Cathepsin B cleavage of I(i) from class II MHC α- and β-chains

Class II MHC-associated invariant chain (I(i)) might regulate binding of digested peptides to the Ag binding site (desetope) of class II MHC proteins by directly or allosterically blocking that site until cleavage and release of I(i) from MHC α- and β-chains at the time of peptide charging. We examined the cleavage and release of I(i) from class II MHC α/β/I(i) trimers by cathepsin B, which has been shown by others to colocalize with class II MHC molecules in intracellular compartments and to generate antigenic peptide fragments. Cathepsin B at pH 5.0 cleaved and released I(i) from class II MHC α- and β-chains. Cathepsin B digested I(i) from α- and β-chains in a dose-dependent fashion, yielding 23-, 21-, and 10-kDa fragments. Blockage of cathepsin B activity with leupeptin restored the 2D(nonequilibrium pH gradient gel electrophoresis/SDS) PAGE patterns of I(i) and sialic acid-derivatized forms of I(i) seen without the protease. The fragmentation pattern of cathepsin D treatment was different from that of cathepsin B, yielding 25-kDa intermediates.