Mechanism of contractile ring formation during cytokinesis

利用裂殖酵母裂殖酵母和非洲爪蟾卵分析了收缩环的形成机制。这似乎是裂变酵母细胞中的收缩环是由核分裂过程中积累在细胞内侧区域的F-肌动蛋白索形成的。首先，在纺锤体杆体附近形成由F-肌动蛋白索组成的紫菀状结构，然后引导索伸长并包围细胞。聚集在这个区域周围的F-肌动蛋白索与这条前导索融合，这样就形成了收缩环。Cdc 12在前导索的延伸中起作用，Cdc 15在F-肌动蛋白索与前导索的融合中起作用，小G蛋白Rho被认为是裂解信号通路的重要组成成员。我们分析了Rho蛋白在裂殖酵母中的功能，发现了一个新的G蛋白Rho 3。Rho 3定位于细胞膜。结果发现，一种新的Rho 3或Cdc 42的下游存在一个新的家族蛋白For 3。Rho 3和For 3控制肌动蛋白细胞骨架和微管，从而调节细胞的形状和胞质分裂的位置。微管毒素对微管的破坏并不影响收缩环的收缩，但会影响其形成。我们还发现，在卵裂沟的下方，从两极发出的微管相互连接。我们测试了一种可能性，钙离子在裂解信号中发挥作用，通过分析钙释放周围的分裂沟。在卵裂沟的前缘既没有钙波，也没有钙泡或钙点等钙释放。

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.