Functional studies on the role of the C-terminal domain of mammalian polo-like kinase

Mammalian polo-like kinase (Pik) acts at various stages in early and late mitosis. Pik is phosphorylated and activated in mitosis, and the proper subcellular localization of Pik is essential for mitotic regulation. We have observed that overexpression of the C-terminal domain of Pik is more effective than wild-type or kinase-defective Pik in causing mitotic delay or arrest. The specific activity of Pik with C-terminal deletions or substitution of aspartate for threo-nine-210 is increased severalfold relative to wild type. We show in this communication that the C-terminal domain can bind to full-length or the catalytic domain of Pik and inhibit its kinase activity, and that this binding is disrupted when threonine-210 is substituted with an aspartic acid residue. The C-terminal domain binds unphosphorylated Pik from G(2) arrested cells, but not phosphorylated Pik from mitotic cells. Green fluorescent protein-C-terminal Pik is localized at the centrosome and the midbody of transfected cells as shown previously for full-length enzyme. These and other data indicate that although the C terminus serves to regulate Pik kinase activity, the localization of the C terminus at the centrosome and other sites in transfected cells may block the correct localization of endogenous Pik.