Contractile ring formation in Xenopus egg and fission yeast.
How actin filaments (F-actin) and myosin II (myosin) assemble to form the contractile ring was investigated with fission yeast and Xenopus egg. In fission yeast cells, an aster-like structure composed of F-actin cables is formed at the medial cortex of the cell during prophase to metaphase, and a single F-actin cable(s) extends from this structure, which seems to be a structural basis of the contractile ring. In early mitosis, myosin localizes as dots in the medial cortex independently of F-actin. Then they fuse with each other and are packed into a thin contractile ring. At the growing ends of the cleavage furrow of Xenopus eggs, F-actin at first assembles to form patches. Next they fuse with each other to form short F-actin bundles. The short bundles then form long bundles. Myosin seems to be transported by the cortical movement to the growing end and assembles there as spots earlier than F-actin. Actin polymerization into the patches is likely to occur after accumulation of myosin. The myosin spots and the F-actin patches are simultaneously reorganized to form the contractile ring bundles. The idea that a Ca signal triggers cleavage furrow formation was tested with Xenopus eggs during the first cleavage. We could not detect any Ca signals such as a Ca wave, Ca puffs or even Ca blips at the growing end of the cleavage furrow. Furthermore, cleavages are not affected by Ca-chelators injected into the eggs at concentrations sufficient to suppress the Ca waves. Thus we conclude that formation of the contractile ring is not induced by a Ca signal at the growing end of the cleavage furrow.
Noguchi
T
T
Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, Japan.
Arai
R
R
Motegi
F
F
Nakano
K
K
Mabuchi
I
I
D000199:Actins / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002455:Cell Division / Q000502:physiology*; D003599:Cytoskeleton / Q000378:metabolism; D024462:Myosin Type II / Q000378:metabolism*; D009865:Oocytes / Q000166:cytology* / Q000378:metabolism; D012568:Schizosaccharomyces / Q000502:physiology*; D029702:Schizosaccharomyces pombe Proteins / Q000378:metabolism; D029867:Xenopus Proteins / Q000378:metabolism; D014982:Xenopus laevis / Q000502:physiology*
D000199:Actins; D029702:Schizosaccharomyces pombe Proteins; D029867:Xenopus Proteins; D024462:Myosin Type II; D002118:Calcium