Localization and signaling of gp subunit ste4p are controlled by a-factor receptor and the a-specific protein asg7p

Haploid yeast cells initiate pheromone signaling upon the binding of pherom one to its receptor and activation of the coupled G protein. A regulatory p rocess termed receptor inhibition blocks pheromone signaling when the a-fac tor receptor is inappropriately expressed in MATa cells. Receptor inhibitio n blacks signaling by inhibiting the activity of the G protein beta subunit , Ste4p. To investigate how Ste4p activity is inhibited, its subcellular lo cation was examined. In wild-type cells, alpha -factor treatment resulted i n localization of Ste4p to the plasma membrane of mating projections. In ce lls expressing the a-factor receptor, alpha -factor treatment resulted in l ocalization of Ste4p away from the plasma membrane to an internal compartme nt. An altered version of Ste4p that is largely insensitive to receptor inh ibition retained its association with the membrane in cells expressing the a-factor receptor. The inhibitory function of the a-factor receptor require d ASG7, an a-specific gene of previously unknown function. ASG7 RNA was ind uced by pheromone, consistent with increased inhibition as the pheromone re sponse progresses. The a-factor receptor inhibited signaling in its ligande d state, demonstrating that the receptor can block the signal that it initi ates. ASG7 was required for the altered localization of Ste4p that occurs d uring receptor inhibition, and the subcellular location of Asg7p was consis tent with its having a direct effect on Ste4p localization. These results d emonstrate that Asg7p mediates a regulatory process that blocks signaling f rom a G protein beta subunit and causes its relocalization within the cell.