too small 60%: appropriate size 15%: too large 20% % of proteasome-generated fragments
||Yewdell JW, Reits E, Neefjes J. Making sense of mass destruction: quantitating MHC class I antigen presentation. Nat Rev Immunol. 2003 Dec3(12):952-61. DOI: 10.1038/nri1250 p.954 right column bottom paragraphPubMed ID14647477
|| Cascio, P., Hilton, C., Kisselev, A. F., Rock, K. L. & Goldberg, A. L. 26S proteasomes and immunoproteasomes produce mainly N-extended versions of an antigenic peptide. EMBO J. 20, 2357–2366 (2001). DOI: 10.1093/emboj/20.10.2357PubMed ID11350924
||P.954 right column bottom paragraph: "Estimates are that 60% of proteasome-generated fragments are too small, 15% are of the appropriate size and 20% are too large for direct presentation by MHC class I molecules [primary source]. This implies that most of the longer peptides need to be trimmed before properly fitting MHC class I molecules. As cells generally lack carboxypeptidase activity [ref 7], only elongated peptides with amino-terminal extensions can be converted into MHC class-I-binding peptides. Several cytosolic peptidases have been implicated in peptide generation [refs 7, 26, 37, 38]. All of these peptidases can generate or even destroy antigenic peptides with different specificities (Table 1) and thereby influence MHC class I antigen presentation."