Association of lysozyme to phospholipid surfaces and vesicle fusion

Lysozyme-induced fusion of phosphatidylserine (PS) vesicles was studied as a function of pH. Fusion, monitored by lipid-mixing, was measured by following the dilution of pyrene-labelled phosphatidylcholine, incorporated in PS vesicles, into unlabelled bilayers. It is demonstrated that lysozyme-induced fusion is pH-dependent and significant fusion is triggered at pH 5 or below. The interaction of lysozyme with the vesicle bilayer was characterized by measuring resonance energy transfer from tryptophane, present in the protein, to pyrene. It is shown that concomitant with fusion, a strong resonance energy transfer signal appears at pH 5 or below. Furthermore, in monolayer experiments it was found that addition of lysozyme to the subphase caused an increase in surface pressure, when the pH was kept below 5.5. Very low concentrations of lysozyme sufficed to bring about the observed effects. The results are taken to indicate that lysozyme-induced fusion results from penetration of protein into the hydrophobic core of the bilayer, occurring at acidic pH.