Moonlighting Proteins Shine New Light on Molecular Signaling Niches

Transcriptomics of temperature-sensitive R gene-mediated resistance identifies a WAKL10 protein interaction network

Understanding temperature-sensitivity of R gene-mediated resistance against apoplastic pathogens is important for sustainable food production in the face of global warming. Here, we show that resistance of Brassica napus cotyledons against Leptosphaeria maculans was temperature-sensitive in introgression line Topas-Rlm7 but temperature-resilient in Topas-Rlm4. A set of 1,646 host genes was differentially expressed in Topas-Rlm4 and Topas-Rlm7 in response to temperature. Amongst these were three WAKL10 genes, including BnaA07g20220D, representing the temperature-sensitive Rlm7-1 allele and Rlm4. Network analysis identified a WAKL10 protein interaction cluster specifically for Topas-Rlm7 at 25 °C. Diffusion analysis of the Topas-Rlm4 network identified WRKY22 as a putative regulatory target of the ESCRT-III complex-associated protein VPS60.1, which belongs to the WAKL10 protein interaction community. Combined enrichment analysis of gene ontology terms considering gene expression and network data linked vesicle-mediated transport to defence. Thus, dysregulation of effector-triggered defence in Topas-Rlm7 disrupts vesicle-associated resistance against the apoplastic pathogen L. maculans.

Maternal Embryo Effect Arrest 31 (MEE31) is a moonlighting protein involved in GDP-D-mannose biosynthesis and KAT1 potassium channel regulation

Due to anthropogenic global warming, droughts are expected to increase and water availability to decrease in the coming decades. For this reason, research is increasingly focused on developing plant varieties and crop cultivars with reduced water consumption. Transpiration occurs through stomatal pores, resulting in water loss. Potassium plays a significant role in stomatal regulation. KAT1 is an inward-rectifying potassium channel that contributes to stomatal opening. Using a yeast high-throughput screening of an Arabidopsis cDNA library, MEE31 was found to physically interact with KAT1. MEE31 was initially identified in a screen for mutants with delayed embryonic development. The gene encodes a conserved phosphomannose isomerase (PMI). We report here that MEE31 interacts with and increases KAT1 activity in yeast and this interaction was also confirmed in plants. In addition, MEE31 complements the function of the yeast homologue, whereas the truncated version recovered in the screening does not, thus uncoupling the enzymatic activity from KAT1 regulation. We show that MEE31 overexpression leads to increased stomatal opening in Arabidopsis transgenic lines. Our data suggest that MEE31 is a moonlighting protein involved in both GDP-D-mannose biosynthesis and KAT1 regulation.

The discovery of hidden guanylate cyclases (GCs) in the Homo sapiens proteome

Recent discoveries have established functional guanylate cyclase (GC) catalytic centers with low activity within kinase domains in plants. These crypto GCs generate guanosine 3',5'-cyclic monophosphate (cGMP) essential for both intramolecular and downstream signaling. Here, we have set out to search for such crypto GCs moonlighting in kinases in the H. sapiens proteome and identified 18 candidates, including the neurotropic receptor tyrosine kinase 1 (NTRK1). NTRK1 shows a domain architecture much like plant receptor kinases such as the phytosulfokine receptor, where a functional GC essential for downstream signaling is embedded within a kinase domain. In vitro characterization of the NTRK1 shows that the embedded NTRK1 GC is functional with a marked preference for Mn2+ over Mg2+. This therefore points to hitherto unsuspected roles of cGMP in intramolecular and downstream signaling of NTRK1 and the role of cGMP in NTRK1-dependent growth and neoplasia.

Plant adenylate cyclases have come full circle

In bacteria, fungi and animals, 3'-5'-cyclic adenosine monophosphate (cAMP) and adenylate cyclases (ACs), enzymes that catalyse the formation of 3',5'-cAMP from ATP, are recognized as key signalling components. In contrast, the presence of cAMP and its biological roles in higher plants have long been a matter of controversy due to the generally lower amounts in plant tissues compared with that in animal and bacterial cells, and a lack of clarity on the molecular nature of the generating and degrading enzymes, as well as downstream effectors. While treatment with 3',5'-cAMP elicited many plant responses, ACs were, however, somewhat elusive. This changed when systematic searches with amino acid motifs deduced from the conserved catalytic centres of annotated ACs from animals and bacteria identified candidate proteins in higher plants that were subsequently shown to have AC activities in vitro and in vivo. The identification of active ACs moonlighting within complex multifunctional proteins is consistent with their roles as molecular tuners and regulators of cellular and physiological functions. Furthermore, the increasing number of ACs identified as part of proteins with different domain architectures suggests that there are many more hidden ACs in plant proteomes and they may affect a multitude of mechanisms and processes at the molecular and systems levels.

