Regulation of the Dual Specificity Protein Phosphatase, DsPTP1, through Interactions with Calmodulin

Revealing the Complete Bispecific Phosphatase Genes (DUSPs) across the Genome and Investigating the Expression Patterns of GH_A11G3500 Resistance against Verticillium wilt

DUSPs, a diverse group of protein phosphatases, play a pivotal role in orchestrating cellular growth and development through intricate signaling pathways. Notably, they actively participate in the MAPK pathway, which governs crucial aspects of plant physiology, including growth regulation, disease resistance, pest resistance, and stress response. DUSP is a key enzyme, and it is the enzyme that limits the rate of cell metabolism. At present, complete understanding of the DUSP gene family in cotton and its specific roles in resistance to Verticillium wilt (VW) remains elusive. To address this knowledge gap, we conducted a comprehensive identification and analysis of four key cotton species: Gossypium arboreum, Gossypium barbadense, Gossypium hirsutum, and Gossypium raimondii. The results revealed the identification of a total of 120 DUSP genes in the four cotton varieties, which were categorized into six subgroups and randomly distributed at both ends of 26 chromosomes, predominantly localized within the nucleus. Our analysis demonstrated that closely related DUSP genes exhibited similarities in terms of the conserved motif composition and gene structure. A promoter analysis performed on the GhDUSP gene promoter revealed the presence of several cis-acting elements, which are associated with abiotic and biotic stress responses, as well as hormone signaling. A tissue expression pattern analysis demonstrated significant variations in GhDUSP gene expression under different stress conditions, with roots exhibiting the highest levels, followed by stems and leaves. In terms of tissue-specific detection, petals, leaves, stems, stamens, and receptacles exhibited higher expression levels of the GhDUSP gene. The gene expression analysis results for GhDUSPs under stress suggest that DUSP genes may have a crucial role in the cotton response to stress in cotton. Through Virus-Induced Gene Silencing (VIGS) experiments, the silencing of the target gene significantly reduced the resistance efficiency of disease-resistant varieties against Verticillium wilt (VW). Consequently, we conclude that GH_A11G3500-mediated bispecific phosphorylated genes may serve as key regulators in the resistance of G. hirsutum to Verticillium wilt (VW). This study presents a comprehensive structure designed to provide an in-depth understanding of the potential biological functions of cotton, providing a strong foundation for further research into molecular breeding and resistance to plant pathogens.

Decrypting Molecular Mechanisms Involved in Counteracting Copper and Nickel Toxicity in Jack Pine (Pinus banksiana) Based on Transcriptomic Analysis

The remediation of copper and nickel-afflicted sites is challenged by the different physiological effects imposed by each metal on a given plant system. Pinus banksiana is resilient against copper and nickel, providing an opportunity to build a valuable resource to investigate the responding gene expression toward each metal. The objectives of this study were to (1) extend the analysis of the Pinus banksiana transcriptome exposed to nickel and copper, (2) assess the differential gene expression in nickel-resistant compared to copper-resistant genotypes, and (3) identify mechanisms specific to each metal. The Illumina platform was used to sequence RNA that was extracted from seedlings treated with each of the metals. There were 449 differentially expressed genes (DEGs) between copper-resistant genotypes (RGs) and nickel-resistant genotypes (RGs) at a high stringency cut-off, indicating a distinct pattern of gene expression toward each metal. For biological processes, 19.8% of DEGs were associated with the DNA metabolic process, followed by the response to stress (13.15%) and the response to chemicals (8.59%). For metabolic function, 27.9% of DEGs were associated with nuclease activity, followed by nucleotide binding (27.64%) and kinase activity (10.16%). Overall, 21.49% of DEGs were localized to the plasma membrane, followed by the cytosol (16.26%) and chloroplast (12.43%). Annotation of the top upregulated genes in copper RG compared to nickel RG identified genes and mechanisms that were specific to copper and not to nickel. NtPDR, AtHIPP10, and YSL1 were identified as genes associated with copper resistance. Various genes related to cell wall metabolism were identified, and they included genes encoding for HCT, CslE6, MPG, and polygalacturonase. Annotation of the top downregulated genes in copper RG compared to nickel RG revealed genes and mechanisms that were specific to nickel and not copper. Various regulatory and signaling-related genes associated with the stress response were identified. They included UGT, TIFY, ACC, dirigent protein, peroxidase, and glyoxyalase I. Additional research is needed to determine the specific functions of signaling and stress response mechanisms in nickel-resistant plants.

