Plo1 kinase recruitment to the spindle pole body and its role in cell division in Schizosaccharomyces pombe

Polo kinases execute multiple roles during cell division. The fission yeast polo related kinase Plo1 is required to assemble the mitotic spindle, the prophase actin ring that predicts the site for cytokinesis and for septation after the completion of mitosis (Ohkura et al., 1995; Bahler et al., 1998). We show that Plo1 associates with the mitotic but not interphase spindle pole body (SPB). SPB association of Plo1 is the earliest fission yeast mitotic event recorded to date. SPB association is strong from mitotic commitment to early anaphase B, after which the Plo1 signal becomes very weak and finally disappears upon spindle breakdown. SPB association of Plo1 requires mitosis-promoting factor (MPF) activity, whereas its disassociation requires the activity of the anaphase-promoting complex. The stf1.1 mutation bypasses the usual requirement for the MPF activator Cdc25 (Hudson et al., 1990). Significantly, Plo1 associates inappropriately with the interphase SPB of stf1.1 cells. These data are consistent with the emerging theme from many systems that polo kinases participate in the regulation of MPF to determine the timing of commitment to mitosis and may indicate that pole association is a key aspect of Plo1 function. Plo1 does not associate with the SPB when septation is inappropriately driven by deregulation of the Spg1 pathway and remains SPB associated if septation occurs in the presence of a spindle. Thus, neither Plo1 recruitment to nor its departure from the SPB are required for septation; however, overexpression of plo1+ activates the Spg1 pathway and causes transient Cdc7 recruitment to the SPB and multiple rounds of septation.