Structure of ssA2. (A) Two views of the overall structure of ssA2. The A2 domain forms a compact structure, with the Tyr1605-Met1606 cleavage site being buried. Residues Tyr1605 and Met1606 are shown as ball-and-stick models and are colored in blue. The region of ssA2 N-terminal to the cleavage site is colored in green and that C-terminal to the site in salmon. The bound Ca²⁺ ion is shown as a golden sphere. The secondary structure elements of ssA2 are labeled. (B) Structural comparison of ssA2 (cyan) and wtA2 (pink; PDB code 3GXB). The major structural difference occurs in the α3-β4 region. The α3-β4 region is labeled and enclosed in a circle. (C) The Ca²⁺-binding site of ssA2. The surrounding residues are shown as ball-and-stick models. The coordination bonds between Ca²⁺ and the ligands are indicated with dashed lines and the bond lengths (Å) are marked. (D-G) The equivalent sites in wtA2 and the ssA2 mutants. The wtA2 protein (D) is colored in magenta, and those of the D1596A (E), N1602A (F), and D1596A/N1602A (G) mutants of ssA2 are colored in white, blue, and orange, respectively. In each of these structures, a molecule (wheat) is bound at this site, although in a geometry different from that of ssA2.