Fig. 3.

Thiosemicarbazide inhibits MERS-CoV infection by blocking the interaction between RBD and hDPP4. a) Schematic presentation of immunofluorescence-based MERS-CoV infection assay. b) Antiviral potency of compound 1 and 1g-8 against MERS-CoV infection. The cells were treated with individual compounds at the time of MERS-CoV infection at an MOI of 0.0625. The cells were further incubated for 24 h followed by immunofluorescence imaging. MERS-CoV infection was determined using a rabbit anti-MERS-CoV spike antibody (green), and cell viability was measured with Hoechst 33342 (red). Concentrations of 3.125 μM and 50 μM were respectively selected as the low and high doses for each compound. Chloroquine (CHQ) at a concentration of 10 μM was included as a control. c-left) Docking models for compounds 1 and 1j bound to the MERS-CoV receptor binding domain (RBD) (PDB: 4L3N). The molecular surface of MERS-CoV RBD is presented in gray while compound 1 is marked in magenta. c-right) Superimposition of the RBD docked with 1 (the model in left) on the X-ray structure of RBD complexed with hDPP4 (PDB: 4L72). The gray molecular surface is MERS-CoV RBD (PDB id: 4L3N, 4L72) and the green ribbon is hDPP4. d-e) Comparison of docked configurations of 1 (d) and 1j (e) to the binding site in the RBD. Key interactions between ligand and RBD are presented by dashed lines: orange is a hydrogen bond, cyan is a pi-pi interaction, and green is a pi-cation interaction. Ligands are colored by atom types, with carbon as magenta in 1, cyan in 1j, and silver in RBD; nitrogen is blue; oxygen is red; and fluorine is green.