Amino acid sequences which promote and prevent the binding and membrane insertion of surface-active peptides: comparison of melittin and promelittin

The temporal sequence of molecular events involved in the interactions of a number of related peptides with membranes are revealed using two complemen tary fluorescence techniques. Comparative studies are reported of the inter actions of melittin, promelittin and a melittin analogue with trp-19 replac ed with lie and the n-terminal gly replaced with a trp residue, with phosph atidylcholine membranes. It is shown that the interaction of the n-terminal region of melittin rapidly binds and inserts into the body of the membrane with a rate constant of around 367 s(-1). This is followed by a slightly s lower membrane insertion of the trp-19 region with a rate constant of aroun d 112 s(-1). The positive charges of the melittin molecule then come into c lose proximity with the membrane with rate constants around 27 s(-1). Final ly, these charged regions insert into the hydrophobic core of the membrane with rate constants of about 0.3 s(-1). The effect of incorporating net neg ative charge onto the membrane surface in the form of 15 mole % phosphatidy lserine, augments by about threefold, the binding of the charged domains of the melittin molecule. The observations of the melittin interactions are c ompared with the melittin-precursor protein, promelittin. Sections of the p romelittin molecule are also found to bind and insert into the body of the phospholipid membrane, although nearly 30 times less rapidly than melittin. No charged sections of promelittin are found to insert into the membrane.