Interaction of water-soluble form of APL of bovine brain myelin with phospholipid vesicles

Abstract

The watere-soluble form of apoproteolipid(( APL) from bovine brain melin was found to bind with phosphdtidylcholine (PC)phosphatidlethanolamine (PE) (6:4) vesicles below pH 5. The protein bound to vesicles was photoactively labeled with 3-(trifluoromethl)-3-(m-[$^{125}I$] iodophenyl)diazirine ([$^{125}I$] TID) and was digested with trpsin. A [$^{125}I$]TID-labeled fragment with an apparent molecular weight of approximatel 2500 Da was extracted. An APL fragment with an identical Mr value was also obtained from the trptic digestion of APL/vesicle complex without a prior labeling with [125I]TID. Determination of amino acid composition and the identification of N-terminal amino acid residue of this unlabeled fragment showed that this protected segment isd amino acid residues from Met-205 to Lys-228. In another experiment, the [125I]TID-labeled APL obtained from the above experiment without proteolsis step was extracted and reconstituted into PC vesicles. Subsequent tryptic digestion of the exposed segment and comparison of the elution profile of the extracted polpeptides on Sephadex LH-60 column with published profile of these polypeptides indicated that the membraneinserted segment of the watersoluble form of APL when bound to vesicles is the C-terminal region of this apoprotein within the amino acid residues between Met-205 and Lys-268.