Synthesis and incorporation of myelin polypeptides into CNS myelin.

Abstract

The distribution of newly synthesized proteolipid protein (PLP, 23 kdaltons) and myelin basic proteins (MBP, 14-21.5 kdaltons) was determined in microsomal and myelin fractions prepared from the brainstems of 10- to 30-day-old rats sacrificed at different times after an intracranial injection of 35S-methionine. Labeled MBP were found in the myelin fraction 2 min after the injection, whereas PLP appeared first in the rough microsomal fraction and only after a lag of 30 min in the myelin fraction. Cell-free translation experiments using purified mRNA demonstrated that PLP and MBP are synthesized in bound and free polysomes, respectively. A mechanism involving the cotranslational insertion into the ER [endoplasmic reticulum] membrane and subsequent passage of the polypeptides through the Golgi apparatus is consistent with the lag observed in the appearance of the in vivo-labeled PLP in the myelin membrane. Newly synthesized PLC and MBP are not proteolytically processed, because the primary translation products synthesized in vitro had the same electrophoretic mobility and N-terminal amino acid sequence as the mature PLP and MBP polypeptides. Crude myelin fractions are highly enriched in mRNA coding for the MBP but not in mRNA coding for PLP. Whereas the bound polysomes synthesizing PLP are largely confined to the cell body, free polysomes synthesizing MBP are concentrated in oligodendrocyte processes involved in myelination, which explains the immediate incorporation of MBP into the developing myelin sheath.