Structure and function of the proline-rich region of myelin basic protein

Myelin basic protein (MBP)--the major extrinsic membrane protein of central nervous system myelin--from several species contains a rarely encountered highly conserved triproline segment as residues 99-101 of its 170-residue sequence. Cis peptide bonds are known to arise at X-Pro junctions in proteins and may be of functional significance in protein folding, chain reversal, and/or maintenance of tertiary structure. We have examined the conformation of this proline-rich region using principally 13C nuclear magnetic resonance spectroscopy (125 MHz) both in intact bovine MBP and in several MBP fragment peptides which we synthesized, including octapeptide 97-104 (Arg-Thr-Pro-Pro-Pro-Ser-Gln-Gly). Results suggested an all-trans conformation in aqueous solution for the triproline segment in MBP hexapeptide (99-104), heptapeptide (98-104), and octapeptide. Comparison with the 13C spectrum of intact MBP (125 MHz) suggested that the proline-rich region, as well as all other X-Pro MBP peptide junctures, was also essentially all trans in aqueous solution. Although experiments in which octapeptide 97-104 was bound to a lipid preparation (4:1 dipalmitoylphosphatidylcholine/dimyristoylphosphatidic acid) demonstrated that cis-proline bonds do arise (to the extent of ca. 5%) in the membrane environment, a role of linear chain propagation is suggested for the triproline segment of myelin basic protein.