[Existence of boundary lipids in reconstituted hydrophobic protein-lipid system]

A fraction of hydrophobic proteins (9, 14, 18 kD) soluble in a chloroform-methanol mixture (2:1) has been isolated from Micrococcus lysodeikticus bacterial membranes. The proteins obtained were introduced into proteoliposomes at a protein/lipid weight ratio ranging from 0.1 to 0.25 in combination with the fluorescent probe pyrene or the spin probe 2-(14-carboxytetradecyl)-2-ethyl-4.4-dimethyl-3-oxasolidinyloxyl. The excimertization of pyrene upon direct excitation of its molecules (gamma excit.=338nm) and under conditions of energy transfer from the excited protein chromophores to pyrene (gamma excit.=286nm) and the spin-spin exchange between the spin probe molecules was investigated. The experimental results suggest that the hydrophobic protein molecules are surrounded by a structurally heterogenous lipid area containing up to 3.3 mg of lipid per l mg of protein. The maximal expression of structural heterogeneity was observed at the minimal content of protein in the proteoliposomes. Treatment with the membranotropic antibiotic gramicidin S resulted in disappearance of lateral heterogeneity of lipids in the constituted system and in lipid aggregation in bacterial membranes. It is assumed that the aggregability of membrane proteins depends on the structural rearrangement of some part of lipid bilayer around them.