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ARS Home » Midwest Area » Urbana, Illinois » Global Change and Photosynthesis Research » Research » Publications at this Location » Publication #344801
Title: Revisiting paradigms of calcium signaling protein kinase regulation in plants

Author
item BENDER, KYLE - UNIVERSITY OF ILLINOIS
item ZIELINSKI, RAYMOND - UNIVERSITY OF ILLINOIS
item Huber, Steven

Submitted to: Biochemical Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2017
Publication Date: 1/5/2018
Citation: Bender, K.W., Zielinski, R.E., Huber, S.C. 2018. Revisiting paradigms of calcium signaling protein kinase regulation in plants. Biochemical Journal. 475(1):202-223.

Interpretive Summary: Calcium signaling is a universal mechanism in eukaryotes for the control of many pathways and processes. In response to a stimulus or stress, intracellular calcium concentration increases 10 to 20-fold from a low resting level and with oscillations that are referred to as calcium signatures that elicit specific cellular responses. Critical to the initiation of the response are calcium sensor proteins that bind calcium at physiologically relevant concentrations and transmit that signal. Two families of protein kinases are thought to play a prominent role in calcium-mediated signal transduction: the calcium-dependent protein kinases (CPKs) and calcineurin B-like protein-interacting protein kinases (CIPKs). With CPKs, the mechanism of activation by calcium may be more complex than generally assumed and if so, the commonly used experimental approach that involves the use of truncated CPK proteins would yield results that cannot be interpreted unambiguously. With CIPKs, some of the underlying assumptions, in particular about the role of calcium, may be more limited than generally recognized and therefore additional experimental verification is recommended.

Technical Abstract: Calcium (Ca2+) serves as a universal second messenger in eukaryotic signal transduction. Understanding the Ca2+ activation kinetics of Ca2+-sensors – proteins that relay the initial Ca2+ signal to downstream targets – is critical to understanding the signalling mechanisms. In plants, the repertoire of Ca2+-sensors is greatly expanded compared to other eukaryotes, and in the present review we focus on the regulatory properties of two classes: the Ca2+-dependent protein kinases (CPKs) and Calcineurin B-Like (CBL) protein-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 Ca2+ concentrations except for those with degenerate EF-hands, and new results suggest that the Ca2+ -dependence of kinase activation may be modulated by both protein-protein interactions and CPK autophosphorylation. Early results suggested that the mechanism of activation of plant CPKs by Ca2+ involved relief of auto-inhibition but recent structural analysis of protist CPKs suggests an allosteric Ca2+-activation mechanism. Further study is required to determine whether allosteric activation may also play a role with plant CPKs. With CBL-CIPKs, the two major mechanisms of activation are thought to be i) binding of Ca2+/CBL and ii) phosphorylation of residues in the CIPK activation loop. However, the relative importance of these two mechanisms is unclear and there is a dearth of information about the activation by physiologically relevant [Ca2+] such that the paradigm of CBLs as Ca2+ -sensors requires critical, experimental validation. Developing detailed models of CPK and CIPK regulation is essential to understand how these kinases function in relation to Ca2+- signalling.