The Specificity of IQGAP1 Toward the PI3K-Akt Pathway is Dependent on the IQ3 motif

Epidermal growth factor receptor (EGFR) and its downstream phosphatidylinositol 3-kinase (PI3K) pathway are commonly deregulated in many cancers including head and neck cancer (HNC). Recently, we have shown that the IQ motif-containing GTPase-activating protein 1 (IQGAP1) provides a molecular platform to scaffold all the components from the PI3K-Akt pathway and results in the sequential generation of phosphatidylinositol-3,4,5-triphosphate (PI3,4,5P 3 ). This makes the IQGAP1-PI3K scaffold a promising therapeutic target. In addition to the PI3K-Akt pathway, IQGAP1 also scaffolds the Ras-ERK pathway. To identify an IQGAP1 mutant that specifically loses IQGAP1-PI3K signaling but not other functions, we have focused on the IQ3 motif since this region binds with both the PIPK1α and PI3K enzymes and a short peptide derived from this sequence blocks binding and PI3K signaling. An IQ3 deletion mutant (ΔIQ3) in IQGAP1 was functionally compared with wild-type (WT) IQGAP1. We found that the IQ3 domain specifically mediates the PI3K-Akt pathway but does not regulate the Ras-ERK pathway. The IQ3 deletion mutant lost interactions with PI3K-Akt components but retained its binding of ERK pathway components and cell surface receptors, EGFR and integrins. In addition, the IQ3 deletion mutant lost regulation of cell migration. Consistently, the IQ3 motif derived peptide blocked Akt activation and led to the blockage of invasion mediated by EGFR organized into an integrin complex by syndecan-4. Taken together, this work has defined the IQGAP1 IQ3 motif as a specific target sequence for the scaffolding of the PI3K-AKT pathway.