POLARIZATION OF YEAST-CELLS IN SPATIAL GRADIENTS OF ALPHA-MATING FACTOR

The process of cell fusion during mating of the yeast Saccharomyces ce revisiae is mediated hy factors secreted by the mating partners. Spati al gradients of one of these mating factors, alpha-factor, polarized t he growth of projections by MATa cells. The site of previous budding d id not affect the direction of polarization, and subsequent budding wa s also polarized if mating factor was removed. Orientation occurred in the presence of nocodazole, suggesting that microtubules were not cri tical. At extremely low concentrations of alpha-factor, sst2-mutants ( which in genetic studies do not discriminate between partners producin g different amounts of alpha-factor) were able to polarize their proje ctions. The sensitivity of this spatial sensing mechanism in wild-type cells is such that differences in receptor occupancy estimated to be about 1% are sufficient for orientation.