Advances in Cytokinesis Research. Contractile Ring Formation in Xenopus Egg and Fission Yeast.

Abstract

ABSTRACT. 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.

Role for Actin Filament Turnover and a Myosin II Motor in Cytoskeleton-driven Disassembly of the Epithelial Apical Junctional Complex

“…Contractile F-actin rings play critical roles in variety of biological processes including cell division (Field et al, 1999;Noguchi et al, 2001), wound closure (Bement et al, 1993(Bement et al, , 1999Mandato and Bement, 2001;Florian et al, 2002), endocytosis (Araki et al, 2002;Sokac et al, 2003), and extrusion of apoptotic cells from epithelial monolayers (Rosenblatt et al, 2001). Two different mechanisms account for generation of force required for ring contraction.…”

“…Another requires production of force by the motor protein, myosin II, (Bement et al, 1993(Bement et al, , 1999Mandato and Bement, 2001;Araki et al, 2002;Florian et al, 2002). Because both mechanisms have been implicated in the biogenesis of F-actin rings during cytokinesis and wound healing (Mandato and Bement, 2001;Noguchi et al, 2001), we analyzed whether they are involved in reorganization of apical F-actin in calcium-depleted epithelial cells.…”

Nonmuscle myosin II exerts tension but does not translocate actin in vertebrate cytokinesis

“…The three NM II paralogs (II-A, II-B, and II-C) are composed of catalytically active heads, which bind to and translocate actin in an ATP-dependent manner, and bipolar filament-forming rods (1). Current concepts of vertebrate cytokinesis favor the notion of translocation of actin filaments by NM II motors to effect cell division (2)(3)(4)(5)(6). Translocation is linked to the hydrolysis of MgATP, resulting in conversion of chemical energy into mechanical movement.…”

Dividing the spoils of growth and the cell cycle: The fission yeast as a model for the study of cytokinesis

“…The CF is thought to be induced by geometrical asymmetry of the cortical tension in the cell. Several mechanisms for CF formation have been proposed [reviewed by Wang,2005]; currently, the most favored one is the purse-string model [reviewed by Mabuchi and Itoh,1992; Noguchi et al,2001; Pollard,2010]. The equatorial CR is composed of F-actin filaments and myosin II, and their interaction produces the force required for membrane ingression at the site of division.…”