Differential regulation of the durum wheat MAPK phosphatase 1 by calmodulin, bivalent cations and possibly mitogen activated protein kinase 3

MAPK phosphatases (MKPs) are relevant negative regulators of MAPKs in eukaryotes as they mediate the feedback control of MAPK cascades in multiple cellular processes. Despite their relevance, our knowledge on the role of cereal MKPs in stress tolerance is scarce and TMKP1 remains today the only studied MKP in wheat. TMKP1 was previously reported to be involved in plant salt stress tolerance. Moreover, TMKP1 was shown to interact with calmodulin (CaM), 14-3-3 and TMPK3/TMPK6 proteins, which regulate its catalytic activity. To further understand the functional properties of TMKP1, we investigate here the contribution of its phosphorylation status, and of TMPK3 together with CaM and bivalent cations on the catalytic activity. In-gel kinase assays revealed that TMKP1 can be phosphorylated by similar wheat and Arabidopsis MAPKs, including most likely MPK3 and MPK6. In addition, we provide evidence for the capacity of wheat TMPK3 to bind to TMKP1 via a conserved Kinase Interacting Domain (KID) located on its C-terminal part. This interaction leads to a stimulation of TMKP1 activity in the presence of Mn²⁺ or Mg²⁺ ions, but to its inhibition in the presence of Ca²⁺ ions. However, the phosphorylation status of TMKP1 seems to be dispensable for TMKP1 activation by TMPK3. Remarkably, in assays combining TMPK3 with CaM/Ca²⁺ complex, we registered rather an inhibition of TMKP1 activity which however can be suppressed by Mn²⁺ cations. Our data are in favor of complex differential regulation of TMKP1 by its MPK substrates, metallic cations that might help in fine-tuning the plant cellular responses to various stresses.

A mitogen-activated protein kinase phosphatase influences grain size and weight in rice

Grain size and weight are directly associated with grain yield in crops. However, the molecular mechanisms that set final grain size and weight remain largely unknown. Here, we characterize two large grain mutants, large grain8-1 (large8-1) and large grain8-2 (large8-2). LARGE8 encodes the mitogen-activated protein kinase phosphatase1 (OsMKP1). Loss of function mutations in OsMKP1 results in large grains, while overexpression of OsMKP1 leads to small grains. OsMKP1 determines grain size by restricting cell proliferation in grain hulls. OsMKP1 directly interacts with and deactivates the mitogen-activated protein kinase 6 (OsMAPK6). Taken together, we identify OsMKP1 as a crucial factor that influences grain size by deactivating OsMAPK6, indicating that the reversible phosphorylation of OsMAPK6 plays important roles in determining grain size in rice.

Central Roles and Regulatory Mechanisms of Dual-Specificity MAPK Phosphatases in Developmental and Stress Signaling

Mitogen-Activated Protein Kinase (MAPK) cascades are conserved signaling modules that integrate multiple signaling pathways. One level of control on the activity of MAPKs is through their negative regulators, MAPK phosphatases (MKPs). Therefore, MKPs also play an integrative role for plants responding to diverse environmental stimulus; but the mechanism(s) by which these phosphatases contribute to specific signals remains largely unknown. In this review, we summarize recent advances in characterizing the biological functions of a sub-class of MKPs, dual-specificity phosphatases (DSPs), ranging from controlling plant growth and development to modulating stress adaptation. We also discuss putative regulatory mechanisms of DSP-type MKPs, which plants may use to control the correct level of responses at the right place and time. We highlight insights into potential regulation of cross-talk between different signaling pathways, facilitating the development of strategies for targeting such cross-talk and to help improve plant resistance against adverse environmental conditions without affecting the growth and development.

