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J. Biol. Chem., Vol. 282, Issue 1, 29-38, January 5, 2007

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A Novel Cdc42-interacting Domain of the Yeast Polarity Establishment Protein Bem1

IMPLICATIONS FOR MODULATION OF MATING PHEROMONE SIGNALING^*

From the Department of Computational Biology, Graduate School of Frontier Sciences, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8561 and the Institute for Bioinformatics Research and Development, Japan Science and Technology Agency, Tokyo 102-0081, Japan

In Saccharomyces cerevisiae, the Rho-type small GTPase Cdc42 is activated by its guanine-nucleotide exchange factor Cdc24 to polarize the cell for budding and mating. A multidomain protein Bem1 interacts not only with Cdc42 but also with Cdc24 and the effectors of Cdc42, including the p21-activated kinase Ste20, to function as a scaffold for cell polarity establishment. Although Bem1 interacts with Cdc24 and Ste20 via its PB1 and the second SH3 domains (SH3b), respectively, it is unclear how Bem1 binds Cdc42. Here we show that a region comprising the SH3b and its C-terminal flanking segment termed CI (SH3b-CI) directly interacts with Cdc42. A dual-bait reverse two-hybrid approach revealed that the CI is critical to the interaction: N253D substitution in the CI abolishes the binding of the SH3b-CI to Cdc42 but not to the proline-rich region of Ste20, whereas W192K substitution in the SH3b has the opposite effect. Nevertheless, the SH3b-CI interacts with Ste20 proline-rich region and Cdc42 in a mutually exclusive manner. The N253D substitution renders cellular growth temperature-sensitive and suppresses mating. The W192K-induced mating defect is exacerbated by the N253D substitution and suppressed by increasing the dosage of Ste20 provided that the CI is intact. Intriguingly, Cdc42 can mediate an indirect interaction of the SH3b-CI to the CRIB domain of Ste20. These results suggest that the SH3b and the CI collaborate in tethering of Ste20 to Bem1 to ensure efficient mating pheromone signaling.

Received for publication, October 2, 2006 , and in revised form, October 26, 2006.

^* This work was supported by grants-in-aid for Scientific Research and the National Project on Protein Structural and Functional Analyses from the Ministry of Education, Culture, Sports, Science and Technology of Japan, and Core Research for Evolutional Science and Technology (CREST) project of the Japan Science and Technology Agency. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed. Tel.: 81-4-7136-3989; Fax: 81-4-7136-3979; E-mail: ito{at}k.u-tokyo.ac.jp.

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