Biosynthesis of the D2-cell adhesion molecule: post-translational modifications, intracellular transport, and developmental changes.

Abstract

Posttranslational modifications and intracellular transport of the D 2-cell adhesion molecule (D2-CAM) were examined in cultured fetal rat neuronal cells. Developmental changes in biosynthesis were studied in rat forebrain explant cultures. Two D2-CAM polypeptides with MW of 187,000-210,000 (A) and 131,000-158,000 (B) were synthesized using radiolabeled precursors in cultured neurons. A and B were found to contain only N-linked complex oligosaccharides, and both polypeptides appeared to be polysialated as determined by [14C]mannosamine incorporation and precipitation with anti-polysialic acid antibody. The 2 polypeptides were sulfated in the trans-Golgi compartment and phosphorylated at the plasma membrane. D2-CAM underwent rapid intracellular transport, appearing at the cell surface within 35 min of synthesis. A and B were shown to be integral membrane proteins as seen by radioiodination by photoactivation employing a hydrophobic labeling reagent. In rat forebrain explant cultures, D2-CAM was synthesized as 4 polypeptides: A (195,000 MW), B (137,000 MW), C (115,000 MW) and a group of polypeptides in the high MW region (HMW) between 250,000 and 350,000. Peptide maps of the 4 polypeptides yielded similar patterns. Biosynthesis of C and HMW increased with age, relative to A and B. A and B were sulfated in embryonic brain sulfation was not noticeable at postnatal ages. Phosphorylation of A and B was observed at all ages examined. D2-CAM function may be modified during development by changes in the relative synthesis of the different polypeptides, as well as by changes in their glycosylation and sulfation.