The role of Plo1 kinase in mitotic commitment and septation in Schizosaccharomyces pombe

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Abstract
Plo1‐associated casein kinase activity peaked during mitosis before septation. Phosphatase treatment abolished this activity. Mitotic Plo1 activation had a requirement for prior activation of M‐phase promoting factor (MPF), suggesting that Plo1 does not act as a mitotic trigger kinase to initiate MPF activation during mitotic commitment. A link between Plo1 and the septum initiating network (SIN) has been suggested by the inability of plo1Δ cells to septate and the prolific septation following plo1+ overexpression. Interphase activation of Spg1, the G protein that modulates SIN activity, induced septation but did not stimulate Plo1‐associated kinase activity. Conversely, SIN inactivation did not affect the mitotic stimulation of Plo1‐associated kinase activity. plo1.ts4 cells formed a misshapen actin ring, but rarely septated at 36°C. Forced activation of Spg1 enabled plo1.ts4 mutant cells, but not cells with defects in the SIN component Sid2, to convert the actin ring to a septum. The ability of plo1+ overexpression to induce septation was severely compromised by SIN inactivation. We propose that Plo1 acts before the SIN to control septation.