Polarization and cell-fate decision facilitated by the adaptor Ste50 in Saccharomyces cerevisiae

In response to pheromone, many proteins localize on the plasma membrane of yeast cell to reform it into a polarized shmoo structure. The adaptor protein Ste50, long known as a pheromone signal enhancer critical for shmoo polarization, has never been explored systematically for its localization and function in the polarization process. Time-lapse single-cell imaging and quantitation shown here characterizes Ste50p involvement in the establishment of cell polarity. We found early localization of Ste50p patches on the cell cortex that mark the point of shmoo initiation, these polarity sites move, and patches remain associated with the growing shmoo tip in a pheromone concentration timedependent manner until shmoo maturation. By quantitative analysis we show that polarization corelates with the rising levels of Ste50p, enabling rapid individual cell responses to pheromone that corresponds to a critical level of Ste50p at the initial G1 phase. We exploited the quantitative differences in the pattern of Ste50p expression to corelate with the cell-cell phenotypic heterogeneity showing Ste50p involvement in the cellular differentiation choice. Taken together, these findings present spatiotemporal localization of Ste50p during yeast polarization, suggesting that Ste50p is a component of the polarisome, and plays a critical role in regulating the polarized growth of shmoo during pheromone response.