Processing Epitope Precursor Recognition and Analogue Provides Insight on Antigenic Aminopeptidase with Bound Substrate Crystal
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The Journal of Immunology Crystal Structure of Insulin-Regulated Aminopeptidase with Bound Substrate Analogue Provides Insight on Antigenic Epitope Precursor Recognition and Processing Anastasia Mpakali,* Emmanuel Saridakis,* Karl Harlos,† Yuguang Zhao,† Athanasios Papakyriakou,* Paraskevi Kokkala,*,‡ Dimitris Georgiadis,‡ and Efstratios Stratikos* Aminopeptidases that generate antigenic peptides influence immunodominance and adaptive cytotoxic immune responses. The mechanisms that allow these enzymes to efficiently process a vast number of different long peptide substrates are poorly understood. In this work, we report the structure of insulin-regulated aminopeptidase, an enzyme that prepares antigenic epitopes for cross- presentation in dendritic cells, in complex with an antigenic peptide precursor analog. Insulin-regulated aminopeptidase is found in a semiclosed conformation with an extended internal cavity with limited access to the solvent. The N-terminal moiety of the peptide is located at the active site, positioned optimally for catalysis, whereas the C-terminal moiety of the peptide is stabilized along the extended internal cavity lodged between domains II and IV. Hydrophobic interactions and shape complementarity enhance peptide affinity beyond the catalytic site and support a limited selectivity model for antigenic peptide selection that may underlie the generation of complex immunopeptidomes. The Journal of Immunology, 2015, 195: 2842–2851. ytotoxic adaptive immune responses rely on the recogni- process, and present external Ags onto MHCI to allow cross-priming tion on the cell surface of complexes of MHC class I of naive CD8+ T cells (2). The cross-presentation pathway is key for C molecules (MHCI) with antigenic peptides. These peptides the immune defense against many viruses, bacteria, and tumors and are generated inside the cell by proteolytic digestion that often can help the immune system avoid immune evasion strategies. Fur- includesthetrimmingofoneormoreaminoacidsfromtheNterminus thermore, cross-presentation is important for inducing cytotoxic re- of antigenic peptide precursors to generate the mature antigenic sponses through vaccination, especially against tumors (3, 4). peptides (1). This trimming is performed by specialized intracellular Insulin-regulated aminopeptidase (IRAP; EC 3.4.11.3) is a aminopeptidases that generate or destroy MHCI peptide ligands and transmembrane enzyme localized in intracellular vesicles, which by guest on October 1, 2021. Copyright 2015 Pageant Media Ltd. can therefore influence the generation of cytotoxic responses. In has been shown to generate antigenic peptides for MHCI for cross- conventional Ag presentation, somatic cells degrade intracellular presentation (5, 6). IRAP (also known as placental leucine amino- proteins and present derived antigenic peptides. In cross-presentation, peptidase and oxytocinase) also has other important biological specialized professional APCs such as dendritic cells (DCs) take up, functions, including the regulation of trafficking of the glucose transporter type 4, the control of oxytocin levels in pregnancy, and the regulation of brain oxytocin and vasopressin levels (7). The *National Center for Scientific Research Demokritos, Agia Paraskevi, Athens 15310, Greece; †Division of Structural Biology, Wellcome Trust Centre for Human Genetics, soluble domain of IRAP carries the aminopeptidase enzymatic ac- Oxford University, Oxford OX3 7BN, United Kingdom; and ‡Department of Chem- tivity and is highly homologous to two other intracellular amino- istry, University of Athens, Athens 15771, Greece peptidases that are important for Ag processing, namely endoplasmic http://classic.jimmunol.org ORCID: 0000-0002-3566-2309 (E.S.). reticulum aminopeptidase (ERAP)1 and ERAP2. The three ami- Received for publication May 13, 2015. Accepted for publication July 13, 2015. nopeptidases have been recently classified under the oxytocinase This work was supported by the European Union (European Social Fund) and Greek subfamily of M1 aminopeptidases and characterized to be primarily national funds through the Operational Program “Education and Lifelong Learning” responsible for antigenic peptide N-terminal trimming in the cell of the National Strategic Reference Framework: Research Funding Program of the General Secretariat for Research & Technology (Grant ERC-14). The research lead- (8). ERAP1 has been shown to be a critical editor of antigenic ing to these results has received support from the European Community’s Seventh peptides and its activity to influence the immunopeptidome and Framework Programme (FP7/2007-2013) under BioStruct-X (Grant Agreement Downloaded