Fungal cell wall phosphomannans facilitate the toxic activity of a plant PR-5 protein

Osmotin is a plant PR-5 protein. It has a broad spectrum of antifungal acti vity, yet also exhibits specificity for certain fungal targets. The structu ral bases for this specificity remain unknown. We show here that full sensi tivity of Saccharomyces cerevisiae cells to the PR-5 protein osmotin is dep endent on the function of MNN2, MNN4 and MNNG. MNN2 is an alpha-1,2-mannosy ltransferase catalyzing the addition of the first mannose to the branches o n the poly 1,6-mannose backbone of the outer chain of cell wall N-linked ma nnans. MNN4 and MNNG are required for the transfer of mannosylphosphate to cell wall mannans. Null mnn2, mnn4 or mnn6 mutants lack phosphomannans and are defective in binding osmotin to the fungal cell wall. Both antimannopro tein antibody and the cationic dye alcian blue protect cells against osmoti n cytotoxicity. MNN1 is an alpha-1,3-mannosyltransferase that adds the term inal mannose to the outer chain branches of N-linked mannan, masking mannos ylphosphate. Null mnn1 cells exhibit enhanced osmotin binding and sensitivi ty. Several cell wall mannoproteins can bind to immobilized osmotin, sugges ting that their polysaccharide constituent determines osmotin binding. Our results demonstrating a causal relationship between cell surface phosphoman nan and the susceptibility of a yeast strain to osmotin suggest that cell s urface polysaccharides of invading pathogens control target specificity of plant PR-5 proteins.