Laminar flow downregulates Notch activity to promote lymphatic sprouting

The major function of the lymphatic system is to drain interstitial fluid from tissue. Functional drainage causes increased fluid flow that triggers lymphatic expansion, which is conceptually similar to hypoxia-triggered angiogenesis. Here, we have identified a mechanotransduction pathway that translates laminar flow-induced shear stress to activation of lymphatic sprouting. While low-rate laminar flow commonly induces the classic shear stress responses in blood endothelial cells and lymphatic endothelial cells (LECs), only LECs display reduced Notch activity and increased sprouting capacity. In response to flow, the plasma membrane calcium channel ORAI1 mediates calcium influx in LECs and activates calmodulin to facilitate a physical interaction between Krüppel-like factor 2 (KLF2), the major regulator of shear responses, and PROX1, the master regulator of lymphatic development. The PROX1/KLF2 complex upregulates the expression of DTX1 and DTX3L. DTX1 and DTX3L, functioning as a heterodimeric Notch E3 ligase, concertedly downregulate NOTCH1 activity and enhance lymphatic sprouting. Notably, overexpression of the calcium reporter GCaMP3 unexpectedly inhibited lymphatic sprouting, presumably by disturbing calcium signaling. Endothelial-specific knockouts of Orai1 and Klf2 also markedly impaired lymphatic sprouting. Moreover, Dtx3l loss of function led to defective lymphatic sprouting, while Dtx3l gain of function rescued impaired sprouting in Orai1 KO embryos. Together, the data reveal a molecular mechanism underlying laminar flow-induced lymphatic sprouting.

CaM/BAG5/Hsc70 signaling complex dynamically regulates leaf senescence

Calcium signaling plays an essential role in plant cell physiology, and chaperone-mediated protein folding directly regulates plant programmed cell death. The Arabidopsis thaliana protein AtBAG5 (Bcl-2-associated athanogene 5) is unique in that it contains both a BAG domain capable of binding Hsc70 (Heat shock cognate protein 70) and a characteristic IQ motif that is specific for Ca(2+)-free CaM (Calmodulin) binding and hence acts as a hub linking calcium signaling and the chaperone system. Here, we determined crystal structures of AtBAG5 alone and in complex with Ca(2+)-free CaM. Structural and biochemical studies revealed that Ca(2+)-free CaM and Hsc70 bind AtBAG5 independently, whereas Ca(2+)-saturated CaM and Hsc70 bind AtBAG5 with negative cooperativity. Further in vivo studies confirmed that AtBAG5 localizes to mitochondria and that its overexpression leads to leaf senescence symptoms including decreased chlorophyll retention and massive ROS production in dark-induced plants. Mutants interfering the CaM/AtBAG5/Hsc70 complex formation leads to different phenotype of leaf senescence. Collectively, we propose that the CaM/AtBAG5/Hsc70 signaling complex plays an important role in regulating plant senescence.

The Tinkerbell (Tink) Mutation Identifies the Dual-Specificity MAPK Phosphatase INDOLE-3-BUTYRIC ACID-RESPONSE5 (IBR5) as a Novel Regulator of Organ Size in Arabidopsis

Mitogen-activated dual-specificity MAPK phosphatases are important negative regulators in the MAPK signalling pathways responsible for many essential processes in plants. In a screen for mutants with reduced organ size we have identified a mutation in the active site of the dual-specificity MAPK phosphatase INDOLE-3-BUTYRIC ACID-RESPONSE5 (IBR5) that we named tinkerbell (tink) due to its small size. Analysis of the tink mutant indicates that IBR5 acts as a novel regulator of organ size that changes the rate of growth in petals and leaves. Organ size and shape regulation by IBR5 acts independently of the KLU growth-regulatory pathway. Microarray analysis of tink/ibr5-6 mutants identified a likely role for this phosphatase in male gametophyte development. We show that IBR5 may influence the size and shape of petals through auxin and TCP growth regulatory pathways.

Phosphatases in plants

Reversible protein phosphorylation is an essential posttranslational modification mechanism executed by opposing actions of protein phosphatases and protein kinases. About 1,000 predicted kinases in Arabidopsis thaliana kinome predominate the number of protein phosphatases, of which there are only ~150 members in Arabidopsis. Protein phosphatases were often referred to as "housekeeping" enzymes, which act to keep eukaryotic systems in balance by counteracting the activity of protein kinases. However, recent investigations reveal the crucial and specific regulatory functions of phosphatases in cell signaling. Phosphatases operate in a coordinated manner with the protein kinases, to execute their important function in determining the cellular response to a physiological stimulus. Closer examination has established high specificity of phosphatases in substrate recognition and important roles in plant signaling pathways, such as pathogen defense and stress regulation, light and hormonal signaling, cell cycle and differentiation, metabolism, and plant growth. In this minireview we provide a compact overview about Arabidopsis protein phosphatase families, as well as members of phosphoglucan and lipid phosphatases, and highlight the recent discoveries in phosphatase research.