from N˚283570). K.H. was supported by the Medical Research Council, and the Wellcome concomitant cellular cytotoxic responses, as well as immunodo- Trust provided administrative support (Grant 075491/Z/04). minance (9, 10). Similarly, IRAP can generate correct-length an- The atomic coordinates and structure factors presented in this article have been tigenic peptides from precursors in vitro, but in distinct patterns submitted to the Protein Data Bank under accession numbers 4Z7I and 5C97 for compared with ERAP1, suggesting differences in specificity and the ligand-bound and ligand-free structure, respectively. mechanism (11). Several studies have suggested that Ag processing Address correspondence and reprint requests to Efstratios Stratikos, National Center can vary significantly between cells and inflammatory states, for Scientific Research Demokritos, Agia Paraskevi, Athens 15310, Greece. E-mail address: [email protected] or [email protected] complicating vaccine design (12–14). As a result, understanding The online version of this article contains supplemental material. the mechanism of antigenic peptide recognition by intracellular Abbreviations used in this article: DC, dendritic cell; ERAP, endoplasmic reticulum aminopeptidases is of great importance. ERAP1 and ERAP2 have aminopeptidase; hPhe, homophenylalanine; IRAP, insulin-regulated aminopeptidase; been shown to be polymorphic, and several single nucleotide PDB, Protein Data Bank; PEG, polyethylene glycol; SNP, single nucleotide poly- polymorphisms (SNPs) in these enzymes have been associated with morphism; TFA, trifluoroacetic acid. predisposition to disease, ranging from viral infections and cancer Copyright Ó 2015 by The American Association of Immunologists, Inc. 0022-1767/15/$25.00 to autoimmunity. These SNPs have been shown to affect enzymatic www.jimmunol.org/cgi/doi/10.4049/jimmunol.1501103 The Journal of Immunology 2843 activity, Ag generation, and cytotoxic responses [reviewed in (15– purification of the expressed protein. All DNA purifications were performed 17)] and to form specific disease-associated functionally distinct with electroelution and phenol/chloroform extraction and precipitated with allotypes (18, 19). Recently, two SNPs in IRAP have also been ethanol/sodium acetate. reported to associate with psoriasis and ankylosing spondylitis, Site-directed mutagenesis suggesting that IRAP polymorphic variation may also contribute to the pathogenesis of autoimmunity (20, 21). Mutagenesis reactions to generate the two SNPs in IRAP (A609Tand I166M) were performed using the Quickchange II kit (Agilent Technologies), To gain insight on the mechanism and determinants of Ag se- according to the manufacturer’s instructions. Primers were designed with the lection in cross-presentation by IRAP, we crystallized IRAP in the Quickchange PrimerDesigntool(http://www.genomics.agilent.com) and were absence and presence of a model antigenic peptide and solved the HPLC purified. The sequences of primers were (59-39) as follows: A609T structures to 3.4A˚ and 3.3A˚ , respectively. The peptide model was FW,GAAATGAGAACCATACTACACCCATCACCGAAGC; A609T REV, built on electron density extending from the catalytic site of the GCTTCGGTGATGGGTGTAGTATGGTTCTCATTTC; I166M FW, GTT- TCCATGGGCACAGATGAGGCTTCCCACTG; and I166M REV, CAGT- enzyme, along the central cavity toward the base of domain IV. The GGGAAGCCTCATCTGTGCCCATGGAAAC. overall conformation of IRAP, compared with the known structures of the homologous ERAP1, represents a semiclosed state, which Stable cell line generation and protein expression in forms an internal cavity with limited external solvent access. The mammalian cells peptide N terminus is anchored at the catalytic site, and its Stable cell lines were created with the help of construct pURD_hsIRAP, C-terminal half is lodged between domains II and IV, primarily because pURD is a stable cell line generation vector (23). Briefly, HEK 293S through hydrophobic interactions and space complementarity. GnTI(2) cells (23, 24) were cotransfected with pURD_hsPLPAP and Structural and biochemical analysis provide insight on the substrate a PhiC31 integrase expression vector (pgk-phic31). The polyclonal cell specificity of IRAP and on the mechanism by which it can process population resulting from puromycin (2 mg/ml) selection was used for the protein production. The secreted protein has additional amino acids from the a very large variety of peptide sequences, but also highlight signif- cloning vector, as follows: N-terminal ETG and C-terminal RTETSQVAPA icant mechanistic differences from ERAP1, a finding that may un- sequences (last nine residues are Rhodopsin 1D4 tag). For protein purifica- derlie differences in Ag processing in different cells. Structural tion, the conditioned media were passed through the anti-1D4 tag Ab (Uni- mapping and functional analysis