Evidence for Boundary Lipid in Membranes

TLDR

Cytochrome oxidase isolated from beef‑heart mitochondria with appropriate phospholipid content forms vesicular structures. Lipid‑protein interactions were studied using the lipid spin label 16‑doxylstearic acid. Varying the phospholipid/protein ratio revealed two spectral components: a highly immobilized lipid spin label at low ratios and a fluid bilayer component at higher ratios, and analysis showed that the amount of phospholipid bound to the protein (~0.2 mg per mg protein) is independent of the fluid bilayer extent, supporting a single‑layer boundary of immobilized lipid around the protein.

Abstract

Cytochrome oxidase (EC 1.9.3.1) isolated from beef-heart mitochondria with an appropriate phospholipid content forms vesicular structures. Lipid-protein interactions in this model membrane system were studied with the lipid spin label, 16-doxylstearic acid. As the phospholipid/protein ratio is varied, two spectral components are observed. At low phospholipid/protein ratios (≤0.19 mg of phospholipid per mg of protein) the lipid spin label is highly immobilized. At higher phospholipid content an additional component characteristic of fluid lipid bilayers is evident. By summation of digitalized spectra and subsequent integration it was shown that all composite spectra could be approximated by assuming only two components are present, and that the amount of phospholipid bound to the protein is independent of the extent of the fluid bilayer region. The experimentally determined amount of phospholipid for maximum occupancy of protein-bound sites is about 0.2 mg of phospholipid per 1.0 mg of protein. Calculations show that this ratio is consistent with a single layer of phospholipid surrounding the protein complex. The data are interpreted as evidence for a boundary of immobilized lipid between the hydrophobic protein and adjacent fluid bilayer regions in this membrane model system.