MOLECULAR-CLONING OF A NOVEL CA2-BINDING PROTEIN THAT IS INDUCED BY NACL STRESS()

Plant responses to high salt stress have been studied for several deca des. However, the molecular mechanisms underlying these responses stil l elude us. In order to understand better the molecular mechanism rela ted to NaCl stress in plants, we initiated the cloning of a large numb er of NaCl-induced genes in Arabidopsis. Here, we report the cloning o f a cDNA encoding a novel Ca2+-binding protein, named AtCP1, which sha res sequence similarities with calmodulins. AtCP1 exhibits, in particu lar, a high degree of amino acid sequence homology to the Ca2+-binding loops of the EF hands of calmodulin. However, unlike calmodulin, AtCP 1 appears to have only three Ca2+-binding loops. We examined Ca2+ bind ing of the protein by a Ca2+-dependent electrophoretic mobility shift assay. A recombinant AtCP1 protein that was expressed in Escherichia c oli did show a Ca2+-dependent electrophoretic mobility shift. To gain insight into the expression of the AtCP1 gene, northern blot analysis was carried out. The AtCP1 gene had a tissue-specific expression patte rn: high levels of expression In flower and root tissues and nearly un detectable levels in leaves and siliques. Also, the expression of the AtCP1 gene was induced by NaCl treatment but not by ABA treatment. Fin ally, subcellular localization experiments using an AtCP1:smGFP fusion gene in soybean suspension culture cells and tobacco leaf protoplasts indicate that AtCP1 is most likely a cytosolic protein.