Fig. S3.

Crystal structure of MERS-CoV S1 NTD. (A) The structure of the MERS-CoV S1 NTD can be separated into top, core and bottom regions (86). The two β-sheets of the NTD core are colored green and magenta, and the rest of the NTD is colored cyan. (B) The NTD structure is colored as a rainbow from blue to red, N to C terminus, respectively. (C) Extracted ion chromatogram (extracted theoretical mass 442.15 at 5 ppm), MS, and MS/MS spectra of ligand isolated from purified MERS-CoV S NTD. Concentrated NTD was heated at 98 °C for 10 min, and the denatured protein was pelleted via centrifugation. The supernatant was desalted and analyzed as previously described (87) on a Q Exactive Plus mass spectrometer with the modification that +1 ions were also selected for fragmentation. Data analysis was performed in QualBrowser. (D) Extracted ion chromatogram (extracted theoretical mass 442.15 at 5 ppm), MS, and MS/MS spectra of purified folic acid (Sigma) resuspended in 5% methanol and 1% formic acid and analyzed as described above. (E) The chemical structure of folic acid. (F) Crystal structure of MERS-CoV S NTD shown as a ribbon with the Fo-Fc map before placement and refinement of folic acid shown at 2.0 sigma (gray mesh). Folic acid (shown in sticks) was modeled into this density and used for further structural refinement. Oxygen atoms are red and nitrogen atoms are blue.