Conservation of mechanisms controlling entry into mitosis: budding yeast wee1 delays entry into mitosis and is required for cell size control.
BACKGROUND
In fission yeast, the Wee1 kinase delays entry into mitosis until a critical cell size has been reached; however, a similar role for Wee1-related kinases has not been reported in other organisms. SWE1, the budding yeast homolog of wee1, is thought to function in a morphogenesis checkpoint that delays entry into mitosis in response to defects in bud morphogenesis.


RESULTS
In contrast to previous studies, we found that budding yeast swe1 Delta cells undergo premature entry into mitosis, leading to birth of abnormally small cells. Additional experiments suggest that conditions that activate the morphogenesis checkpoint may actually be activating a G2/M cell size checkpoint. For example, actin depolymerization is thought to activate the morphogenesis checkpoint by inhibiting bud morphogenesis. However, actin depolymerization also inhibits bud growth, suggesting that it could activate a cell size checkpoint. Consistent with this possibility, we found that actin depolymerization fails to induce a G2/M delay once daughter buds pass a critical size. Other conditions that activate the morphogenesis checkpoint block bud formation, which could also activate a size checkpoint if cell size at G2/M is monitored in the daughter bud. Previous work reported that Swe1 is degraded during G2, which was proposed to account for failure of large-budded cells to arrest in response to actin depolymerization. However, we found that Swe1 is present throughout G2 and undergoes hyperphosphorylation as cells enter mitosis, as found in other organisms.


CONCLUSIONS
Our results suggest that the mechanisms known to coordinate entry into mitosis in other organisms have been conserved in budding yeast.
Harvey
Stacy L
SL
Department of Molecular, Cellular, and Developmental Biology, Sinsheimer Labs, University of California, Santa Cruz, CA 95064, USA.
Kellogg
Douglas R
DR
D015153:Blotting, Western; D018797:Cell Cycle Proteins; D048429:Cell Size; D005455:Fluorescent Antibody Technique; D008938:Mitosis / Q000502:physiology*; D009687:Nuclear Proteins; D011505:Protein-Tyrosine Kinases / Q000502:physiology*; D012441:Saccharomyces cerevisiae / Q000166:cytology* / Q000378:metabolism; D029702:Schizosaccharomyces pombe Proteins
D018797:Cell Cycle Proteins; D009687:Nuclear Proteins; D029702:Schizosaccharomyces pombe Proteins; C068689:wee1 protein, S pombe; D011505:Protein-Tyrosine Kinases