Abstract
The process of cell fusion during mating of the yeast Saccharomyces cerevisiae is mediated by factors secreted by the mating partners. Spatial gradients of one of these mating factors, alpha-factor, polarized the growth of projections by MATa cells. The site of previous budding did not affect the direction of polarization, and subsequent budding was also polarized if mating factor was removed. Orientation occurred in the presence of nocodazole, suggesting that microtubules were not critical. At extremely low concentrations of alpha-factor, sst2-mutants (which in genetic studies do not discriminate between partners producing different amounts of alpha-factor) were able to polarize their projections. 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.