Characterization of PDZ-binding kinase, a mitotic kinase

hDlg, the human homologue of the Drosophila Discs-large (Dlg) tumor suppres sor protein, is known to interact with the tumor suppressor protein APC and the human papillomavirus E6 transforming protein. In a two-hybrid screen, we identified a 322-aa serine/threonine kinase that binds to the PDZ2 domai n of hDlg. The mRNA for this PDZ-binding kinase, or PBK, is most abundant i n placenta and absent from adult brain tissue. The protein sequence of PBK has all the characteristic protein kinase subdomains and a C-terminal PDZ-b inding T/SXV motif. In vitro, PBK binds specifically to PDZ2 of hDlg throug h its C-terminal T/SXV motif. PBK and hDlg are phosphorylated at mitosis in HeLa cells, and the mitotic phosphorylation of PBK is required for its kin ase activity. In vitro, cdc2/cyclin B phosphorylates PBK. This evidence sho ws how PBK could link hDlg or other PDZ-containing proteins to signal trans duction pathways regulating the cell cycle or cellular proliferation.