The Yeast Claudin Dcv1 Is Essential for the Maintenance of Distinct Membrane Domains and Polarized Mating Functions
Authors: Sukumar Madhushalini E , Stone David E 
Published: June 2021

Chemotropic and chemotactic cells exhibit a remarkable ability to interpret chemical gradients and sense direction. The mating response of the budding yeast S. cerevisiae is a model chemotropic system. The two haploid mating types, MATa and MATα, sense the pheromone secreted by cells of the opposite type, polarize their growth to form a mating projection towards the closest mating partner, and fuse to form diploids. In MATa cells, the Ste2 pheromone receptor is the primary gradient sensor. Ste2 is uniformly distributed on the plasma membrane (PM) in vegetative cells, but upon ligand binding, it is rapidly internalized. It then reappears as a polarized crescent that coincides with the incipient mating projection site. In mating mixtures, the newly emerged receptor crescents orient towards the closest mating partner. Although actin‐dependent directed secretion stabilizes and amplifies receptor polarity, we have shown that receptor polarization precedes the polarization of actin cables and occurs in the absence of actin‐directed secretion. In contrast, internalization of the receptor is essential for its polarization. How is receptor polarity established upstream of actin‐directed secretion and what are the key players?

In a directed genetic screen, we found that deletion of the yeast claudin homolog, DCV1, conferred a significant defect in receptor polarization without affecting actin‐directed secretion. Unlike wild type (WT) cells, dcv1Δ cells were unable to establish a receptor polarization site that could be amplified independently of f‐actin. Based on its primary structure, Dcv1 was predicted to be an integral membrane protein. Immuno‐fluorescence (IF) microscopy in cells expressing Dcv1‐HA suggested that the protein localizes uniformly to the PM in vegetative cells and away from the mating projection in pheromone‐treated cells. Studies of cells expressing fluorescently‐tagged Dcv1 confirmed the IF results. Furthermore, Dcv1 appeared to localize as a ring at the base of newly formed mating projections. Cells co‐expressing fluorescently‐tagged forms of Dcv1 and the receptor exhibited inverse localization of these proteins concurrent with morphogenesis. This result is consistent with the observation that claudins promote the formation of membrane domains and act as membrane barriers. dcv1Δ cells also exhibited abnormal localization of various lipids in the PM. These include phosphotidylinositol, phosphotidylserine, and sterols. Preliminary lipidomic studies using mass spectrometry suggest differences in the phospholipid composition of the PM in WT and dcv1Δ cells. Consistent with its effect on receptor polarization, dcv1Δ conferred a defect in pheromone‐gradient sensing. Time‐lapse imaging of mating mixtures also revealed that Dcv1 is required for a variety of polarized mating functions in addition to receptor redistribution, and for efficient mating. In summary, our data implicate a gene of previously unknown function in the polarization of proteins and plasma membrane lipids that contribute to efficient chemotropism and mating. We propose that the yeast claudin Dcv1 plays a role in organizing mating‐specific membrane domains essential for the polarization of the receptor and other mating‐specific proteins.

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