Phosphorylation near nuclear localization signal regulates nuclear import of adenomatous polyposis coli protein

Mutation of the adenomatous polyposis coli (APC) gene is an early step in t he development of colorectal carcinomas. APC protein is located in both the cytoplasm and the nucleus. The objective of this study was to define the n uclear localization signals (NLSs) in APC protein. APC contains two potenti al NLSs comprising amino acids 1767-1772 (NLS1(APC)) and 2048-2053 (NLS2(AP C)). Both APC NLSs are well conserved among human, mouse, rat, and fly. NLS 1(APC) and NLS2(APC) each were sufficient to target the cytoplasmic protein beta -galactosidase to the nucleus. Mutational analysis of APC demonstrate d that both NLSs were necessary for optimal nuclear import of full-length A PC protein. Alignment of NLS2(APC) with the simian virus 40 large T antigen NLS (NLSSV40 (T-ag)) revealed sequence similarity extending to adjacent ph osphorylation sites. Changing a serine residue (Ser(2054)) to aspartic acid mutated the potential protein kinase A site adjacent to NLS2(APC), resulti ng in both inhibition of the NLS2(APC)-mediated nuclear import of a chimeri c beta -galactosidase fusion protein and a reduction of full-length APC nuc lear localization. Our data provide evidence that control of APC's nuclear import through phosphorylation is a potential mechanism for regulating APC' s nuclear activity.