Exogenous antigens are thought to be processed for cross-presentation in much the same manner as endogenous proteins once they enter the cytosolic pathway (Rock et al. 2010). Immunoproteasome components are the major proteases involved in generating the antigenic fragments. The precurssor peptides are further trimmed by cytosolic aminopeptidases and shuttled to ER through TAP for MHC class I loading.
endopeptidase activity of 26S proteasome [cytosol]