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© The Rockefeller University Press, 0021-9525/1998//1947 $5.00
The Journal of Cell Biology, Volume 143, Number 7, , 1998 1947-1960

Regular Articles

Dual Function of Cyk2, a cdc15/PSTPIP Family Protein, in Regulating Actomyosin Ring Dynamics and Septin Distribution



John Lippincott and Rong Li

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.

Key Words: cytokinesis • actin • myosin • septins • yeast

Abbreviations used in this paper: Cyk2, cytokinesis 2; GFP, green fluorescent protein; HA, hemagglutinin; ORF, open reading frame; YPD, yeast extract, peptone, dextrose; YPG, yeast extract, peptone, galactose, raffinose.

The authors are extremely grateful to A. Mallavarapu for providing custom software and help with video microscopy. We thank C. Field (Harvard Medical School) and J. Frazier (University of California, San Francisco, CA) for their gift of anti-Cdc3 antibodies, A. Straight (Harvard Medical School) for providing tubulin-GFP, D. Winter (Harvard Medical School) for providing Arp2–GFP, and J. Pringle (University of North Carolina, Chapel Hill, NC) for septin mutant strains. We are indebted to J. Swedlow (University of Dundee, Dundee, Scotland, UK), W. Prinz, and J. Yarrow (both from Harvard Medical School) for their help with three-dimensional deconvolution imaging and to S. Storms (Harvard Medical School) for computer assistance. We thank S. Shepard for preparation of media and solutions, and K. Shannon, D. Winter, T. Lechler, N. Tolliday, C. Field, A. Straight (all from Harvard Medical School), and F. Lippincott (Boston, MA) for support, thoughtful discussion, and critical reading of the manuscript.


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