Autocrine activation of the pheromone response pathway in matα2 - cells is attenuated by SST2- and ASG7-dependent mechanisms

Abstract

Yeast mat alpha2 mutants express both mating pheromones and both mating pheromone receptors. They show modest signaling in the pheromone response pathway, as revealed by increased levels of FUS1 transcript, yet are resistant to pheromone treatment. Together, these phenotypes suggest that alpha2- cells undergo autocrine activation of the pheromone response pathway, which is subsequently attenuated. We constructed a regulatable version of the alpha2 gene (GALalpha2) and showed that, upon loss of alpha2 activity, cells exhibit an initial robust response to pheromone that is attenuated within 3 h. We reasoned that the viability of alpha2- cells might be due to attenuation, and therefore performed a genome-wide synthetic lethal screen to identify potential adaptation components. We identified two genes, SST2 and ASG7. Loss of either of these attenuation components results in activation of the pheromone pathway in alpha2- cells. Loss of both proteins causes a more severe phenotype. Sst2 functions as a GTPase activating protein (GAP) for the Galpha subunit of the trimeric G protein. Asg7 is an a -cell specific protein that acts in concert with the alpha-cell specific a -factor receptor, Ste3, to inhibit signaling by Gbetagamma. Hence, our results suggest that mat alpha2 mutants mimic the intracellular signaling events that occur in newly fused zygotes.

Interspecies variation reveals a conserved repressor of α-specific genes in Saccharomyces yeasts

“…In contrast, deletion of STE3 enhanced wrinkling, bimating, and expression of FIG2 (data not shown). This result was consistent with expression of STE3 in a cells dampening the pheromonesignaling response (Roth et al 2000;Rivers and Sprague 2003). Taken together, these data established that the cell and colony morphology defects in MATa sum1⌬ mutants were caused by production of ␣-factor and subsequent stimulation by the same or neighboring cells within a growing colony.…”

“…Without active repression of ␣-specific genes, this newly formed a cell might mate with another a cell, generating a MATa/a diploid unable to undergo meiosis and destined for meiotic oblivion should it mate with an ␣ cell to form a triploid. A complementary role for Sum1 was maintenance of robust a cell mating ability, as expression of ␣-specific genes in a cells causes decreased mating ability (Roth et al 2000;Rivers and Sprague 2003). Furthermore, our results show that ␣-specific genes can be expressed at a substantial level even in the absence of their primary activator.…”

A walk-through of the yeast mating pheromone response pathway

“…Special mechanisms have evolved to accomplish this quickly [74,75,121,124]. A slower, but more permanent solution is then implemented when the transcription of many pathway components is repressed by the a1/α2 diploid-specific heterodimer [61].…”

Magnificent seven: roles of G protein-coupled receptors in extracellular sensing in fungi

“…Msg5 is a MAPK phosphatase that inhibits the MAPK Fus3 to inactivate pheromone signaling (Andersson et al, 2004). Asg7 is an a -cell-specific protein that acts in concert with the a -factor receptor Ste3 to inhibit G protein signaling (Rivers & Sprague, 2003). Detailed studies on the regulation and activation of pheromone receptor controlled pheromone response pathway have prompted the utilization of this pathway as a model system for studying other GPCRs in yeast (Minic et al, 2005b).…”