Protein stability and function of p73 are modulated by a physical interaction with RanBPM in mammalian cultured cells

Upon a certain DNA damage including cisplatin treatment, p73 is stabilized and exerts its growth-suppressive and/or proapoptotic function. However, the precise molecular basis by which the intracellular levels of p73 are regulated remains unclear. In the present study, we have identified RanBPM as a novel binding partner of p73alpha by yeast-based two-hybrid screening, and also found that RanBPM has an ability to stabilize p73alpha. GST pull-down assays and co-immunoprecipitation experiments revealed that RanBPM directly bound to the extreme COOH-terminal region of p73alpha, whereas it failed to interact with p53. Co-expression of RanBPM with p73alpha resulted in the nuclear translocation of RanBPM, and both proteins co-localized in cell nucleus as examined by indirect immunofluorescent staining. It is worth noting that the expression of RanBPM inhibited the ubiquitination of p73alpha, and thereby prolonged its half-life. Subsequent studies demonstrated that the proapoptotic activity of p73alpha was significantly enhanced in the presence of RanBPM. Taken together, our present findings implicate a novel role for RanBPM in the regulation of p73 stability and function.