IQ-domain GTPase-activating protein 1 regulates β-catenin at membrane ruffles and its role in macropinocytosis of N-cadherin and adenomatous polyposis coli

Abstract

β-Catenin is an integral component of E-cadherin dependent cell-cell junctions. Here we show that β-catenin co-localizes with IQ-domain GTPase-activating protein 1 (IQGAP1), adenomatous polyposis coli (APC), and N-cadherin at actin-positive membrane ruffles in NIH 3T3 fibroblasts. We used deletion mapping to identify the membrane ruffle-targeting region of β-catenin, localizing it to amino acids 47-217, which overlap the IQGAP1 binding site. Knockdown by small interference RNA (siRNA) revealed IQGAP1-dependent membrane targeting of β-catenin, APC, and N-cadherin. Transient overexpression of IQGAP1 or N-cadherin increased β-catenin at membrane ruffles. IQGAP1/APC regulates cell migration, and using a wound healing assay we demonstrate that siRNA-mediated loss of β-catenin also caused a modest reduction in the rate of cell migration. More significantly, we discovered that β-catenin is internalized by Arf6-dependent macropinocytosis near sites of membrane ruffling. The β-catenin macropinosomes co-stained for APC, N-cadherin, and to a lesser extent IQGAP1, and internalization of each binding partner was abrogated by siRNA-dependent knockdown of β-catenin. In addition, β-catenin macropinosomes co-localized with the lysosomal marker, lysosome associated membrane protein 1, consistent with their recycling by the late endosomal machinery. Our findings expand on current knowledge of β-catenin function. We propose that in motile cells β-catenin is recruited by IQGAP1 and N-cadherin to active membrane ruffles, wherein β-catenin mediates the internalization and possible recycling of the membrane-associated proteins N-cadherin and APC. © 2007 by The American Society for Biochemistry and Molecular Biology, Inc.