IQGAP1 Is a Scaffold for Mitogen-Activated Protein Kinase Signaling

Abstract

IQGAP1 modulates many cellular functions such as cell-cell adhesion, transcription, cytoskeletal architecture, and selected signaling pathways. We previously documented that IQGAP1 binds extracellular signal-regulated kinase (ERK) 2 and regulates growth factor-stimulated ERK activity. Here we show that MEK, the molecule immediately upstream of ERK in the Ras/mitogen-activated protein (MAP) kinase signaling cascade, also interacts directly with IQGAP1. Both MEK1 and MEK2 bound IQGAP1 in vitro and coimmunoprecipitated with IQGAP1. The addition of ERK2 enhanced by fourfold the in vitro interaction of MEK2 with IQGAP1 without altering binding of MEK1. Similarly, ERK1 promoted MEK binding to IQGAP1, but either MEK protein altered the association between IQGAP1 and ERK. Epidermal growth factor (EGF) differentially regulated binding, enhancing MEK1 interaction while reducing MEK2 binding to IQGAP1. In addition, both knockdown and overexpression of IQGAP1 reduced EGF-stimulated activation of MEK and ERK. Analyses with selective IQGAP1 mutant constructs indicated that MEK binding is crucial for IQGAP1 to modulate EGF activation of ERK. Our data strongly suggest that IQGAP1 functions as a molecular scaffold in the Ras/MAP kinase pathway.

ACKNOWLEDGMENTS

We are grateful to Natalie Ahn (University of Colorado) for generously donating the HA-tagged and His₆-tagged MEK1 and MEK2 constructs, to Melanie Cobb (University of Texas) for the gift of His₆-tagged ERK1 and ERK2, and to David Turner (University of Michigan) for the mU6pro vector. We thank Chris French for the use of Nucleofector (Amaxa) and Angela Lam for technical assistance. We appreciate Rob Krikorian's expert help in the preparation of the manuscript.

This study was supported in part by grants from the National Institutes of Health (to D.B.S.) and a United States Army Fellowship grant DAMD17-02-1-0304 (to M.R.).