Mechanisms Underlying Pheromone Gradient Tracking in Mating Yeast

Chemotaxis and chemotropism play essential roles in a wide range of biological processes, during which cells interpret extracellular chemical gradients and orient their movement or growth. How cells translate the extracellular chemical gradients into intracellular signaling gradients and accurately orient their cytoskeletons remain unclear. The mating of the budding yeast, Saccharomyces cerevisiae, is the best-studied chemotropic process to date. Haploid yeast cells signal their positions to cells of the opposite mating type by secreting mating pheromones. Upon pheromone binding, the pheromone-responding G protein-coupled receptor (GPCR) activates its cognit Gα subunit, which releases the Gβγ subunit. Free Gβγ gets phosphorylated, signals to the nucleus through a MAP kinase cascade to arrest the cell cycle in G1. Gβγ also positions the polarity complex at the chemotropic growth site (CS). The polarity complex (PC) nucleates actin cables for polarized growth. When cells are treated with isotropic pheromone or pheromone-stimulated cells are unable to sense a gradient, they form mating projections at the default polarity site (DS) determined in G1. How do cells accurately decode the shallow pheromone gradients and orient their growth from the cell-cycle determined DS to the gradient-aligned CS? Numerous models have been proposed to explain yeast gradient sensing, but none of them fully explained how yeast cells overcome the challenges from the shallow and complicated pheromone gradients and the strong intrinsic DS. Here we propose a deterministic gradient sensing model which answers these questions. We demonstrate that yeast cells accomplish gradient sensing in four phases. Following global internalization of the receptor and G protein (Phase I), mating cells use the DS to assemble a gradient tracking machine (GTM) composed of signaling, polarity, and trafficking proteins (Phase II). Within the GTM, differential activation of the receptor triggers feedback mechanisms that segregate vesicle delivery upgradient and endocytosis downgradient. The segregation of the trafficking machinery redistributes the GTM towards the pheromone source (Phase III, tracking). The GTM stabilizes (Phase IV) when the receptor peak and vesicle delivery align with the pheromone source and are surrounded by the negative GPCR signaling regulator and endocytosis. We also showed that tracking depends on actin-independent – but not actin-dependent – vesicle delivery, and that the DS must be inactivated for budding to allow tracking to start. Together, this model answers the questions of how yeast cells solve the environmental and intrinsic challenges and accurately position the CS. It also provides an additional function of the cell-cycle determined DS. Besides positioning the daughter cell emergence, the DS functions as a platform to assemble the GTM.