Department of Microbiology and Immunology, The University of Melbourne, The Peter Doherty Institute for Infection and Immunity, Melbourne, VIC 3000, Australia;
Department of Biochemistry and Molecular Biology, The University of Melbourne, Bio21 Molecular Science and Biotechnology Institute, Parkville, VIC 3010, Australia.
Proc Natl Acad Sci U S A. 2020 Oct 6;117(40):24974-24985. doi: 10.1073/pnas.2011260117. Epub 2020 Sep 21.
The antigen-presenting molecule MR1 (MHC class I-related protein 1) presents metabolite antigens derived from microbial vitamin B synthesis to activate mucosal-associated invariant T (MAIT) cells. Key aspects of this evolutionarily conserved pathway remain uncharacterized, including where MR1 acquires ligands and what accessory proteins assist ligand binding. We answer these questions by using a fluorophore-labeled stable MR1 antigen analog, a conformation-specific MR1 mAb, proteomic analysis, and a genome-wide CRISPR/Cas9 library screen. We show that the endoplasmic reticulum (ER) contains a pool of two unliganded MR1 conformers stabilized via interactions with chaperones tapasin and tapasin-related protein. This pool is the primary source of MR1 molecules for the presentation of exogenous metabolite antigens to MAIT cells. Deletion of these chaperones reduces the ER-resident MR1 pool and hampers antigen presentation and MAIT cell activation. The MR1 antigen-presentation pathway thus co-opts ER chaperones to fulfill its unique ability to present exogenous metabolite antigens captured within the ER.
抗原呈递分子 MR1（MHC Ⅰ类相关蛋白 1）可将微生物维生素 B 合成衍生的代谢物抗原呈递给黏膜相关不变 T（MAIT）细胞。该进化保守途径的关键方面仍未得到充分描述，包括 MR1 获得配体的位置以及辅助配体结合的辅助蛋白。我们使用荧光标记的稳定 MR1 抗原类似物、特异性 MR1 mAb、蛋白质组学分析和全基因组 CRISPR/Cas9 文库筛选来回答这些问题。我们表明内质网 (ER) 中存在两种无配体的 MR1 构象，它们通过与伴侣蛋白 tapasin 和 tapasin 相关蛋白的相互作用而稳定。该池是 MR1 分子呈现外源性代谢物抗原给 MAIT 细胞的主要来源。这些伴侣蛋白的缺失会减少 ER 中驻留的 MR1 池，并阻碍抗原呈递和 MAIT 细胞激活。因此，MR1 抗原呈递途径利用 ER 伴侣蛋白来实现其独特的呈现 ER 内捕获的外源性代谢物抗原的能力。
