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Vol. 14, Issue 9, 3617-3627, September 2003

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Gef1p and Scd1p, the Two GDP-GTP Exchange Factors for Cdc42p, Form a Ring Structure that Shrinks during Cytokinesis in Schizosaccharomyces pombe

Kouji Hirota ^* , Kayoko Tanaka ^*, Kunihiro Ohta  , and Masayuki Yamamoto ^* 

^* Department of Biophysics and Biochemistry, Graduate School of Science, University of Tokyo, Hongo, Tokyo 113-0033, Japan; Genetic Dynamics Research Unit-Laboratory, The Institute of Physical and Chemical Research (RIKEN), Wako-shi, Saitama 351-0198, Japan; and Cellular & Molecular Biology Laboratory, The Institute of Physical and Chemical Research (RIKEN)/CREST of the JST, Wako-shi, Saitama 351-0198, Japan

Submitted October 17, 2002; Revised May 14, 2003; Accepted May 14, 2003
Monitoring Editor: John Pringle

Fission yeast Cdc42p, a small GTPase of the Rho family, is essential for cell proliferation and maintenance of the rod-like cell morphology. Scd1/Ral1p is a GDP-GTP exchange factor (GEF) for Cdc42p. This study and a parallel study by others establish that Gef1p is another GEF for Cdc42p. Deletions of gef1 and scd1 are synthetically lethal, generating round dead cells, and hence mimic the phenotype of cdc42 deletion. Gef1p is localized mainly to the cell division site. Scd1p is also there, but it is also detectable in other parts of the cell, including the nucleus, growing ends, and the tips of conjugation tubes. Gef1p and Scd1p form a ring structure at the cell division site, which shrinks during cytokinesis following the contraction of the actomyosin ring. Formation of the Gef1p/Scd1p ring apparently depends on the integrity of the actomyosin ring. In turn, recruitment of Cdc42p to the cell division site follows the shrinking Gef1p/Scd1p ring; the Cdc42p accumulates like a closing iris. These observations suggest that Gef1p and Scd1p may have a role in mediating between contraction of the actomyosin ring and formation of the septum, by recruiting active Cdc42p to the septation site.

Corresponding author. E-mail address: myamamot{at}ims.u-tokyo.ac.jp.

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