Characterization of the calmodulin gene family in wheat: structure, chromosomal location, and evolutionary aspects

Genome-wide identification and expression analysis of calmodulin and calmodulin-like genes in wheat (Triticum aestivum L.)

Calmodulin (CaM) and calmodulin-like (CML) genes are widely involved in plant growth and development and mediating plant stress tolerance. However, the whole genome scale studies about CaM and CML gene families have not been done in wheat, and the possible functions of most wheat CaM/CML gene members are still unknown. In this study, a total of 18 TaCaM and 230 TaCML gene members were identified in wheat genome. Among these genes, 28 TaCaM/CML gene members have 74 duplicated copies, while 21 genes have 48 transcript variants, resulting in 321 putative TaCaM/CML transcripts totally. Phylogenetic tree analysis showed that they can be classified into 7 subfamilies. Similar gene structures and protein domains can be found in members of the same gene cluster. The TaCaM/CML genes were spread among all 21 chromosomes with unbalanced distributions, while most of the gene clusters contained 3 homoeologous genes located in the same homoeologous chromosome group. Synteny analysis showed that most of TaCaM/CMLs gene members can be found with 1-4 paralogous genes in T. turgidum and Ae. Tauschii. High numbers of cis-acting elements related to plant hormones and stress responses can be observed in the promoters of TaCaM/CMLs. The spatiotemporal expression patterns showed that most of the TaCaM/TaCML genes can be detected in at least one tissue. The expression levels of TaCML17, 21, 30, 50, 59 and 75 in the root or shoot can be up-regulated by abiotic stresses, suggesting that TaCML17, 21, 30, 50, 59 and 75 may be related with responses to abiotic stresses in wheat. The spatiotemporal expression patterns of TaCaM/CML genes indicated they may be involved widely in wheat growth and development. Our results provide important clues for exploring functions of TaCaMs/CMLs in growth and development as well as responses to abiotic stresses in wheat in the future.

Calmodulin Is the Fundamental Regulator of NADK-Mediated NAD Signaling in Plants

Calcium (Ca2+) signaling and nicotinamide adenine dinucleotide (NAD) signaling are two basic signal regulation pathways in organisms, playing crucial roles in signal transduction, energy metabolism, stress tolerance, and various developmental processes. Notably, calmodulins (CaMs) and NAD kinases (NADKs) are important hubs for connecting these two types of signaling networks, where CaMs are the unique activators of NADKs. NADK is a key enzyme for NADP (including NADP+ and NADPH) biosynthesis by phosphorylating NAD (including NAD+ and NADH) and therefore, maintains the balance between NAD pool and NADP pool through an allosteric regulation mode. In addition, the two respective derivatives from NAD+ (substrate of NADK) and NADP+ (product of NADK), cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP), have been considered to be the important messengers for intracellular Ca2+ homeostasis which could finally influence the combination between CaM and NADK, forming a feedback regulation mechanism. In this review article, we briefly summarized the major research advances related to the feedback regulation pathway, which is activated by the interaction of CaM and NADK during plant development and signaling. The theories and fact will lay a solid foundation for further studies related to CaM and NADK and their regulatory mechanisms as well as the NADK-mediated NAD signaling behavior in plant development and response to stress.

Abiotic stress responses in plants: roles of calmodulin-regulated proteins

Intracellular changes in calcium ions (Ca(2+)) in response to different biotic and abiotic stimuli are detected by various sensor proteins in the plant cell. Calmodulin (CaM) is one of the most extensively studied Ca(2+)-sensing proteins and has been shown to be involved in transduction of Ca(2+) signals. After interacting with Ca(2+), CaM undergoes conformational change and influences the activities of a diverse range of CaM-binding proteins. A number of CaM-binding proteins have also been implicated in stress responses in plants, highlighting the central role played by CaM in adaptation to adverse environmental conditions. Stress adaptation in plants is a highly complex and multigenic response. Identification and characterization of CaM-modulated proteins in relation to different abiotic stresses could, therefore, prove to be essential for a deeper understanding of the molecular mechanisms involved in abiotic stress tolerance in plants. Various studies have revealed involvement of CaM in regulation of metal ions uptake, generation of reactive oxygen species and modulation of transcription factors such as CAMTA3, GTL1, and WRKY39. Activities of several kinases and phosphatases have also been shown to be modulated by CaM, thus providing further versatility to stress-associated signal transduction pathways. The results obtained from contemporary studies are consistent with the proposed role of CaM as an integrator of different stress signaling pathways, which allows plants to maintain homeostasis between different cellular processes. In this review, we have attempted to present the current state of understanding of the role of CaM in modulating different stress-regulated proteins and its implications in augmenting abiotic stress tolerance in plants.

