A role for the yeast actin cytoskeleton in pheromone receptor clustering and signalling.

The development of cell polarity in response to external stimuli is a feature of most eukaryotic cell types. Haploid cells of the budding yeast Saccharomyces cerevisiae secrete peptide pheromones to induce conjugation with cells of the opposite mating type. Pheromone binding triggers transcription of mating-specific genes, cell cycle arrest in the G1 phase and the formation of a mating projection oriented toward the source of pheromone [1-2]. Based on a multitude of studies in diverse eukaryotic cells, it has been hypothesized that hierarchies of proteins function to govern the generation of cell polarity [3-4]. Numerous proteins have been identified in yeast that accumulate both at a position on the cell cortex that will develop into a mating projection in response to pheromone binding and at the site of bud formation in response to an intrinsic cue during mitotic growth. When the actin cytoskeleton is disrupted before bud formation by the addition of latrunculin-A (LAT-A), several proteins involved in budding, including the GTPase Cdc42p, are still able to achieve their appropriate polarized localization [5]. In contrast, we show here that following pheromone addition, an intact actin cytoskeleton is required for localization of several proteins to a discrete position on the cell cortex. We also demonstrate a role for actin in pheromone-induced receptor clustering and signalling. We propose that actin-mediated pheromone receptor clustering might consolidate signalling from Cdc42p to one region of the cell cortex so that small differences in receptor occupancy across the cell surface can be amplified into dramatic cellular polarity.