Endosomal aspartic proteinases are required for invariant-chain processing

Immunogenic peptides are displayed in the context of class II histocompatibility proteins on the surface of antigen-presenting cells. Class II alpha and beta subunits bind the invariant chain (I-chain), a transmembrane glycoprotein which must dissociate prior to peptide presentation. Proteolytic release of I-chain in an acidic compartment is followed by class II alpha beta surface expression. Two distinct proteinases sequentially catalyze I-chain dissociation in B-lymphoblastoid cell lines. An aspartic proteinase initiates processing whereas a cysteine proteinase catalyzes the final stages of I-chain release. Inactivation of these enzymes prevents class II alpha beta maturation, demonstrating that acidic proteinases are essential for the generation of functional class II complexes. I-chain processing was localized to a dense endosomal compartment, suggesting this is the first site where class II alpha beta become accessible to peptides. I-chain fragments complexed with class II alpha beta accumulate in dense endosomes of B-lymphoblastoid cells treated with cysteine proteinase inhibitors. A signal for endosomal retention/targeting present in the cytoplasmic tail of these fragments may sequester class II alpha beta in this compartment until I-chain processing is complete.