Novel Protein Kinases Associated with Calcineurin B–like Calcium Sensors in Arabidopsis

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Abstract
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 signals, 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 interaction with the AtCBL-type but not calmodulin-type Ca2+ binding proteins from plants. Interactions between the kinases and AtCBLs require micromolar concentrations of Ca2+, suggesting that increases in cellular Ca2+ concentrations may trigger the formation of AtCBL–kinase complexes in vivo. Unlike most serine/threonine kinases, the AtCBL-interacting kinase efficiently uses Mn2+ to Mg2+ as a cofactor and may function as a Mn2+ binding protein in the 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.