Involvement of calmodulin in regulation of primary root elongation by N-3-oxo-hexanoyl homoserine lactone in Arabidopsis thaliana

Many bacteria use signal molecules of low molecular weight to monitor their local population density and to coordinate their collective behavior in a process called "quorum sensing" (QS). N-acyl-homoserine lactones (AHLs) are the primary QS signals among Gram-negative bacteria. AHL-mediated QS plays an essential role in diverse bacterial physiological processes. Recent evidence shows that plants are able to sense bacterial AHLs and respond to them appropriately. However, little is known about the mechanism by which plants perceive and transduce the bacterial AHLs within cells. In this study, we found that the stimulatory effect of N-3-oxo-hexanoyl homoserine lactone (3OC6-HSL) on primary root elongation of Arabidopsis was abolished by the calmodulin (CaM) antagonists N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide (W-7) and trifluoperazine (TFP). Western-blot and ELISA analysis revealed that the concentration of CaM protein in Arabidopsis roots increased after treatment with 1 μM 3OC6-HSL. Results from quantitative RT-PCR demonstrated that the transcription of all nine CaM genes in Arabidopsis genome was up-regulated in the plants treated with 3OC6-HSL. The loss-of-function mutants of each AtCaM gene (AtCaM1-9) were insensitive to 3OC6-HSL-stimulation of primary root elongation. On the other hand, the genetic evidence showed that CaM may not participates the inhibition of primary root length caused by application of long-chained AHLs such as C10-HSL and C12-HSL. Nevertheless, our results suggest that CaM is involved in the bacterial 3OC6-HSL signaling in plant cells. These data offer new insight into the mechanism of plant response to bacterial QS signals.

On the complexity of miRNA-mediated regulation in plants: novel insights into the genomic organization of plant miRNAs

MicroRNAs (miRNAs) are endogenous small non-coding RNAs of about 20–24 nt, known to play key roles in post-transcriptional gene regulation, that can be coded either by intergenic or intragenic loci. Intragenic (exonic and intronic) miRNAs can exert a role in the transcriptional regulation and RNA processing of their host gene. Moreover, the possibility that the biogenesis of exonic miRNAs could destabilize the corresponding protein-coding transcript and reduce protein synthesis makes their characterization very intriguing and suggests a possible novel mechanism of post-transcriptional regulation of gene expression.

This work was designed to carry out the computational identification of putative exonic miRNAs in 30 plant species and the analysis of possible mechanisms involved in their regulation.

The results obtained represent a useful starting point for future studies on the complex networks involved in microRNA-mediated gene regulation in plants.

Functional analysis of the stress-inducible soybean calmodulin isoform-4 (GmCaM-4) promoter in transgenic tobacco plants

The transcription of soybean (Glycine max) calmodulin isoform-4 (GmCaM-4) is dramatically induced within 0.5 h of exposure to pathogen or NaCl. Core cis-acting elements that regulate the expression of the GmCaM-4 gene in response to pathogen and salt stress were previously identified, between -1,207 and -1,128 bp, and between -858 and -728 bp, in the GmCaM-4 promoter. Here, we characterized the properties of the DNA-binding complexes that form at the two core cis-acting elements of the GmCaM-4 promoter in pathogen-treated nuclear extracts. We generated GUS reporter constructs harboring various deletions of approximately 1.3-kb GmCaM-4 promoter, and analyzed GUS expression in tobacco plants transformed with these constructs. The GUS expression analysis suggested that the two previously identified core regions are involved in inducing GmCaM-4 expression in the heterologous system. Finally, a transient expression assay of Arabidopsis protoplasts showed that the GmCaM-4 promoter produced greater levels of GUS activity than did the CaMV35S promoter after pathogen or NaCl treatments, suggesting that the GmCaM-4 promoter may be useful in the production of conditional gene expression systems.