Moonlight-like proteins are actually cell wall components in Pseudocercospora fijiensis

The fungal cell wall protects fungi against threats, both biotic and abiotic, and plays a role in pathogenicity by facilitating host adhesion, among other functions. Although carbohydrates (e.g. glucans, chitin) are the most abundant components, the fungal cell wall also harbors ionic proteins, proteins bound by disulfide bridges, alkali-extractable, SDS-extractable, and GPI-anchored proteins, among others; the latter consisting of suitable targets which can be used for fungal pathogen control. Pseudocercospora fijiensis is the causal agent of black Sigatoka disease, the principal threat to banana and plantain worldwide. Here, we report the isolation of the cell wall of this pathogen, followed by extensive washing to eliminate all loosely associated proteins and conserve those integrated to its cell wall. In the HF-pyridine protein fraction, one of the most abundant protein bands was recovered from SDS-PAGE gels, electro-eluted and sequenced. Seven proteins were identified from this band, none of which were GPI-anchored proteins. Instead, atypical (moonlight-like) cell wall proteins were identified, suggesting a new class of atypical proteins, bound to the cell wall by unknown linkages. Western blot and histological analyses of the cell wall fractions support that these proteins are true cell wall proteins, most likely involved in fungal pathogenesis/virulence, since they were found conserved in many fungal pathogens.

Mutations in the Vicinity of the IRAK3 Guanylate Cyclase Center Impact Its Subcellular Localization and Ability to Modulate Inflammatory Signaling in Immortalized Cell Lines

Interleukin-1 receptor-associated kinase 3 (IRAK3) modulates the magnitude of cellular responses to ligands perceived by interleukin-1 receptors (IL-1Rs) and Toll-like receptors (TLRs), leading to decreases in pro-inflammatory cytokines and suppressed inflammation. The molecular mechanism of IRAK3's action remains unknown. IRAK3 functions as a guanylate cyclase, and its cGMP product suppresses lipopolysaccharide (LPS)-induced nuclear factor kappa-light-chain-enhancer of activated B cell (NFκB) activity. To understand the implications of this phenomenon, we expanded the structure-function analyses of IRAK3 through site-directed mutagenesis of amino acids known or predicted to impact different activities of IRAK3. We verified the capacity of the mutated IRAK3 variants to generate cGMP in vitro and revealed residues in and in the vicinity of its GC catalytic center that impact the LPS-induced NFκB activity in immortalized cell lines in the absence or presence of an exogenous membrane-permeable cGMP analog. Mutant IRAK3 variants with reduced cGMP generating capacity and differential regulation of NFκB activity influence subcellular localization of IRAK3 in HEK293T cells and fail to rescue IRAK3 function in IRAK3 knock-out THP-1 monocytes stimulated with LPS unless the cGMP analog is present. Together, our results shed new light on the mechanism by which IRAK3 and its enzymatic product control the downstream signaling, affecting inflammatory responses in immortalized cell lines.

Interplay between membrane proteins and membrane protein-lipid pertaining to plant salinity stress

High salinity in agricultural lands is one of the predominant issues limiting agricultural yields. Plants have developed several mechanisms to withstand salinity stress, but the mechanisms are not effective enough for most crops to prevent and persist the salinity stress. Plant salt tolerance pathways involve membrane proteins that have a crucial role in sensing and mitigating salinity stress. Due to a strategic location interfacing two distinct cellular environments, membrane proteins can be considered checkpoints to the salt tolerance pathways in plants. Related membrane proteins functions include ion homeostasis, osmosensing or ion sensing, signal transduction, redox homeostasis, and small molecule transport. Therefore, modulating plant membrane proteins' function, expression, and distribution can improve plant salt tolerance. This review discusses the membrane protein-protein and protein-lipid interactions related to plant salinity stress. It will also highlight the finding of membrane protein-lipid interactions from the context of recent structural evidence. Finally, the importance of membrane protein-protein and protein-lipid interaction is discussed, and a future perspective on studying the membrane protein-protein and protein-lipid interactions to develop strategies for improving salinity tolerance is proposed.

