PROTON NMR-STUDY OF PEPTIDES FROM MYELIN BASIC-PROTEIN - EVIDENCE FORLYS(74)-HIS(77) INTERACTION REVEALED FROM HISTIDINE LINE BROADENING

Residues 69-84 of guinea pig myelin basic protein contain the encephal itogenic determinant for the Lewis rat. Insertion of histidine and gly cine at positions 77 and 78 in bovine MBP greatly reduces the encephal itogenicity of the protein. Synthetic peptides analogous to this regio n of MBP containing glycine and histidine are encephalitogenic if they lack the N-terminal half, residues 69-74. However, if they contain bo th histidine plus the N-terminal half, encephalitogenicity is abolishe d, suggesting that an interaction of histidine with an amino acid in t he N-terminal half changes the conformation or the properties of the p eptide. This was investigated by measuring the H-1-NMR spectra of synt hetic peptides analogous to this region of MBP, both containing histid ine but with and without the N-terminal half. The major difference in the spectra of the two peptides was the pH dependence of line broadeni ng of the histidine resonances. The histidine C2H and C4H resonances w ere broadened at intermediate pH values in both peptides. However, sha rpening of the lines at high pH showed a different pH dependence in th e two peptides. For the longer peptide containing the N-terminal half, the lines did not sharpen until the pH was increased above 10.2, coin ciding with the pK(a) of Lys-74. Acetylation of this peptide caused th e pH at which the lines began to sharpen to drop to 8.8. In the shorte r peptide, lacking the N-terminal half and Lys-74, the lines also shar pened at pH 8.8. The greater broadening which persisted up above pH 10 for the longer peptide suggests slow exchange between two different c onformations or environments of the histidine. One of these could be a conformation in which the deprotonated histidine hydrogen bonds with Lys-74. The Lys side-chain resonances indicated a decrease in rotation al freedom above the pK(a) of histidine, consistent with this conclusi on. Although this putative interaction between His and Lys-74 did not appear to have a significant effect on the overall conformation of the peptide, it could result in a reduction in encephalitogenicity by alt ering the properties of the peptide, This could affect processing and presentation of this determinant by antigen presenting cells.