Unusual processing of GP280, a protein associated with the intermicrovillar areas of yolk sac epithelial cells: plasma membrane delivery of immature protein.

We have previously shown the close association with endocytosis of 2 glycoproteins, gp330 and gp280, expressed within the clathrin coated intermicrovillar areas of epithelial cells lining the proximal tubule and visceral layer of the yolk sac. Exposure at 37 degrees C of yolk sac epithelial cells in the presence of antibodies to gp280 induced disruption of the early endocytic pathway whereas anti-gp330 antibodies did not. In this study, using yolk sac epithelial cells in culture, we show that, in contrast to gp330 which is processed by the Golgi enzymes before expression on the plasma membrane, gp280 follows an unusual processing pathway: indeed, although the bulk of the plasma membrane gp280 is endoglycosidase H resistant, the protein is initially targeted to the plasma membrane in an endoglycosidase H sensitive form. This implies that processing takes place mainly, if not exclusively, by recycling in the Golgi apparatus.