Synthesis of the myelin proteolipid protein in the developing mouse brain.

Mice ranging in age from 16 to 44 days were injected intracerebrally with 3H-leucine, and incorporation into total brain proteolipids and the myelin proteolipid protein was measured. All proteolipids were isolated from whole brain by ether precipitation and separated into their individual components by SDS polyacrylamide gel electrophoresis. Two major proteolipids with apparent molecular weights of 20,700 and 25,400 were observed in these preparations, and their proportion increased over the developmental period examined. A Ferguson plot analysis comparing these proteins with those of isolated myelin showed that the 25,400-dalton proteolipid component from whole brain was the myelin proteolipid protein. Rates of incorporation of 3H-leucine into total brain proteolipids peaked at 22 days of age. Synthesis of the myelin proteolipid protein increased rapidly to a maximum value at 22 days and decreased rather slowly until at 44 days it was about 83% of its maximum rate of synthesis. The data indicate that the developmental pattern of synthesis of the myelin proteolipid protein is unlike that of the myelin basic proteins. Synthesis of the major myelin proteins is developmentally asynchronous in that peak synthesis of the myelin proteolipid appears to occur several days later than the basic proteins. In addition, it maintains its maximum rate of synthesis over a longer period of time than do the basic proteins.