Productive association between MHC class I and tapasin requires the tapasin transmembrane/cytosolic region and the tapasin C-terminal Ig-like domain

Abstract

The current model of antigen assembly with major histocompatibility complex (MHC) class I molecules posits that interactions between the tapasin N-terminal immunoglobulin (Ig)-like domain and the MHC class I peptide-binding groove permit tapasin to regulate antigen selection. Much less is known regarding interactions that might involve the tapasin C-terminal Ig-like domain. Additionally, the tapasin transmembrane/cytoplasmic region enables tapasin to bridge the MHC class I molecule to the transporter associated with antigen processing (TAP). In this investigation, we made use of two tapasin mutants to determine the relative contribution of the tapasin C-terminal Ig-like domain and the tapasin transmembrane/cytoplasmic region to the assembly of MHC class I molecules. Deletion of a loop within the tapasin C-terminal Ig-like domain (Δ334-342) prevented tapasin association with the MHC class I molecule Kd. Although tapasin Δ334-342 did not increase the efficiency of Kd folding, Kd surface expression was enhanced on cells expressing this mutant relative to tapasin-deficient cells. In contrast to tapasin Δ334-342, a soluble tapasin mutant lacking the transmembrane/cytoplasmic region retained the ability to bind to Kd molecules, but did not facilitate Kd surface expression. Furthermore, when soluble tapasin and tapasin Δ334-342 were co-expressed, soluble tapasin had a dominant negative effect on the folding and surface expression of not only Kd, but also Db and Kb. In addition, our molecular modeling of the MHC class I-tapasin interface revealed novel potential interactions involving tapasin residues 334-342. Together, these findings demonstrate that the tapasin C-terminal and transmembrane/cytoplasmic regions are critical to tapasin's capacity to associate effectively with the MHC class I molecule.

Mechanistic Basis for Epitope Proofreading in the Peptide-Loading Complex

“…To establish these contacts, the C-terminal domain of Tsn has to rotate compared with its initial orientation in the X-ray crystal structure. The C-terminal Tsn residues observed in our MD simulations have previously been suggested to be involved in the interaction and have been shown to influence assembly and surface expression of MHC I molecules (52)(53)(54)(55).…”

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

“…An additional MHC class I interaction site has been suggested on the membrane proximal domain of tapasin consisting of residues 333-342 (18)(19)(20)(21). The amino acid alignment suggests that, although R333 and S341 are conserved between the two proteins, there is considerable variation between the two molecules in this region.…”

“…Residues in the predicted contact site in MHC class I (e.g., T134) are essential for incorporation of MHC class I into the peptide loading complex and efficient peptide loading (13)(14)(15)(16)(17). A second interaction point between tapasin and the MHC class I H chain involves residues 333-342 in the C-terminal Ig-like domain of tapasin (18)(19)(20)(21), which are predicted to bind residues 222-229, situated in a b strand in the a3 domain of the MHC class I heterodimer (20,(22)(23)(24)(25).…”

F pocket flexibility influences the tapasin dependence of two differentially disease‐associated MHC Class I proteins

“…the end of the binding groove that includes the F pocket, is the proposed binding region of tapasin, according to experimental, and theoretical investigations [17][18][19][20]; but as yet, no crystal structure of a complex between class I and tapasin exists. In cells, tapasin reduces the conformational disorder of empty or suboptimally loaded class I molecules, and it mediates iterative peptide exchange by accelerating dissociation [1,[21][22][23].…”