Letter | Published:

Structure of the human MHC-I peptide-loading complex

Nature volume 551, pages 525528 (23 November 2017) | Download Citation


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 cells1,2. 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 calreticulin3,4. 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.

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This research was supported by the German Research Foundation (SFB 807 and GRK 1986 to R.T.). C.S. acknowledges funding from the Federal Ministry for Education and Research (BMBF, ZIK program, 03Z22HN22), the European Regional Development Funds (EFRE, ZS/2016/04/78115) and the MLU Halle-Wittenberg. We thank K. Zehl for technical support; all members of the Institute of Biochemistry (Goethe University Frankfurt) for comments on the manuscript; and especially W. Kühlbrandt, D. Mills, M. Wilkes, and the staff at the Department of Structural Biology (MPI of Biophysics, Frankfurt/Main) for discussions, cryo-EM infrastructure and support. E. D’Imprima and R. Sanchez provided the code for RecenterParticles.

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Author notes

    • Andreas Blees
    •  & Dovile Januliene

    These authors contributed equally to this work.


  1. Institute of Biochemistry, Biocenter, Goethe University Frankfurt, Max-von-Laue Strasse 9, 60438 Frankfurt/Main, Germany

    • Andreas Blees
    • , Nicole Koller
    • , Simon Trowitzsch
    •  & Robert Tampé
  2. Department of Structural Biology, Max Planck Institute of Biophysics, Max-von-Laue Strasse 3, 60438 Frankfurt/Main, Germany

    • Dovile Januliene
    •  & Arne Moeller
  3. Interdisciplinary Research Center HALOmen, Martin Luther University Halle-Wittenberg, Kurt-Mothes-Strasse 3, 06120 Halle/Saale, Germany

    • Tommy Hofmann
    •  & Carla Schmidt


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A.B. isolated the PLC and performed all biochemical experiments. D.J. and A.M. carried out all EM imaging and single-particle analyses. T.H. and C.S. performed the mass spectrometry experiments. N.K. implemented the single-cell-based transport analyses. S.T. and A.B. designed the purification strategy for the PLC. S.T. and D.J. built the PLC model. A.B., D.J., S.T., A.M., and R.T. interpreted the data and wrote the manuscript with contributions from all authors. A.M., S.T., and R.T. conceived the study, designed the research, and planned the experiments. R.T. initiated and planned the project.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to Simon Trowitzsch or Arne Moeller or Robert Tampé.

Reviewer Information Nature thanks P. Cresswell, G. Skiniotis and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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