FIGURE 2.

AtTTM3 is a short-chain inorganic polyphosphatase. A, ribbon diagrams of the yeast inorganic polyphosphate polymerase Vtc4p (left panel) and Arabidopsis TTM3 (right panel). Tunnel domain β-strands are shown in yellow, and the α-helices are in blue. The TTM domain is topologically closed on one side by a C-terminal plug helix. Conserved residues in the tunnel center are shown (in bond representation, in yellow) alongside with the triphosphate substrates and Mn²⁺ ions (magenta spheres). B, DAPI stained UREA page gel reveals that only Vtc4p can generate inorganic polyphosphate from ATP. C, substrate specificity of AtTTM3. P_i release is measured for a range of different di- and triphosphate containing potential substrates. D, a one-dimensional ³¹P NMR time-course experiment reveals that AtTTM3 specifically generates PP_i and P_i from PPP_i. t = 0 min (black line), 94 min (red line), and 141 min (blue line). E, decay of the PPP_i substrate concentration during the NMR time-course experiment indicates a turnover number of ∼10/s.