Evidence for Specific Polypeptide Chain Folding in Myelin Basic Protein from Reactions Between Fragments of the Protein and Monoclonal Antibodies

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Abstract
The specificities of two monoclonal IgM antibodies (18.25 and 21.142) evoked in mice with guinea-pig myelin basic protein were examined and interpreted in terms of a specific folding of the protein''s polypeptide chain. Studies with guinea-pig and rabbit myelin basic protein fragments showed that a region encompassing the central Phe-Phe (87-88) sequence is obligatory, but not sufficient, for reactivity with antibody 18.25. Appreciable reactivity was observed for rabbit peptides 22-95 and 45-151, and lower, but significant, reactivity was shown by peptide 32-95. Only very weak reactivity was seen with peptide 44-95. No reactivity was observed with peptide 1-95 after its lysine residues were acetylated, acetamidinated, or guanidinated. These results have been interpreted in terms of a polypeptide chain folding that creates an epitope within sequence Val-Val-His-Phe-Phe-Lys-Asn-Ile-Val (84-92). The specific conformation of this epitope, which includes probably the Lys-89 and possibly the Asn-90 and Val-92 side chains, could be formed by the association of sequence 84-92 with either sequence Ile-Leu-Asp-Ser-Ile-Gly-Arg-Phe-Phe (37-45) or without sequence Val-Leu-Ser-Arg-Phe (108-112) to form .beta.-sheet structures essentially identical with those that appear to be present in the intact BP [Martenson R. E. J. Neurochem. 46, 1612-1622 (1986)]. The second monoclonal antibody, no. 21.14.2, reacts only with guinea-pig myelin basic protein and fragments containing the species-restricted sequence Arg-Ala-Asp-Tyr-Lys-Ser-Lys (129-135). The smallest available fragment that displayed full reactivity with peptide 22-164; peptides 45-167, 89-167, and 119-167, on the other hand, were only about one-fourth as reactive as the intact protein. These results suggest that sequence 22-44 constitutes a region in peptide 22-164 and in the intact protein which stabilizes sequence 129-135 in the appropriate antibody-binding conformation.