The intramembranous domains of lipophilin in phosphatidylcholine vesicles are similar to those in the myelin membrane.

A membrane-permeable photolabel 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine (125I-TID) has been used to label lipophilin in normal human myelin and after incorporation of purified lipophilin into phosphatidylcholine (PC) vesicles. The labelled protein was isolated and the specific activities for lipophilin from myelin and from PC vesicles was found to be 1.2 X 10(11) and 1.5 X 10(11) cpm/mol, respectively. The chromatographic profiles of tryptic peptides were similar in both cases and the specific activities of the C-terminal intramembranous fragments (residues 205-268) the same. We concluded that the organization of lipophilin in PC vesicles was similar to its organization in myelin and that the PC-vesicle system represents a good system in which to study the orientation and interaction of lipophilin with lipids.