Do plants and animals differ in phenotypic plasticity?

This paper compares the flexibility in the nexus between phenotype and genotype in plants and animals. These taxa although considered to be fundamentally different are found to be surprisingly similar in the mechanisms used to achieve plasticity. Although non-cognitive behaviour occurs in plants, its range is limited, while morphological and developmental plasticity also occur to a considerable extent in animals. Yet both plants and animals are subject to unique constraints and thus need to find unique solutions to functional problems. A true comparison between the plant and animal phenotype would be a comparison between plants and sessile photosynthesizing colonial invertebrates. Such comparisons are lacking. However, they would provide important insights into the adaptive significance of plasticity in these groups. It is also suggested that a comparison of inflexible traits in these groups would provide an understanding of the constraints, as well as the costs and benefits,of a plastic versus non-plastic phenotype in plants and animals.

Pathogen- and NaCl-induced expression of the SCaM-4 promoter is mediated in part by a GT-1 box that interacts with a GT-1-like transcription factor

The Ca(2+)-binding protein calmodulin mediates cellular Ca(2+) signals in response to a wide array of stimuli in higher eukaryotes. Plants express numerous CaM isoforms. Transcription of one soybean (Glycine max) CaM isoform, SCaM-4, is dramatically induced within 30 min of pathogen or NaCl stresses. To characterize the cis-acting element(s) of this gene, we isolated an approximately 2-kb promoter sequence of the gene. Deletion analysis of the promoter revealed that a 130-bp region located between nucleotide positions -858 and -728 is required for the stressors to induce expression of SCaM-4. A hexameric DNA sequence within this region, GAAAAA (GT-1 cis-element), was identified as a core cis-acting element for the induction of the SCaM-4 gene. The GT-1 cis-element interacts with an Arabidopsis GT-1-like transcription factor, AtGT-3b, in vitro and in a yeast selection system. Transcription of AtGT-3b is also rapidly induced within 30 min after pathogen and NaCl treatment. These results suggest that an interaction between a GT-1 cis-element and a GT-1-like transcription factor plays a role in pathogen- and salt-induced SCaM-4 gene expression in both soybean and Arabidopsis.

Calcium/calmodulin-mediated signal network in plants

Various extracellular stimuli elicit specific calcium signatures that can be recognized by different calcium sensors. Calmodulin, the predominant calcium receptor, is one of the best-characterized calcium sensors in eukaryotes. In recent years, completion of the Arabidopsis genome project and advances in functional genomics have helped to identify and characterize numerous calmodulin-binding proteins in plants. There are some similarities in Ca(2+)/calmodulin-mediated signaling in plants and animals. However, plants possess multiple calmodulin genes and many calmodulin target proteins, including unique protein kinases and transcription factors. Some of these proteins are likely to act as "hubs" during calcium signal transduction. Hence, a better understanding of the function of these calmodulin target proteins should help in deciphering the Ca(2+)/calmodulin-mediated signal network and its role in plant growth, development and response to environmental stimuli.

Calmodulin is involved in heat shock signal transduction in wheat

The involvement of calcium and calcium-activated calmodulin (Ca(2+)-CaM) in heat shock (HS) signal transduction in wheat (Triticum aestivum) was investigated. Using Fluo-3/acetoxymethyl esters and laser scanning confocal microscopy, it was found that the increase of intracellular free calcium ion concentration started within 1 min after a 37 degrees C HS. The levels of CaM mRNA and protein increased during HS at 37 degrees C in the presence of Ca(2+). The expression of hsp26 and hsp70 genes was up-regulated by the addition of CaCl(2) and down-regulated by the calcium ion chelator EGTA, the calcium ion channel blockers LaCl(3) and verapamil, or the CaM antagonists N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide and chlorpromazine. Treatment with Ca(2+) also increased, and with EGTA, verapamil, chlorpromazine, or trifluoperazine decreased, synthesis of HS proteins. The temporal expression of the CaM1-2 gene and the hsp26 and hsp70 genes demonstrated that up-regulation of the CaM1-2 gene occurred at 10 min after HS at 37 degrees C, whereas that of hsp26 and hsp70 appeared at 20 min after HS. A 5-min HS induced expression of hsp26 after a period of recovery at 22 degrees C after HS at 37 degrees C. Taken together, these results indicate that Ca(2+)-CaM is directly involved in the HS signal transduction pathway. A working hypothesis about the relationship between upstream and downstream of HS signal transduction is presented.

