CONFORMATIONAL STUDIES ON C-13-ENRICHED HUMAN AND BOVINE MYELIN BASIC-PROTEIN, IN SOLUTION AND INCORPORATED INTO LIPOSOMES

Carbon-13 nuclear magnetic resonance (NMR) spin-lattice relaxation (T1) and line-width studies have been performed on human and bovine myelin basic proteins in solution and incorporated into liposomes. These studies were facilitated by carbon-13 enrichment of the two methionyl residues in the protein which provided an S-methylated form and a noncovalently modified “intact” methionine carbon-13 enriched form. Values of T1 for side chain methionine S-CH3 carbons, in solution, found to be 0.5 to 0.6 s for the S-methylated proteins, and 0.7 to 0.8 s for the intact protein, displayed no significant variation over a range of pH or protein concentration. Line widths of corresponding spectra were essentially constant. The data were interpreted in terms of a relatively loose overall structure for the protein in which local (segmental) motion, at least in the vicinity of methionyl residues, dominated relaxation phenomena. Experiments were performed wherein 13C-enriched myelin basic protein (both in the S-methylated form and the intact form) was incorporated into lipid vesicles (50:50 phosphatidylcholine:phosphatidic acid containing 30% protein by weight). Carbon-13 enriched Met S-CH3 resonances corresponding to the protein interacting with the liposome were clearly visible and displayed line widths of about 15 Hz, as compared with about 1 Hz for the protein in free solution. © 1978, American Chemical Society. All rights reserved.