To gain structural insight into SsTroA function, we determined the crystal structure of Zn-bound SsTroA. The X-ray crystal structure was solved by molecular replacement using TpTroA as the search model (PDB code: 1K0F) [22] . The final structure was refined to a resolution of 2.6 Å and consists of 278 amino acids (residues 40-317). Phasing and refinement statistics are reported in Table S3 . The overall structure of SsTroA is typical of SBPs in the A-1 family [19] . The peptide chain crosses between the N-terminal and C-terminal domains only once linking the two (b/a) 4 domains via a 33-residue kinked rigid a-helical backbone that runs the full length of the molecule (Fig. 5A ) and the metal binding site is buried in the interdomain interface. The N-terminal domain (residues 40-164) is a b/a sandwich-like domain, consisting of a four-stranded b-sheet with 2-1-3-4 linking topology, and with an a-helix connecting each strand. The C-terminal domain (residues 196-317) is also a b/a sandwich composed of a four stranded parallel b-sheet with 2-1-3-4 linking topology, with both sheets sandwiched between layers of a-helices. The two independent domains interact with each other across a large interface that creates the binding pocket for the metal ligand.