VESICLE MEMBRANE-FUSION INDUCED BY THE CONCERTED ACTIVITIES OF SPHINGOMYELINASE AND PHOSPHOLIPASE-C

When vesicles composed of an equimolar mixture of sphingomyelin, phosp hatidylcholine, phosphatidylethanolamine, and cholesterol are treated with phospholipase C, phospholipid hydrolysis occurs without major cha nges in vesicle architecture. In the same way, addition of sphingomyel inase leads only to sphingomyelin cleavage. However, when both enzymes are added together, their joint hydrolytic activities give rise to le akage-free vesicle aggregation, lipid mixing, and aqueous contents mix ing, i.e. vesicle fusion, The contribution of both enzymes is unequal, the main role of sphingomyelinase being the production of relatively large amounts of ceramide that will facilitate the lamellar-to-nonlame llar transition in the formation of the fusion pore, whereas phospholi pase C provides mainly a localized, asymmetric, high concentration of diacylglycerol that constitutes the trigger for the fusion process. Th e lipidic endproducts of both enzymes cooperate in destabilizing and f using the membranes in a way that is never achieved through the action of any of the enzymes individually, nor by the products themselves wh en premixed with the other lipids during liposome preparation. Thus th e enzymes appear to be coupled through their reaction products. This i s the first observation of membrane fusion induced by the concerted ac tivities of two enzymes. Besides, considering that both diacylglycerol and ceramide are important metabolites involved in cell signaling, it may also provide new ideas in the exploration of ''cross-talk'' pheno mena between different signal transduction pathways.