The Primary Structure of Bovine Brain Myelin Lipophilin (Proteolipid Apoprotein)

Abstract

The amino-acid sequence of bovine myelin lipophilin (proteolipid apoprotein, Folch-protein) was completed. Lipophilin is a 276 amino acid residues containing, extremely hydrophobic membrane protein with molecular mass 30,000 [dalton]. The sequence determination was based on automated Edman degradation of 4 tryptophan and 4 cyanogen bromide fragments and of proteolytic peptides of complete lipophilin as well as the fragments obtained by chemical cleavage. Four addition sequences were determined which led to the completion of the primary structure. Lipophilin is esterified at threonine-198 by long chain fatty acids (palmitic stearic and oleic acid). The attachment site was established at the same threonine residue in 3 different peptides isolated from thermolysinolytic, papainolytic and chymotrypsinolytic hydrolysates. This threonine residue is part of a hydrophilic segment of lipophilin. The covalent fatty acyl bond is being discussed together with important structural and functional properties of this membrane protein which can be derived from sequence information. New separation and purification methods of hydrophobic and hydrophilic polypeptides for this sequence determination (fractional solubilization, silica gel exclusion, high-performance liquid chromatography) had to be elaborated as indispensable tools. They are generally applicable to the structural analysis of hydrophobic membrane proteins. Four long (26, 29, 40 and 30 residues) and 1 medium long (12 residues) hydrophobic segments are separated by 4 predominantly positive and 1 negatively charged hydrophilic segment(s). On the basis of structural data a model for the membrane integration of lipophilin is proposed.