Murine Ia-associated invariant chain's processing to complex oligosaccharide forms and its dissociation from the I-A(k) complex

The processing of murine invariant chain (Ii) to a cell surface form bearing complex N-linked oligosaccharides has been demonstrated in the B cell lymphoma, AKTB-1b. In addition, the rate of processing of pulse-labeled Ii has been determined relative to its rate of dissociation from the α/β complex of I-A(k). Ii, α-, and β-chains were immunoprecipitated with anti-I-A(k) or anti-Ii monoclonal antibodies. The heretofore uncharacterized complex oligosaccharide form of Ii (Ii-c) was identified in gel-purified immunoprecipitates by peptide mapping with reverse-phase HPLC. Ii-c is resistant to deglycosylation by Endo H, which is specific for high-mannose N-linkages, but can be digested with Endo F, a glycosidase capable of cleaving both complex and high-mannose N-linked oligosaccharides. Immunoprecipitation of surface iodinated cells indicates that Ii-c is expressed on the plasma membrane. Pulse-chase metabolic labeling data show that the processing of Ii to Ii-c occurs with a t( 1/2 ) of about 120 min. In contrast, the processing of both α- and β-chains of I-A(k) to complex forms occurs with a t( 1/2 ) of 15 to 20 min. Our data sow that Ii-hm begins to dissociate rapidly from the I-A(k) complex after 100 to 120 min of chase. Only a small amount (less than 5% on a per mole basis) of Ii-c was found associated with the I-A(k) complexes after 300 min of continuous metabolic labeling. These results are consistent with Ii serving as a carrier for Ia antigens as they are transported to the cell surface. In addition, they suggest that the processing of Ii to Ii-c, or a late processing event of the α- and β-chains, such as their sialylation, may be a possible mechanism for inducing the dissociation of Ii from the I-A(k) complex.