A novel interaction between atrophin-interacting protein 4 and β-p21-activated kinase-interactive exchange factor is mediated by an SH3 domain

Abstract

Cross-talk between G protein-coupled receptors and receptor tyrosine kinase signaling pathways is crucial to the efficient relay and integration of cellular information. Here we identify and define the novel binding interaction of the E3 ubiquitin ligase atrophin-interacting protein 4 (AIP4) with the GTP exchange factor β-p21-activated kinase-interactive exchange factor (βPIX). We demonstrate that this interaction is mediated in part by the βPIX-SH3 domain binding to a proline-rich stretch of AIP4. Analysis of the interaction by isothermal calorimetry is consistent with a heterotrimeric complex with one AIP4-derived peptide binding to two βPIX-SH3 domains. We determined the crystal structure of the βPIX-SH3·AIP4 complex to 2.0-Å resolution. In contrast to the calorimetry results, the crystal structure shows a monomeric complex in which AIP4 peptide binds the βPIX-SH3 domain as a canonical Class I ligand with an additional type II polyproline helix that makes extensive contacts with another face of βPIX. Taken together, the novel interaction between AIP4 and βPIX represents a new regulatory node for G protein-coupled receptor and receptor tyrosine kinase signal integration. Our structure of the βPIX-SH3·AIP4 complex provides important insight into the mechanistic basis for βPIX scaffolding of signaling components, especially those involved in cross-talk. © 2007 by The American Society for Biochemistry and Molecular Biology, Inc.