Characteristics of the interaction of melittin with sarcoplasmic reticulummembranes

Addition of an amphipathic bee venom peptide, melittin, to sarcoplasmic ret iculum (SR) vesicles isolated from rabbit skeletal muscles resulted in a fa st (<1 min) blue shift in the fluorescence maximum of the melittin-SR membr ane complex. Over the following 45 min the position of the fluorescence max imum did not change, but the fluorescence intensity of the melittin-SR memb rane complex decreased by similar to 35% with rate constant 0.14 min(-1). M elittin rapidly quenched the isotropic signal in the EPR spectrum of spin-l abeled stearic acid added to SR membranes. Further changes in the spectral parameters of the spin probe bound to SR membranes in the presence of melit tin indicated an increase of the viscosity of the probe microenvironment (e mpiric parameter T/eta was decreased by similar to 35% with rate constant 0 .11 min(-1)). The surface potential of SR membranes measured using a pH-sen sitive dye, neutral red, decreased after melittin addition from -60 to -30 mV. It was demonstrated with the use of a cross-linking agent, cupric o-phe nanthroline, that melittin induced slow aggregation of Ca-ATPase protein in SR membranes; the content of enzyme in the monomeric form decreased with r ate constant 0.14 min(-1). It is concluded that melittin binds rapidly to S R membranes, inducing slow changes in Ca-ATPase conformation and oligomeric state as well as structural transitions in the lipid bilayer of SR membran es.