| Popis: |
The transporter associated with antigen processing (TAP) translocates antigenic peptides into the ER for binding onto MHC class I molecules. Tapasin organizes a peptide-loading complex (PLC) by recruiting MHC class I and accessory chaperones to the N-terminal regions (N-domains) of the TAP subunits TAP1 and TAP2. To investigate the function of the tapasin-docking sites of TAP in MHC class I processing, N-terminally truncated variants of TAP1 and TAP2 were expressed in combination with wild-type chains, as fusion proteins or as single subunits. The results indicate that TAP interacts at steady state with MHC class I solely via one N-domain whereas the opposing tapasin-docking site concomitantly associates with calnexin. Strikingly, TAP variants lacking the N-domain in TAP2, but not in TAP1, build PLCs that fail to generate stable MHC class I-peptide complexes. This correlates with a substantially reduced recruitment of accessory chaperones into the PLC demonstrating their important role in the quality control of MHC class I loading. However, stable surface expression of MHC class I is rescued in compartments of the late secretory route by a mechanism that depends on the proteolytical activity of the proprotein convertase family. Finally, it is shown that proprotein convertase expression is differentially modulated in response to interferons indicating that individual members of this protease family may make distinct contributions to an antiviral immune response. |
