Allelic variation in the mouse Tap-1 and Tap-2 transporter genes.

The TAP1 and TAP2 proteins form a heterodimer that transports short peptides from the cytosol into the endoplasmic reticulum lumen. Extensive allelic polymorphism of the rat TAP transporter has been shown to affect the repertoire of peptides presented by MHC class I molecules. Structural polymorphism in the human TAP genes is much more limited and has not been observed to have functional consequences. We have examined the polymorphism in mouse TAP1 and TAP2 in inbred mice. While the number of polymorphic positions in these molecules is more similar to that in human than to that in the rat, all strains examined have a structurally unique TAP transporter, suggesting the possibility of functional polymorphism. Furthermore, allelic variations in the mouse transporter are predominantly located in or adjacent to membrane-spanning domains, although no significant bias in the ratio of nonsynonymous to synonymous substitutions is observed. We also report that mouse TAP1 begins 172 amino acids upstream of the previously published start site and report the genomic organization of mouse Tap-1 and Tap-2.

Novel peptide-binding proteins and peptide transport in normal and TAP-deficient microsomes.

Most major histocompatibility complex (MHC) class I-binding peptides are translocated by TAP heterodimers, but some enter the ER lumen by alternative pathways. To further define mechanisms of peptide handling, we developed a system for the analysis of peptide-binding components in the ER membrane and lumen using iodinated cross-linkable peptide derivatives. Here we demonstrate that at least three proteins bind peptides in the ER lumen. Peptide cross-linking to these lumenal proteins can be used as an alternative method to monitor peptide transport. TAP and one other protein bind peptides on the cytoplasmic face of the ER. The presence of multiple peptide-binding proteins necessitates caution in interpreting traditional peptide-binding and transport assays. Finally, we demonstrate sequence-specific peptide transport in TAP-deficient cells transfected with only rat TAP1.

Peptide transport in antigen presentation.

The TAP1/2 complex translocates peptides from the cytosol into the endoplasmic reticulum. In the rat, TAP polymorphism affects the pool of peptides presented by major histocompatibility complex class I molecules, whereas in mouse and humans the functional consequences of observed structural polymorphism have not yet been determined. Peptide binding to TAP precedes ATP binding, and ATP hydrolysis is required to release and translocate peptides. Cytosolic peptides entering the class I pathway via the TAP complex may differ significantly in size and sequence. Not all peptides that are able to bind to TAP can be translocated into the endoplasmic reticulum, and TAP-binding affinity does not directly correlate with antigenicity of the peptide. Most translocated peptides are released from the endoplasmic reticulum by an incompletely defined ATP-dependent mechanism. TAP interaction with class I molecules stimulates peptide binding and transport by TAP and may also facilitate efficient loading of class I with peptides. Pathogenic microorganisms, such as herpes simplex virus, may encode inhibitors of TAP-mediated peptide transport in order to evade immune surveillance.