The activity of the wheat MAP kinase phosphatase 1 is regulated by manganese and by calmodulin

MAPK phosphatases (MKPs) are negative regulators of MAPKs in eukaryotes and play key roles in the regulation of different cellular processes. However in plants, little is known about the regulation of these Dual Specific Phosphatases (DSPs) by Ca(2+) and calmodulin (CaM). Here, we showed that the wheat MKP (TMKP1) harboring a calmodulin (CaM) binding domain, binds to CaM in a Ca(2+)-dependent manner. In addition, TMKP1 exhibited a phosphatase activity in vitro that is specifically enhanced by Mn(2+) and to a lesser extent by Mg(2+), but without any synergistic effect between the two bivalent cations. Most interestingly, CaM/Ca(2+) complex inhibits the catalytic activity of TMKP1 in a CaM-dose dependent manner. However, in the presence of Mn(2+) this activity is enhanced by CaM/Ca(2+) complex. These dual regulatory effects seem to be mediated via interaction of CaM/Ca(2+) to the CaM binding domain in the C-terminal part of TMKP1. Such effects were not reported so far, and raise a possible role for CaM and Mn(2+) in the regulation of plant MKPs during cellular response to external signals.

Phosphorylation and stabilization of Arabidopsis MAP kinase phosphatase 1 in response to UV-B stress

MAP kinase phosphatases (MKPs) are important regulators of the activation levels and kinetics of MAP kinases. This is crucial for a large number of physiological processes during development and growth, as well as interactions with the environment, including the response to ultraviolet-B (UV-B) stress. Arabidopsis MKP1 is a key regulator of MAP kinases MPK3 and MPK6 in response to UV-B stress. However, virtually nothing is presently known about the post-translational regulation of plant MKPs in vivo. Here, we provide evidence that MKP1 is a phosphoprotein in vivo and that MKP1 accumulates in response to UV-B stress. Moreover, proteasome inhibitor experiments suggest that MKP1 is constantly turned-over under non-stress conditions and that MKP1 is stabilized upon stress treatment. Stress-responsive phosphorylation and stabilization of MKP1 demonstrate the post-translational regulation of a plant MKP in vivo, adding an additional regulatory layer to MAP kinase signaling in plants.

Functional characterization of orchardgrass cytosolic Hsp70 (DgHsp70) and the negative regulation by Ca2+/AtCaM2 binding

When plants are exposed to extreme temperature, stress-inducible proteins are highly induced and involved in subcellular defence mechanisms. Hsp70, one of stress-inducible proteins, functions as an ATP-dependent molecular chaperone in broad organisms to process such as the inhibition of protein denaturation, promotion of protein folding, and renaturation of denatured proteins. In this study, we isolated a heat-inducible orchardgrass Hsp70 (DgHsp70) that is a homolog of cytosolic Hsp70 that possesses a CaM-binding domain. Purified DgHsp70 protein displayed dose-dependent ATPase, holdase, and ATP-dependent foldase activities. To investigate functional roles of DgHsp70 by the association of Arabidopsis calmodulin-2 (AtCaM2), showing heat-sensitive reduction on transcription, we first characterized the binding activity by gel-overlay assay. DgHsp70 binds to AtCaM2 in the presence of Ca(2+) via a conserved CaM-binding domain. Ca(2+)/AtCaM2 binding decreased ATPase activity of DgHsp70, and concomitantly, reduced foldase activity. Based on the protein structure of bovine Hsc70, which is the closest structural homolog of DgHsp70, a CaM-binding domain is located near the ATP-binding site and CaM may span the ATP-binding pocket of Hsp70. Its decreased functional foldase activity may be caused by blocking ATP hydrolysis after Ca(2+)/AtCaM2 binding. It may associate with inhibition of functional activity of DgHsp70 in the absence of stress and/or de novo protein synthesis of DgHsp70 in the presence of thermal stress condition.

