Spatial regulation of cytokinesis by the Kin1 and Pom1 kinases in fission yeast

Cytokinesis requires a tight spatio-temporal coordination with mitosis to ensure proper segregation of the genetic information during cell division. In fission yeast, an actomyosin contractile ring is assembled in mitosis and dictates the site of cytokinesis. Here we investigated the functions of Kin1 and Pom1, two conserved fission yeast kinases, in cell division. We found that kin1Δ is synthetically lethal with pom1Δ because double mutant cells fail to spatially organize the actomyosin ring during mitosis, leading to aberrant septum synthesis and accumulation of post-mitotic nuclei in the same cell compartment. Assembly of an Rlc1-GFP ring in the cell center at mitosis is also compromised. Similar cytokinetic defects are observed in a tea1Δ kin1Δ mutant. Furthermore, aberrant septation and nuclear accumulation are observed in a pom1Δ strain in which the Kin1 level is either down or up-regulated. Thus, a tight control of Kin1 level is critical for ensuring accurate cell division in a pom1Δ background. Since none of the kinases can substitute for each other, Kin1 and Pom1 have distinct complementary functions. We show that Kin1 is required for F-actin polarization in interphase and after completion of mitosis and this function may be essential for cytokinesis in a pom1Δ background.