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© The Rockefeller University Press, 0021-9525/1998//1301 $5.00
The Journal of Cell Biology, Volume 142, Number 5, , 1998 1301-1312

Articles

Involvement of an Actomyosin Contractile Ring in Saccharomyces cerevisiae Cytokinesis



Erfei Bi*, Paul Maddox*, Daniel J. Lew{ddagger}, E.D. Salmon*, John N. McMillan{ddagger}, Elaine Yeh*, and John R. Pringle*

* Department of Biology, University of North Carolina, Chapel Hill, North Carolina 27599-3280; and {ddagger} Department of Pharmacology and Cancer Biology, Duke University Medical Center, Durham, North Carolina 27710

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{Delta} strains divide nearly as efficiently as wild type; other myo1{Delta} 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.

Key Words: actin • cytokinesis • myosin • septins • yeast

Abbreviations used in this paper: DIC, differential interference contrast; GFP, green fluorescent protein; LAT-A, latrunculin-A; ORF, open reading frame; SC, synthetic complete medium.

Address all correspondence to John R. Pringle, Department of Biology, CB #3280, Coker Hall, University of North Carolina, Chapel Hill, NC 27599-3280. Tel.: (919) 962-2293. Fax: (919) 962-0320. E-mail: jpringle{at}email.unc.edu

E. Bi's present address is Department of Cell and Developmental Biology, University of Pennsylvania School of Medicine, 133 Anatomy-Chemistry Building, Philadelphia, PA 19104-6058.


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