A POINT MUTATION IN THE HUMAN TRANSPORTER ASSOCIATED WITH ANTIGEN-PROCESSING (TAP2) ALTERS THE PEPTIDE-TRANSPORT SPECIFICITY

The heterodimeric transporter associated with antigen processing (TAP1 /TAP2) translocates peptides from the cytosol into the endoplasmic ret iculum where loading of major histocompatibility complex class I molec ules takes place. TAP transporters from different species are known to exhibit distinct transport specificities with regard to the C-termina l amino acid (aa) of peptides. Thus, human TAP (hTAP), and rat TAP (rT AP) containing the rTAP2(a) allele are rather promiscuous, whereas mou se TAP (mTAP), and rTAP containing the rTAP2(a) allele are restrictive and select against peptides with C-terminal small polar/hydrophobic o r positively charged aa. The structural basis for this selectivity is not clear. To assess the relative contribution of the TAP1 and TAP2 su bunits to transport specificity, we have constructed and analyzed inte rspecies TAP hybrids and point mutants of hTAP2 expressed in Sf9 insec t cells and in TAP-deficient T2 cells. Transport assays with 20 C-term inal variants of the peptide RYWA- NATRSX showed that first, transport specificity with regard to C-terminal aa is mainly influenced by TAP2 , but TAP1 can also contribute. Second, the selective transport of pep tides with C-terminal positively charged aa is critically controlled b y the amino-terminal region (1-361) on the TAP2 chain, while transport of peptides with C-terminal small polar/hydrophobic aa is determined by residues located within as well as outside the region 1-361. Third, a single point mutation in hTAP2 (374A --> D) resulted in a drastic a lteration of the transport pattern. These results indicate that both T AP1 and TAP2 contribute to efficient peptide transport and that single paint mutations in hTAP2 are able to alter the peptide transport spec ificity. This opens the possibility that naturally occurring mutations in one of the hTAP subunits may alter epitope selection in vivo.