Novel protein kinases associated with calcineurin B-like calcium sensors in Arabidopsis

Members of the Arabidopsis calcineurin B-like Ca2+ binding protein (AtCBL) family are differentially regulated by stress conditions. One AtCBL plays a role in salt stress; another is implicated in response to other stress sig nals, including drought, cold, and wounding. In this study, we identified a group of novel protein kinases specifically associated with AtCBL-type Ca2 + sensors. In addition to a typical protein kinase domain, they all contain a unique C-terminal region that is both required and sufficient for intera ction with the AtCBL-type but not calmodulin-type Ca2+ binding proteins fro m plants. Interactions between the kinases and AtCBLs require micromolar co ncentrations of Ca2+, suggesting that increases in cellular Ca2+ concentrat ions may trigger the formation of AtCBL-kinase complexes in vivo. Unlike mo st serine/threonine kinases, the AtCBL-interacting kinase efficiently uses Mn2+ to Mg2+ as a cofactor and may function as a Mn2+ binding protein in th e cell. These findings link a new type of Ca2+ sensors to a group of novel protein kinases, providing the molecular basis for a unique Ca2+ signaling machinery in plant cells.