Sucrose induces rapid activation of CfSAPK, a mitogen-activated protein kinase, in Cephalostachyum fuchsianum Gamble cells

Sucrose was recently demonstrated to function as a molecular signal. However, sucrose-specific sensing and signalling pathways remain largely undefined. Here, we show that Cephalostachyum fuchsianum sucrose-activated protein kinase (CfSAPK) is transiently and specifically activated by sucrose in C. fuchsianum Gamble suspension cells. The result suggested that CfSAPK participates in a sucrose-signalling pathway. CfSAPK was partially purified from sucrose-treated cells and further analysed. Kinase activity assays revealed that CfSAPK preferentially used myelin basic protein (MBP) as substrate in vitro and strongly phosphorylate MBP threonine residue(s) and weakly phosphorylated MBP serine residue(s). Of the divalent cations tested, Mg(2+) was required for CfSAPK activation. Phosphatase treatment of CfSAPK abolished its kinase activity, indicating that phosphorylation is required for CfSAPK activation. Seven internal tryptic peptides identified from CfSAPK matched mitogen-activated protein kinases (MAPKs) in plants. CfSAPK cDNA was cloned using RT-PCR and rapid amplification of cDNA ends (RACE). CfSAPK cDNA encodes a 382-amino acid protein with a calculated molecular mass of 43,466.9 Da. The CfSAPK protein contains all 11 conserved kinase subdomains found in other Ser/Thr kinases. The amino acids sequence of CfSAPK is highly homologous to group A MAPKs in monocotyledon plants.

AtIQM1, a novel calmodulin-binding protein, is involved in stomatal movement in Arabidopsis

We recently identified a novel IQ motif-containing protein family, IQM, which shares sequence homology with a pea heavy metal-induced protein 6 and a ribosome inactivating protein, trichosanthin. Distinct expression patterns for each gene suggest that each IQM family member may play a different role in plant development and response to environmental cues. However functions of the IQM family members remain to be analyzed. IQM1 bound with calmodulin 5 (CaM5) in yeast two-hybrid assay via its IQ-motif. The CaM binding was Ca(2+)-independent in vitro, and was also observed in bimolecular fluorescence complementation analyses in onion epidermal cells. IQM1 was found to express strongly in guard cells and the cortex of roots. The T-DNA insertion mutants of IQM1 displayed a smaller stomatal aperture, a decreased water loss rate and a shorter primary root. Moreover, iqm1 did not change its stomatal aperture when treated with light, dark, ABA and chitin obviously. Microarray analyses showed that 243 and 28 genes were up- and down-regulated by more than twofold in iqm1-1, respectively. Interesting, 34 of 117 and 7 of 30 chitin-responsive transcriptional factor and ubiquitin ligase genes were up-regulated, respectively. Stomatal guard cells of iqm1-1 also showed enhanced expression of genes involved in production and signaling of reactive oxygen species (ROS). Consistently, increased ROS level was observed in the iqm1 guard cells.

TMKP1 is a novel wheat stress responsive MAP Kinase phosphatase localized in the nucleus

The regulation of plant signalling responses by Mitogen-Activated Protein Kinases (MAPKs)-mediated protein phosphorylation is well recognized. MAP kinase phosphatases (MKPs) are negative regulators of MAPKs in eukaryotes. We report here the identification and the characterization of TMKP1, the first wheat MKP (Triticum turgidum L. subsp. Durum). Expression profile analyses performed in two durum wheat cultivars showing a marked difference in salt and drought stress tolerance, revealed a differential regulation of TMKP1. Under salt and osmotic stress, TMKP1 is induced in the sensitive wheat variety and repressed in the tolerant one. A recombinant TMKP1 was shown to be an active phosphatase and capable to interact specifically with two wheat MAPKs (TMPK3 and TMPK6). In BY2 tobacco cells transiently expressing GFP::TMKP1, the fusion protein was localized into the nucleus. Interestingly, the deletion of the N-terminal non catalytic domain results in a strong accumulation of the truncated fusion protein in the cytoplasm. In addition, when expressed in BY2 cells, TMPK3 and TMPK6 fused to red fluorescent protein (RFP) were shown to be present predominantly in the nucleus. Surprisingly, when co-expressed with the N-terminal truncated TMKP1 fusion protein; both kinases are excluded from the nuclear compartment and accumulate in the cytoplasm. This strongly suggests that TMKP1 interacts in vivo with TMPK3 and TMPK6 and controls their subcellular localization. Taken together, our results show that the newly isolated wheat MKP might play an active role in modulating the plant cell responses to salt and osmotic stress responses.

