Genes for calcineurin B-like proteins in Arabidopsis are differentially regulated by stress signals

An important effector of Ca2+ signaling in animals and yeast is the Ca2+/ca lmodulin dependent protein phosphatase calcineurin. However, the biochemica l identity of plant calcineurin remained elusive. Here we report the molecu lar characterization of AtCBL (Arabidopsis thaliana calcineurin B-like prot ein) from Arabidopsis. The protein is most similar to mammalian calcineurin B, the regulatory subunit of the phosphatase, AtCBL also shows significant similarity with another Ca2+-binding protein, the neuronal calcium sensor in animals. It contains typical EF-hand motifs with Ca2+-binding capability , as confirmed by in vitro Ca2+-binding assays, and it interacts in vivo wi th rat calcineurin A in the yeast two-hybrid system. Interaction of AtCBL1 and rat calcineurin A complemented the salt-sensitive phenotype in a yeast calcineurin B mutant. Cloning of cDNAs revealed that AtCBL proteins are enc oded by a family of at least six genes in Arabidopsis, Genes for three isof orms were identified in this study. AtCBL1 mRNA was preferentially expresse d in stems and roots and its mRNA levels strongly increased in response to specific stress signals such as drought, cold, and wounding. In contrast, A tCBL2 and AtCBL3 are constitutively expressed under all conditions investig ated, Our data suggest that AtCBL1 may act as a regulatory subunit of a pla nt calcineurin-like activity mediating calcium signaling under certain stre ss conditions.