UFD2a mediates the proteasomal turnover of p73 without promoting p73 ubiquitination

p73 protein level is kept extremely low in mammalian cultured cells and its stability may be regulated by not only the ubiquitin/proteasome-dependent proteolysis but also through other unidentified mechanisms. Here, we found for the first time that p73 is physically as well as functionally associated with the U-box-type E3/E4 ubiquitin ligase UFD2a. The immunoprecipitation experiments demonstrated that this interaction is mediated by the COOH-terminal region of p73 alpha containing SAM domain. During the cisplatin-induced apoptosis in SHSY5Y neuroblastoma cells, p73a accumulated at a protein level, whereas the endogenous UFD2a was significantly reduced in response to cisplatin. Ectopic expression of UFD2a decreased the half-life of p73 alpha in association with a significant inhibition of the p73 alpha-mediated transactivation as well as proapoptotic activity. Downregulation of endogenous UFD2a by antisense strategy resulted in a remarkable accumulation of p73a. Unexpectedly, UFD2a-mediated degradation of p73 alpha was sensitive to the proteasomal inhibitor, however, UFD2a did not affect the ubiquitination levels of p73 alpha. Taken together, our present findings imply that UFD2a might promote the proteasomal turnover of p73 in a ubiquitination-independent manner, and also suggest that UFD2a might play an important role in the regulation of cisplatin-induced apoptosis mediated by p73.