Identification of calmodulin isoform-specific binding peptides from a phage-displayed random 22-mer peptide library

Plants express numerous calmodulin (CaM) isoforms that exhibit differential activation or inhibition of CaM-dependent enzymes in vitro; however, their specificities toward target enzyme/protein binding are uncertain. A random peptide library displaying a 22-mer peptide on a bacteriophage surface was constructed to screen peptides that specifically bind to plant CaM isoforms (soybean calmodulin (ScaM)-1 and SCaM-4 were used in this study) in a Ca2+-dependent manner. The deduced amino acid sequence analyses of the respective 80 phage clones that were independently isolated via affinity panning revealed that SCaM isoforms require distinct amino acid sequences for optimal binding. SCaM-1-binding peptides conform to a 1-5-10 ((FILVW)XXX(FILV) XXXX(FILVW)) motif (where X denotes any amino acid), whereas SCaM-4-binding peptide sequences conform to a 1-8-14 ((FILVW)XXXXXX(FAILVW)XXXXX(FILVW)) motif. These motifs are classified based on the positions of conserved hydrophobic residues. To examine their binding properties further, two representative peptides from each of the SCaM isoform-binding sequences were synthesized and analyzed via gel mobility shift assays, Trp fluorescent spectra analyses, and phosphodiesterase competitive inhibition experiments. The results of these studies suggest that SCaM isoforms possess different binding sequences for optimal target interaction, which therefore may provide a molecular basis for CaM isoform-specific function in plants. Furthermore, the isolated peptide sequences may serve not only as useful CaM-binding sequence references but also as potential reagents for studying CaM isoform-specific function in vivo.

Mlo, a modulator of plant defense and cell death, is a novel calmodulin-binding protein. Isolation and characterization of a rice Mlo homologue

Transient influx of Ca(2+) constitutes an early event in the signaling cascades that trigger plant defense responses. However, the downstream components of defense-associated Ca(2+) signaling are largely unknown. Because Ca(2+) signals are mediated by Ca(2+)-binding proteins, including calmodulin (CaM), identification and characterization of CaM-binding proteins elicited by pathogens should provide insights into the mechanism by which Ca(2+) regulates defense responses. In this study, we isolated a gene encoding rice Mlo (Oryza sativa Mlo; OsMlo) using a protein-protein interaction-based screening of a cDNA expression library constructed from pathogen-elicited rice suspension cells. OsMlo has a molecular mass of 62 kDa and shares 65% sequence identity and scaffold topology with barley Mlo, a heptahelical transmembrane protein known to function as a negative regulator of broad spectrum disease resistance and leaf cell death. By using gel overlay assays, we showed that OsMlo produced in Escherichia coli binds to soybean CaM isoform-1 (SCaM-1) in a Ca(2+)-dependent manner. We located a 20-amino acid CaM-binding domain (CaMBD) in the OsMlo C-terminal cytoplasmic tail that is necessary and sufficient for Ca(2+)-dependent CaM complex formation. Specific binding of the conserved CaMBD to CaM was corroborated by site-directed mutagenesis, a gel mobility shift assay, and a competition assay with a Ca(2+)/CaM-dependent enzyme. Expression of OsMlo was strongly induced by a fungal pathogen and by plant defense signaling molecules. We propose that binding of Ca(2+)-loaded CaM to the C-terminal tail may be a common feature of Mlo proteins.

