Binding of human normal and multiple sclerosis-derived myelin basic protein to phospholipid vesicles: Effects on membrane head group and bilayer regions

Abstract

The detailed interaction of human myelin basic protein (MBP) with charged lipids may be critical in organizing the myelin sheath into its biologically functional structure. Carbon-13 and phosphorus-31 nuclear magnetic resonance spectroscopy has been used to study this interaction by examining spectral consequences of additions of MBP to membrane preparations of the negatively charged lipid phosphatidylglycerol (PG). Lipid head group 13C and 31P linewidths were found to narrow upon addition of protein, while concomitant broadening was noted for bilayer carbon resonances. At intermediate MBP PG ratios, two components in slow exchange on the NMR time scale (bulk PG and a protein-induced PG domain) were observed for the 13C resonance of the head group carbon atom adjacent to phosphate. These results, and other spectral evidence, suggested that head groups in free PG vesicles are motionally restricted by intermolecular interactions which are disrupted by competition with MBP Lys and Arg positively charged side chains. Titration of PG with the homopolypeptide poly- l-lysine produced comparable effects on PG 13C head group spectra, indicating that electrostatic attractions constitute the primary basis of the observed interactions. Vicinal and/or geminal 13C- 31P coupling constants measured from the spectra of PG head group carbons were found to be essentially invariant for free PG in dimethyl sulfoxide solution, free PG vesicles, PG vesicles + MBP, and PG vesicles + poly- l-lysine. Comparison of the spectral effects induced in PG head group resonances by normal vs multiple sclerosis-derived MBP (MS-MBP) indicated that the MS-MBP is relatively less effective in converting PG to the protein-induced domain, a result which was attributed to increased protein self-aggregation arising from the reduced net positive character of the MS protein samples.