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Interaction between HLA-DM and HLA-DR involves regions that undergo conformational changes at lysosomal pH
H J Ullrich1, K Döring, U Grüneberg
1Human Immunogenetics Laboratory, Imperial Cancer Research Fund, Holborn, London, United Kingdom.
Summary
Major histocompatibility complex class II (MHC II) peptide loading is pH-dependent and HLA-DM-catalyzed. HLA-DM stabilizes MHC II conformations at acidic pH, enhancing antigenic peptide exchange.
Area of Science:
- Immunology
- Molecular Biology
- Biochemistry
Background:
- Major histocompatibility complex class II (MHC II) molecules present antigenic peptides to T helper cells.
- HLA-DM is a non-classical MHC II molecule that catalyzes the exchange of peptides bound to classical MHC II molecules.
- Lysosomal pH is crucial for MHC II peptide loading.
Purpose of the Study:
- To investigate the physical mechanism of HLA-DM catalysis in MHC II peptide loading.
- To elucidate the role of pH in the interaction between HLA-DM and HLA-DR3.
Main Methods:
- Time-resolved fluorescence anisotropy (TRFA).
- Fluorescence binding studies using 8-anilino-1-naphthalenesulfonic acid (ANS).
Main Results:
- Both HLA-DM and HLA-DR3 conformations are pH-sensitive, exposing hydrophobic regions upon protonation.
- HLA-DM interaction stabilizes HLA-DR3 conformations by shielding hydrophobic domains.
- Increased DM-DR association occurs at lysosomal pH, favoring peptide exchange.
- DM preferentially associates with DR/CLIP complexes over stable DR/peptide complexes.
Conclusions:
- HLA-DM catalyzes peptide loading by stabilizing the low-pH conformation of HLA-DR.
- The dissociation of DM from DR is facilitated by peptide binding or neutralization of the complex.