Structure of the human MHC-I peptide-loading complex

Abstract

The peptide-loading complex (PLC) is a transient, multisubunit membrane complex in the endoplasmic reticulum that is essential for establishing a hierarchical immune response. The PLC coordinates peptide translocation into the endoplasmic reticulum with loading and editing of major histocompatibility complex class I (MHC-I) molecules. After final proofreading in the PLC, stable peptide-MHC-I complexes are released to the cell surface to evoke a T-cell response against infected or malignant cells. Sampling of different MHC-I allomorphs requires the precise coordination of seven different subunits in a single macromolecular assembly, including the transporter associated with antigen processing (TAP1 and TAP2, jointly referred to as TAP), the oxidoreductase ERp57, the MHC-I heterodimer, and the chaperones tapasin and calreticulin. The molecular organization of and mechanistic events that take place in the PLC are unknown owing to the heterogeneous composition and intrinsically dynamic nature of the complex. Here, we isolate human PLC from Burkitt's lymphoma cells using an engineered viral inhibitor as bait and determine the structure of native PLC by electron cryo-microscopy. Two endoplasmic reticulum-resident editing modules composed of tapasin, calreticulin, ERp57, and MHC-I are centred around TAP in a pseudo-symmetric orientation. A multivalent chaperone network within and across the editing modules establishes the proofreading function at two lateral binding platforms for MHC-I molecules. The lectin-like domain of calreticulin senses the MHC-I glycan, whereas the P domain reaches over the MHC-I peptide-binding pocket towards ERp57. This arrangement allows tapasin to facilitate peptide editing by clamping MHC-I. The translocation pathway of TAP opens out into a large endoplasmic reticulum lumenal cavity, confined by the membrane entry points of tapasin and MHC-I. Two lateral windows channel the antigenic peptides to MHC-I. Structures of PLC captured at distinct assembly states provide mechanistic insight into the recruitment and release of MHC-I. Our work defines the molecular symbiosis of an ABC transporter and an endoplasmic reticulum chaperone network in MHC-I assembly and provides insight into the onset of the adaptive immune response.

Molecular determinants of chaperone interactions on MHC-I for folding and antigen repertoire selection

“…The discovery that TAPBPR can function as a peptide exchange catalyst outside the peptideloading complex and can maintain empty MHC-I molecules in a peptide-receptive conformation has opened a new window to study the peptide loading process in a range of detailed functional and mechanistic studies (4,12,13). Structural insights into static snapshots of MHC-I/chaperone complexes have been gleaned by X-ray crystallography (14,15) and cryoEM (16). However, MHC-I molecules are inherently dynamic at multiple sites (17,18) and transitions between different "open" and "closed" conformations enable peptide loading, as shown by molecular dynamics (MD) simulation (19,20) and kinetic modeling (21).…”

ER‐aminopeptidase 1 determines the processing and presentation of an immunotherapy‐relevant melanoma epitope

“…Both in vitro kinetic analyses and in cellulo inhibitor and siRNA experiments demonstrated that ERAP1 generates and stabilizes the minimal gp100 209-217 epitope. This is in contrast to the MART-1 [26][27][28][29][30][31][32][33][34][35] (melanoma antigen recognized by T cells) epitope being destroyed by ERAP1 [24]. ERAP1 has been shown to be constitutively expressed in tumor cells, even in the absence of IFN-γ [19,24].…”

Mechanism of pharmacochaperoning in KATP channels revealed by cryo-EM

“…An exciting finding here is how the Kir6.2 N-terminus can act as a plug or handle to regulate channel gating and assembly by inserting itself into the ABC core central cavity formed by the two TM bundles. The exploitation of this structural space is akin to the mechanism by which viral peptide ICP47 enables immuneevasion of the pathogens (Blees et al, 2017;Oldham et al, 2016;Parcej and Tampe, 2010). In this case, ICP47 secreted by viruses such as Herpes simplex virus and Cytomegalovirus inserts into the inner vestibule formed by the ABC peptide transporters TAP-1 and TAP-2 (transporter associated with antigen processing) (Blees et al, 2017;Oldham et al, 2016), thus preventing the transport of cytosolic peptides into the ER for MHC complex loading and immune surveillance.…”