INVOLVEMENT OF AN ACTOMYOSIN CONTRACTILE RING IN SACCHAROMYCES-CEREVISIAE CYTOKINESIS

In Saccharomyces cerevisiae, the mother cell and bud are connected by a narrow neck. The mechanism by which this neck is closed during cytok inesis has been unclear. Here we report on the role of a contractile a ctomyosin ring in this process. Myo1p (the only type II myosin in S. c erevisiae) forms a ring at the presumptive bud site shortly before bud emergence. Myo1p ring formation depends on the septins but not on F-a ctin, and preexisting Myo1p rings are stable when F-actin is depolymer ized. The Myo1p ring remains in the mother-bud neck until the end of a naphase, when a ring of F-actin forms in association with it. The acto myosin ring then contracts to a point and disappears. In the absence o f F-actin, the Myo1p ring does not contract. After ring contraction, c ortical actin patches congregate at the mother-bud neck, and septum fo rmation and cell separation rapidly ensue. Strains deleted for MYO1 ar e viable; they fail to form the actin ring but show apparently normal congregation of actin patches at the neck. Some myo1 Delta strains div ide nearly as efficiently as wild type; other myo1 Delta strains divid e less efficiently, but it is unclear whether the primary defect is in cytokinesis, septum formation, or cell separation. Even cells lacking F-actin can divide, although in this case division is considerably de layed. Thus, the contractile actomyosin ring is not essential for cyto kinesis in S. cerevisiae. In its absence, cytokinesis can still be com pleted by a process (possibly localized cell-wall synthesis leading to septum formation) that appears to require septin function and to be f acilitated by F-actin.