Proteolipid protein dimerization at cysteine 108: Implications for protein structure

Proteolipid protein (PLP) and its alternatively spliced isoform DM20 comprise ∼50% of central nervous system (CNS) myelin protein. The two proteins are identical in sequence except for the presence of a 35 amino sequence within the intracellular loop of PLP that is absent in DM20. In this work, we compared the expression of PLP/DM20 in transfected cells, oligodendrocytes and brain. In all 3 tissues, PLP exists as both a monomer and a disulfide-linked dimer; in contrast, DM20 is found mainly as a monomer. PLP dimers were increased by both chemical crosslinking and incubation with hydrogen peroxide, and were mediated by a cysteine at amino acid 108, located within the proximal intracellular loop of both PLP and DM20. The PLP-specific sequence thus influences the accessibility of this cysteine to chemical modification, perhaps as a result of altering protein structure. Consistent with these findings, several mutant PLPs known to cause Pelizaeus-Merzbacher disease form predominantly disulfide-linked, high molecular weight aggregates in transfected COS7 cells that are arrested in the ER and are associated with increased expression of CHOP, a part of the cellular response to unfolded proteins. In contrast, the same mutations in DM20 accumulate fewer high molecular weight disulfide-linked species that are expressed at the cell surface, and are not associated with increased CHOP. Taken together, these data suggest that mutant PLP multimerization, mediated in part by way of cysteine 108, may be part of the pathogenesis of Pelizaeus-Merzbacher disease.