Two developmentally regulated messenger RNAs differing in their coding region may exist for the myelin-associated glycoprotein.

Mouse and rat brain RNA were translated in vitro in a rabbit reticulocyte lysate translation system, and myelin-associated glycoprotein-related polypeptides were immunoprecipitated. Two products of Mr = 72,000 (p72MAG) and Mr = 67,000 (p67MAG) were specifically immunoprecipitated. These polypeptides were characterized as follows: 1) exogenously added purified myelin-associated glycoprotein (MAG) competitively eliminates their immunoprecipitation; 2) peptide maps obtained from them are very similar; 3) they are both destined to become glycoproteins, as assayed by insertion into dog pancreas microsomes and their subsequent sensitivity to endoglycosidase H. Translatable mRNA for the two polypeptides have different developmental expressions. p72MAG mRNA appears at an earlier age than p67MAG mRNA and remains the dominant MAG mRNA species throughout the period of rapid myelination. As the rate of myelination decreases, p67MAG mRNA becomes the dominant MAG mRNA species. Finally, two endoglycosidase H-sensitive polypeptides were specifically immunoprecipitated from mouse brain microsomes. Therefore, there exist two MAG proteins that differ slightly in their polypeptide structure and that are developmentally regulated. The possibility that the two polypeptides are synthesized de novo from two coordinately regulated mRNAs that differ in their coding region is discussed.