Swe1p responds to cytoskeletal perturbation, not bud size, in S. cerevisiae.
BACKGROUND
S. cerevisiae cells must grow to a critical size in G1 in order to pass start and enter the cell cycle. A recent study proposed that in addition to the mother size control in G1, the bud must grow to a critical bud size in G2 in order to enter mitosis. Insufficient bud size would cause G2 arrest enforced by the mitotic inhibitor Swe1p, explaining previous findings that some perturbations that block bud growth also trigger Swe1p-dependent cell-cycle arrest.


RESULTS
We tested the critical-bud-size hypothesis. We found that halting bud growth by inactivation of the myosin Myo2p did not trigger Swe1p-dependent arrest in budded cells, even when the buds were very small. Moreover, Swe1p did not affect cell-cycle progression in unstressed cells, even when bud size was decreased by overriding G1 size control. Actin depolymerization did cause Swe1p-dependent arrest in small-budded but not large-budded cells, as previously reported. However, we found that the key determinant of cell-cycle arrest in those circumstances was not bud size, but rather the relative abundance of the Swe1p mitotic inhibitor and the mitosis-promoting cyclins.


CONCLUSIONS
Swe1p does not respond to insufficient bud size. Instead, actin stress empowers Swe1p to promote arrest. The effectiveness of Swe1p in promoting that arrest declines as cells progress through the cell cycle.
McNulty
John J
JJ
Department of Pharmacology and Cancer Biology, Duke University Medical Center, Durham, North Carolina 27710, USA.
Lew
Daniel J
DJ
D000818:Animals; D018797:Cell Cycle Proteins / Q000378:metabolism*; D003599:Cytoskeleton / Q000378:metabolism*; D017931:DNA Primers; D005434:Flow Cytometry; D005656:Fungal Proteins / Q000235:genetics; D016195:G2 Phase / Q000502:physiology*; D008856:Microscopy, Fluorescence; D011505:Protein-Tyrosine Kinases / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000235:genetics* / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism*
D018797:Cell Cycle Proteins; D017931:DNA Primers; D005656:Fungal Proteins; D029701:Saccharomyces cerevisiae Proteins; C099685:SWE1 protein, S cerevisiae; D011505:Protein-Tyrosine Kinases