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J. Cell Biol.,
Volume 142, Number 5, September 7, 1998 1301-1312
* Department of Biology, University of North Carolina, Chapel Hill, North Carolina 27599-3280; and 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 myo1 Department of
Pharmacology and Cancer Biology, Duke University Medical Center, Durham, North Carolina 27710
strains divide
nearly as efficiently as wild type; other myo1 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.
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