EFFECT OF HIGH-PRESSURE ON THE ASSOCIATION OF MELITTIN TO MEMBRANES

To determine the underlying basis for the sensitivity of peripheral pe ptides to lipid packing, we monitored the change in association of mel ittin to different membranes under hydrostatic pressure by fluorescenc e polarization and by fluorescence intensity in the presence of aqueou s quenchers. Association to lysophosphatidylcholine micelles or to mem branes composed of dimyristoylphosphatidylcholine, dipalmitoylphosphat idylcholine, palmitoyloleoylphosphatidylcholine, or dioleoylphosphatid ylcholine was found to be stable from 1 to 2000 atm. Similar results w ere obtained using multilamellar vesicles, small unilamellar vesicles, or large unilamellar vesicles. Thus, the increase in lipid chain pack ing induced by pressure does not alter the association of bound comple xes. This result indicates similar compressibilities of the peptide an d the head group binding region. Increasing the ionic strength to incr ease the charge of the free peptide also resulted in a pressure-insens itive complex showing that the hydration does not change upon binding. This conclusion is substantiated by a lack of van't Hoff DELTAH to di oleoylphosphatidylcholine large unilamellar vesicles. To gain a more m olecular picture of these associations, the rotational properties of t he tryptophan side chain of bound melittin as a function of lipid pack ing was also studied. These data indicate subtle differences in peptid e orientation in different lipids.