Mechanisms of formation and destruction of the contractile ring during cytokinesis

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We analyzed mechanism of formation of the contractile ring using various cells. In fission yeast cells, the contractile ring is formed from F-actin cables which accumulate at the medial region of the cell during nuclear division. The aster-like structure consisting of F-actin cables and the leading cable which elongates from the structure are important in this process. Cdcl2 plays a role in elongation of the leading cable, while Cdc15 plays a role in the fusion of the F-actin cables to the leading cable. We also analyzed function of Rho proteins in fission yeast, and found a novel G-protein Rho3. Rho3 is localized to the cell membrane .It was found that a novel formin family protein For3 is present downstream of either Rho3 or Cdc42. Rho3 and For3 controls both actin cytoskeleton and microtubules, and thereby regulate shape of the cell and position of cytokinesis. Cla4(PAK) and Bni1 (formin) cooperatively function in regulation of the septin ring in budding yeast. F-actin plays an impo … More rtant role in this process. In Tetrahymena, dimeric EF1a bundles F-actin. Ca/calmodulin dissociates this EF1a dimer and thus inhibit the bundling of F-actin. Tetrahymena fimbrin binds and bundles F-actin in a Ca-insensitive manner. It is localized to the cleavage furrow. We found that actin is less concentrated in the cortex near the centrosome while concentrated in the cortex away from it, where the furrow is formed, in the fourth division in sea urchin embryos. Role of microtubules in formation of the contractile ring in Xenopus eggs was analyzed. Disruption of microtubules by microtubule poisons did not interfere with contraction of the contractile ring, but interfered with its formation. We also found that the microtubules emanated from the both poles link with each other beneath the cleavage furrow. We tested a possibility that Ca ions play a role in cleavage signaling by analyzing Ca release around the cleavage furrow. Neither Ca waves nor small Ca releases such as Ca puffs or Ca blips were found at the leading edge of the cleavage furrow. Rho-kinase in HeLa cells binds to filamin A. Thus, it is possible that these proteins co-localize in the contractile ring. Less