The hierarchical importance of components of the peptide-loading complex

TAP2, tapasin and calreticulin (CRT) are three crucial components of the peptide loading complex (PLC) in MHC class I antigen processing and presentation. While the functions of the proteins within the complex have been progressively defined, the hierarchical importance of the components has not been clearly and quantitatively analyzed. To compare the efficiency of MHC class I antigen processing in different cell lines, we established a novel suilysin-mediated antigen delivery method to quantitatively analyze antigen processing in a number of cell lines, including K41, the CRT-deficient line K42, the tapasin-deficient line 90a and the tapasin/TAP2-deficient cell line 91a. We ranked the importance of the components of MHC class I antigen processing such as CRT, TAP and tapasin using OVA challenge as the antigen model. Surprisingly, CRT was shown to be the most important component of the PLC, even though TAP was speculated to be the most crucial protein involved in the pathway. This is the first report that ranks the components of the antigen-processing pathway. Further investigation is required for elucidation of the roles played by the individual proteins within the complex.