A MORPHOGENESIS CHECKPOINT MONITORS THE ACTIN CYTOSKELETON IN YEAST

A morphogenesis checkpoint in budding yeast delays cell cycle progress ion in response to perturbations of cell polarity that prevent bud for mation (Lew, D.J., and S.I. Reed. 1995. J. Cell Biol. 129:739-749). Th e cell cycle delay depends upon the tyrosine kinase Swe1p, which phosp horylates and inhibits the cyclin-dependent kinase Cdc28p (Sia, R.A.L. , H.A. Herald, and D.J. Lew. 1996. Mel. Biol. Cell. 7:1657-1666). In t his report, we have investigated the nature of the defect(s) that trig ger this checkpoint. A Swe1p dependent cell cycle delay was triggered by direct perturbations of the actin cytoskeleton, even when polarity establishment functions remained intact. Furthermore, actin perturbati on could trigger the checkpoint even in cells that had already formed a bud, suggesting that the checkpoint directly monitors actin organiza tion, rather than (or in addition to) polarity establishment or bud fo rmation. In addition,we show that the checkpoint could detect actin pe rturbations through most of the cell cycle. However, the ability to re spond to such perturbations by delaying cell cycle progression was res tricted to a narrow window of the cell cycle, delimited by the periodi c accumulation of the checkpoint effector, Swe1p.