Summary
It remains unknown when, where, and how the site of abscission is generated during cytokinesis. Here, we show that the sites of constriction, i.e., the sites of future abscission, are initially formed at the ends of the intercellular bridge during early midbody stage, and that these sites are associated with the non-muscle myosin-IIB (not myosin-IIA), actin filaments, and septin 9 until abscission. The ESCRT-III component CHMP4B localizes to the midbody and “spreads” to the site of abscission only during late midbody stage. Strikingly, inhibition of myosin-II motor activity by a low dose of Blebbistatin completely abolishes the formation of the constriction sites, resulting in the localization of all the above-mentioned components to the midbody region. These data strongly suggest that a secondary actomyosin ring provides the primary driving force for the thinning of the intercellular bridge to allow ESCRT-mediated membrane fission.
Highlightsd The transitional septin hourglass possesses a zonal architecture d Bud3 (RhoGEF) and Bud4 (anillin) sandwich myosin-II on the transitional hourglass d Bud3 stabilizes the single septin filaments in the transitional hourglass d Bud4 promotes paired-single filament interactions in the transitional hourglass
The contractile ring, which plays critical roles in cytokinesis in fungal and animal cells, has fascinated biologists for decades. However, the basic question of how the non-muscle myosin-II and actin filaments are assembled into a ring structure to drive cytokinesis remains poorly understood. It is even more mysterious why and how the budding yeast Saccharomyces cerevisiae, the fission yeast Schizosaccharomyces pombe, and humans construct the ring structure with one, two, and three myosin-II isoforms, respectively. Here, we provide a comparative analysis of the roles of the non-muscle myosin-IIs in cytokinesis in these three model systems, with the goal of defining the common and unique features and highlighting the major questions regarding this family of proteins.
How cells adopt a different morphology to cope with stress is not well understood. Here, we show that budding yeast Ecm25 associates with polarized endocytic sites and interacts with the polarity regulator Cdc42 and several late-stage endocytic proteins via distinct regions, including an actin filament-binding motif. Deletion of ECM25 does not affect Cdc42 activity or cause any strong defects in fluid-phase and clathrin-mediated endocytosis but completely abolishes hydroxyurea-induced cell elongation. This phenotype is accompanied by depolarization of the spatiotemporally coupled exo-endocytosis in the bud cortex while maintaining the overall mother-bud polarity. These data suggest that Ecm25 provides an essential link between the polarization signal and the endocytic machinery to enable adaptive morphogenesis under stress conditions.
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