Determination of a native proteolytic site in myelin-associated glycoprotein.

Myelin-associated glycoprotein (MAG) is a transmembrane structural protein that is thought to be involved in the formation and/or maintenance of the myelin sheath. MAG is proteolyzed at a discrete location near its transmembrane domain by a calcium activated myelin-associated cysteine protease in the central nervous system. The soluble proteolysis product, dMAG, can be found in the cerebrospinal fluid. The proteolysis of MAG may be involved in the molecular mechanism of demyelination, as the proteolytic degradation of myelin proteins has been observed in disease states. The site for the proteolysis of MAG to dMAG was identified. This was accomplished by developing a protocol for the purification of soluble dMAG and by protein sequencing of short peptides containing the carboxy-terminus of dMAG. The results from these experiments indicated that the native proteolytic site in MAG was located extracellularly and occurred between residues 512 (Ala) and 513 (Lys), with a large hydrophobic residue at the P2 position (Trp-511). This finding in turn indicated that the protease for which MAG was a substrate had cathepsin L-like activity. Cathepsin L-like activity in myelin was confirmed by peptidolysis experiments using known cathepsin L substrates. Additional experiments are in progress to determine the identity of this protease.