Deterministic model of local gradient sensing. Feedback interactions between the receptor, its kinases, and its G protein steepen the gradient of signaling-competent (active-unphosphorylated) and signaling-incompetent (active-phosphorylated and inactive) receptors on the cell surface. (A) During tracking, signaling-competent receptors concentrate vesicle delivery upgradient, while signaling-incompetent receptors concentrate receptor-driven endocytosis downgradient, thereby directing the GTM toward the pheromone source. (B) During stabilization, signaling-competent receptors concentrate vesicle delivery at the center of the incipient CS, while signaling-incompetent receptors concentrate endocytosis around it, thereby directing chemotropic growth toward the mating partner. Because the GTM is positioned by the dynamic pattern of receptor species, it rapidly reflects changes in gradient profile and direction.