Regulation of cell wallβ-glucan assembly:PTC1 Negatively affectsPBS2 Action in a pathway that includes modulation ofEXG1 transcription
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Analysis of genes involved in yeast cell wallβ-glucan assembly has led to the isolation ofEXG1,PBS2 andPTC1.EXG1 andPBS2 were isolated as genes that, when expressed from multicopy plasmids, led to a dominant killer toxin-resistant phenotype. ThePTC1 gene was cloned by functional complementation of the calcofluor white-hypersensitive mutantcwh47-1.PTC1/CWH47 is the structural gene for a type 2C serine/threonine phosphatase,EXG1 codes for an exo-β-glucanase, andPBS2 encodes a MAP kinase kinase in the Pbs2p-Hog1p signal transduction pathway. Overexpression ofEXG1 on a 2μ plasmid led to reduction in a cell wallβ1,6-glucan and caused killer resistance in wild type cells; while theexg1Δ mutant displayed modest increases in killer sensitivity andβ1,6-glucan levels. Disruption ofPTC1/CWH47 and overexpression ofPBS2 gave rise to similarβ-glucan related phenotypes, with higher levels ofEXG1 transcription, increased exo-β-glucanase activity, reducedβ1,6-glucan levels, and resistance to killer toxin. Genetic analysis revealed that loss of function of thePBS2 gene was epistatic toPTC1/CWH47 disruption, indicating a functional role for the Ptc1p/Cwh47p phosphatase in the Pbs2p-Hog1p signal transduction pathway. These results suggest that Ptc1p/Cwh47p and Pbs2p play opposing regulatory roles in cell wall glucan assembly, and that this is effected in part by modulating Exg1p activity.
Key wordCell wall Killer toxin resistance glucanase Protein kinase Protein phosphatase
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