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

Abstract

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.

An actin–myosin-II interaction is involved in maintaining the contractile ring in fission yeast

“…We recently reported that several components of the contractile ring of the fission yeast S. pombe, including Myo2, can localize to this structure through both F-actin-dependent and -independent mechanisms, even after the completion of ring assembly, such that when F-actin is lost a fraction of the component remains at the cortical ring (Takaine et al, 2014a). Consistent with our previous findings, we found that mYFP-Myo2 colocalized with F-actin in the medial region as a node structure at early anaphase and as a ring at late anaphase (Fig.…”

“…A 3mYFP-hphMX6 module flanked by homologous sequences (400-500 bp) was amplified using MTP19 (Takaine et al, 2014a) as a template for this purpose. The DNA fragment was transformed into the wt strain MTY10, and transformants were selected for hygromycin resistance.…”

“…Myo3-3mYFP ring intensities were quantified as previously described (Takaine et al, 2014a). In brief, we imaged myo3-3mYFP sid4-3mYFP cells in 13 z-sections at 0.38-µm intervals using the spinning disk confocal microscope averaging two images.…”

IQGAP1 is associated with nuclear envelope reformation and completion of abscission

“…IQGAP1 is an integral component of the yeast actinomyosin contractile ring and is essential for yeast cytokinesis. [22][23][24][25][26][27][28][29][30][31] In the budding yeast S. cerevisiae, Iqg1 co-localizes with filamentous actin and Myo1p, a myosin II homolog, at the mother-bud junction after the initiation of anaphase. 24,25 Here, it regulates formation of the contractile ring, as its overexpression results in it premature formation, 24 whereas its depletion prevents accumulation of actin in the cytokinetic ring.…”

“…26,27 Rng2 bundles actin filaments 29 and is essential for maintaining myosin-II at the contractile ring. 30 In mammalian cells there are 3 IQGAP proteins, IQGAP1, -2 and -3 among which IQGAP1 and -3 have been linked to cytokinesis. In mouse embryos treated with toxin B, which inhibits Cdc42, the IQGAP upstream GTPase, IQGAP1 localization is disrupted, the actin cytoskeleton is rearranged and cytokinesis is inhibited.…”