Mechanism of contractile ring formation during cytokinesis

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We analyzed mechanism of formation of the contractile ring using the fission yeast Schizosaccharomyces pombe and Xenopus laevis eggs. It seemed that the contractile ring in fission yeast cells are formed from F-actin cables which accumulate at the medial region of the cell during nuclear division. First, the aster-like structure consisting of F-actin cables is formed during prophase near the spindle pole body, and then the leading cable elongates and encircles the cell. F-actin cables accumulated around this region are fused to this leading cable so that the contractile ring is estabIished. Cdc12 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.The small G-protein Rho has been considered to be an important component member of the cleavage signaling pathway. We 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.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.