MAPK Scaffold IQGAP1 Binds the EGF Receptor and Modulates Its Activation

Abstract

Cellular responses produced by EGF are mediated through the receptor (EGFR) and by various enzymes and scaffolds. Recent studies document IQGAP1 as a scaffold for the MAPK cascade, binding directly to B-Raf, MEK, and ERK and regulating their activation in response to EGF. We previously showed that EGF is unable to activate B-Raf in cells lacking IQGAP1. However, the mechanism by which IQGAP1 links B-Raf to EGFR was unknown. Here we report that endogenous EGFR and IQGAP1 co-localize and co-immunoprecipitate in cells. EGF has no effect on the association, but Ca 2؉ attenuates binding. In vitro analysis demonstrated a direct association mediated through the IQ and kinase domains of IQGAP1 and EGFR, respectively. Calmodulin disrupts this interaction. Using a mass spectrometry-based assay, we show that EGF induces phosphorylation of IQGAP1 Ser 1443 , a residue known to be phosphorylated by PKC. This phosphorylation is eliminated by pharmacological inhibition of either EGFR or PKC and transfection with small interfering RNA directed against the PKC␣ isoform. In IQGAP1-null cells, EGF-stimulated tyrosine phosphorylation of EGFR is severely attenuated. Normal levels of autophosphorylation are restored by reconstituting wild type IQGAP1 and enhanced by an IQGAP1 S1443D mutant. Collectively, these data demonstrate a functional interaction between IQGAP1 and EGFR and suggest that IQGAP1 modulates EGFR activation.

IQGAP1 Binds to Estrogen Receptor-α and Modulates Its Function

“…The effects of ligand binding on the interaction of receptors with IQGAP1 are diverse. For example, EGF has no effect on the association between IQGAP1 and epidermal growth factor receptor (17). By contrast, stimulation of Madin-Darby bovine kidney cells with FGF2 promotes binding of fibroblast growth factor recep-FIGURE 7.…”

“…For example, IQGAP1 binds to several components of the epidermal growth factor receptorRaf-MAPK cascade, regulating activation of ERK (13,17,28). Work from a number of groups has demonstrated that IQGAP1 associates with selected transmembrane receptors and participates in their normal function (13,14).…”

“…Moreover, the observation that deletion of amino acids 763-864 from IQGAP1 eliminated its association with ER␣ confirms that the IQ region of IQGAP1 is the site to which ER␣ binds. Several other proteins, including calmodulin (29,30), myosin light chain (31), Rap1 (32), human epidermal growth factor receptor-2 (16), epidermal growth factor receptor (17), and S100B (33), also bind to the IQ region of IQGAP1. Protein binding to IQGAP1 may be regulated.…”

“…These range from signaling proteins and small GTPases to cytoskeletal components and kinases (13)(14)(15). IQGAP1 also binds to and regulates the function of selected growth factor receptors, such as human epidermal growth factor receptor-2 (16), epi-dermal growth factor receptor (17), fibroblast growth factor receptor (18) and vascular endothelial growth factor receptor-2 (19).…”

The IQGAP1 Protein Is a Calmodulin-regulated Barbed End Capper of Actin Filaments

“…The binding of CaM has been reported to inhibit the interaction of IQGAP1 with F-actin (20) as well as with Cdc42 (24,25) and B-Raf (26). Recent results further indicate that IQGAP1 co-immunoprecipitates with EGFR in a calmodulinsensitive fashion (27), and that the S100 protein also regulates IQGAP1 by binding to its IQ domains (28).…”