Perturbations of phospholipid head groups by membrane proteins in biological membranes and recombinants

P-31 nuclear magnetic resonance (NMR) spin-lattice relaxation times (T1) have been used to probe the behavior of phospholipid head groups in the presence of membrane proteins. Measurements have been made on rabbit muscle sarcoplasmic reticulum and recombinants of the Ca2+ Mg2+ ATPase, rod outer segment disk membranes and recombinants of rhodopsin, and human erythrocyte ghosts and recombinants of human erythrocyte glycophorin. Recombined membranes with lipid/protein ratios greater than or equal to that found in biological membranes showed T1 behavior similar to the biological membranes and pure phosphatidylcholine. However, recombined membranes with a low lipid/protein ratio exhibited a T1 that was dramatically shorter than any of the other systems. Analysis of the relaxation mechanism and the factors contributing to it implicate a phospholipid head group conformation change at high protein content. It is suggested that this is due to trapping of phospholipid between proteins and is not the same phenomenon as motional restriction at the lipid-protein interface at higher lipid contents.