Intrinsic polarization cues interfere with pheromone gradient sensing in S. cerevisiae

Polarity decisions are central to many processes, including mitosis and chemotropism. In S. cerevisiae, budding and mating projection (MP) formation use an overlapping system of cortical landmarks that converge on the small G-protein Cdc42. However, pheromone gradient sensing must override the Rsr1-dependent internal polarity cues used for budding. Using this model system, we asked what happens when intrinsic and extrinsic spatial cues are misaligned. Is there competition, or collaboration? By live cell microscopy and microfluidics technics we uncovered three previously overlooked features of this signaling system. First, the cytokinesis-associated polarization patch serves as a polarity landmark independently of all known cues. Second, the Rax1-Rax2 complex functions as novel pheromone promoted polarity cue in the distal pole of the cells. Finally, we showed that internal cues remain active during pheromone gradient tracking and that they interfere with this process biasing the location of MPs, since yeast defective in internal cue utilization align significantly better than wild- type with artificially generated pheromone gradients.