Co-evolution of rat TAP transporters and MHC class I RT1-A molecules.

The genes for rat major histocompatibility complex (MHC) class I molecules are associated either with those for the A allele of the transporter associated with antigen processing (TAP-A), which can transport peptides with basic carboxy-terminal residues, or with those for TAP-B, which cannot [1-5]. To explore whether these associations have a functional basis, we compared the sequences of 13 rat MHC class la RT1-A cDNAs from nine MHC haplotypes. Of seven TAP-A- linked RT1-A molecules, six possess strongly acidic F pockets, and these bind a high proportion of peptides with basic carboxy-terminal residues. The F pockets of TAP-B-linked molecules, by contrast, were more basic. Furthermore, we identified six positions at the 'righthand end' of the peptide-binding groove, at which a majority of TAP-B-linked molecules diverge from the consensus sequence for class la molecules whereas, at these positions, all the TAP-A-linked molecules reflect the consensus sequence. Our results suggest that the linked rat class la and TAP genes have co-evolved to maximize the supply of appropriate peptides to the presenting molecules.