Solid-State NMR Investigations of the MHC II Transmembrane Domains: Topological Equilibria and Lipid Interactions
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The major histocompatibility complex class II (MHC II) membrane proteins are key players in the adaptive immune response. An aberrant function of these molecules is associated with a large number of autoimmune diseases such as diabetes type I and chronic inflammatory diseases. The MHC class II is assembled from DQ alpha 1 and DQ beta 1 which come together as a heterodimer through GXXXG-mediated protein–protein interactions and a highly specific protein-sphingomyelin-C18 interaction motif located on DQA1. This association can have important consequences in regulating the function of these membrane proteins. Here, we investigated the structure and topology of the DQA1 and DQB1 transmembrane helical domains by CD-, oriented 2H and 15N solid-state NMR spectroscopies. The spectra at peptide-to-lipid ratios of 0.5 to 2 mol% are indicative of a topological equilibrium involving a helix crossing the membrane with a tilt angle of about 20° and another transmembrane topology with around 30° tilt. The latter is probably representing a dimer. Furthermore, at the lowest peptide-to-lipid ratio, a third polypeptide population becomes obvious. Interestingly, the DQB1 and to a lesser extent the DQA1 transmembrane helical domains exhibit a strong fatty acyl chain disordering effect on the inner segments of the 2H-labelled palmitoyl chain of POPC bilayers. This phosphatidylcholine disordering requires the presence of sphingomyelin-C18 suggesting that the ensemble of transmembrane polypeptide and sphingolipid exerts positive curvature strain.
KeywordsTransmembrane dimer Highly specific protein–lipid interaction Supported lipid bilayer Solid-state NMR Helix topology Fatty acyl chain order parameter
KK TETVV CALGL SVGLV GIVVG TVFII RGLRS KK
KK QSKML SGIGG FVLGL IFLGL GLIIH HRSQK K
Line width at half height
Major histocompatibility complex
Nuclear magnetic resonance
The discussions with Britta Brügger and Thomas Kupke on p24, MHC II and in particular DQA1 and its specific interactions with sphingomyelin are gratefully acknowledged. We thank Arnaud Marquette for his help with first CD spectra.
ES and CA designed and performed the experiments and analysed data, BB helped in the design the experiments and in the analysis and wrote the paper.
We are grateful for the financial contributions of the Agence Nationale de la Recherche (Projects ProLipIn 10-BLAN-731, membraneDNP 12-BSV5-0012, MemPepSyn 14-CE34-0001-01, InMembrane 15-CE11-0017-01 and the LabEx Chemistry of Complex Systems 10-LABX-0026_CSC), the University of Strasbourg, the CNRS, the Région Alsace and the RTRA International Center of Frontier Research in Chemistry. BB thanks the Institut Universitaire de France for support and for providing additional time for research.
Compliance with Ethical Standards
Conflict of interest
All authors declare that they have no conflict of interest.
This article does not contain any studies with human participants or animals performed by any of the authors.
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