An official website of the United States government
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.
more »
« less
Gradient tracking in mating yeast depends on Bud1 inactivation and actin-independent vesicle delivery
The mating of budding yeast depends on chemotropism, a fundamental cellular process. Haploid yeast cells of opposite mating type signal their positions to one another through mating pheromones. We have proposed a deterministic gradient sensing model that explains how these cells orient toward their mating partners. Using the cell-cycle determined default polarity site (DS), cells assemble a gradient tracking machine (GTM) composed of signaling, polarity, and trafficking proteins. After assembly, the GTM redistributes up the gradient, aligns with the pheromone source, and triggers polarized growth toward the partner. Since positive feedback mechanisms drive polarized growth at the DS, it is unclear how the GTM is released for tracking. What prevents the GTM from triggering polarized growth at the DS? Here, we describe two mechanisms that are essential for tracking: inactivation of the Ras GTPase Bud1 and positioning of actin-independent vesicle delivery upgradient.
more »
« less
- Award ID(s):
- 1818067
- PAR ID:
- 10372281
- Publisher / Repository:
- DOI PREFIX: 10.1083
- Date Published:
- Journal Name:
- Journal of Cell Biology
- Volume:
- 221
- Issue:
- 12
- ISSN:
- 0021-9525
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Budding yeast cells interpret shallow pheromone gradients from cells of the opposite mating type, polarize their growth toward the pheromone source, and fuse at the chemotropic growth site. We previously proposed a deterministic, gradient-sensing model that explains how yeast cells switch from the intrinsically positioned default polarity site (DS) to the gradient-aligned chemotropic site (CS) at the plasma membrane. Because phosphorylation of the mating-specific Gβ subunit is thought to be important for this process, we developed a biosensor that bound to phosphorylated but not unphosphorylated Gβ and monitored its spatiotemporal dynamics to test key predictions of our gradient-sensing model. In mating cells, the biosensor colocalized with both Gβ and receptor reporters at the DS and then tracked with them to the CS. The biosensor concentrated on the leading side of the tracking Gβ and receptor peaks and was the first to arrive and stop tracking at the CS. Our data showed that the concentrated localization of phosphorylated Gβ correlated with the tracking direction and final position of the G protein and receptor, consistent with the idea that gradient-regulated phosphorylation and dephosphorylation of Gβ contributes to gradient sensing. Cells expressing a nonphosphorylatable mutant form of Gβ exhibited defects in gradient tracking, orientation toward mating partners, and mating efficiency.more » « less
-
ABSTRACT Cell polarization in response to chemical gradients is important in development and homeostasis across eukaryota. Chemosensing cells orient toward or away from gradient sources by polarizing along a front–rear axis. Using the mating response of budding yeast as a model of chemotropic cell polarization, we found that Dcv1, a member of the claudin superfamily, influences front–rear polarity. Although Dcv1 localized uniformly on the plasma membrane (PM) of vegetative cells, it was confined to the rear of cells responding to pheromone, away from the pheromone receptor. dcv1Δ conferred mislocalization of sensory, polarity and trafficking proteins, as well as PM lipids. These phenotypes correlated with defects in pheromone-gradient tracking and cell fusion. We propose that Dcv1 helps demarcate the mating-specific front domain primarily by restricting PM lipid distribution.more » « less
-
Morphogenesis in fungi and animals is directed by polarization of small GTPases Cdc42 and Rac. In the budding yeastSaccharomyces cerevisiaecompetition between polarity patches results in one polarized patch and the growth of a single bud. Here, we describe cell polarity in the yeastAureobasidium pullulans, which establishes multiple coexisting polarity sites yielding multiple buds during a single cell division cycle. Polarity machinery components oscillate in their abundance in these coexisting sites but do so independently of one another, pointing to a lack of global coupling between sites. Previous theoretical work has demonstrated that negative feedback in a polarity circuit could promote coexistence of multiple polarity sites, and time-delayed negative feedback is known to cause oscillations. We show that both these features of negative feedback depend on a protein we identified as Pak1, and that Pak1 requires Rac1 but not Cdc42 for its localization. This work shows how conserved signaling networks can be modulated for distinct morphogenic programs even within the constraints of fungal budding.more » « less
-
The Cdc42 guanosine triphosphatase (GTPase) plays a central role in polarity development in species ranging from yeast to humans. In budding yeast, a specific growth site is selected in the G1 phase. Rsr1, a Ras GTPase, interacts with Cdc42 and its associated proteins to promote polarized growth at the proper bud site. Yet how Rsr1 regulates cell polarization is not fully understood. Here, we show that Rsr1-GDP interacts with the scaffold protein Bem1 in early G1, likely hindering the role of Bem1 in Cdc42 polarization and polarized secretion. Consistent with these in vivo observations, mathematical modeling predicts that Bem1 is unable to promote Cdc42 polarization in early G1 in the presence of Rsr1-GDP. We find that a part of the Bem1 Phox homology domain, which overlaps with a region interacting with the exocyst component Exo70, is necessary for the association of Bem1 with Rsr1-GDP. Overexpression of the GDP-locked Rsr1 interferes with Bem1-dependent Exo70 polarization. We thus propose that Rsr1 functions in spatial and temporal regulation of polarity establishment by associating with distinct polarity factors in its GTP- and GDP-bound states.more » « less
