POLO-LIKE KINASE IS A CELL-CYCLE-REGULATED KINASE ACTIVATED DURING MITOSIS

Previously, we demonstrated that expression of polo-like kinase (PLK) is required for cellular DNA synthesis and that overexpression of PLK is sufficient to induce DNA synthesis. We now report that the endogeno us levels of PLK, its phosphorylation status, and protein kinase activ ity are tightly regulated during cell cycle progression. PLK protein i s low in G(1), accumulates during S and G(2)M, and is rapidly reduced after mitosis. During mitosis, PLK is phosphorylated on serine, and it s serine threonine kinase function is activated at a time close to tha t of p34(cdc2). The phosphorylated form of PLK migrates with reduced m obility on SDS-polyacrylamide gel electrophoresis, and dephosphorylati on by purified protein phosphatase 2A converts it to the more rapidly migrating form and reduces the total amount of PLK kinase activity. Pu rified p34(cdc2)-cyclin B complex can phosphorylate PLK protein in vit ro but causes little increase in PLK kinase activity.