Involvement of an Actomyosin Contractile Ring in Saccharomyces cerevisiae Cytokinesis

Abstract

In Saccharomyces cerevisiae, the mother cell and bud are connected by a narrow neck. The mechanism by which this neck is closed during cytokinesis has been unclear. Here we report on the role of a contractile actomyosin ring in this process. Myo1p (the only type II myosin in S. cerevisiae) forms a ring at the presumptive bud site shortly before bud emergence. Myo1p ring formation depends on the septins but not on F-actin, and preexisting Myo1p rings are stable when F-actin is depolymerized. The Myo1p ring remains in the mother–bud neck until the end of anaphase, when a ring of F-actin forms in association with it. The actomyosin ring then contracts to a point and disappears. In the absence of F-actin, the Myo1p ring does not contract. After ring contraction, cortical actin patches congregate at the mother–bud neck, and septum formation and cell separation rapidly ensue. Strains deleted for MYO1 are viable; they fail to form the actin ring but show apparently normal congregation of actin patches at the neck. Some myo1Δ strains divide nearly as efficiently as wild type; other myo1Δ strains divide 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 delayed. Thus, the contractile actomyosin ring is not essential for cytokinesis in S. cerevisiae. In its absence, cytokinesis can still be completed by a process (possibly localized cell–wall synthesis leading to septum formation) that appears to require septin function and to be facilitated by F-actin.

The Role of Cdc42p GTPase-activating Proteins in Assembly of the Septin Ring in Yeast

“…Taken together, the evidence suggests that the septins are the major constituents of the neck filaments. One major role for the septins in various organisms is in cytokinesis (Hartwell, 1971;Neufeld and Rubin, 1994;Kinoshita et al, 1997;Bi et al, 1998;Lippincott and Li, 1998). Consistent with this, the septins typically localize to the division site (cleavage furrow) at least during the cytokinesis phase of the cell cycle (Longtine et al, 1996;Field and Kellogg, 1999;Trimble, 1999;Nguyen et al, 2000;Westfall and Momany, 2002).…”

“…Instead of the normal ring structure at the mother-bud neck ( Figure 3B, left panel), the septins often formed parallel bars in the neck (Figure 3, B and C, arrowheads) or a cap on the bud (Figure 3, B and C, arrows). This disorganization of the septins was paralleled by disorganization of the type II myosin Myo1p, a contractile-ring component that normally localizes to the mother-bud neck throughout the cell cycle in a septin-dependent manner (Bi et al, 1998;Lippincott and Li, 1998; Figure 4A). In many bem3⌬ rga1⌬ rga2⌬ cells, Myo1p formed a ring that looked normal or nearly so ( Figure 4B, cells 1 and 2).…”

Roles of a Fimbrin and an α-Actinin-like Protein in Fission Yeast Cell Polarization and Cytokinesis

“…However, one significant difference is in the relative importance of the actomyosin contractile ring during cytokinesis. Although myosin II and the contractile ring are essential for cytokinesis in S. pombe (Bezanilla et al, 1997;Kitayama et al, 1997;May et al, 1997;Motegi et al, 1997), they are not essential in S. cerevisiae (Bi et al, 1998;Lippincott and Li, 1998). A possible explanation for this difference is that the primary role of the contractile ring is to guide septum deposition (Hales et al, 1999;Vallen et al, 2000) and that the need for such guidance is greater in forming the ϳ3-m-diameter septum in S. pombe than in forming the ϳ1-m-diameter septum in S. cerevisiae.…”

Stable and Dynamic Axes of Polarity Use Distinct Formin Isoforms in Budding Yeast

“…Septins are recruited to the nascent bud site in a Cdc42p-dependent manner, and then remain at that location as a ring around the neck of the emerging and growing bud, and finally divide into two rings at cell separation ( Figure S2A and B; for review, see Field and Kellogg, 1999;Gladfelter et al, 2001;Faty et al, 2002;Longtine and Bi, 2003). However, the septins play a variety of roles, promoting the proper activation of the B-cyclins for cell cycle progression (Barral et al, 1999;McMillan et al, 1999;Shulewitz et al, 1999;Longtine et al, 2000;Hanrahan and Snyder, 2003), maintaining a diffusion barrier between the bud and mother cortices , and recruiting cytokinetic (Bi et al, 1998;Lippincott and Li, 1998) and cell wall-synthesizing machinery (DeMarini et al, 1997) to the bud neck. To determine whether septins influence actin cable organization, we examined Tpm1p distribution in yeast with a temperaturesensitive CDC10 septin (cdc10-1) (Hartwell, 1971).…”