MONOCLONAL-ANTIBODY O-10 DEFINES A CONFORMATIONALLY SENSITIVE CELL-SURFACE EPITOPE OF PROTEOLIPID PROTEIN (PLP) - EVIDENCE THAT PLP MISFOLDING UNDERLIES DYSMYELINATION IN MUTANT MICE

Mutations in the gene for proteolipid protein (PLP) have been associat ed with CNS dysmyelination and abnormal oligodendrocyte death in spont aneous mouse mutants and in Pelizaeus-Merzbacher disease; however, the effect of mutations on PLP structure and function are little understo od. We have identified a monoclonal antibody directed against a novel cell surface epitope of PLP, termed O10. By immunofluorescence analysi s, COS-7 cells transiently transfected to express PLP (or its isoform DM20) can be stained with antibody O10 and another antibody (A431) dir ected against the C terminus of PLP/DM20. The subcellular distribution of immunofluorescence labels for the two antibodies is not identical, suggesting that the O10 epitope is acquired post-translationally. Whe n PLP/DM20 from jimpy, jimpy(msd), and rumpshaker mutant mice is expre ssed in COS-7 cells and compared with wild-type PLP/DM20, none of the mutant isoforms displays the O10 epitope, whereas the C-terminal epito pe is detected. Because the O10 but not the A431 epitope is also sensi tive to SDS and reducing agents, this strongly suggests abnormal prote in folding in the PLP mutants. PLP from jimpy(msd) mice is obviously m isfolded, because the amino acid substitution (Ala(242) --> Val) is lo cated within a transmembrane domain to which the O10 antibody does not bind. We propose that the O10 epitope emerges as the full length prot ein reaches a functional tertiary structure and that the absence of th is epitope marks a structural defect of PLP that leads to dysmyelinati on.