Revisiting paradigms of Ca2+ signaling protein kinase regulation in plants

Calcium (Ca 2+ ) serves as a universal second messenger in eukaryotic signal transduction. Understanding the Ca 2+ activation kinetics of Ca 2+ sensors is critical to understanding the cellular signaling mechanisms involved. In this review, we discuss the regulatory properties of two sensor classes: the Ca 2+ -dependent protein kinases (CPKs/CDPKs) and the calcineurin B-like (CBL) proteins that control the activity of CBL-interacting protein kinases (CIPKs) and identify emerging topics and some foundational points that are not well established experimentally. Most plant CPKs are activated by physiologically relevant Ca 2+ concentrations except for those with degenerate EF hands, and new results suggest that the Ca 2+ -dependence of kinase activation may be modulated by both protein–protein interactions and CPK autophosphorylation. Early results indicated that activation of plant CPKs by Ca 2+ occurred by relief of autoinhibition. However, recent studies of protist CDPKs suggest that intramolecular interactions between CDPK domains contribute allosteric control to CDPK activation. Further studies are required to elucidate the mechanisms regulating plant CPKs. With CBL–CIPKs, the two major activation mechanisms are thought to be (i) binding of Ca 2+ -bound CBL to the CIPK and (ii) phosphorylation of residues in the CIPK activation loop. However, the relative importance of these two mechanisms in regulating CIPK activity is unclear. Furthermore, information detailing activation by physiologically relevant [Ca 2+ ] is lacking, such that the paradigm of CBLs as Ca 2+ sensors still requires critical, experimental validation. Developing models of CPK and CIPK regulation is essential to understand how these kinases mediate Ca 2+ signaling and to the design of experiments to test function in vivo .