Structure and Molecular Characterization of Diadenosine Polyphosphate Hydrolase in Brachypodium distachyon

Putative diadenosine polyphosphate (ApnA) hydrolase gene, which encodes an amino acid sequence showing homology with that of Arabidopsis long-chain (ApnA) (n=5–6) hydrolase (AtNUDX13) and which conserves nudix motif and glycine tripeptide motif, was identified from Brachypodium distachyon. The mature form of Brachypodium (ApnA) hydrolase (BraNUDX15) catalyzed long-chain (ApnA) and Ap4A, Ap4G, Gp4G, and dCTP, showing different substrate specificity from Arabidopsis (ApnA) hydrolases AtNUDX13, 25, 26, and 27. BraNUDX15 required Mg2+ and produced ATP from (ApnA), indicating asymmetrical (ApnA) hydrolase as the same as Arabidopsis (ApnA) hydrolases. Results show that BraNUDX15 gene was up-regulated by UV-A, UV-B, and UV-C irradiation and downregulated by drought stress, but it was left unchanged by salt stress. Subcellular localization indicated that the BraNUDX15 protein was colocalized with the surface of the chloroplasts. These results suggest BraNUDX15 as a unique (ApnA) hydrolase with different substrate specificity from those of Arabidopsis (ApnA) hydrolases. It might play a role in regulating (ApnA) levels in chloroplasts under conditions of drought stress and UV irradiation.