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.