesion. Proteins that bind IQGAP1 include calmodulin, Cdc42, Rac1, actin, ƒÒ-catenin, E-cadherin, and CLIP-170. IQGAP1 also functions as a scaffold for the mitogen-activated protein kinase (MAPK) cascade, binding directly to B-Raf, MEK1/2, and ERK1/2 and regulating their activation. IQGAP1 binding partners have well documented roles in neoplastic transformation and metastasis, and emerging data suggests that IQGAP1 is an oncogene.
IQGAP1 function is modulated by several mechanisms, including protein binding, subcellular localization, self-association and post-translational modification. Post-translational modification of proteins through reversible phosphorylation is used in a vast number of highly regulated pathways and functions. The focus of my research is the application of mass spectrometry-based approaches to enable the identification and characterization of IQGAP1 phosphorylation. These studies have revealed that IQGAP1 is heavily phosphorylated in asynchronous MCF-7 breast epithelial cells and that incubation with phorbol 12-myristate 13-acetate (PMA) robustly stimulates phosphorylation of IQGAP1 on Ser-1443. In vitro and in vivo experiments have confirmed IQGAP1 to be a protein kinase CƒÕ (PKCƒÕƒw substrate. Overexpression of a phosphomimetic IQGAP1 construct in N1E-115 neuroblastoma cells showed a marked enhancement in the ability of IQGAP1 to induce neurite outgrowth.
Further research has linked IQGAP1 with signaling through the epidermal growth factor receptor (EGFR), the prototypal member of the receptor tyrosine kinase family. EGFR interacts directly with IQGAP1 and when activated by EGF catalyzes the phosphorylation of IQGAP1 at Ser-1443. Endogenous EGFR and IQGAP1 co-immunoprecipitate in a Ca2+-sensitive manner. In cells, EGFR and IQGAP1 co-localize at cell-cell contacts during serum starvation, and together relocalize at the membrane periphery in lamellipodia following EGF activation. In vitro, IQGAP1 binds directly to the kinase domain of EGFR, mediated through the IQ domain in IQGAP1, and this interaction is disrupted by calmodulin binding. Expression of IQGAP1 in IQGAP1-/- MEF cells augments EGF-stimulated activation of the EGFR. Collectively, these data newly reveal that IQGAP1 phosphorylation plays a role in signal transduction pathways.