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

Abstract

An important effector of Ca²⁺ signaling in animals and yeast is the Ca²⁺/calmodulin-dependent protein phosphatase calcineurin. However, the biochemical identity of plant calcineurin remained elusive. Here we report the molecular characterization of AtCBL (Arabidopsisthalianacalcineurin B-like protein) 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 with 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 encoded by a family of at least six genes in Arabidopsis. Genes for three isoforms were identified in this study. AtCBL1 mRNA was preferentially expressed in stems and roots and its mRNA levels strongly increased in response to specific stress signals such as drought, cold, and wounding. In contrast, AtCBL2 and AtCBL3 are constitutively expressed under all conditions investigated. Our data suggest that AtCBL1 may act as a regulatory subunit of a plant calcineurin-like activity mediating calcium signaling under certain stress conditions.