Protein associations and basic protein conformation in the myelin membrane. The use of difluorodinitrobenzene as a cross-linking reagent.

The near-neighbor relationships of proteins in the myelin membrane were examined using dinitrodifluorobenzene and other cross-linking reagents. When intact cat dorsal column or isolated myelin fragments were treated with cross-linking reagents, up to 20% of the myelin basic protein dimerized. The only other cross-linked product formed in the intact cat dorsal column was a heterodimer consisting of myelin basic protein and either the major or minor proteolipid protein. The remaining myelin proteins, including the major proteolipid protein, were cross-linked into very high molecular weight aggregates. In contrast, when the myelin membrane was dipersed in sodium dodecyl sulfate before the addition of cross-linking reagent, all the proteins remained essentially monomeric, with the exception of myelin basic protein which dimerized to some extent. In the absence of cross-linking reagent, it was shown by radioimmunoassay that small amounts of myelin basic protein dimer and the heterodimer were normally present in sodium dodecyl sulfate-polyacrylamide gels. We found no evidence of intramolecular cross-links between the two peptides formed by N-bromosuccinimide cleavage or between the two peptides formed by cyanogen bromide cleavage of the basic protein monomer. The regions of the myelin basic protein molecule involved in dimerization were also determined by similar cleavage of the cross-linked dimer. A rudimentary model for the structure of basic protein dimer in myelin is presented.