Isolation and characterization of a novel calmodulin-binding protein from potato

Tuberization in potato is controlled by hormonal and environmental signals. Ca(2+), an important intracellular messenger, and calmodulin (CaM), one of the primary Ca(2+) sensors, have been implicated in controlling diverse cellular processes in plants including tuberization. The regulation of cellular processes by CaM involves its interaction with other proteins. To understand the role of Ca(2+)/CaM in tuberization, we have screened an expression library prepared from developing tubers with biotinylated CaM. This screening resulted in isolation of a cDNA encoding a novel CaM-binding protein (potato calmodulin-binding protein (PCBP)). Ca(2+)-dependent binding of the cDNA-encoded protein to CaM is confirmed by (35)S-labeled CaM. The full-length cDNA is 5 kb long and encodes a protein of 1309 amino acids. The deduced amino acid sequence showed significant similarity with a hypothetical protein from another plant, Arabidopsis. However, no homologs of PCBP are found in nonplant systems, suggesting that it is likely to be specific to plants. Using truncated versions of the protein and a synthetic peptide in CaM binding assays we mapped the CaM-binding region to a 20-amino acid stretch (residues 1216-1237). The bacterially expressed protein containing the CaM-binding domain interacted with three CaM isoforms (CaM2, CaM4, and CaM6). PCBP is encoded by a single gene and is expressed differentially in the tissues tested. The expression of CaM, PCBP, and another CaM-binding protein is similar in different tissues and organs. The predicted protein contained seven putative nuclear localization signals and several strong PEST motifs. Fusion of the N-terminal region of the protein containing six of the seven nuclear localization signals to the reporter gene beta-glucuronidase targeted the reporter gene to the nucleus, suggesting a nuclear role for PCBP.

Calmodulin as a versatile calcium signal transducer in plants

The complexity of Ca²⁺ patterns observed in eukaryotic cells, including plants, has led to the hypothesis that specific patterns of Ca²⁺ propagation, termed Ca²⁺ signatures, encode information and relay it to downstream elements (effectors) for translation into appropriate cellular responses. Ca²⁺ -binding proteins (sensors) play a key role in decoding Ca²⁺ signatures and transducing signals by activating specific targets and pathways. Calmodulin is a Ca²⁺ sensor known to modulate the activity of many mammalian proteins, whose targets in plants are now being actively characterized. Plants possess an interesting and rapidly growing list of calmodulin targets with a variety of cellular roles. Nevertheless, many targets appear to be unique to plants and remain uncharacterized, calling for a concerted effort to elucidate their functions. Moreover, the extended family of calmodulin-related proteins in plants consists of evolutionarily divergent members, mostly of unknown function, although some have recently been implicated in stress responses. It is hoped that advances in functional genomics, and the research tools it generates, will help to explain themultiplicity of calmodulin genes in plants, and to identify their downstream effectors. This review summarizes current knowledge of the Ca²⁺ -calmodulin messenger system in plants and presents suggestions for future areas of research. Contents I. Introduction 36 II. CaM isoforms and CaM-like proteins 37 III. CaM-target proteins 42 IV. CaM and nuclear functions 46 V. Regulation of ion transport 49 VI. CaM and plant responses to environmental stimuli 52 VII. Conclusions and future studies 58 Acknowledgements 59 References 59.

Calcium: silver bullet in signaling

Accumulating evidence suggests that Ca(2+) serves as a messenger in many normal growth and developmental process and in plant responses to biotic and abiotic stresses. Numerous signals have been shown to induce transient elevation of [Ca(2+)](cyt) in plants. Genetic, biochemical, molecular and cell biological approaches in recent years have resulted in significant progress in identifying several Ca(2+)-sensing proteins in plants and in understanding the function of some of these Ca(2+)-regulated proteins at the cellular and whole plant level. As more and more Ca(2+)-sensing proteins are identified it is becoming apparent that plants have several unique Ca(2+)-sensing proteins and that the downstream components of Ca(2+) signaling in plants have novel features and regulatory mechanisms. Although the mechanisms by which Ca(2+) regulates diverse biochemical and molecular processes and eventually physiological processes in response to diverse signals are beginning to be understood, recent studies have raised many interesting questions. Despite the fact that Ca(2+) sensing proteins are being identified at a rapid pace, progress on the function(s) of many of them is limited. Studies on plant 'signalome' - the identification of all signaling components in all messengers mediated transduction pathways, analysis of their function and regulation, and cross talk among these components - should help in understanding the inner workings of plant cell responses to diverse signals. New functional genomics approaches such as reverse genetics, microarray analyses coupled with in vivo protein-protein interaction studies and proteomics should not only permit functional analysis of various components in Ca(2+) signaling but also enable identification of a complex network of interactions.

