The RGS Protein Sst2 is Essential for Gradient Sensing in Mating Yeast

Chemotropism, or directed cell growth, depends on the ability of cells to accurately sense chemical gradients to properly establish cell polarity. During mating, the budding yeast Saccharomyces cerevisiae uses GPCRs to sense the pheromone secreted by mating partners. In response, yeast form a mating projection towards the nearest partner. The GPCR Ste2 is distributed uniformly on the plasma membrane in vegetative cells. Upon pheromone binding to the receptor, the activated G protein dissociates into Gα-GTP and Gβγ. It was previously thought that the receptor and G protein polarize directly to the chemotropic site (CS), in the direction of the pheromone source. We discovered that mating cells first polarize the receptor and G protein to the default polarity site (DS) before redistributing, or tracking, these proteins to the CS. The receptor and G protein then stabilize at the CS prior to morphogenesis. Sst2 stimulates the GTPase activity of Gα-GTP and preferentially binds to unphosphorylated receptor (Ballon et. al., 2006). Sst2 has been implicated in gradient sensing and proposed to act as a signaling barrier (Dixit et. al., 2014). We used time-lapse fluorescent microscopy to follow Sst2-GFP and Ste2-GFP localization in mating cells and ask how receptor tracking is affected in sst2∆ cells. Sst2 appeared at the DS after the receptor, started tracking before the receptor, and tracked from the DS to the CS like the receptor. Upon morphogenesis, Sst2 surrounded the receptor at the mating projection. Sst2 localization is likely determined by active-unphosphorylated receptor and Gα-GTP. This distribution is consistent with Sst2 serving as a signaling barrier. Notably, the receptor failed to track in sst2∆ cells, suggesting that the spatiotemporal dynamics of Sst2-catalyzed G-protein recycling is critical to gradient sensing.