Identification of a novel nuclear-localized adenylate kinase 6 from Arabidopsis thaliana as an essential stem growth factor

Adenylate kinase (AK; EC 2.7.4.3) is highly conserved across a wide range of organisms, including Saccharomyces cerevisiae, Caenorhabditis elegans, Drosophila melanogaster, Arabidopsis thaliana, and Homo sapiens. While AK6 orthologs play important roles in the growth of yeast and worms, the physiological function of AK6 in A. thaliana is still unknown. In this study, we first cloned and expressed Arabidopsis adenylate kinase 6 (AAK6). Enzyme assays revealed that AAK6 has characteristic adenylate kinase properties, with ATP as the preferred phosphate donor and AMP as the best phosphate acceptor. A subcellular localization assay demonstrated that AAK6 had a predominant nuclear localization. Through biochemical purification and mass spectrometry analysis, a putative homolog of the S. cerevisiae Rps14 protein was identified as a partner of AAK6. Most importantly, we observed that aak6 T-DNA insertion mutants had decreased stem growth compared with wild-type plants. These data indicate that AAK6 exhibits adenylate kinase activity and is an essential growth factor in Arabidopsis.