LATE EVENTS IN THE INTRACELLULAR SORTING OF MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-II MOLECULES ARE REGULATED BY THE 80-82 SEGMENT OF THE CLASS-II BETA-CHAIN

The molecular mechanisms that regulate sorting of major histocompatibi lity complex (MHC) class II molecules into the endocytic pathway are p oorly understood. For many proteins, access to endosomal compartments is regulated by cytosolically expressed sequences. We present evidence that a sequence in the lumenal domain of the MHC class II molecule re gulates a very late event in class II biogenesis. Class II molecules c ontaining single amino acid changes in the highly conserved 80-82 regi on of the beta chain were introduced into invariant chain (Ii)-negativ e fibroblasts with wild-type alpha chain, and the derived transfectant s were analyzed biochemically. Using an endosomal isolation technique, we have quantified the level of class II molecules expressed in endoc ytic compartments and found that in the absence of Ii, approximately 1 5 % of total cellular class II molecules can be isolated from endosoma l compartments. Mutation at position 80 enhances this localization, wh ile changes at positions 81 and 82 ablate class II expression in endos omal compartments. In addition, we have evaluated whether the induced changes in intracellular distribution of class II molecules were due t o alterations in early biosynthetic events, indicative of misfolding o f the molecules, or to modulation of later trafficking events more lik ely to be a consequence of the modulation of a specific transport even t. Despite the dramatic effects on endosomal localization induced by t he mutations, early biosynthetic events and maturation of class II wer e unaffected by the mutations. Collectively, our data argue that late trafficking events that control the ability of the class II molecule t o access antigens is regulated by the 80-82 segment of the MHC class I I beta chains.