STRESS PROTEINS ON THE YEAST-CELL SURFACE DETERMINE RESISTANCE TO OSMOTIN, A PLANT ANTIFUNGAL PROTEIN

Strains of the yeast Saccharomyces cerevisiae differ in their sensitiv ities to tobacco osmotin, an antifungal protein of the PR-5 family. Ho wever, cells sensitive to tobacco osmotin showed resistance to osmotin -like proteins purified from the plant Atriplex nummularia, indicating a strict specificity between the antifungal protein and its target ce ll. A member of a gene family encoding stress proteins induced by heat and nitrogen limitation, collectively called Pir proteins, was isolat ed among the genes that conveyed resistance to tobacco osmotin to a su sceptive strain, We show that overexpression of Pir proteins increased resistance to osmotin, whereas simultaneous deletion of all PIR genes in a tolerant strain resulted in sensitivity, Pir proteins have been immunolocalized to the cell wall, The enzymatic digestion of the cell wall of sensitive and resistant cells rendered spheroplasts equally su sceptible to the cytotoxic action of tobacco osmotin but not to other osmotin-like proteins, indicating that the cell membrane interacts spe cifically with osmotin and facilitates its action, Our results demonst rate that fungal cell wall proteins are determinants of resistance to antifungal PR-5 proteins.