Biosynthesis of the D2 cell adhesion molecule: pulse-chase studies in cultured fetal rat neuronal cells.

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Abstract
D2 is a membrane glycoprotein that apparently functions as a cell adhesion molecule (CAM) in neural cells. Its biosynthesis in cultured fetal rat brain neurons was studied. D2-CAM was synthesized initially as 2 polypeptides: MW 186,000 (A) and MW 136,000 (B). With increasing chase times the MW of both molecules increased to 187,000-201,000 (A) and 137,000-158,000 (B). These were similar to the sizes of D2-CAM labeled with [14C]glucosamine, [3H]fucose and [14C]mannosamine, indicating that the higher MW species are glycoproteins. In the presence of tunicamycin, which specifically blocks the synthesis of high mannose cores, MW were reduced to 175,000 (A) and 124,000 (B). Newly synthesized A and B are susceptible to degradation by endo-.beta.-N-acetyl-glucosaminidase H, which specifically degrades high mannose cores, but they are resistant to such degradation after 150 min of posttranslational processing. Apparently A and B are initially synthesized with 4-5 high mannose cores which are later converted into N-linked complex oligosaccharides attached to asparagine residues. However, no shift of [35S]methionine radioactivity between A and B was detected with different pulse or chase times, showing that these molecules are not interconverted. The neuronal D2-CAM glycoproteins apparently are derived from 2 mRNA.