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Mol. Cell. Biol. 20 (23): 8815-8825

Copyright © 2000 by the American Society for Microbiology. All rights reserved.

Molecular and Cellular Biology, December 2000, p. 8815-8825, Vol. 20, No. 23
Copyright © 2000, American Society for Microbiology. All rights reserved.

Asg7p-Ste3p Inhibition of Pheromone Signaling: Regulation of the Zygotic Transition to Vegetative Growth

Amy F. Roth,1 Bryce Nelson,2 Charlie Boone,2 and Nicholas G. Davis3,*

Department of Surgery1 and Departments of Surgery and Pharmacology,3 Wayne State University School of Medicine, Detroit, Michigan 48201, and Banting and Best Department of Medical Research, University of Toronto, Toronto, Ontario, Canada M5G 1L62

Received 28 June 2000/Returned for modification 22 August 2000/Accepted 15 September 2000

The inappropriate expression of the a-factor pheromone receptor (Ste3p) in the MATa cell leads to a striking inhibition of the yeast pheromone response, the result of a functional interaction between Ste3p and some MATa-specific protein. The present work identifies this protein as Asg7p. Normally, expression of Ste3p and Asg7p is limited to distinct haploid mating types, Ste3p to MATalpha cells and Asg7p to MATa cells. Artificial coexpression of the two in the same cell, either a or alpha , leads to dramatic inhibition of the pheromone response. Ste3p-Asg7p coexpression also perturbs the membrane trafficking of Ste3p: Ste3p turnover is slowed, a result of an Asg7p-mediated retardation of the secretory delivery of the newly synthesized receptor to the plasma membrane. However, in the absence of ectopic Ste3p expression, the asg7Delta mutation is without consequence either for pheromone signaling or overall mating efficiency of a cells. Indeed, the sole phenotype that can be assigned to MATa asg7Delta cells is observed following zygotic fusion to its alpha  mating partner. Though formed at wild-type efficiency, zygotes from these pairings are morphologically abnormal. The pattern of growth is deranged: emergence of the first mitotic bud is delayed, and, in its place, growth is apparently diverted into a novel structure superficially resembling the polarized mating projection characteristic of haploid cells responding to pheromone. Together these results suggest a mechanism in which, following the zygotic fusion event, Ste3p and Asg7p gain access to one another and together act to repress the pheromone response, promoting the transition of the new diploid cell to vegetative growth.

* Corresponding author. Mailing address: Departments of Surgery and Pharmacology, Wayne State University School of Medicine, Elliman Building, Room 1205, 421 E. Canfield, Detroit, MI 48201. Phone: (313) 577-7807. Fax: (313) 577-7642. E-mail: ndavis{at}

Molecular and Cellular Biology, December 2000, p. 8815-8825, Vol. 20, No. 23
Copyright © 2000, American Society for Microbiology. All rights reserved.

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