pH-dependence of interaction between melittin and bacteriorhodopsin

Melittin differentially slowed down the fast (M_412f) and the slow (M_412s) decay components of the photocyde intermediate M of trimeric bacteriorhodopsin in purple membrane while it accelerated the M_412s of Triton X-100-solubilized bacteriorhodopsin monomers. Raising the bulk pH could enhance the effect of melittin on the M_412s of bacteriorhodopsin in these two states. From pH 5.5 to 8.8, melittin slightly influenced the yield of intermediate M in purple membrane, whereas the yield of M_412s decreased and subsequently reversed with the addition of melittin. Moreover, the monomeric bacteriorhodopsin bleached more readily in the presence of melittin and the higher pH made the bleaching effect of melittin more intensive as well. These results re-certify our former suggestions that there was electrostatic interaction between melittin and bacteriorhodopsin, and indicate that the biphasic M decay may not result from the well-known linear kinetic scheme (M→N →BR). At last the mechanisms underlying the interaction of melittin with purple membranes and bacteriorhodopsin monomers are analyzed.