The interaction of a proteolipid from sarcoplasmic reticulum membranes with phospholipids. A spin label study

Abstract

A homogeneous proteolipid fraction ( M r ~ 12,000) was isolated from sarcoplasmic reticulum membranes of rabbit skeletal muscle. Covalent spin labeling of the protein and incorporation into egg yolk lecithin dispersions revealed its location in the low-polarity inferior of the phospholipid bilayer. By introducing stearic acid spin labels 2-(3-carboxypropyl)-4,4-dimethyl-2-tridecyl-3-oxazolidinoxyl and 2-(10-carboxydecyl)-2-hexyl-4,4-dimethyl-3-oxazolidinoxyl into bilayers reconstituted from membrane phospholipids and proteolipid we found that the proteolipid reduces the motional freedom of the alkyl chains in the centre of the bilayer whereas the glycerol backbone region remains unaffected. Macroscopically ordered preparations of stearic acid spin-labeled phospholipids containing proteolipid showed no marked effect of the protein on the predominant orientation of the lipid alkyl chains perpendicular to the bilayer. The results suggest a two-dimensional arrangement of the polypeptide chains in the hydrocarbon interior of the bilayer.