Predicted Folding of β-Structure in Myelin Basic Protein

Predictions of myelin basic protein secondary structure have not previously considered a major role for beta-structure in the organization of the native molecule because optical rotatory dispersion and circular dichroism studies have provided little, if any, evidence for beta-structure, and because a polycationic protein is generally considered to resist folding into a compact structure. However, the Chou-Fasman, Lim, and Robson algorithms identify a total of five beta-strands in the amino acid sequence. Four of these hydrophobic amino acid sequences (37-45, 87-95, 110-118, and 150-158) could form a hairpin intermediate that initiates folding of a Greek-key-type beta-structure. A second fold on the more hydrophobic side, with the addition of a strand from the N-terminus (residues 13-21), would complete the five-stranded antiparallel beta-sheet. A unique strand alignment can be predicted by phasing the hydrophobic residues. The unusual triproline sequence of myelin basic protein (100-102) is enclosed in the 14-residue hairpin loop. If these prolines are in the trans conformation, models show that a reverse turn could occur at residues 102-105 (Pro-Ser-Gln-Gly). Algorithms do not agree on the prediction of alpha-helices, but each of the two large loops could accommodate an alpha-helix. Myelin basic protein is known to be phosphorylated in vivo on as many as five Ser/Thr residues. Phosphorylation might alter the dynamics of folding if the nascent polypeptide were phosphorylated in the cytoplasm. In particular, phosphorylation of Thr-99 could neutralize cationic residues Lys-106 and Arg-108 within the hairpin loop. In addition, the methylation of Arg-108 might stabilize the hairpin loop structure through hydrophobic interaction with the side chain of Pro-97. The cationic side chains of arginine and lysine residues located on the faces of the beta-sheet (Arg-43, Arg-114, Lys-13, Lys-92, Lys-153, and Lys-156) could provide sites for interaction with phospholipids and other anionic structures on the surface of the myelin lipid bilayer.