Membrane Proteins in Plant Salinity Stress Perception, Sensing, and Response

Plants have several mechanisms to endure salinity stress. The degree of salt tolerance varies significantly among different terrestrial crops. Proteins at the plant's cell wall and membrane mediate different physiological roles owing to their critical positioning between two distinct environments. A specific membrane protein is responsible for a single type of activity, such as a specific group of ion transport or a similar group of small molecule binding to exert multiple cellular effects. During salinity stress in plants, membrane protein functions: ion homeostasis, signal transduction, redox homeostasis, and solute transport are essential for stress perception, signaling, and recovery. Therefore, comprehensive knowledge about plant membrane proteins is essential to modulate crop salinity tolerance. This review gives a detailed overview of the membrane proteins involved in plant salinity stress highlighting the recent findings. Also, it discusses the role of solute transporters, accessory polypeptides, and proteins in salinity tolerance. Finally, some aspects of membrane proteins are discussed with potential applications to developing salt tolerance in crops.

Amino acid motifs for the identification of novel protein interactants

Biological systems consist of multiple components of different physical and chemical properties that require complex and dynamic regulatory loops to function efficiently. The discovery of ever more novel interacting sites in complex proteins suggests that we are only beginning to understand how cellular and biological functions are integrated and tuned at the molecular and systems levels. Here we review recently discovered interacting sites which have been identified through rationally designed amino acid motifs diagnostic for specific molecular functions, including enzymatic activities and ligand-binding properties. We specifically discuss the nature of the latter using as examples, novel hormone recognition and gas sensing sites that occur in moonlighting protein complexes. Drawing evidence from the current literature, we discuss the potential implications at the cellular, tissue, and/or organismal levels of such non-catalytic interacting sites and provide several promising avenues for the expansion of amino acid motif searches to discover hitherto unknown protein interactants and interaction networks. We believe this knowledge will unearth unexpected functions in both new and well-characterized proteins, thus filling existing conceptual gaps or opening new avenues for applications either as drug targets or tools in pharmacology, cell biology and bio-catalysis. Beyond this, motif searches may also support the design of novel, effective and sustainable approaches to crop improvements and the development of new therapeutics.

Twin cyclic mononucleotide cyclase and phosphodiesterase domain architecture as a common feature in complex plant proteins

The majority of proteins in both prokaryote and eukaryote proteomes consist of two or more functional centers, which allows for intramolecular tuning of protein functions. Such architecture, as opposed to animal orthologs, applies to the plant cyclases (CNC) and phosphodiesterases (PDEs), the vast majority of which are part of larger multifunctional proteins. In plants, until recently, only two cases of combinations of CNC-PDE in one protein were reported. Here we propose that in plants, multifunctional proteins in which the PDE motif has been identified, the presence of the additional CNC center is common. Searching the Arabidopsis thaliana proteome with a combined PDE-CNC motif allowed the creation of a database of proteins with both activities. One such example is methylenetetrahydrofolate dehydrogenase, in which we determined the activities of adenylate cyclase (AC) and PDE. Based on biochemical and mutagenesis analyses we assessed the impact of the AC and PDE catalytic centers on the dehydrogenase activity. This allowed us to propose additional regulatory mechanism that govern folate metabolism by cAMP. It is therefore conceivable that the combined CNC-PDE architecture is a common regulatory configuration, where control of the level of cyclic nucleotides (cNMP) influences other catalytic activities of the protein.

New Horizons in Plant Cell Signaling

Responding to environmental stimuli with appropriate molecular mechanisms is essential to all life forms and particularly so in sessile organisms such as plants [...].

Ca2+ signals in plant immunity

Calcium ions function as a key second messenger ion in eukaryotes. Spatially and temporally defined cytoplasmic Ca²⁺ signals are shaped through the concerted activity of ion channels, exchangers, and pumps in response to diverse stimuli; these signals are then decoded through the activity of Ca²⁺ -binding sensor proteins. In plants, Ca²⁺ signaling is central to both pattern- and effector-triggered immunity, with the generation of characteristic cytoplasmic Ca²⁺ elevations in response to potential pathogens being common to both. However, despite their importance, and a long history of scientific interest, the transport proteins that shape Ca²⁺ signals and their integration remain poorly characterized. Here, we discuss recent work that has both shed light on and deepened the mysteries of Ca²⁺ signaling in plant immunity.

