Yeast G-proteins mediate directional sensing and polarization behaviors in response to changes in pheromone gradient direction

Yeast cells polarize by projecting up mating pheromone gradients, a classic cell polarity behavior. However, these chemical gradients may shift direction. We examine how yeast cells sense and respond to a 180° switch in the direction of microfluidically generated pheromone gradients. We identify two behaviors: at low concentrations of a-factor, the initial projection grows by bending, whereas at high concentrations, cells form a second projection toward the new source. Mutations that increase heterotrimeric G-protein activity expand the bending-growth morphology to high concentrations;mutations that increase Cdc42 activity result in second projections at low concentrations. Gradient-sensing projection bending requires interaction between Gβγand Cdc24, whereas gradient-nonsensing projection extension is stimulated by Bem1 and hyperactivated Cdc42. Of interest, a mutation in Ga affects both bending and extension. Finally, we find a genetic perturbation that exhibits both behaviors. Overexpression of the formin Bni1, a component of the polarisome, makes both bending- growth projections and second projections at low and high a-factor concentrations, suggesting a role for Bni1 downstream of the heterotrimeric G-protein and Cdc42 during gradient sensing and response. Thus we demonstrate that G-proteins modulate in a ligand-dependent manner two fundamental cell-polarity behaviors in response to gradient directional change.