Saccharomyces cerevisiae Mid2p Is a Potential Cell Wall Stress Sensor and Upstream Activator of thePKC1-MPK1 Cell Integrity Pathway
ABSTRACT The MID2 gene of Saccharomyces cerevisiaeencodes a protein with structural features indicative of a plasma membrane-associated cell wall sensor. MID2 was isolated as a multicopy activator of the Skn7p transcription factor. Deletion ofMID2 causes resistance to calcofluor white, diminished production of stress-induced cell wall chitin under a variety of conditions, and changes in growth rate and viability in a number of different cell wall biosynthesis mutants. Overexpression ofMID2 causes hyperaccumulation of chitin and increased sensitivity to calcofluor white. α-Factor hypersensitivity ofmid2Δ mutants can be suppressed by overexpression of upstream elements of the cell integrity pathway, includingPKC1, RHO1, WSC1, andWSC2. Mid2p and Wsc1p appear to have overlapping roles in maintaining cell integrity since mid2Δ wsc1Δ mutants are inviable on medium that does not contain osmotic support. A role for MID2 in the cell integrity pathway is further supported by the finding that MID2 is required for induction of Mpk1p tyrosine phosphorylation during exposure to α-factor, calcofluor white, or high temperature. Our data are consistent with a role for Mid2p in sensing cell wall stress and in activation of a response that includes both increased chitin synthesis and the Mpk1p mitogen-activated protein kinase cell integrity pathway. In addition, we have identified an open reading frame, MTL1, which encodes a protein with both structural and functional similarity to Mid2p.