Proteins related to the phosphoinositide-dependent protein kinase family have been identified in the majority of eukaryotes. Although much is known about upstream mechanisms that regulate the PDK1-family of kinases in metazoans, how these kinases regulate cell growth and division remains unclear. Here, we characterize a fission yeast protein related to members of this family, which we have termed Pdk1p. Pdk1p localizes to the spindle pole body and the actomyosin ring in early mitotic cells. Cells deleted for pdk1 display multiple defects in mitosis and cytokinesis, all of which are exacerbated when the function of fission yeast polo kinase, Plo1p, is partially compromised. We conclude that Pdk1p functions in concert with Plo1p to regulate multiple processes such as the establishment of a bipolar mitotic spindle, transition to anaphase, placement of the actomyosin ring and proper execution of cytokinesis. We also present evidence that the effects of Pdk1p on cytokinesis are likely mediated via the fission yeast anillin-related protein, Mid1p, and the septation initiation network.
INTRODUCTIONThe segregation of chromosomes during mitosis and the physical division of cells during cytokinesis represent two irreversible events in the cell cycle. Not surprisingly, mitosis and cytokinesis are tightly regulated in the cell cycle in such a way that mitosis is initiated only upon completion of DNA synthesis and cytokinesis is executed only after completion of mitosis.The fission yeast Schizosaccharomyces pombe has been used extensively for the study of mitosis and cytokinesis, due to the structural and molecular conservation of these events in fission yeast and metazoans (Nurse 1990;Gould and Simanis, 1997;Feierbach and Chang, 2001). S. pombe has three chromosomes that condense during mitosis and are separated along an intranuclear mitotic spindle (Yanagida, 1998). After chromosome segregation, S. pombe cells divide through the use of an actomyosin-based contractile ring. Gene products important for mitotic entry, mitotic progression, and cytokinesis have been identified from a number of genetic screens, as well as from reverse genetic approaches (Nurse et al., 1976;Hirano et al., 1986;Chang et al., 1996;Balasubramanian et al., 1998). Central to the regulation of mitosis and cytokinesis are the S. pombe polo-related protein kinase Plo1p (Ohkura et al., 1995;BĂ€hler et al., 1998;Mulvihill et al., 1999) and members of the septation initiation network (SIN), a signaling pathway comprised of three protein kinases (Cdc7p, Sid1p, and Sid2p) and a GTPase (Spg1p) Rajagopalan et al., 2003). Members of the polo-kinase family are conserved across all eukaryotes (Nigg, 1998). In addition, several components of the SIN are highly conserved, including an analogous signaling complex in the budding yeast referred to as the mitotic exit network (Bardin and Amon, 2001;McCollum and Gould, 2001;Simanis, 2003). The fission yeast Plo1p-kinase and SIN components localize to the spindle pole body and coordinate mitotic events with cytokinesis .To...