Identification of a mitogen-activated protein kinase site in human myelin basic protein in situ

Ultrastructural localization of a specific phosphorylated isomer of myelin basic protein (MBP) has been achieved with a monoclonal antibody specific for human MBP sequence, 89–105, in which Thr⁹⁸ was phosphorylated. Cryosections of human brain white matter revealed that gold particles were found localized almost exclusively to the major dense line demonstrating that threonine 98 in the sequence Thr-Pro-Arg-Thr-Pro-Pro-Pro, a mitogen-activated protein kinase-specific site, was phosphorylated in vivo. In two cases of multiple sclerosis, the density of gold particles in myelin was reduced by about 30%, in one case by 42%, and by 80% in a fourth case. However, gold labelling was seen in areas of demyelination suggesting that the phosphorylated threonyl peptide was protected from degradation.