Fission yeast IQGAP maintains F-actin-independent localization of myosin-II in the contractile ring.

During cytokinesis in many eukaryotic cells, myosin-II concentrates at the equatorial cortex with actin filaments (F-actin) and is supposed to generate forces to divide the cell into two, which is called the contractile ring (CR) hypothesis. Several lines of evidence indicate that the myosin-II is recruited independently of F-actin and interacts specifically with the equatorial F-actin. Molecular details of these mechanisms are still unknown. We used the fission yeast Schizosaccharomyces pombe to investigate the regulation of myosin-II localization. We demonstrate that the CR myosin-II was composed of F-actin-dependent and -independent fractions by simultaneously observing F-actin and myosin. The F-actin-independent fraction was visualized as cortical dots in the absence of F-actin. IQGAP Rng2, an indispensable element of CR, was implicated in maintenance of the F-actin-independent fraction of myosin-II, whereas anillin Mid1 was required for assembly but not for maintenance of the fraction. In the CR of the rng2 mutant, myosin-II was less concentrated, unstable, and nonhomogeneous, which often resulted in cytokinesis failure. These results suggest that Rng2 tethers myosin-II to the cortex along the CR independently of F-actin to provide a sufficient concentration. The robust localization of myosin-II would ensure successful cytokinesis.