Calcium/Calmodulin-Dependent Protein Kinase Is Negatively and Positively Regulated by Calcium, Providing a Mechanism for Decoding Calcium Responses during Symbiosis Signaling  

Abstract

The establishment of symbiotic associations in plants requires calcium oscillations that must be decoded to invoke downstream developmental programs. In animal systems, comparable calcium oscillations are decoded by calmodulin (CaM)–dependent protein kinases, but symbiotic signaling involves a calcium/CaM–dependent protein kinase (CCaMK) that is unique to plants. CCaMK differs from the animal CaM kinases by its dual ability to bind free calcium, via calcium binding EF-hand domains on the protein, or to bind calcium complexed with CaM, via a CaM binding domain. In this study, we dissect this dual regulation of CCaMK by calcium. We find that calcium binding to the EF-hand domains promotes autophosphorylation, which negatively regulates CCaMK by stabilizing the inactive state of the protein. By contrast, calcium-dependent CaM binding overrides the effects of autophosphorylation and activates the protein. The differential calcium binding affinities of the EF-hand domains compared with those of CaM suggest that CCaMK is maintained in the inactive state at basal calcium concentrations and is activated via CaM binding during calcium oscillations. This work provides a model for decoding calcium oscillations that uses differential calcium binding affinities to create a robust molecular switch that is responsive to calcium concentrations associated with both the basal state and with oscillations.

A Medicago truncatula Cystathionine-β-Synthase-like Domain-Containing Protein Is Required for Rhizobial Infection and Symbiotic Nitrogen Fixation

“…CCaMK is required for the formation of ITs and nodules, and constitutively active variants of CCaMK can initiate spontaneous nodule organogenesis Lévy et al, 2004;Gleason et al, 2006;Tirichine et al, 2006;Miller et al, 2013). In addition to Interacting Protein of DMI3/Cyclops, acting downstream of CCaMK are two transcription factors NSP1 and NSP2, and VAPYRIN, an ankyrin domain-containing protein of unknown function (Kaló et al, 2005;Smit et al, 2005;Messinese et al, 2007;Yano et al, 2008;Oldroyd et al, 2011).…”

The EF-Hand Ca2+ Binding Protein MICU Choreographs Mitochondrial Ca2+ Dynamics in Arabidopsis

“…Rapid adjustment is particularly important for the endosymbiotic organelles, i.e., the mitochondria and the chloroplasts, since perturbation of the finely tuned metabolic, redox, and chemiosmotic processes can severely affect cellular homeostasis or even trigger programmed cell death (Kim et al, 2006;Logan, 2007). Dynamic changes in free Ca 2+ concentrations have been recognized to act in the transduction within the cell of both endogenous and external signals in order to regulate biotic and abiotic stress responses (Knight et al, 1991;Kiegle et al, 2000;Whalley et al, 2011), developmental programs (Foreman et al, 2003;Monshausen et al, 2008), stomatal dynamics (Gilroy et al, 1991;Allen et al, 2000;Pei et al, 2000;Kim et al, 2010), protein import into cell compartments (Chigri et al, 2006), and symbiotic plant-microbe interactions (Wais et al, 2000;Miller et al, 2013).…”

Phosphorylation of a NAC Transcription Factor by a Calcium/Calmodulin-Dependent Protein Kinase Regulates Abscisic Acid-Induced Antioxidant Defense in Maize

“…CCaMK is a central regulator of nodule organogenesis, rhizobia infection, and arbuscular mycorrhizal signaling (Gobbato, 2015;Miller et al, 2013;Shimoda et al, 2012). Recently, a growing body of evidence reveals additional functions of CCaMK, such as response to ABA signaling and stress tolerance in wheat, maize, and rice Shi et al, 2012;Yang et al, 2011a).…”

“…This model includes a two-step activation of CCaMK by Ca 2+ (Miller et al, 2013;Poovaiah et al, 2013). CCaMK is a central decoder of Ca 2+ -spiking in the common symbiosis pathway for regulating root nodules and arbuscular mycorrhizae (Gobbato, 2015;Miller et al, 2013;Shimoda et al, 2012). However, CCaMK is highly expressed outside of roots, indicating other roles besides the regulation of symbiosis.…”

“…A model for the activation of CCaMK has been proposed based on work with CCaMK from lily (Sathyanarayanan et al, 2000): Ca 2+ binding to the EF-hands of CCaMK promotes autophosphorylation, which in turn promotes CaM binding, relieving autoinhibition of the kinase and thus promoting substrate phosphorylation. This model includes a two-step activation of CCaMK by Ca 2+ (Miller et al, 2013;Poovaiah et al, 2013). CCaMK is a central decoder of Ca 2+ -spiking in the common symbiosis pathway for regulating root nodules and arbuscular mycorrhizae (Gobbato, 2015;Miller et al, 2013;Shimoda et al, 2012).…”