Modulation of Inflammatory Cytokine Production in Human Monocytes by cGMP and IRAK3

Interleukin-1 receptor-associated kinase-3 (IRAK3) is a critical checkpoint molecule of inflammatory responses in the innate immune system. The pseudokinase domain of IRAK3 contains a guanylate cyclase (GC) centre that generates small amounts of cyclic guanosine monophosphate (cGMP) associated with IRAK3 functions in inflammation. However, the mechanisms of IRAK3 actions are poorly understood. The effects of low cGMP levels on inflammation are unknown, therefore a dose–response effect of cGMP on inflammatory markers was assessed in THP-1 monocytes challenged with lipopolysaccharide (LPS). Sub-nanomolar concentrations of membrane permeable 8-Br-cGMP reduced LPS-induced NFκB activity, IL-6 and TNF-α cytokine levels. Pharmacologically upregulating cellular cGMP levels using a nitric oxide donor reduced cytokine secretion. Downregulating cellular cGMP using a soluble GC inhibitor increased cytokine levels. Knocking down IRAK3 in THP-1 cells revealed that unlike the wild type cells, 8-Br-cGMP did not suppress inflammatory responses. Complementation of IRAK3 knockdown cells with wild type IRAK3 suppressed cytokine production while complementation with an IRAK3 mutant at GC centre only partially restored this function. Together these findings indicate low levels of cGMP form a critical component in suppressing cytokine production and in mediating IRAK3 action, and this may be via a cGMP enriched nanodomain formed by IRAK3 itself.

Elucidation of the Translation Initiation Factor Interaction Network of Haloferax volcanii Reveals Coupling of Transcription and Translation in Haloarchaea

Translation is an important step in gene expression. Initiation of translation is rate-limiting, and it is phylogenetically more diverse than elongation or termination. Bacteria contain only three initiation factors. In stark contrast, eukaryotes contain more than 10 (subunits of) initiation factors (eIFs). The genomes of archaea contain many genes that are annotated to encode archaeal homologs of eukaryotic initiation factors (aIFs). However, experimental characterization of aIFs is scarce and mostly restricted to very few species. To broaden the view, the protein-protein interaction network of aIFs in the halophilic archaeon Haloferax volcanii has been characterized. To this end, tagged versions of 14 aIFs were overproduced, affinity isolated, and the co-isolated binding partners were identified by peptide mass fingerprinting and MS/MS analyses. The aIF-aIF interaction network was resolved, and it was found to contain two interaction hubs, (1) the universally conserved factor aIF5B, and (2) a protein that has been annotated as the enzyme ribose-1,5-bisphosphate isomerase, which we propose to rename to aIF2Bα. Affinity isolation of aIFs also led to the co-isolation of many ribosomal proteins, but also transcription factors and subunits of the RNA polymerase (Rpo). To analyze a possible coupling of transcription and translation, seven tagged Rpo subunits were overproduced, affinity isolated, and co-isolated proteins were identified. The Rpo interaction network contained many transcription factors, but also many ribosomal proteins as well as the initiation factors aIF5B and aIF2Bα. These results showed that transcription and translation are coupled in haloarchaea, like in Escherichia coli. It seems that aIF5B and aIF2Bα are not only interaction hubs in the translation initiation network, but also key players in the transcription-translation coupling.

CD46 Genetic Variability and HIV-1 Infection Susceptibility

CD46 is the main receptor for complement protein C3 and plays an important role in adaptive immune responses. CD46 genetic variants are associated with susceptibility to several infectious and autoimmune diseases. Additionally, CD46 function can be subverted by HIV-1 to evade attack by complement, a strategy shared by viruses of other families. We sought to determine the association between CD46 gene variants and HIV-1 acquired through intravenous drug use (IDU) and sexual routes (n = 823). Study subjects were of European ancestry and were HIV-1 infected (n = 438) or exposed but seronegative (n = 387). Genotyping of the rs2796265 SNP located in the CD46 gene region was done by allele-specific real-time PCR. A meta-analysis merging IDU and sexual cohorts indicates that the minor genotype (CC) was associated with increased resistance to HIV-1 infection OR = 0.2, 95% CI (0.07–0.61), p = 0.004. The HIV-1-protective genotype is correlated with reduced CD46 expression and alterations in the ratio of CD46 mRNA splicing isoforms.

