Two new proteases in the MHC class I processing pathway

The Proteasome generates exact major histocompatibility complex (MHC) class I ligands as well as NH2-terminal-extended precursor Peptides. The proteases responsible for the final NH2-terminal trimming of the precursor Peptides had, until now, not been determined. By using specific selective criteria we purified two cytosolic proteolytic activities, puromycin-sensitive Aminopeptidase and bleomycin hydrolase. These proteases could remove NH2-terminal Amino acids from the vesicular stomatitis virus nucleoprotein cytotoxic T cell epitope 52-59 (RGYVYQGL) resulting, in combination with proteasomes, in the generation of the correct epitope. Our data provide evidence for the existence of redundant systems acting downstream of the Proteasome in the antigen-processing pathway for MHC class I molecules.