Regulation of cell wallβ-glucan assembly:PTC1 Negatively affectsPBS2 Action in a pathway that includes modulation ofEXG1 transcription

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Abstract
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.