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Membrane Permeabilizing Activity of Pathogenesis-Related Protein Linusitin from Flax Seed

Linusitin is a 25-kDa pathogenesis-related (PR) protein of class 5 isolated from flax seeds. It has been proposed that PR-5 proteins exert their antifungal activity by permeabilizing fungal membranes. Using a fluorescent method that has been used for antimicrobial proteins other than PR proteins, we tested this hypothesis. The method is based on the release of the dye calcein from the inside of either small or large unilamellar lipid vesicles. The amount of calcein release induced by the protein was studied as a function of protein and lipid concentration and of membrane composition. All the results could be accounted for with an available statistical model. The model predicts that calcein release from the vesicles is a result of the tetrameric protein aggregation. Whether this aggregate corresponds to a transmembrane pore or to another protein complex that perturbs the membrane remains to be established. The lipid composition of the vesicles affected the permeabilization activity of linusitin. An increase of activity was observed by increasing the content of negatively charged phospholipids in the vesicles, inclusion of sterols into the membrane, or decreasing the pH of the solution. The possible roles of the observed changes in permeabilizing activity in actual plant-fungus interactions are discussed.