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J. Cell Biol.,
Volume 143, Number 7, December 28, 1998 1947-1960
Department of Cell Biology, Harvard Medical School, Boston, Massachusetts 02115
We previously showed that the budding
yeast Saccharomyces cerevisiae assembles an actomyosin-based ring that undergoes a contraction-like size
change during cytokinesis. To learn more about the
biochemical composition and activity of this ring, we
have characterized the in vivo distribution and function
of Cyk2p, a budding yeast protein that exhibits significant sequence similarity to the cdc15/PSTPIP family of
cleavage furrow proteins. Video microscopy of cells expressing green fluorescent protein (GFP)-tagged
Cyk2p revealed that Cyk2p forms a double ring that coincides with the septins through most of the cell cycle.
During cytokinesis, however, the Cyk2 double ring
merges with the actomyosin ring and exhibits a contraction-like size change that is dependent on Myo1p. The septin double ring, in contrast, does not undergo the
contraction-like size change but the separation between
the two rings increases during cytokinesis. These observations suggest that the septin-containing ring is dynamically distinct from the actomyosin ring and that Cyk2p transits between the two types of structures.
Gene disruption of CYK2 does not affect the assembly
of the actomyosin ring but results in rapid disassembly
of the ring during the contraction phase, leading to incomplete cytokinesis, suggesting that Cyk2p has an important function in modulating the stability of the actomyosin ring during contraction. Overexpression of
Cyk2p also blocks cytokinesis, most likely due to a loss
of the septins from the bud neck, indicating that Cyk2p
may also play a role in regulating the localization of the
septins.
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