Deuterium nuclear magnetic resonance investigation of bacteriophage M13 coat protein in dimyristoylphosphatidylcholine liposomes using palmitic acid as a probe

The effect of incorporation of various amounts of M13 bacteriophage coat protein on the bilayer order and acyl chain motion in dimyristoylphosphatidylcholine (DMPC) liposomes has been investigated using deuterium NMR of specifically deuterated palmitic acid as a bilayer probe, phosphorus NMR and additional spin-label electron spin resonance (ESR). The secondary structure of the M13 coat protein in these bilayers was determined from circular dichroism spectra. Phosphorus NMR spectra of the mixed liposomes are characteristic for DMPC organized in bilayers, also after incorporation of various levels of M13 protein. Circular dichroism spectra of the coat protein indicate that the protein conformation is predominantly a β-structure (more than 75%). Various incorporation levels of M13 coat protein do not affect the order of the deuterium-labelled positions along the acyl chain at the carbon-2, 9 and 16 positions. In contrast, the spin-spin relaxation times decrease at higher protein levels, especially at the carbon-16 position. The spin-label ESR spectra of the same system using 14-doxylstearic acid as a label show a second, motionally restricted component, that is not observed by deuterium NMR. The NMR and ESR results are consistent with a model in which the fatty acid molecules are in a fast two-site exchange (at a rate of approx. 10 7 Hz) between the sites in the bulk of the lipid bilayer and the motionally restricted sites on the coat protein.