Structural studies of soybean calmodulin isoform 4 bound to the calmodulin-binding domain of tobacco mitogen-activated protein kinase phosphatase-1 provide insights into a sequential target binding mode

The calcium regulatory protein calmodulin (CaM) binds in a calcium-dependent manner to numerous target proteins. The calmodulin-binding domain (CaMBD) region of Nicotiana tabacum MAPK phosphatase has an amino acid sequence that does not resemble the CaMBD of any other known Ca(2+)-CaM-binding proteins. Using a unique fusion protein strategy, we have been able to obtain a high resolution solution structure of the complex of soybean Ca(2+)-CaM4 (SCaM4) and this CaMBD. Complete isotope labeling of both parts of the complex in the fusion protein greatly facilitated the structure determination by NMR. The 12-residue CaMBD region was found to bind exclusively to the C-lobe of SCaM4. A specific Trp and Leu side chain are utilized to facilitate strong binding through a novel "double anchor" motif. Moreover, the orientation of the helical peptide on the surface of Ca(2+)-SCaM4 is distinct from other known complexes. The N-lobe of Ca(2+)-SCaM4 in the complex remains free for additional interactions and could possibly act as a calcium-dependent adapter protein. Signaling through the MAPK pathway and increases in intracellular Ca(2+) are both hallmarks of the plant stress response, and our data support the notion that coordination of these responses may occur through the formation of a unique CaM-MAPK phosphatase multiprotein complex.

An S-locus receptor-like kinase in plasma membrane interacts with calmodulin in Arabidopsis

Calmodulin-regulated protein phosphorylation plays a pivotal role in amplifying and diversifying the action of calcium ion. In this study, we identified a calmodulin-binding receptor-like protein kinase (CBRLK1) that was classified into an S-locus RLK family. The plasma membrane localization was determined by the localization of CBRLK1 tagged with a green fluorescence protein. Calmodulin bound specifically to a Ca(2+)-dependent calmodulin binding domain in the C-terminus of CBRLK1. The bacterially expressed CBRLK1 kinase domain could autophosphorylate and phosphorylates general kinase substrates, such as myelin basic proteins. The autophosphorylation sites of CBRLK1 were identified by mass spectrometric analysis of phosphopeptides.

Regulation of MAPK phosphatase 1 (AtMKP1) by calmodulin in Arabidopsis

The mitogen-activated protein kinases (MAPKs) are key signal transduction molecules, which respond to various external stimuli. The MAPK phosphatases (MKPs) are known to be negative regulators of MAPKs in eukaryotes. We screened an Arabidopsis cDNA library using horseradish peroxidase-conjugated calmodulin (CaM), and isolated AtMKP1 as a CaM-binding protein. Recently, tobacco NtMKP1 and rice OsMKP1, two orthologs of Arabidopsis AtMKP1, were reported to bind CaM via a single putative CaM binding domain (CaMBD). However, little is known about the regulation of phosphatase activity of plant MKP1s by CaM binding. In this study, we identified two Ca(2+)-dependent CaMBDs within AtMKP1. Specific binding of CaM to two different CaMBDs was verified using a gel mobility shift assay, a competition assay with a Ca(2+)/CaM-dependent enzyme, and a split-ubiquitin assay. The peptides for two CaMBDs, CaMBDI and CaMBDII, bound CaM in a Ca(2+)-dependent manner, and the binding affinity of CaMBDII was found to be higher than that of CaMBDI. CaM overlay assays using mutated CaMBDs showed that four amino acids, Trp(453) and Leu(456) in CaMBDI and Trp(678) and Ile(684) in CaMBDII, play a pivotal role in CaM binding. Moreover, the phosphatase activity of AtMKP1 was increased by CaM in a Ca(2+)-dependent manner. Our results suggest that two important signaling pathways, Ca(2+) signaling and the MAPK signaling cascade, are connected in plants via the regulation of AtMKP1 activity. To our knowledge, this is the first report to show that the biochemical activity of MKP1 in plants is regulated by CaM.

Calmodulin7 plays an important role as transcriptional regulator in Arabidopsis seedling development

Although calmodulin (CaM) is known to play multiple regulatory roles in eukaryotes, its direct function as transcriptional regulator is unknown. Furthermore, the physiological functions of CaM are largely unknown in plants. Here, we show that one of the four Arabidopsis thaliana CaM isoforms, CAM7, is a transcriptional regulator that directly interacts with the promoters of light-inducible genes and promotes photomorphogenesis. CAM7 overexpression causes hyperphotomorphogenic growth and an increase in the expression of light-inducible genes. Mutations in CAM7 produce no visible effects on photomorphogenic growth, indicating likely redundant gene functions. However, cam7 mutants display reduced expression of light-inducible genes, and cam7 hy5 double mutants show an enhancement of the hy5 phenotype. Moreover, overexpression of CAM7 can partly suppress the hy5 phenotype, indicating that the two factors work together to control light-induced seedling development. The mutational and transgenic studies, together with physiological analyses, illustrate the concerted function of CAM7 and HY5 basic leucine zipper transcription factor in Arabidopsis seedling development.

