ON THE REGULATION AND FUNCTION OF HUMAN POLO-LIKE KINASE-1 (PLK1) - EFFECTS OF OVEREXPRESSION ON CELL-CYCLE PROGRESSION

The human protein kinase Plk1, a member of the polo-like kinase family , is known to function at mitosis. Here we show that the relative spec ific activity of Plk1 increases in mitosis, that Plk1 is specifically phosphorylated during mitosis, and that phosphatase treatment reduces mitotic Plk1 activity to interphase levels. To identify domains involv ed in the regulation of Plk1 activity, deletion mutants of Plk1 were c onstructed and their activities examined. Deletion of the extreme C-te rminus of Plk1 substantially increased kinase activity, indicating tha t the C-terminus harbors an inhibitory domain. Finally, the consequenc es of over-production of wild-type and mutant Plk1 protein were analyz ed, using transient transfection assays. Cells overexpressing Plk1 pro tein were able to enter mitosis and establish an apparently normal bip olar spindle. In contrast, progression through mitosis was transiently delayed, and cytokinesis appeared to be disturbed, as reflected by a significant increase in large cells with multiple, often fragmented nu clei. These results are relevant to recently proposed roles for Plks d uring both entry into and exit from mitosis. (C) 1997 Academic Press.