Topographical arrangement of membrane proteins in the intact myelin sheath. Lactoperoxidase incorproation of iodine into myelin surface proteins.

The lactoperoxidase-catalyzed iodination technique was utilized to incorporate radioactive iodine into membrane proteins which lie on the outer surface of the myelin sheath. An intact, myelinated nerve bundle, the dorsal column of the cat spinal cord, was employed. The enzymatically iodinated proteins were identified by polyacrylamide gel electrophoresis, and the specific radioactivity was determined. Results indicated that several high molecular weight proteins were predominantly labeled by the nonpenetrating lactopreoxidase. Proteolipid protein was also labeled, although to a lesser extent; basic protein was not labeled under these conditions. The data suggest that several high molecular weight proteins are exposed on the outer surface of the myelin sheath. Proteolipid protein is at least partially exposed on the outer surface, although it could be present at both membrane surfaces. Evidence is presented which suggests that the basic protein is located at the inner surface of the membrane, corresponding to the major dense line of myelin.