Fig. 8. Truncated forms of high-mannose N-glycans are not recognized by N-glycanase. Class I MHC heavy chains were obtained from U373 cells by immunoprecipitation. After digestion with the indicated glycosidases, samples were analyzed either directly by SDS–PAGE (top panel) or IEF (bottom panel), or a subsequent digestion with mammalian PNG1 was performed prior to gel electrophoresis where indicated. Incubation with mannosidase yielded a doublet band, which is presumably due to incomplete digestion of the substrate as indicated by the two structures shown on the left. The faster migrating protein (closed arrowhead for lane 1) is a glycosidase-sensitive proteolytic degradation product (open arrowhead for lanes 2–5) of the heavy chain molecule.