Low-ultraviolet circular dichroism spectroscopy of oligopeptides 1-95 and 96-168 derived from myelin basic protein of rabbit

Abstract

Myelin basic protein (MBP) is a major protein constituent of the myelin sheath of the central nervous system, where it is believed to have functional .alpha.-helical segments. One element of the function of the protein might be "conformational adaptability" of specific regions of its amino acid sequence, since the purified protein appears to be largely devoid of ordered structure. To pursue this question, low-ultraviolet circular dichroism (CD) spectroscopy was conducted on the sequential thrombic peptides 1-95 and 96-168 of the protein in the presence of 0-92% trifluoroethanol (TFE), a solvent known to promote stable secondary structures in polypeptides. The series of CD spectra of the oligopeptides were subjected to a computerized best-fit analysis of four peptide conformations, the .alpha.-helix, .beta.-structure, .beta.-turn, and nonordered form. Agreement between experimental and best-fit composite spectra was achieved when standard CD curves of peptide conformations were derived from known theoretical spectra and experimental spectra of polypeptides. In dilute buffer alone, oligopeptides 1-95 and 96-168 evidenced no .alpha.-helix but significant .beta.-structure (18% and 23%, respectively), as well as a predominant, extended nonordered conformation. However, the two parts of the protein differed in conformational adaptability. From 0% to 30% TFE, 96-168 exhibited concomitant transitions to 10% helix and 32% .beta.-structure from the nonordered form. In contrast, in 10%-30% TFE, 1-95 underwent a transition to .apprx. 21% helix with partial loss of .beta.-structure as well as nonordered form; higher concentrations of TFE (40-75%) promoted additional transitions to both helix and .beta.-structure (totaling 33% and 25%, respectively). A maximum of .apprx. 50-55% of the amino acid residues of both oligopeptides ultimately resided in sequences of ordered structures from 50-92% TFE. In contrast, .apprx. 45% of their amino acids retained the nonordered conformation apparently by virtue of sequences that contain up to 50% Pro, Gly, Ser, and Asp. The locations of the ordered conformations could be tentatively assigned within the amino acid sequence on the basis of hydropathic and predictive analyses. .beta.-Structure .fwdarw. .alpha.-helix transitions occurred between 75% and 92% TFE, which apparently rendered the ordered sequences mainly helical. These patterns of conformational change underscore the involvement of .beta.-structure and highly adaptable amino acid sequences in the conformations of MBP.