The interaction of bothropstoxin-I (Lys49-PLA2) with liposome membranes

Bothropstoxin-I (BthTx-I) is a Lys49-PLA₂ from the venom of the snake Bothrops jararacussu, which permeabilizes biological and artificial membranes by a mechanism independent of lipid hydrolysis. This mechanism has been investigated by studying the interaction of nine single tryptophan BthTx-I mutants with negatively charged phospholipid membranes. Changes in the solvent exposure of the tryptophan in each mutant were evaluated comparing the rate of chemical modification (k_mod) by bromosuccinamide with the maximum intrinsic tryptophan fluorescence emission wavelength (λ_max) in buffer and in the presence of 10% DMPA/90% DPPC liposomes. No changes in λ_max were observed, whereas k_mod values for tryptophans at positions 7, 10, 31 and 125 were significantly reduced in the presence of lipids, suggesting that bound phospholipid decreases solvent accessibility at these positions. Since the half-lives of the fluorescence and chemical modification effects differ by at least six orders of magnitude, these results suggest that the bound phospholipid may interact with multiple locations on the protein surface over micro- to millisecond timescales.