FIGURE 6.
c-di-AMP/PstASA hydrogen bonding network. A, c-di-AMP-binding site with amino acid residues from monomer A shown in green and monomer B shown in yellow. Residue Phe-36 forms a face-to-face π-stacking with adenine, and the other residues Leu-37, Asn-41 (monomer B), Thr-28, Gly-47, and Gln-108 (monomer A) form hydrogen bonds with different parts of the c-di-AMP ligand. Additional residues shown in the structure are Phe-99 (monomer B) and Arg-26 (monomer A), forming an edge-to-face π-stacking and a cation-π interaction, respectively. Hydrogen bond interactions are indicated with dotted lines and π interactions with double-head arrows. B, schematic representation of the hydrogen bond interactions between c-di-AMP-binding site residues. Residues from monomer A are shown in green and from monomer B in yellow. The water molecules (indicated as red W) forming hydrogen bonds are also shown in this schematic diagram. The details of these interactions are described under “Results,” and adenines are labeled Aα and Aβ, and PO4 groups 1 and 2, and will be referred to as such in the text.