Single-Chain Expression and Crystallization of an Antigenic C-Terminus in Complex with the Regulatory Domain of ER Aminopeptidase 1

Human endoplasmic reticulum aminopeptidase 1 (ERAP1) is one of two ER luminal aminopeptidases that participate in the final processing of peptide precursors and generates the N-termini of the MHC class I-restricted epitopes. In order to investigate the interactions of its binding site with substrate peptides, X-ray crystallographic analyses have been carried out to study structures of ERAP1 regulatory (ERAP1_R) domain in complex with antigenic peptides. Single-chain bimodular constructs with various antigenic peptides linked to the C-terminal end of ERAP1_R domain are designed to facilitate crystallization process of these complexes. These recombinant proteins have been purified and crystalized, and x-ray diffraction data of one crystal have been processed to a resolution of 2.8 . The crystal belongs to the space group P21, with unit cell parameters a =64.2, b = 66.8, c = 66.3 , β = 110.2°. A Refmac-refined omit map reveals a clear density for the antigenic peptide’s carboxylate-end that is in contact with the ERAP1 regulatory domain of neighboring molecule. Thus the single-chain bimodular constructs have provided an expedited approach to study sequence-specific interactions between the ERAP1 regulatory domain and antigen peptide’s C-terminal ends.