Nuclear export of α-catenin: overlap between nuclear export signal sequences and the β-catenin binding site

Abstract

Mutations in APC or in β-catenin, which are common in colon cancer, lead to constitutive activation of β-catenin/Tcf-dependent signaling. α-Catenin is also found in some colon cancer cell nuclei, and loss of its expression correlates with increased β-catenin/Tcf transcriptional activity. Moreover, targeted expression of α-catenin in the nucleus inhibits β-catenin/Tcf-dependent transcription. Thus, an understanding of the regulation of α-catenin localization could provide insight into the control of β-catenin signaling. While the β-catenin/Tcf complex can promote nuclear import of α-catenin, the mechanism for its nuclear export is not known. We found that leptomycin B (LMB) inhibited nuclear export of GFP-α-catenin in HCT116 colon cancer cells, suggesting that α-catenin localization is regulated by CRM-1-dependent nuclear export. We identified two putative nuclear export signals in a domain of α-catenin that overlaps with the β-catenin binding domain. Using a nuclear export assay, we determined that one of these (NES1) is a weak LMB-insensitive NES, whereas the other (NES2) is strong and LMB-sensitive. Mutations in either NES reduced nuclear export of α-catenin in HCT116 cells. In addition, mutations in NES1, but not NES2, reduced binding of α-catenin to β-catenin and impaired the ability of α-catenin to repress β-catenin/Tcf-dependent transcription. Therefore, NES1 is required both for repression of β-catenin signaling and for nuclear export, while NES2 is required only for nuclear export.