Study on Molecular Structure, Expression and Degradation of Myelin-Associated Glycoprotein (MAG)

cDNA cloning of human myelin-associated glycoprotein (MAG) revealed complete amino acid sequence. In rodents, expression of the two forms of mRNA is developmentary regulated; the mRNA without exon 12 portion is expressed mainly in the active myelinating stage of development. Although the cDNA library used here was prepared from adult human brain poly (A)+ RNA all clones obtained corresponded to the mRNA without exon 12 portion. It has been demonstrated that the myelin membrane contains an endogenous calcium activated neutral protease (CANP) which degrades myelin basic protein (MBP) and MAG. The myelin endogenous CANP cleaves the 100KDa MAG molecule to a 90KDa derivative called derivative of MAG (dMAG). Since dMAG does not appear to be as tightly bound to membranes as intact MAG, the CANP is supposed (not proven) to cleave C-terminus of MAG. To confirm the proteolytic cleavage sites in MAG molecules, we raised antisera against the sequences of C-terminus of MAG. The peptides unique to large form of MAG (L-MAG) and small form of MAG (S-MAG) were synthesized using an automatic peptide synthesizer. We incubated purified myelin to produce dMAG and examined each anti-MAG antibody. Both anti-L-MAG and anti-S-MAG antibodies did not recognize dMAG. These results indicate that dMAG does not contain the sequences of C-termini in both L-MAG and S-MAG. Phosphorylation sites are thought to be important for functions of MAG as a recognition molecule and located near C-terminus of the cytoplasmic sequence. As dMAG lacks C-terminus, production of dMAG may result in the disturbance of myelin-axon interactions during demyelinating process