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J. Cell Biol.,
Volume 140, Number 2, January 26, 1998 355-366
Department of Cell Biology, Harvard Medical School, Boston, Massachussetts 02115
We have identified a Saccharomyces cerevisiae protein, Cyk1p, that exhibits sequence similarity to
the mammalian IQGAPs. Gene disruption of Cyk1p results in a failure in cytokinesis without affecting other
events in the cell cycle. Cyk1p is diffused throughout most of the cell cycle but localizes to a ring structure at
the mother-bud junction after the initiation of
anaphase. This ring contains filamentous actin and
Myo1p, a myosin II homologue. In vivo observation
with green fluorescent protein-tagged Myo1p showed that the ring decreases drastically in size during cell division and therefore may be contractile. These results
indicate that cytokinesis in budding yeast is likely to involve an actomyosin-based contractile ring. The assembly of this ring occurs in temporally distinct steps:
Myo1p localizes to a ring that overlaps the septins at
the G1-S transition slightly before bud emergence;
Cyk1p and actin then accumulate in this ring after the
activation of the Cdc15 pathway late in mitosis. The localization of myosin is abolished by a mutation in
Cdc12p, implicating a role for the septin filaments in
the assembly of the actomyosin ring. The accumulation
of actin in the cytokinetic ring was not observed in cells
depleted of Cyk1p, suggesting that Cyk1p plays a role
in the recruitment of actin filaments, perhaps through a
filament-binding activity similar to that demonstrated
for mammalian IQGAPs.
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