Polo-like Kinase Is a Cell Cycle-regulated Kinase Activated during Mitosis

Abstract

Previously, we demonstrated that expression of pololike kinase (PLK) is required for cellular DNA synthesis and that overexpression of PLK is sufficient to induce DNA synthesis. We now report that the endogenous levels of PLK, its phosphorylation status, and protein kinase activity are tightly regulated during cell cycle progression. PLK protein is 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 its serine threonine kinase function is activated at a time close to that of p34 cdc2. The phosphorylated form of PLK migrates with reduced mobility on SDS-polyacrylamide gel electrophoresis, and dephosphorylation by purified protein phosphatase 2A converts it to the more rapidly migrating form and reduces the total amount of PLK kinase activity. Purified p34 cdc2 -cyclin B complex can phosphorylate PLK protein in vitro but causes little increase in PLK kinase activity.The processes of cell growth and division are stringently regulated to ensure fidelity of DNA replication and correct segregation of genetic information (1, 2). So essential are these processes to cellular homeostasis that many of the gene products regulating passage through the cell cycle are highly conserved both in amino acid sequence and function between organisms as evolutionarily divergent as yeast and man (3,4). Within the last few years, a variety of related enzymes known as cyclin-dependent protein kinases (CdKs) 1 have been identified that are activated and inactivated at specific times during cell cycle progression (5-8). The CdKs must form complexes with a cyclin family member to be activated (9, 10) and are also subject to both positive and negative phosphorylation by other protein kinases and dephosphorylation by protein phosphatases (11-17). Together, the family of CdKs constitutes critical components of the engine that propels the cell through the cycle. Many proteins that are phosphorylated in a cell cyclespecific fashion have been identified as putative targets of CdK regulation including nuclear lamins (18, 19), nucleolin (20, 21), and other matrix proteins, as well as cytoskeletal proteins (7,22). In addition to such architectural proteins, the tumor suppressor gene products p53 (23) and RB (24 -28) are both subject to regulatory phosphorylation by CdK family members (7). Thus, it is clear that reversible phosphorylation reactions play a major role in regulating cell cycle progression, and the list of enzymes known to be involved is rapidly expanding.A number of cell cycle-regulated kinases unrelated to the CdKs have also been identified in lower eukaryotes, including the homologous yeast CDC5 (29) and Drosophila polo (30). Polo was first identified as an embryonic lethal Drosophila mutation causing formation of monopolar and multipolar mitotic spindles and abnormal segregation of chromosomes (31). Saccharomyces cerevisiae CDC5 mutants likewise are impaired in mitotic spindle formation, but the CDC5 kinase also appears to have a function during S pha...

Adhesion induced expression of the serine/threonine kinase Fnk in human macrophages

“…The Drosophila gene polo and the yeast cell cycle genes Cdc5 and plo1 encode related protein kinases which are required for progression through mitosis (Sunkel and Glover, 1988;Llamazares et al, 1991;Fenton and Glover, 1993;Kitada et al, 1993;Ohkura et al, 1995;Toczyski et al, 1997). In recent studies mammalian protein kinases have been identi®ed which are homologous to the Drosophila gene polo (Simmons et al, 1992;Clay et al, 1993;Lake and Jelinek, 1993;Golsteyn et al, 1994;Hamanaka et al, 1994;Holtrich et al, 1994). To date the subfamily of polo-related protein kinases encompasses the murine kinases Snk, Plk, Sak and Fnk (Simmons et al, 1992;Clay et al, 1993;Fode et al, 1994;Donohue et al, 1995) as well as the human orthologues Sak and Plk (Lake and Jelinek, 1993;Golsteyn et al, 1994;Hamanaka et al, 1994;Holtrich et al, 1994;Karn et al, 1997).…”

“…In recent studies mammalian protein kinases have been identi®ed which are homologous to the Drosophila gene polo (Simmons et al, 1992;Clay et al, 1993;Lake and Jelinek, 1993;Golsteyn et al, 1994;Hamanaka et al, 1994;Holtrich et al, 1994). To date the subfamily of polo-related protein kinases encompasses the murine kinases Snk, Plk, Sak and Fnk (Simmons et al, 1992;Clay et al, 1993;Fode et al, 1994;Donohue et al, 1995) as well as the human orthologues Sak and Plk (Lake and Jelinek, 1993;Golsteyn et al, 1994;Hamanaka et al, 1994;Holtrich et al, 1994;Karn et al, 1997). The expression of the Plk gene is tightly linked to cell proliferation and regulated during cell cycle progression (Clay et al, 1993;Lake and Jelinek, 1993;Holtrich et al, 1994).…”

The physical association and phosphorylation of Cdc25C protein phosphatase by Prk

“…In the past few years, an emerging family of protein kinases (the polo kinase family) has been described in yeast (Kitada et al, 1993), Drosophila (Llamazares et al, 1991), Xenopus (Kumagai and Dunphy, 1996), mouse (Simmons et al, 1992;Golsteyn et al, 1995;Donohue et al, 1995), and human (Hamanaka et al, 1994;Li et al, 1996; also see recent review Nigg, 1998). The Drosophila polo gene, homologous to the budding yeast Cdc5, encodes a serine/threonine (Ser/Thr) kinase and is required for mitosis in this species; mutations in this gene result in abnormal mitotic and meiotic division (Fenton and Glover, 1993).…”

“…The human has at least three genes, plk (Hamanaka et al, 1994), prk (Li et al, 1996) and snk (GenBank accession #AF059617), whose structures are related to Drosophila. We have previously reported the cloning and characterization of a cDNA coding for human prk (proliferation-related kinase or polo-related kinase) (Li et al, 1996).…”

Cell Cycle Regulation of the Human Polo-like Kinase (PLK) Promoter

“…The kinetics of Plk kinase activation (9), its localization at the spindle poles and postmitotic midbody (10), and its association with CHO1/MKLP-1 (11) suggest that Plk, like polo, CDC5, and Plo1, is involved in chromosome segregation.…”