The Interface Between Tapasin and MHC Class I Identification of Amino Acid Residues in Both Proteins That Influence Their Interaction

The transporter associated with Ag processing (TAP) translocates proteasomally derived cytosolic peptides into the endoplasmic reticulum. TAP is a central component of the peptide-loading complex (PLC), to which tapasin (TPN) recruits MHC class I (MHC I) and accessory chaperones. The PLC functions to facilitate and optimize MHC I–mediated Ag presentation. The heterodimeric peptide transporter consists of two homologous subunits, TAP1 and TAP2, each of which contains an N-terminal domain (N-domain) in addition to a conserved transmembrane (TM) core segment. Each N-domain binds to the TM region of a single TPN molecule, which recruits one MHC I molecule to TAP1 and/or TAP2. Although both N-domains act as TPN-docking sites, various studies suggest a functional asymmetry within the PLC resulting in greater significance of the TAP2/TPN interaction for MHC loading. In this study, we demonstrate that the leucine-rich hydrophobic sequence stretches (with the central leucine residues L20 and L66) in the first and second TM helix of TAP2 form a functional unit acting as a docking site for optimal TPN/MHC I recruitment, whereas three distinct highly conserved arginine and/or aspartate residues inside or flanking these TM helices are dispensable. Moreover, we show that the physical interaction between TAP2 and TPN is disrupted by benzene, a compound known to interfere with hydrophobic interactions, such as those between pairing leucine zippers. No such effects were observed for the TAP1/TAP2 interaction or the complex formation between TPN and MHC I. We propose that TAP/TPN complex formation is driven by hydrophobic interactions via leucine zipper–like motifs.

Section: Discussionsupporting

confidence: 90%

Hydrophobic Interactions Are Key To Drive the Association of Tapasin with Peptide Transporter Subunit TAP2

Rufer

Kägebein

Leonhardt

et al. 2015

“…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).…”

Section: Mechanism Of Differential Bindingmentioning

confidence: 60%

Mechanistic Basis for Epitope Proofreading in the Peptide-Loading Complex

Fleischmann

Fisette

Thomas

et al. 2015

The peptide-loading complex plays a pivotal role in Ag processing and is thus central to the efficient immune recognition of virally and malignantly transformed cells. The underlying mechanism by which MHC class I (MHC I) molecules sample immunodominant peptide epitopes, however, remains poorly understood. In this article, we delineate the interaction between tapasin (Tsn) and MHC I molecules. We followed the process of peptide editing in real time after ultra-fast photoconversion to pseudoempty MHC I molecules. Tsn discriminates between MHC I loaded with optimal and MHC I bound to suboptimal cargo. This differential interaction is key to understanding the kinetics of epitope proofreading. To elucidate the underlying mechanism at the atomic level, we modeled the Tsn/MHC I complex using all-atom molecular dynamics simulations. We present a catalytic working cycle, in which Tsn binds to MHC I with suboptimal cargo and thereby adjusts the energy landscape in favor of MHC I complexes with immunodominant epitopes.

“…2 and 4), this tapasin region probably contacts L d . Previous studies found evidence for interaction sites on both the MHC class I ␣2 and ␣3 domains for tapasin (7,38,54,55). Given that aa 334 -342 of tapasin are modeled to be at a membrane-distal position in the three-dimensional structure of the tapasin Ig-like domain (Fig.…”

Section: Discussionmentioning

confidence: 99%

The Ig-Like Domain of Tapasin Influences Intermolecular Interactions

Turnquist

Petersen²,

Vargas

et al. 2004

Presentation of antigenic peptides to T lymphocytes by MHC class I molecules is regulated by events involving multiple endoplasmic reticulum proteins, including tapasin. By studying the effects of substitutions in the tapasin Ig-like domain, we demonstrated that H-2Ld/tapasin association can be segregated from reconstitution of folded Ld surface expression. This finding suggests that peptide acquisition by Ld is influenced by tapasin functions that are independent of Ld binding. We also found that the presence of a nine-amino acid region in the Ig-like domain of mouse or human tapasin is required for association with Ld, and certain point substitutions in this sequence abrogate human, but not mouse, tapasin association with Ld. These data are consistent with a higher overall affinity between Ld and mouse tapasin compared with human tapasin. In addition, we found that other point mutations in the same region of the tapasin Ig-like domain affect MHC class I surface expression and Ag presentation. Finally, we showed that the cysteine residues in the Ig-like domain of tapasin influence tapasin’s stability, its interaction with the MHC class I H chain, and its stabilization of TAP. Mutagenesis of these cysteines decreases tapasin’s electrophoretic mobility, suggesting that these residues form an intramolecular disulfide bond. Taken together, these results reveal a critical role for the tapasin Ig-like domain in tapasin function.