Abstract

Truncation mutations in the adenomatous polyposis coli protein (APC) are responsible for familial polyposis, a form of inherited colon cancer. In addition to its role in mediating β-catenin degradation in the Wnt signaling pathway, APC plays a role in regulating microtubules. This was suggested by its localization to the end of dynamic microtubules in actively migrating areas of cells and by the apparent correlation between the dissociation of APC from polymerizing microtubules and their subsequent depolymerization [1Mimori-Kiyosue Y. Shiina N. Tsukita S. Adenomatous Polyposis Coli (APC) protein moves along microtubules and concentrates at their growing ends in epithelial cells.J Cell Biol. 2000; 148: 505-517Crossref PubMed Scopus (240) Google Scholar, 2Näthke I.S. Adams C.L. Polakis P. Sellin J.H. Nelson W.J. The adenomatous polyposis coli tumor suppressor protein localizes to plasma membrane sites involved in active cell migration.J Cell Biol. 1996; 134: 165-179Crossref PubMed Scopus (437) Google Scholar]. The microtubule binding domain is deleted in the transforming mutations of APC [3Polakis P. Mutations in the APC gene and their implications for protein structure and function.Curr Opin Genet Dev. 1995; 5: 66-71Crossref PubMed Scopus (170) Google Scholar, 4Polakis P. The adenomatous polyposis coli (APC) tumor suppressor.Biochim Biophys Acta. 1997; 1332: F127-F147PubMed Google Scholar]; however, the direct effect of APC protein on microtubules has never been examined. Here we show that binding of APC to microtubules increases microtubule stability in vivo and in vitro. Deleting the previously identified microtubule binding site from the C-terminal domain of APC does not eliminate its binding to microtubules but decreases the ability of APC to stabilize them significantly. The interaction of APC with microtubules is decreased by phosphorylation of APC by GSK3β. These data confirm the hypothesis that APC is involved in stabilizing microtubule ends. They also suggest that binding of APC to microtubules is mediated by at least two distinct sites and is regulated by phosphorylation.