Molecular Characterization of a Novel Armadillo Repeat-Like Protein Gene Differentially Induced by High-Salt Stress and Dehydration from the Model Legume Lotus Japonicus

Armadillo (ARM) repeat proteins have tandem repeats of a degenerate sequence motif required for protein-protein interaction and are distributed widely in eukaryotes. In this study, we isolated and characterized a novel ARM repeat-like protein gene from the model legume Lotus japonicus that is differentially induced by abiotic stress. The gene, LjTDF-5, encodes a hypothetical protein of 369 aa with a protein signature "ARM-type fold". Three-dimensional protein structure was predicted to consist of 23 alpha-helices and no beta-sheets by homology modeling. The LjTDF-5-homologous genes were distributed broadly in the plant kingdom and the C-terminal region, around 60 amino acids in length, was highly conserved in all of the homologs examined although any known functional domains or protein signatures in this region were not detected in silico analyses. Subcellular localization assays revealed that the sGFP-fused LjTDF-5 protein localized to the nuclei of onion epidermis cells, despite the protein not containing a typical nuclear localization signal. In quantitative real-time RT-PCR, the expression of LjTDF-5 was highly induced by 100 mM NaCl in the roots and by dehydration in the shoot, but not by abscisic acid (ABA, 10 mu M). These results suggest that the ARM repeat-like protein LjTDF-5 functions in or around the nucleus in response to high-salt stress and dehydration in L. japonicus.