Identification of prelarge and presmall basic proteins in mouse myelin and their structural relationship to large and small basic proteins

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Abstract
A new technique was described to identify antigenically related proteins by radioimmunoassay after sodium dodecyl sulfate/polyacrylamide gel fractionation. When adult mouse myelin was examined by this technique, 4 proteins, antigenically related to the small myelin basic protein, were identified. They were designated: prelarge (MW 21,500), large (18,500), presmall (17,000), and small (14,000). The 4 proteins were isolated by elution from polyacrylamide gels, and each protein migrated as a single band when analyzed by either sodium dodecyl sulfate or acidic polyacrylamide gel elctrophoresis. Serial dilutions of the purified proteins were measured by radioimmunoassay. Both the slope of the inhibition curve and the level of maximal inhibition for each protein were the same as for the small myelin basic protein, indicating that each of the 4 proteins contained all of the antigenic sites present in the small basic protein. Structural relationships among the 4 proteins were examined by using 2- dimensional analysis of tryptic digests. Large was similar in amino acid sequence to the major myelin basic protein from other species; small was identical in sequence to large, except for an internal deletion of approximately 40 amino acid residues; prelarge contained the sequence of large plus an additional sequence of 25-35 amino acid residues; and presmall contained the sequence of small plus the same additional sequence as in prelarge. The 4 proteins were also treated with 2-(2-nitrophenylsulfenyl)-3-methyl-3''-bromoindolinine (BNPS-skatole) which cleaved proteins specifically at tryptophan residues. Analysis of the cleavage products indicated that the additional amino acid sequence in both prelarge and presmall extends from the amino terminus of the molecule.