Identification of a calmodulin-binding NAC protein as a transcriptional repressor in Arabidopsis

Calmodulin (CaM), a ubiquitous calcium-binding protein, regulates diverse cellular functions by modulating the activity of a variety of proteins. However, little is known about how CaM directly regulates transcription. Screening of an Arabidopsis cDNA expression library using horseradish peroxidase-conjugated calmodulin as a probe identified a calmodulin-binding NAC protein (CBNAC). Using gel overlay assays, a Ca2+-dependent CaM-binding domain was identified in the C terminus of this protein. Specific binding of CaM to CaM-binding domain was corroborated by site-directed mutagenesis and a split-ubiquitin assay. Using a PCR-mediated random binding site selection method, we identified a DNA-binding sequence (CBNACBS) for CBNAC, which consisted of a GCTT core sequence flanked on both sides by other frequently repeating sequences (TTGCTTANNNNNNAAG). CBNAC was able to bind to CBNACBS, which resulted in the repression of transcription in Arabidopsis protoplasts. Interestingly, the transcriptional repression mediated by CBNAC was enhanced by CaM. These results suggest that CBNAC may be a CaM-regulated transcriptional repressor in Arabidopsis.

AtBAG6, a novel calmodulin-binding protein, induces programmed cell death in yeast and plants

Calmodulin (CaM) influences many cellular processes by interacting with various proteins. Here, we isolated AtBAG6, an Arabidopsis CaM-binding protein that contains a central BCL-2-associated athanogene (BAG) domain. In yeast and plants, overexpression of AtBAG6 induced cell death phenotypes consistent with programmed cell death (PCD). Recombinant AtBAG6 had higher affinity for CaM in the absence of free Ca2 + than in its presence. An IQ motif (IQXXXRGXXXR, where X denotes any amino-acid) was required for Ca2 +-independent CaM complex formation and single amino-acid changes within this motif abrogated both AtBAG6-activated CaM-binding and cell death in yeast and plants. A 134-amino-acid stretch, encompassing both the IQ motif and BAG domain, was sufficient to induce cell death. Agents generating oxygen radicals, which are known to be involved in plant PCD, specifically induced the AtBAG6 transcript. Collectively, these results suggest that AtBAG6 is a stress-upregulated CaM-binding protein involved in plant PCD.

Catalytic activation of the plant MAPK phosphatase NtMKP1 by its physiological substrate salicylic acid-induced protein kinase but not by calmodulins

MAPK phosphatases (MKPs) are negative regulators of MAPKs. Previously, we identified NtMKP1 as a novel calmodulin (CaM)-binding protein (Yamakawa, H., Katou, S., Seo, S., Mitsuhara, I., Kamada, H., and Ohashi, Y. (2004) J. Biol. Chem. 279, 928-936). In this study, we characterized the interaction of NtMKP1 with substrate MAPKs and CaM. NtMKP1 (produced by in vitro transcription/translation) inactivated salicylic acid-induced protein kinase (SIPK) through dephosphorylation of the TEY motif of SIPK. CaM bound but unexpectedly did not activate the phosphatase activity of NtMKP1. NtMKP1 has four characteristic domains, viz. a dual-specificity phosphatase catalytic domain, a gelsolin homology domain, a CaM-binding domain, and C-terminal domain. Deletion analysis revealed that the N-terminal non-catalytic region of NtMKP1 bound SIPK and was essential for inactivating SIPK, whereas the CaM-binding and C-terminal domains were dispensable. Moreover, the phosphatase activity of NtMKP1 was increased strongly by the binding of SIPK, but weakly by another MAPK, wound-induced protein kinase. Swapping and site-directed mutagenesis of SIPK and wound-induced protein kinase revealed that the strong activation of NtMKP1 phosphatase activity by SIPK partially depended on the putative common docking domain of SIPK. On the other hand, conversion of Lys(41) and Arg(43) of NtMKP1 to Ala (K41A/R43A) abolished the interaction with SIPK. Expression of constitutively active MAPK kinase in Nicotiana benthamiana induced activation of SIPK and cell death. Simultaneous expression of either NtMKP1 or NtMKP1 L443R, which was unable to bind CaM, compromised the constitutively active MAPK kinase-induced responses, whereas that of NtMKP1 K41A/R43A did not. These results indicate that the regulation of NtMKP1 activity by SIPK binding, but not by CaM binding, is important for the function of NtMKP1.

