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Structure of human MR1-ethylvanillin in complex with human MAIT A-F7 TCR
Structural highlights
FunctionHMR1_HUMAN Has antigen presentation function. Involved in the development and expansion of a small population of T-cells expressing an invariant T-cell receptor alpha chain called mucosal-associated invariant T-cells (MAIT). MAIT cells are preferentially located in the gut lamina propria and therefore may be involved in monitoring commensal flora or serve as a distress signal. Expression and MAIT cell recognition seem to be ligand-dependent.[1] Publication Abstract from PubMedThe Major Histocompatibility Complex class I (MHC-I) related protein 1 (MR1) presents small molecule metabolites, drugs, and drug-like molecules that are recognized by MR1-reactive T cells. While we have an understanding of how antigens bind to MR1 and upregulate MR1 cell surface expression, a quantitative, cell-free, assessment of MR1-ligand binding affinity was lacking. Here, we developed a fluorescence polarization (FP)-based assay in which fluorescent MR1 ligand was loaded into MR1 protein in vitro, and competitively displaced by candidate ligands over a range of concentrations. Using this assay, ligand affinity for MR1 could be differentiated as strong (IC(50) < 1 muM), moderate (1 muM < IC(50) < 100 muM), and weak (IC(50) > 100 muM). We demonstrated a clear correlation between ligand binding affinity for MR1, the presence of a covalent bond between MR1 and ligand, and the number of salt bridge and hydrogen bonds formed between MR1 and ligand. Using this newly developed FP-based assay to screen for candidate ligands, we identified the dietary molecules vanillin and ethylvanillin as weak bona fide MR1 ligands. Both upregulated MR1 on the surface of C1R.MR1 cells and the crystal structure of a MAIT cell T cell receptor (TCR)-MR1-ethylvanillin complex revealed that ethylvanillin formed a Schiff base with K43 of MR1 and was buried within the A'-pocket. Collectively, we developed and validated a method to quantitate the binding affinities of ligands for MR1 that will enable an efficient and rapid screening of candidate MR1 ligands. Quantitative affinity measurement of small molecule ligand binding to Major Histocompatibility Complex class-I related protein 1 MR1.,Wang CJH, Awad W, Liu L, Mak JYW, Veerapen N, Illing PT, Purcell AW, Eckle SBG, McCluskey J, Besra GS, Fairlie DP, Rossjohn J, Le Nours J J Biol Chem. 2022 Nov 17:102714. doi: 10.1016/j.jbc.2022.102714. PMID:36403855[2] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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