Class I MHC:B2M Antigen peptide bound class I MHC Antigen peptide Loading of antigen peptide onto MHC class I molecule Authored: Garapati, P V, 2011-03-28 Edited: Garapati, P V, 2011-03-28 Peptides generated by Cathepsin S or IRAP-mediated proteolysis in the endosomes are loaded onto MHC class I molecules, which are internalized and transported to early and late endosomal compartments where antigenic peptides can be loaded. A fraction of the internalized cell surface class I molecules enter MHC class II compartments (MIICs) within endocytic vesicles (Gromme et al. 1999, Kleijmeer et al. 2001). Microscopic analysis has revealed that surface MHC-I molecules are internalized and transported to early and late endosomal compartments (Basha et al. 2008, Lizee et al. 2003). A tyrosine-based endocytic trafficking motif (YXXA) is required for the constitutive internalization of MHC-I molecules from the cell surface into early/late endosomes for peptide loading (Basha et al. 2008, Lizee et al. 2003). Upon entry in to these endosomal compartments the MHC class I complexes exchange their pre-bound peptides with exogenously derived antigenic peptides. Pubmed10468607 Pubmed11247303 Pubmed14566337 Pubmed18802471 Reactome Database ID Release 431236943 Reactome, http://www.reactome.org ReactomeREACT_111077 Reviewed: Desjardins, M, English, L, 2011-05-13