Fluorescence studies on a membrane-embedded peptide from the carboxy terminus of lipophilin.

Fluorescence of an intramembranous polypeptide (T-3) derived from the carboxy-terminal sequence of lipophilin was studied in aqueous solution, detergent micelles, and lipid vesicles. In all cases, the fluorescence of the only Trp (211) was indicative of a hydrophobic, buried residue. Addition of lysophosphatidylcholine (LPC) or phosphatidylcholine (PC) gave Trp-211 a more hydrophobic, less quenching environment as compared to that in aqueous solution. Energy transfer between Trp and Tyr observed in aqueous solution was decreased by the addition of lipid or detergent. There was limited quenching by acrylamide both in the aqueous and in the lipid or detergent environments. However, PC or LPC further decreased this quenching. Cs+ and I- were even less accessible than acrylamide to Trp, further proving that the Trp was located inside the lipid bilayer. The quenching indicated that I- binds to positive charges of the protein located on the surface of the membrane. This, combined with knowledge of the sequence of lipophilin, suggested that Trp-211 was located within the membrane but was close to amino acid residues that are external to the bilayer.