The Binding of TAPBPR and Tapasin to MHC Class I Is Mutually Exclusive

Abstract

The loading of peptide antigens onto MHC class I molecules is a highly controlled process in which the MHC class I dedicated chaperone tapasin is a key player. We recently identified a tapasin related molecule, TAPBPR, as an additional component in the MHC class I antigen presentation pathway. Here we show that the amino acid residues important for tapasin to interact with MHC class I are highly conserved on TAPBPR. We identify specific residues in the N-terminal and C-terminal domains of TAPBPR involved in associating with MHC class I. Furthermore, we demonstrate that residues on MHC class I crucial for its association with tapasin, such as T134, are also essential for its interaction with TAPBPR. Taken together, the data indicate that TAPBPR and tapasin bind in a similar orientation to the same face of MHC class I. In the absence of tapasin, the association of MHC class I with TAPBPR is increased. However, in the absence of TAPBPR, the interaction between MHC class I and tapasin does not increase. In light of our findings, previous data determining the function of tapasin in the MHC class I antigen processing and presentation pathway must be re-evaluated.

Interaction of TAPBPR, a tapasin homolog, with MHC-I molecules promotes peptide editing

“…TAPBPR is highly conserved among vertebrates (mouse and human proteins are 69% identical), is widely expressed, is inducible by IFN-γ, and coprecipitates with MHC-I molecules containing β 2 -microglobulin (β 2 m) (30), but not with components of the PLC. Results of site-directed mutagenesis of both HLA-A2 and TAPBPR are consistent with a view that tapasin and TAPBPR share a similar mode of MHC-I binding (31). The amino acid sequence and biochemical similarities to tapasin suggest that examination of the direct interactions of TAPBPR with MHC-I would not only reflect on the normal function of this molecule, but would also provide insight into the molecular mechanisms that govern tapasin.…”

“…Because previous studies indicated that TAPBPR first interacts with MHC-I early in its biosynthesis (30,31,33), we investigated whether recombinant TAPBPR would bind peptide-free MHC-I with high affinity.…”

“…In addition, the determination of the structural similarity of TAPBPR to tapasin by SAXS and the behavior of mutants in predicted homologous positions to those observed for tapasin suggest that the mechanism of binding and stabilization and the site of binding of MHC-I by TAPBPR is similar to that of tapasin. The chaperone/editing function of TAPBPR, like tapasin, also shows important similarities to the functions of the MHC-II chaperone, HLA-DM, which stabilizes MHC-II molecules by binding to a region of MHC-II on the α1-helix of MHC-II, a position symmetrically related to the site mapped by mutagenesis to MHC-I on the α2-helix (22,31). A role of the MHC-I lightchain β 2 m in facilitating peptide exchange has been recognized for many years (37,38), likely reflecting the dynamic and cooperative nature of MHC/β 2 m/peptide interactions.…”

“…A human gene encoding a tapasin homolog, TAP binding protein, related (TAPBPR), was identified on chromosome 12 (12p13.3) (28,29). The encoded type I membrane protein is expressed in the ER and Golgi and interacts with MHC-I during its maturation (30)(31)(32)(33). TAPBPR is highly conserved among vertebrates (mouse and human proteins are 69% identical), is widely expressed, is inducible by IFN-γ, and coprecipitates with MHC-I molecules containing β 2 -microglobulin (β 2 m) (30), but not with components of the PLC.…”

The ongoing saga of the mechanism(s) of MHC class I-restricted cross-presentation

“…Hence, it is unlikely that recycling MHC-I is able to functionally interact with the PLC in phagosomes as it does in the ER, leaving open the possibility that alternate accessory molecules may play a role. The tapasin homologue TAPBPR, which interacts with MHC-I independently of the PLC [36,37] and can mediate peptide exchange [38*,39*], is a strong candidate for an accessory role in cross-presentation.…”