Peptide transporters and antigen processing.

Our knowledge about intracellular pathways involved in the presentation of antigens was considerably broadened with the recent discovery of peptide transporters encoded in the major histocompatibility complex. The transporter associated with antigen processing (TAP) belongs to an evolutionary conserved family of multimembrane-spanning translocators that bind ATP and show specificity for a variety of different substrates. TAP mediates the translocation of peptides, generated by cytosolic degradation of protein antigens, into the lumen of the endoplasmic reticulum where they bind to newly synthesized MHC class I molecules. A novel assay has been employed to elucidate the details of TAP-mediated peptide transport. The results indicate that TAP selects peptides of sequence and length according to the requirements of MHC class I molecules in different species.