SPIN LABEL SATURATION TRANSFER EPR DETERMINATIONS OF THE STOICHIOMETRY AND SELECTIVITY OF LIPID PROTEIN INTERACTIONS IN THE GEL PHASE

Lipid-protein interactions with the myelin proteolipid protein incorpo rated in the gel phase of dimyristoylphosphatidylcholine bilayers have been studied by saturation transfer EPR spectroscopy of spin-labelled phospholipids. The integrated intensities of the saturation transfer EPR spectra from spin-labelled phosphatidylcholine are linearly depend ent on the protein/lipid ratio, and correspond to a fixed stoichiometr y of approximately 11 lipids per monomer associated with the protein i n the gel phase. The normalized saturation transfer intensities of spi n-labelled phosphatidic acid, on the other hand, display a non-linear dependence on the protein/lipid ratio that can be described well by a selectivity for interaction with the protein in the gel phase with an average association constant relative to phosphatidylcholine of approx . 5.2. These values for the stoichiometry and selectivity of lipid-pro tein interaction in the lipid gel phase obtained from saturation trans fer EPR spectroscopy are comparable to those found previously in fluid phase lipids by conventional EPR spectroscopy.