IdentPMP: identification of moonlighting proteins in plants using sequence-based learning models

Background

A moonlighting protein refers to a protein that can perform two or more functions. Since the current moonlighting protein prediction tools mainly focus on the proteins in animals and microorganisms, and there are differences in the cells and proteins between animals and plants, these may cause the existing tools to predict plant moonlighting proteins inaccurately. Hence, the availability of a benchmark data set and a prediction tool specific for plant moonlighting protein are necessary.

Methods

This study used some protein feature classes from the data set constructed in house to develop a web-based prediction tool. In the beginning, we built a data set about plant protein and reduced redundant sequences. We then performed feature selection, feature normalization and feature dimensionality reduction on the training data. Next, machine learning methods for preliminary modeling were used to select feature classes that performed best in plant moonlighting protein prediction. This selected feature was incorporated into the final plant protein prediction tool. After that, we compared five machine learning methods and used grid searching to optimize parameters, and the most suitable method was chosen as the final model.

Results

The prediction results indicated that the eXtreme Gradient Boosting (XGBoost) performed best, which was used as the algorithm to construct the prediction tool, called IdentPMP (Identification of Plant Moonlighting Proteins). The results of the independent test set shows that the area under the precision-recall curve (AUPRC) and the area under the receiver operating characteristic curve (AUC) of IdentPMP is 0.43 and 0.68, which are 19.44% (0.43 vs. 0.36) and 13.33% (0.68 vs. 0.60) higher than state-of-the-art non-plant specific methods, respectively. This further demonstrated that a benchmark data set and a plant-specific prediction tool was required for plant moonlighting protein studies. Finally, we implemented the tool into a web version, and users can use it freely through the URL: http://identpmp.aielab.net/.

Functional Crypto-Adenylate Cyclases Operate in Complex Plant Proteins

Adenylyl cyclases (ACs) and their catalytic product cAMP are regulatory components of many plant responses. Here, we show that an amino acid search motif based on annotated adenylate cyclases (ACs) identifies 12 unique Arabidopsis thaliana candidate ACs, four of which have a role in the biosynthesis of the stress hormone abscisic acid (ABA). One of these, the 9-cis-epoxycarotenoid dioxygenase (NCED3 and At3g14440), was identified by sequence and structural analysis as a putative AC and then tested experimentally with two different methods. Given that the in vitro activity is low (fmoles cAMP pmol-1 protein min-1), but highly reproducible, we term the enzyme a crypto-AC. Our results are consistent with a role for ACs with low activities in multi-domain moonlighting proteins that have at least one other distinct molecular function, such as catalysis or ion channel activation. We propose that crypto-ACs be examined from the perspective that considers their low activities as an innate feature of regulatory ACs embedded within multi-domain moonlighting proteins. It is therefore conceivable that crypto-ACs form integral components of complex plant proteins participating in intra-molecular regulatory mechanisms, and in this case, potentially linking cAMP to ABA synthesis.

A Hypothesis: Fructosamine-3-Kinase-Related-Protein (FN3KRP) Catalyzes Deglycation of Maillard Intermediates Directly Downstream from Fructosamines

Non-enzymatic glycation (a.k.a. Maillard reaction) is a series of random spontaneous reactions between reducing sugars and amines, resulting in the formation of irreversible advanced glycation endproducts (AGE's). In food chemistry, this process is beneficial by contributing to the flavor, aroma, texture, and appearance of cooked foods. In vivo, however, Maillard reaction is deleterious because uncontrolled modification and crosslinking of biological macromolecules impairs their function. Consequently, chronic hyperglycemia of diabetes mellitus, for instance, leads to increased non-enzymatic glycation and diverse, multi-organ pathologies of diabetic complications. Based on the fact that toxic compounds, such as free radicals, are detoxified in vivo by specific defense mechanisms, one would expect to find mechanisms to control glucose toxicity as well. Thus far, only one such enzyme, fructosamine-3-kinase (FN3K), has been characterized. It operates intracellularly by catalyzing ATP-dependent removal of Maillard adducts, D-fructoselysines, from proteins, thereby reducing the Maillard reaction flux from glucose to AGE's. When FN3K was isolated, a closely related but distinct protein copurified with it. Unlike FN3K, however, this enzyme, fructosamine-3-kinase-related protein (FN3KRP), does not phosphorylate D-fructoselysines but it does phosphorylate several other (non-physiological) substrates. Interestingly, the distribution of FN3KRP in nature appears to be nearly universal whereas that of FN3K is limited to endotherms. In this article, it is suggested that the function of FN3KRP is deglycation of Maillard adducts downstream from fructoselysines. Such a mechanism, if proven correct, would be valuable given reports on apparent correlations between FN3KRP and some chronic conditions and/or diseases, such as a recent publication which proposes that the FN3KRP gene may be a longevity gene.

