The Morphogenesis Checkpoint in Saccharomyces cerevisiae: Cell Cycle Control of Swe1p Degradation by Hsl1p and Hsl7p

Abstract

In Saccharomyces cerevisiae, the Wee1 family kinase Swe1p is normally stable during G₁ and S phases but is unstable during G₂ and M phases due to ubiquitination and subsequent degradation. However, perturbations of the actin cytoskeleton lead to a stabilization and accumulation of Swe1p. This response constitutes part of a morphogenesis checkpoint that couples cell cycle progression to proper bud formation, but the basis for the regulation of Swe1p degradation by the morphogenesis checkpoint remains unknown. Previous studies have identified a protein kinase, Hsl1p, and a phylogenetically conserved protein of unknown function, Hsl7p, as putative negative regulators of Swe1p. We report here that Hsl1p and Hsl7p act in concert to target Swe1p for degradation. Both proteins are required for Swe1p degradation during the unperturbed cell cycle, and excess Hsl1p accelerates Swe1p degradation in the G₂-M phase. Hsl1p accumulates periodically during the cell cycle and promotes the periodic phosphorylation of Hsl7p. Hsl7p can be detected in a complex with Swe1p in cell lysates, and the overexpression of Hsl7p or Hsl1p produces an effective override of the G₂arrest imposed by the morphogenesis checkpoint. These findings suggest that Hsl1p and Hsl7p interact directly with Swe1p to promote its recognition by the ubiquitination complex, leading ultimately to its destruction.

ACKNOWLEDGMENTS

We thank Y. Barral, D. Kellogg, A. Myers, M. Snyder, and J. Thorner for communicating results prior to publication. We thank Sally Kornbluth, Robin Wharton, John York, and Jake Harrison for critical reading of the manuscript, and the members of the Lew and Pringle labs for stimulating interactions.

J.N.M. and M.S.L. were supported by NIH postdoctoral fellowships GM18455 and GM15766, respectively. This work was supported by NIH grant GM31006 to J.R.P. and by NIH grant GM53050 and funds from the Searle Scholars Program/The Chicago Community Trust to D.J.L.