Abstract
The ubiquitin conjugating enzyme Rad6B is overexpressed in breast cancer and induces β-catenin transcriptional activation and stabilization via K63-linked polyubiquitination. Here we identify β-catenin and Rad6B interacting regions, identify potential Rad6B ubiquitination sites in β-catenin, and characterize their breast cancer tissue expression. β-catenin and Rad6B colocalize in breast carcinoma and coimmunoprecipitate from MDA-MB-231 cells. Pull-down assays using GST-β-catenin and His-Rad6B deletion mutants identified amino acids 131-181 and 50-116, respectively, as necessary for their interaction. Ubiquitination assays using β-catenin deletion mutants mapped Rad6B-induced ubiquitination within β-catenin 181-422 encompassing Armadillo repeats 2-7. Lysine to arginine mutations within repeats 5-7 identified K394 as the major Rad6B ubiquitination site in vitro and in vivo, and confirmed by Rad6B ubiquitination of a β-catenin peptide encompassing K394. Ubiquitination of wild type- but not K394R-β-catenin was decreased by Rad6B silencing. Compared to wild type-, K312R-, K335R-, K345R-, or K354R-β-catenin, K394R mutation caused ~50% drop in TOP/Flash activity in Wnt-silent MCF-7 cells. Consistent with these data, expression of Rad6B, itself a β-catenin/TCF transcriptional target, was also reduced in K394R-β-catenin transfected cells. Steady-state K394R-β-catenin levels are decreased compared to wild type-β-catenin. The decreased expression is not due to proteasomal degradation as treatment with MG132 failed to rescue its levels. Lymph node-positive breast carcinomas express higher levels of Rad6 protein and Rad6 activity, and K63-linked ubiquitinated β-catenin than reduction mammoplasties. These data suggest that K394 is a novel site of β-catenin ubiquitination that may be important for the stability and activity of β-catenin in breast cancer.