The Roles of Lipids in Yeast Pheromone Gradient Sensing

Chemotaxis (directed cell movement) and chemotropism (directed cell growth) are essential to a variety of biological processes, including development, neutrophil migration, cancer metastasis, angiogenesis, axon guidance, and fungal life cycles. During all these processes, cells exhibit a remarkable ability to interpret external chemical gradients and efficiently orient their movement or growth in response to the gradient source. How cells decode shallow and complex extracellular signals and properly polarize proteins and lipids to generate a correct directional response remains unclear. Saccharomyces cerevisiae, commonly known as budding yeast, is a well-established model for studying chemotropism. The two haploid mating types, MATa and MATα, can sense the pheromone secreted by the opposite type, polarize their growth toward the closest mating partner, and fuse to form diploid zygotes. The G protein-coupled receptor (GPCR) Ste2 is the primary gradient sensor in MATa cells. After pheromone binding to the receptor, Gβγ dissociates from activated Gα and transmits the signal to the nucleus via a mitogen-activated protein kinase (MAPK) cascade, which results in cell-cycle arrest in late G1 and mating-specific gene activation. Free Gβγ also marks the chemotropic growth site (CS) and recruits a polarity complex that nucleates actin cables for polarized growth. To explain how mating yeast sense and respond to external chemical gradients, our research group proposed a deterministic gradient tracking model in which cells take advantage of the intrinsic default polarity site (DS) to assemble a gradient tracking machine (GTM) that tracks pheromone gradients and stabilizes at the CS. A key postulate of this model is that differential activation of the receptor and Gβγ positions actin-independent vesicle delivery (AI-VD) upgradient while inactive receptor-G protein complexes are preferentially endocytosed downgradient, driving GTM redistribution toward the CS. Moreover, direct interaction of the exocyst pioneer proteins, Sec3 and Exo70, with the GTM components Cdc42 and Bem1, respectively, is necessary for gradient tracking. Asymmetric distribution of proteins and lipids has been reported in pheromone-treated yeast cells. Specific lipids concentrated in the plasma membrane (PM) of mating projections include phosphatidylserine (PS), phosphatidylinositol-4,5-bisphosphate (PIP2), and ergosterol. Studies have shown that these polarized lipids affect pheromone signaling, cell polarization, protein retention, and membrane fluidity. Additionally, PS and PIP2 have been recently reported to polarize at the DS after cytokinesis in isotropic pheromone-treated cells, suggesting that both lipids and proteins comprise the GTM. In this study, I present evidence that PS, PIP2, and ergosterol are lipid components of the GTM. During gradient sensing, the spatiotemporal distribution of these lipids is the same as that of the GTM proteins – they concentrate at the DS, redistribute upgradient, and stabilize at the CS in mating cells. My study is the first in which GTM lipids have been identified and examined in mating cells. I also characterize the mating phenotypes conferred by mutations that block lipid synthesis and disrupt lipid-protein interactions. My results suggest that polarized PS and ergosterol have different functions in the GTM. PS is essential for GTM assembly and mating efficiency, and it contributes to the AI-VD function by localizing Bem1-Exo70 during GTM tracking. On the other hand, ergosterol is required for proper polarization of the GTM during tracking and stabilization. Taking my findings together, I propose that lipids play a key role in GTM function, contributing to protein polarization and directed vesicle delivery during pheromone gradient sensing. Additionally, my results indicate that a member of the claudin superfamily, Dcv1, is required to maintain the polarity of PS and PIP2 during gradient tracking.