DELIVERY OF NASCENT MHC CLASS II-INVARIANT CHAIN COMPLEXES TO LYSOSOMAL COMPARTMENTS AND PROTEOLYSIS OF INVARIANT CHAIN BY CYSTEINE PROTEASES PRECEDES PEPTIDE BINDING IN B-LYMPHOBLASTOID CELLS

The intracellular trafficking, proteolysis, and dissociation of invari ant chain (Ii) associated with nascent class II molecules was examined in B-lymphoblastoid cells. Metabolic labeling and Percoll gradient ce ntrifugation was used to assess the kinetics of delivery and processin g of class II-Ii complexes within the endocytic pathway. Catabolism of class II-Ii complexes rapidly followed their delivery from post-Golgi compartments to dense lysosome-like compartments distinct from early and late endosomes. Direct peptide binding assays revealed that class II molecules associated with even small N-terminal fragments of Ii fai led to bind peptide. Cysteine protease inhibitors alone blocked Ii pro teolysis/dissociation and accumulation of class II-Ii biosynthetic int ermediates within lysosome-containing compartments. Active-site labeli ng of cysteine proteases in B cells was used to identify cysteine prot eases capable of mediating Ii proteolysis within endosomal compartment s. Our results indicate rapid, possibly direct, transport of nascent c lass Il-ti complexes from the Golgi/trans-Golgi network to dense lysos omal compartments wherein cysteine protease(s), likely including cathe psin B, mediate complete removal of Ii. Inhibition of cysteine proteas e activity results in the accumulation of incompletely processed class II-Ii complexes, which lack peptide binding ability, within lysosomal compartments.