Invariant chain associates with HLA class II antigens via its extracytoplasmic region.

Complexes of HLA class II alpha- and beta-chains with invariant chain were proteolytically digested to study domain interactions between these molecules. Detergent extracts of metabolically labeled monensin-treated B lymphoblastoid cells (B-LCL) were digested with proteinase K and immunoprecipitated with anti-HLA-DR or anti-invariant chain antibodies. Subsequent two-dimensional polyacrylamide amide gel electrophoresis showed that proteinase K treatment results in the sequential generation of three polypeptides of approximately 21,500, 19,500, and 18,000 daltons respectively. All are proteolytic fragments derived from invariant chain, and all remain associated with class II antigens. Two-dimensional gels of endoglycosidase H-treated immunoprecipitates showed that all three fragments contain two N-linked oligosaccharides. Neuraminidase treatment of immunoprecipitates and Bandeiraea simplicifolia lectin binding of cell extracts showed that the largest fragment, but not the smallest fragment, also contains O-linked oligosaccharides. None of the fragments possess the transmembrane region; fragments were released in soluble form when biosynthetically labeled B-LCL were ruptured by freezing and thawing and intact membranes were separated from aqueous components by ultracentrifugation. Lack of the transmembrane sequence was confirmed on the 18,000 dalton fragment by demonstrating through specific peptide cleavage at tryptophanyl residues that this fragment retains a substantial portion of the C-terminal region of I chain beyond trp162. Retention of the C-terminal region excludes the presence of the transmembrane region when m.w. are considered. Our data, taken in context of the amino acid sequence of the invariant chain predicted by the cDNA clone, demonstrate that invariant chain interacts with class II antigens via its extracytoplasmic region.