The mouse t complex distorter/sterility candidate, Dnahc8, expresses a γ-type axonemal dynein heavy chain isoform confined to the principal piece of the sperm tail

Heterozygosity for a t haplotype (t) in male mice results in distorted transmission (TRD) of the t-bearing chromosome 17 homolog to their offspring. However, homozygosity for t causes male sterility, thus limiting the spread of t through the population at large. The Ca(2+)-dependent sperm tail curvature phenotypes, "fishhook", where abnormally high levels of sperm exhibit sharp bends in the midpiece, and "curlicue", where motile sperm exhibit a chronic negative curving of the entire tail, have been tightly linked to t-associated male TRD and sterility traits, respectively. Genetic studies have indicated that homozygosity for the t allele of Dnahc8, an axonemal gamma-type dynein heavy chain (gammaDHC) gene, is partially responsible for expression of "curlicue"; however, its involvement in "fishhook"/TRD, if any, is unknown. Here we report that the major isoform of DNAHC8 is copiously expressed, carries an extended N-terminus and full-length C-terminus, and is stable and equally abundant in both testis and sperm from +/+ and t/t animals. By in silico analysis we also demonstrate that at least three of the seventeen DNAHC8(t) mutations at highly conserved positions in wild-type DHCs may be capable of substantially altering normal DNAHC8 function. Interestingly, DNAHC8 is confined to the principal piece of the sperm tail. The combined results of this study suggest possible mechanisms of DNAHC8(t) dysfunction and involvement in "curlicue", and support the hypothesis that "curlicue" is a multigenic phenomenon. They also demonstrate that the accelerated "fishhook" phenotype of sperm from +/t males is not directly linked to DNAHC8(t) dysfunction.

WRKY group IId transcription factors interact with calmodulin

Calmodulin (CaM) is a ubiquitous Ca(2+)-binding protein known to regulate diverse cellular functions by modulating the activity of various target proteins. We isolated a cDNA encoding AtWRKY7, a novel CaM-binding transcription factor, from an Arabidopsis expression library with horseradish peroxidase-conjugated CaM. CaM binds specifically to the Ca(2+)-dependent CaM-binding domain (CaMBD) of AtWRKY7, as shown by site-directed mutagenesis, a gel mobility shift assay, a split-ubiquitin assay, and a competition assay using a Ca2+/CaM-dependent enzyme. Furthermore, we show that the CaMBD of AtWRKY7 is a conserved structural motif (C-motif) found in group IId of the WRKY protein family.

Proteomics of calcium-signaling components in plants

Calcium functions as a versatile messenger in mediating responses to hormones, biotic/abiotic stress signals and a variety of developmental cues in plants. The Ca(2+)-signaling circuit consists of three major "nodes"--generation of a Ca(2+)-signature in response to a signal, recognition of the signature by Ca2+ sensors and transduction of the signature message to targets that participate in producing signal-specific responses. Molecular genetic and protein-protein interaction approaches together with bioinformatic analysis of the Arabidopsis genome have resulted in identification of a large number of proteins at each "node"--approximately 80 at Ca2+ signature, approximately 400 sensors and approximately 200 targets--that form a myriad of Ca2+ signaling networks in a "mix and match" fashion. In parallel, biochemical, cell biological, genetic and transgenic approaches have unraveled functions and regulatory mechanisms of a few of these components. The emerging paradigm from these studies is that plants have many unique Ca2+ signaling proteins. The presence of a large number of proteins, including several families, at each "node" and potential interaction of several targets by a sensor or vice versa are likely to generate highly complex networks that regulate Ca(2+)-mediated processes. Therefore, there is a great demand for high-throughput technologies for identification of signaling networks in the "Ca(2+)-signaling-grid" and their roles in cellular processes. Here we discuss the current status of Ca2+ signaling components, their known functions and potential of emerging high-throughput genomic and proteomic technologies in unraveling complex Ca2+ circuitry.