Lipophilin (proteolipid apoprotein) of brain white matter. Purification and amino acid sequence studies of the four tryptophan fragments.

The chemical cleavage of lipophilin (proteolipid apoprotein) from bovine brain white matter with HBr/dimethyl sulfoxide at the tryptophan residues, under conditions adapted to this hydrophobic protein, releases four fragments with approximate molecular masses 14 kDa (Trp I), 6.8 kDa (Trp IV), 5.2 kDa (Trp III) and 2.1 kDa (Trp II). These fragments have been separated and purified by a combination of solvent distribution, molecular sieve chromatography (Bio-Gel P-150) and high-performance liquid chromatography for automated Edman degradation and combined gas-liquid chromatography/mass spectroscopy. The complete amino acid sequences of Trp II and III and large sequences of Trp I are reported in this communication. The amino acid sequence of Trp IV and the sequences of peptides releasable from lipophilin by proteolytic enzymes (trypsin, thermolysin, subtilisin, chymotrypsin) have been described in previous reports from this laboratory. Despite two small gaps in the complete primary structure of lipophilin from myelin of central nervous system, our sequence data suggest the arrangement of four long hydrophobic sequences (30-40 apolar amino acid residues) within the hydrophobic core of the myelin lipid bilayer, linked by three hydrophilic regions at the aqueous membrane interphase. These features lend lipophilin the properties of a polytopic membrane protein.