An early ethylene up-regulated gene encoding a calmodulin-binding protein involved in plant senescence and death

35S-Labeled calmodulin (CaM) was used to screen a tobacco anther cDNA library. A positive clone (NtER1) with high homology to an early ethylene-up-regulated gene (ER66) in tomato, and an Arabidopsis homolog was isolated and characterized. Based on the helical wheel projection, a 25-mer peptide corresponding to the predicted CaM-binding region of NtER1 (amino acids 796-820) was synthesized. The gel-mobility shift assay showed that the peptide formed a stable complex with CaM only in the presence of Ca(2+). CaM binds to NtER1 with high affinity (K(d) approximately 12 nm) in a calcium-dependent manner. Tobacco flowers at different stages of development were treated with ethylene or with 1-methylcyclopropene for 2 h before treating with ethylene. Northern analysis showed that the NtER1 was rapidly induced after 15 min of exposure to ethylene. However, the 2-h 1-methylcyclopropene treatment totally blocked NtER1 expression in flowers at all stages of development, suggesting that NtER1 is an early ethylene-up-regulated gene. The senescing leaves and petals had significantly increased NtER1 induction as compared with young leaves and petals, implying that NtER1 is developmentally regulated and acts as a trigger for senescence and death. This is the first documented evidence for the involvement of Ca(2+)/CaM-mediated signaling in ethylene action.

Molecular and biochemical evidence for the involvement of calcium/calmodulin in auxin action

The use of (35)S-labeled calmodulin (CaM) to screen a corn root cDNA expression library has led to the isolation of a CaM-binding protein, encoded by a cDNA with sequence similarity to small auxin up RNAs (SAURs), a class of early auxin-responsive genes. The cDNA designated as ZmSAUR1 (Zea mays SAURs) was expressed in Escherichia coli, and the recombinant protein was purified by CaM affinity chromatography. The CaM binding assay revealed that the recombinant protein binds to CaM in a calcium-dependent manner. Deletion analysis revealed that the CaM binding site was located at the NH(2)-terminal domain. A synthetic peptide of amino acids 20-45, corresponding to the potential CaM binding region, was used for calcium-dependent mobility shift assays. The synthetic peptide formed a stable complex with CaM only in the presence of calcium. The CaM affinity assay indicated that ZmSAUR1 binds to CaM with high affinity (K(d) approximately 15 nM) in a calcium-dependent manner. Comparison of the NH(2)-terminal portions of all of the characterized SAURs revealed that they all contain a stretch of the basic alpha-amphiphilic helix similar to the CaM binding region of ZmSAUR1. CaM binds to the two synthetic peptides from the NH(2)-terminal regions of Arabidopsis SAUR-AC1 and soybean 10A5, suggesting that this is a general phenomenon for all SAURs. Northern analysis was carried out using the total RNA isolated from auxin-treated corn coleoptile segments. ZmSAUR1 gene expression began within 10 min, increased rapidly between 10 and 60 min, and peaked around 60 min after 10 microM alpha-naphthaleneacetic acid treatment. These results indicate that ZmSAUR1 is an early auxin-responsive gene. The CaM antagonist N-(6-aminohexyl)5-chloro-1-naphthalenesulfonamide hydrochloride inhibited the auxin-induced cell elongation but not the auxin-induced expression of ZmSAUR1. This suggests that calcium/CaM do not regulate ZmSAUR1 at the transcriptional level. CaM binding to ZmSAUR1 in a calcium-dependent manner suggests that calcium/CaM regulate ZmSAUR1 at the post-translational level. Our data provide the first direct evidence for the involvement of calcium/CaM-mediated signaling in auxin-mediated signal transduction.

Distinct calcium signaling pathways regulate calmodulin gene expression in tobacco

