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Title: Positive and Negative Regulation of Cell Fusion in Saccharomyces cerevisiae
Authors: Hall, Allison
Advisors: Rose, Mark D
Contributors: Molecular Biology Department
Keywords: Cdc42
cell fusion
Subjects: Molecular biology
Cellular biology
Issue Date: 2018
Publisher: Princeton, NJ : Princeton University
Abstract: Cell fusion is ubiquitous in eukaryotic organisms. Saccharomyces cerevisiae cells fuse during mating; opposite mating types polarize their growth, come into contact, forming the zone of cell fusion (ZCF), degrade their cell walls, and fuse their plasma membranes. Because improper cell wall degradation would lead to cell lysis, the timing and location of fusion is tightly regulated. Both positive and negative regulation of cell wall degradation take place during yeast mating. The highly conserved Rho-GTPase, Cdc42p, functions in polarized growth during pheromone response and prezygote formation. Cdc42p binds the heterodimer amphiphysin, Fus2p/Rvs161p, facilitating cell wall degradation. Cdc42p is recruited to a focus at the center of the ZCF, dependent on Fus2p and ZCF curvature. Mutants with an abnormally curved ZCF do not localize Cdc42p, suggesting that flatness of the ZCF allows Fus2p/Rvs161p to recruit Cdc42p. In shmoos, Cdc42p neither forms a focus nor colocalizes with Fus2. A hyperactive component of the cell wall integrity (CWI) pathway, Pkc1p (PKC1*), causes a cell fusion defect, suggesting a negative regulatory role in cell fusion. The cell wall is monitored by five transmembrane proteins, Wsc1-3p, Mid2p, and Mtl1p, which detect cell wall damage and activate the CWI pathway. Deletion of MID2 uniquely causes pheromone-induced death, due to unregulated cell wall degradation. The Cdc42p/Fus2p fusion complex forms in mid2 shmoos and mutations in cell fusion genes (FUS1, FUS2, RVS161, CDC42) suppress mid2, implying that death is due to premature cell fusion in the absence of a partner. PKC1* decreased Cdc42p focus formation in prezygotes and mild over-expression of PKC1* suppressed the defects of both mid2Δ andwsc1Δ. Paradoxically, PKC1* increased Cdc42p focus formation in shmoos, neither colocalizing with Fus2p nor leading to premature lysis. We conclude that Mid2p and Pkc1p negatively regulate cell fusion by disrupting the localization of Fus2p and/or its interaction with Cdc42p. In the course of these studies, we found that WSC1 has a zygote/diploid specific role differing from the role of MID2. Diploids die at a high rate without WSC1, due to cell lysis during budding. These differences underscore the distinct functions of two similar CWI pathway mechanosensors, MID2 and WSC1.
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Type of Material: Academic dissertations (Ph.D.)
Language: en
Appears in Collections:Molecular Biology

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