Fig. 1: Structure and catalytic activity of MERS-CoV ExoN complex.

a Domain organization of MERS-CoV nsp10 and nsp14. Domain boundaries are shown by residue number. Active site residues are indicated as red dots and labeled. b Sequence and numbering of the T20P15 RNA used in exoribonuclease assay and cryo-EM analysis. T-RNA and P-RNA regions are colored in red and blue, respectively, and connected by a UUCG tetraloop. c Exonucleolytic digestion of T20P15 RNA by MERS-CoV ExoN WT, MERS-CoV ExoN E191A mutant, or SARS-CoV-2 ExoN WT. Reactions were stopped at indicated time points, and RNA products were resolved by denaturing polyacrylamide gel electrophoresis (PAGE) and stained by SYBR-Gold. A representative result from three biological replicates is shown. d Cryo-EM map and atomic model of MERS-CoV ExoN•T20P15 RNA complex. e Overview of the interaction between MERS-CoV ExoN and T20P15 RNA. Nucleotide residues in T-RNA and P-RNA are indicated with a subscript “T” or “P”, respectively. Hydrogen bonds are shown as gray dashed lines. Nucleotides bound in the shallow RNA-binding pocket of ExoN and RNA backbone-interacting amino acid residues of ExoN are superimposed with their cryo-EM densities contoured at 7σ. f Active site conformation of the MERS-CoV ExoN•T20P15 RNA complex. Mg²⁺ ions, green spheres. +1C_P, −1C_P, Mg²⁺ ion, and two active site residues are superimposed with their cryo-EM densities contoured at 7σ. Source data are provided as a Source Data file.