The Specificity of EGF-Stimulated IQGAP1 Scaffold Towards the PI3K-Akt Pathway is Defined by the IQ3 motif

Epidermal growth factor receptor (EGFR) and its downstream phosphoinositide 3-kinase (PI3K) pathway are commonly deregulated in cancer. Recently, we have shown that the IQ motif-containing GTPase-activating protein 1 (IQGAP1) provides a molecular platform to scaffold all the components of the PI3K-Akt pathway and results in the sequential generation of phosphatidylinositol-3,4,5-trisphosphate (PI3,4,5P₃). In addition to the PI3K-Akt pathway, IQGAP1 also scaffolds the Ras-ERK pathway. To define the specificity of IQGAP1 for the control of PI3K signaling, we have focused on the IQ3 motif in IQGAP1 as PIPKIα and PI3K enzymes bind this region. An IQ3 deletion mutant loses interactions with the PI3K-Akt components but retains binding to ERK and EGFR. Consistently, blocking the IQ3 motif of IQGAP1 using an IQ3 motif-derived peptide mirrors the effect of IQ3 deletion mutant by reducing Akt activation but has no impact on ERK activation. Also, the peptide disrupts the binding of IQGAP1 with PI3K-Akt pathway components, while IQGAP1 interactions with ERK and EGFR are not affected. Functionally, deleting or blocking the IQ3 motif inhibits cell proliferation, invasion, and migration in a non-additive manner to a PIPKIα inhibitor, establishing the functional specificity of IQ3 motif towards the PI3K-Akt pathway. Taken together, the IQ3 motif is a specific target for suppressing activation of the PI3K-Akt but not the Ras-ERK pathway. Although EGFR stimulates the IQGAP1-PI3K and -ERK pathways, here we show that IQGAP1-PI3K controls migration, invasion, and proliferation independent of ERK. These data illustrate that the IQ3 region of IQGAP1 is a promising therapeutic target for PI3K-driven cancer.