PHOSPHORYLATION OF THE ADENOMATOUS POLYPOSIS-COLI PROTEIN AND ITS POSSIBLE REGULATORY EFFECTS IN CELLS

The adenomatous polyposis coli (APC) gene is etiologically associated with familial adenomatous polyposis and gastrointestinal malignancies, but its cellular function and role in tumorigenesis are unclear. Rece nt reports indicate that wild-type, but not mutant, APC gene product ( APC) is associated with and promotes the assembly of cytoskeletal micr otubules in vitro, suggesting that this mechanism has importance in tu mor development. Because other microtubule-associated proteins (MAPs) undergo phosphorylation in their normal functioning, we postulated tha t APC is a phosphoprotein. HCT116 cells, containing full-length APC pr otein, were [P-32]-prelabeled, and a 300-kDa band corresponding to pho sphorylated APC was immunoprecipitated using each of three different a nti-APC antibodies. High voltage electrophoresis of [P-32]-labeled APC showed the presence of phospho-serine and phospho-threonine residues. Further immunoprecipitation analyses showed phosphorylation of i) ful l-length APC in human lymphoblastoid cells and ii) carboxyl-truncated APC in SW480 and DiFi colon carcinoma cells. Thus, APC is probably a p hosphoprotein in normal and malignant tissues. We hypothesize a mechan ism whereby phosphorylation of APC may play a regulatory role in its i nteraction with microtubules. This may involve phosphorylation of (Ser /Thr)-Pro amino acid motifs in APC's basic domain. We propose that del etion of this domain disrupts APC binding to microtubules, explaining how APC mutations are linked to cancer development. (C) 1995 Academic Press, Inc.