p73-mediated transcriptional activity is negatively regulated by Polo-Like Kinase 1

Trans-activating (TA) p73 is a member of the p53 family of transcription factors and has been implicated in cell cycle regulation, apoptosis and developmental processes. Although TAp73 positively regulates an overlapping repertoire of genes regulated by p53, TAp73 has been observed to be paradoxically overexpressed in a number of tumor cell types arousing much interest in the post-translational regulation of TAp73 transcriptional activity. Here, we present novel findings that show TAp73 can interact and co-localise, with Polo-Like Kinase 1 (PLK1) and that TAp73 is phosphorylated by this kinase on Threonine-27 (Thr-27) within the TA domain. Using reporter assays and Electrophoretic Mobility Shift Assays (EMSA), our findings suggest that TAp73-mediated activation of the p21(cip/waf), 14-3-3 sigma and Bax gene promoters is abrogated by expressed PLK1 for which post-translational modification of TAp73 Thr-27 appears to be a key step in MCF7 cells. Thus highlighting a potential mechanism that uniquely contributes to PLK1-mediated and phosphor-dependent transcriptional deactivation of expressed TAp73.