The ACA4 gene of arabidopsis encodes a vacuolar membrane calcium pump thatimproves salt tolerance in yeast

Several lines of evidence suggest that regulation of intracellular Ca2+ lev els is crucial for adaptation of plants to environmental stress. We have cl oned and characterized Arabidopsis auto-inhibited Ca2+-ATPase, isoform 4 (A CA4), a calmodulin-regulated Ca2+-ATPase. Confocal laser scanning data of a green fluorescent protein-tagged version of ACA4 as well as western-blot a nalysis of microsomal fractions obtained from two-phase partitioning and Su c density gradient centrifugation suggest that ACA4 is localized to small v acuoles. The N terminus of ACA4 contains an auto-inhibitory domain with a b inding site for calmodulin as demonstrated through calmodulin-binding studi es and complementation experiments using the calcium transport yeast mutant K616. ACA4 and PMC1, the yeast vacuolar Ca2+-ATPase, conferred protection against osmotic stress such as high NaCl, KCl, and mannitol when expressed in the K616 strain. An N-terminally modified form of ACA4 specifically conf erred increased NaCl tolerance, whereas full-length ATPase had less effect.