Cold shock and wind stimuli initiate Ca(2+) transients in transgenic tobacco (Nicotiana plumbaginifolia) seedlings (named MAQ 2.4) containing cytoplasmic aequorin. To investigate whether these stimuli initiate Ca(2+) pathways that are spatially distinct, stress-induced nuclear and cytoplasmic Ca(2+) transients and the expression of a stress-induced calmodulin gene were compared. Tobacco seedlings were transformed with a construct that encodes a fusion protein between nucleoplasmin (a major oocyte nuclear protein) and aequorin. Immunocytochemical evidence indicated targeting of the fusion protein to the nucleus in these plants, which were named MAQ 7.11. Comparison between MAQ 7.11 and MAQ 2.4 seedlings confirmed that wind stimuli and cold shock invoke separate Ca(2+) signaling pathways. Partial cDNAs encoding two tobacco calmodulin genes, NpCaM-1 and NpCaM-2, were identified and shown to have distinct nucleotide sequences that encode identical polypeptides. Expression of NpCaM-1, but not NpCaM-2, responded to wind and cold shock stimulation. Comparison of the Ca(2+) dynamics with NpCaM-1 expression after stimulation suggested that wind-induced NpCaM-1 expression is regulated by a Ca(2+) signaling pathway operational predominantly in the nucleus. In contrast, expression of NpCaM-1 in response to cold shock is regulated by a pathway operational predominantly in the cytoplasm.

Interaction of a kinesin-like protein with calmodulin isoforms from Arabidopsis

In Arabidopsis and other plants there are multiple calmodulin isoforms. However, the role of these isoforms in regulating the activity of target proteins is obscure. Here, we analyzed the interaction between a kinesin-like calmodulin-binding motor protein (Reddy, A. S. N., Safadi, F., Narasimhulu, S. B., Golovkin, M., and Hu, X. (1996) J. Biol. Chem. 271, 7052-7060) and three calmodulin isoforms (calmodulin-2, -4, and -6) from Arabidopsis using different approaches. Gel mobility and fluorescence shift assays revealed that the motor binds to all calmodulin isoforms in a calcium-dependent manner. Furthermore, all calmodulin isoforms were able to activate bovine calcium/calmodulin-dependent phosphodiesterase. However, the concentration of calmodulin-2 required for half-maximal activation of phosphodiesterase is 2- and 6-fold lower compared with calmodulin-4 and -6, respectively. The dissociation constants of the motor to calmodulin-2, -4, and -6 are 12.8, 27.0, and 27.8 nM, respectively, indicating that calmodulin-2 has 2-fold higher affinity for the motor than calmodulin-4 and -6. Similar results were obtained using another assay that involves the binding of (35)S-labeled calmodulin isoforms to the motor. The binding saturation curves of the motor with calmodulin isoforms have confirmed that calmodulin-2 has 2-fold higher affinity to the motor. However, the affinity of calmodulin-4 and -6 isoforms for the motor was about the same. Based on these studies, we conclude that all calmodulin isoforms bind to the motor protein but with different affinities.

Loss of conformational stability in calmodulin upon methionine oxidation

We have used electrospray ionization mass spectrometry (ESI-MS), circular dichroism (CD), and fluorescence spectroscopy to investigate the secondary and tertiary structural consequences that result from oxidative modification of methionine residues in wheat germ calmodulin (CaM), and prevent activation of the plasma membrane Ca-ATPase. Using ESI-MS, we have measured rates of modification and molecular mass distributions of oxidatively modified CaM species (CaMox) resulting from exposure to H2O2. From these rates, we find that oxidative modification of methionine to the corresponding methionine sulfoxide does not predispose CaM to further oxidative modification. These results indicate that methionine oxidation results in no large-scale alterations in the tertiary structure of CaMox, because the rates of oxidative modification of individual methionines are directly related to their solvent exposure. Likewise, CD measurements indicate that methionine oxidation results in little change in the apparent alpha-helical content at 28 degrees C, and only a small (0.3 +/- 0.1 kcal mol(-1)) decrease in thermal stability, suggesting the disruption of a limited number of specific noncovalent interactions. Fluorescence lifetime, anisotropy, and quenching measurements of N-(1-pyrenyl)-maleimide (PMal) covalently bound to Cys26 indicate local structural changes around PMal in the amino-terminal domain in response to oxidative modification of methionine residues in the carboxyl-terminal domain. Because the opposing globular domains remain spatially distant in both native and oxidatively modified CaM, the oxidative modification of methionines in the carboxyl-terminal domain are suggested to modify the conformation of the amino-terminal domain through alterations in the structural features involving the interdomain central helix. The structural basis for the linkage between oxidative modification and these global conformational changes is discussed in terms of possible alterations in specific noncovalent interactions that have previously been suggested to stabilize the central helix in CaM.