Nucleoside diphosphate kinase (NDPK) is a ubiquitous enzyme that catalyzes the transfer of the γ-phosphoryl group from a nucleoside triphosphate to a nucleoside diphosphate. In this study, we examined the subcellular localization, tissue-specific gene expression, and enzymatic characteristics of three rice NDPK isozymes (OsNDPK1-OsNDPK3). Sequence comparison of the three OsNDPKs suggested differential subcellular localization. Transient expression of green fluorescence protein-fused proteins in onion cells indicated that OsNDPK2 and OsNDPK3 are localized to plastid and mitochondria respectively, while OsNDPK1 is localized to the cytosol. Expression analysis indicated that all the OsNDPKs are expressed in the leaf, leaf sheath, and immature seeds, except for OsNDPK1, in the leaf sheath. Recombinant OsNDPK2 and OsNDPK3 showed lower optimum pH and higher stability under acidic pH than OsNDPK1. In ATP formation, all the OsNDPKs displayed lower K(m) values for the second substrate, ADP, than for the first substrate, NTP, and showed lowest and highest K(m) values for GTP and CTP respectively.
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