A GTP-exchange factor required for cell orientation

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The Rho-family of GTPases and their regulators are essential for cytoskeletal reorganization and transcriptional activation in response to extracellular signals1,2. Little is known about what links these molecules to membrane receptors. In the budding yeast Saccharomyces cerevisiae, haploid cells respond to mating pheromone through a G-protein-coupled receptor and the βγ subunit of the G protein3, resulting in arrest of the cell cycle, transcriptional activation, and polarized growth towards a mating partner4,5. The Rho-family GTPase Cdc42 and its exchange factor Cdc24 have been implicated in the mating process6,7, but their specific role is unknown. Here we report the identification of cdc24 alleles that do not affect vegetative growth but drastically reduce the ability of yeast cells to mate. When exposed to mating pheromone, these mutants arrest growth, activate transcription, and undergo characteristic morphological and actin-cytoskeleton polarization. However, the mutants are unable to orient towards a pheromone gradient, and instead position their mating projection adjacent to their previous bud site. The mutants are specifically defective in the binding of Cdc24 to the G-protein βγ subunit. Our results demonstrate that the association of an exchange factor and the βγ subunit of a hetero-trimeric G protein links receptor-mediated activation to oriented cell growth.

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Figure 1: Cdc24-m1 phenotypes.
Figure 2: Cdc24-m1 cells are unable to orient in a pheromone gradient.
Figure 3: Cdc24-m mutants are defective in mating and Ste4 binding.
Figure 4: Model of the signal transduction pathway required for cell orientation.


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We thank Y. Anraku, R. Deschenes, G. Fink, E. Jacobs, P. James and P. Philippsen for strains and plasmids; M. Bassilana, M. Bienz, T. Bretscher, H. Pelham and S. Munro for comments and suggestions; and N. Lowe for assistance with sequencing. This work was supported by the Medical Research Council. A.N. was supported by fellowships from the Fonds der Chemischen Industrie and the EC.

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Correspondence to Robert A. Arkowitz.

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