In Vitro Characterization of Guanylyl Cyclase BdPepR2 from Brachypodium distachyon Identified through a Motif-Based Approach

In recent years, cyclic guanosine 3′,5′-cyclic monophosphate (cGMP) and guanylyl cyclases (GCs), which catalyze the formation of cGMP, were implicated in a growing number of plant processes, including plant growth and development and the responses to various stresses. To identify novel GCs in plants, an amino acid sequence of a catalytic motif with a conserved core was designed through bioinformatic analysis. In this report, we describe the performed analyses and consider the changes caused by the introduced modification within the GC catalytic motif, which eventually led to the description of a plasma membrane receptor of peptide signaling molecules—BdPepR2 in Brachypodium distachyon. Both in vitro GC activity studies and structural and docking analyses demonstrated that the protein could act as a GC and contains a highly conserved 14-aa GC catalytic center. However, we observed that in the case of BdPepR2, this catalytic center is altered where a methionine instead of the conserved lysine or arginine residues at position 14 of the motif, conferring higher catalytic activity than arginine and alanine, as confirmed through mutagenesis studies. This leads us to propose the expansion of the GC motif to cater for the identification of GCs in monocots. Additionally, we show that BdPepR2 also has in vitro kinase activity, which is modulated by cGMP.

Computational Identification of Functional Centers in Complex Proteins: A Step-by-Step Guide With Examples

In proteins, functional centers consist of the key amino acids required to perform molecular functions such as catalysis, ligand-binding, hormone- and gas-sensing. These centers are often embedded within complex multi-domain proteins and can perform important cellular signaling functions that enable fine-tuning of temporal and spatial regulation of signaling molecules and networks. To discover hidden functional centers, we have developed a protocol that consists of the following sequential steps. The first is the assembly of a search motif based on the key amino acids in the functional center followed by querying proteomes of interest with the assembled motif. The second consists of a structural assessment of proteins that harbor the motif. This approach, that relies on the application of computational tools for the analysis of data in public repositories and the biological interpretation of the search results, has to-date uncovered several novel functional centers in complex proteins. Here, we use recent examples to describe a step-by-step guide that details the workflow of this approach and supplement with notes, recommendations and cautions to make this protocol robust and widely applicable for the discovery of hidden functional centers.

Damage Signaling by Extracellular Nucleotides: A Role for Cyclic Nucleotides in Elevating Cytosolic Free Calcium?

Extracellular ATP (eATP) is now held to be a constitutive damage-associated molecular pattern (DAMP) that is released by wounding, herbivory or pathogen attack. The concentration of eATP must be tightly regulated as either depletion or overload leads to cell death. In Arabidopsis thaliana, sensing of eATP is by two plasma membrane legume-like lectin serine-threonine receptor kinases (P2K1 and P2K2), although other receptors are postulated. The transcriptional response to eATP is dominated by wound- and defense-response genes. Wounding and pathogen attack can involve the cyclic nucleotides cyclic AMP (cAMP) and cyclic GMP (cGMP) which, in common with eATP, can increase cytosolic-free Ca²⁺ as a second messenger. This perspective on DAMP signaling by eATP considers the possibility that the eATP pathway involves production of cyclic nucleotides to promote opening of cyclic nucleotide-gated channels and so elevates cytosolic-free Ca²⁺. In silico analysis of P2K1 and P2K2 reveals putative adenylyl and guanylyl kinase sequences that are the hallmarks of "moonlighting" receptors capable of cAMP and cGMP production. Further, an Arabidopsis loss of function cngc mutant was found to have an impaired increase in cytosolic-free Ca²⁺ in response to eATP. A link between eATP, cyclic nucleotides, and Ca²⁺ signaling therefore appears credible.