Mating yeast cells use an intrinsic polarity site to assemble a pheromone-gradient tracking machine

Abstract

The mating of budding yeast depends on chemotropism, a fundamental cellular process. The two yeast mating types secrete peptide pheromones that bind to GPCRs on cells of the opposite type. Cells find and contact a partner by determining the direction of the pheromone source and polarizing their growth toward it. Actin-directed secretion to the chemotropic growth site (CS) generates a mating projection. When pheromone-stimulated cells are unable to sense a gradient, they form mating projections where they would have budded in the next cell cycle, at a position called the default polarity site (DS). Numerous models have been proposed to explain yeast gradient sensing, but none address how cells reliably switch from the intrinsically determined DS to the gradient-aligned CS, despite a weak spatial signal. Here we demonstrate that, in mating cells, the initially uniform receptor and G protein first polarize to the DS, then redistribute along the plasma membrane until they reach the CS. Our data indicate that signaling, polarity, and trafficking proteins localize to the DS during assembly of what we call the gradient tracking machine (GTM). Differential activation of the receptor triggers feedback mechanisms that bias exocytosis upgradient and endocytosis downgradient, thus enabling redistribution of the GTM toward the pheromone source. The GTM stabilizes when the receptor peak centers at the CS and the endocytic machinery surrounds it. A computational model simulates GTM tracking and stabilization and correctly predicts that its assembly at a single site contributes to mating fidelity.

Exploratory polarization facilitates mating partner selection inSaccharomyces cerevisiae

“…A recent study suggested that initial polarity sites move gradually and directionally toward the partner cell in S. cerevisiae (Wang et al, 2019). In our mating conditions, the movement was more chaotic, without obvious unidirectional tracking in most cells.…”

“…Following initial polarization, Ste20 displayed indecisive behavior similar to that of Bem1 (Movie 1), in contrast to the directional tracking described for the α-factor receptor Ste2 (Wang et al, 2019). To compare the behavior of Ste2 and polarity probes, we imaged mating in cells harboring both STE2-sfGFP and either BEM1-tdTomato or SPA2-mCherry, a formin regulator that colocalizes with indecisive polarity sites (Henderson et al, 2019).…”

“…However, imaging of probes for sensing/signaling proteins ( Fig. 2A) suggested that cells undergo a more stereotyped behavior, with initial polarization always adjacent to the previous site of cytokinesis, followed by steady unidirectional tracking toward the eventual mating partner (Wang et al, 2019). We considered the possibility that probe selection might affect conclusions about cells' behavior prior to commitment.…”

“…For successful mating, the pheromone landscape must be decoded to orient polarity toward the partner. However, imaging of polarity factors revealed that initial polarity sites were not always oriented toward the eventual mating partner in mating mixes (Henderson et al, 2019;Wang et al, 2019) or up the gradient in cells exposed to artificial pheromone gradients (Dyer et al, 2013;Hegemann et al, 2015;Jin et al, 2011;Kelley et al, 2015;Vasen et al, 2020). Rather, the location of the polarity site changed over time, stabilizing at better-oriented locations.…”

“…Thus, some spatial information was available before any polarity sites were visible. Other studies reported that initial polarity sites were frequently oriented toward bud siteselection cues rather than pheromone sources (Vasen et al, 2020;Wang et al, 2019). Regardless of initial orientation, global sensing during the indecisive phase (when polarity sites are weak and mobile) could promote the selection of optimal locations for stable "committed" polarity sites.…”