Biosynthesis of lymphocyte surface IgD in the mouse.

The synthesis of IgD was studied in mouse spleen cells by using [35S]methionine labeling followed by immunoprecipitation with monoclonal antibody and two-dimensional gel electrophoresis. After a 15-min pulse of [35S]methionine, a relatively basic form (IgD1) of apparent m.w. 59,000 was precipitated. Conversion into more acidic forms of m.w. 63 to 72,000 (IgD2) took place during a chase period of several hours. The acidic form was identical in mobility to that of IgD labeled by surface radioiodination, and was almost completely removed by treatment of intact cells with pronase. Neuraminidase treatment of the surface form (IgD2) produced a form resembling IgD1 in charge, but with no detectable change in m.w. Treatment of IgD1 with endoglycosidase H resulted in a form with an apparent m.w. of 50,000, whereas IgD2 was resistant to this enzyme. Both IgD1 and IgD2 bound to lentil lectin, whereas only IgD2 bound to Ricinus communis hemagglutinin, which binds to terminal galactose residues. These results indicate that IgD is synthesized as an incompletely glycosylated precursor possessing "high mannose" type oligosaccharide moieties, and passes relatively slowly through the cell. Shortly before surface appearance, galactose and sialic acid are added. No specific association with any other labeled protein was observed, and any IgD secretion was below the limits of detectability.