FIGURE 2.

Three-dimensional structure of the AHK5_RD-AHP1 complex from Arabidopsis thaliana (PDB 4EUK), consisting of the histidine-containing phosphotransfer (AHP1, green) and kinase (AHK5_RD, yellow) region, is shown in two orthogonal orientations. The structure of the complex in panel A is rotated by approximately 90 degrees to produce a view shown in panel B. The mechanism of intermolecular phosphotransfer mediated by the Arabidopsis AHK5_RD-AHP1 complex. Maize ZmHP2 (PDB 1WN0, smudge green), Medicago truncatula MtHPT1 (PDB 3US6, limon green) and rice OsHPT (PDB 1YVI, forest green) are superposed over the Arabidopsis AHP1. The His in AHP1 and Asp in AHK5_RD residues that respectively donate and accept a phosphoryl group are shown in sticks in atomic green and yellow colors, respectively. The octahedral coordination geometry of Mg²⁺ (green sphere) participating in the phosphotransfer reaction is indicated by black dashes (atomic distances between 1.9 Å and 2.0 Å), where Mg²⁺ is coordinated by Asp from AHK5_RD, three water molecules (red spheres) and two other residues (Asp and Cys) of AHK5_RD. The distance of 3.4 Å between His from AHP1 and one of the water molecules is also shown.