- Several intramembranous peptides have been isolated from the major myelin proteolipid protein (lipophilin) isolated from normal human myelin membrane after labelling the protein with a membrane-permeable photolabel, 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine. Peptide T-3, comprising residues 205-268, represents the C-terminal portion of the protein. Reconstitution of peptide T-3 into lipid vesicles prepared from egg phosphatidylcholine (PC) or into lysoPC micelles yielded visually transparent preparations, free of scattering artifacts, which were used for circular dichroism studies to assess the extent of secondary structure in the peptide. Peptide T-3 had a high degree of alpha-helix in various environments. In aqueous environment, the secondary structure was 45% alpha-helix, 33% beta-structure and 9% beta-turns. Transfer of the peptide to PC vesicles or lysoPC micelles increased the proportion of alpha-helix and decreased that of beta-structure. In PC vesicles, the alpha-helical content was 80% with little or no beta-structure. Small amounts of other structures such as beta-turns and unordered structures were also present. The partitioning of this C-terminal section of lipophilin into membranes may have an important role initiating and/or stabilizing the native conformation of lipophilin in the myelin membrane.