Controlling association of vesicle embedded peptides by alteration of the physical state of the lipid matrix

We report here the reversible association of a designed peptide embedded in a lipid membrane through a stimulus-sensitive trigger that changes the physical state of the bilayer matrix. A peptide designed with the classical 4-3 heptad repeat of coiled coils, equipped with leucine residues at all canonical interface positions, TH1, was rendered membrane soluble by replacement of all exterior residues with randomly selected hydrophobic amino acids. Insertion of TH1 into large unilamellar phosphatidylcholine vesicles was followed by monitoring tryptophan fluorescence. Peptide insertion was observed when the lipids were in the liquid-crystalline state [1-palmitoyl-2-oleoyl-siz-glycero-3-phosphocholine (POPC)] but not when they were in the crystalline phase [1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)]. Formation of a trimeric a-helical bundle in lipid bilayers was followed by fluorescence resonance energy transfer. Global fit analysis revealed a monomer-trimer equilibrium with a dissociation constant of around 10(-6) MF2. A lipid mixture composed of DPPC and POPC exhibiting a phase transition at 34 degrees C between a crystalline/liquid-crystalline coexistence region and a completely miscible liquid-crystalline phase was used to control the formation of the trimeric peptide bundle. TH1 is phase excluded in crystalline DPPC domains below 34 degrees C, leading to a larger number of trimers. However, when the DPPC domains are dispersed at temperatures above 34 degrees C, the number of trimers is reduced.