Isolation and sequence analysis of a cDNA clone for a carrot calcium-dependent protein kinase: homology to calcium/calmodulin-dependent protein kinases and to calmodulin

Ca2+-independent ZmCPK2 is inhibited by Ca2+-dependent ZmCPK17 during drought response in maize

Calcium oscillations are induced by different stresses. Calcium-dependent protein kinases (CDPKs/CPKs) are one major group of the plant calcium decoders that are involved in various processes including drought response. Some CPKs are calcium-independent. Here, we identified ZmCPK2 as a negative regulator of drought resistance by screening an overexpression transgenic maize pool. We found that ZmCPK2 does not bind calcium, and its activity is mainly inhibited during short term abscisic acid (ABA) treatment, and dynamically changed in prolonged treatment. Interestingly, ZmCPK2 interacts with and is inhibited by calcium-dependent ZmCPK17, a positive regulator of drought resistance, which is activated by ABA. ZmCPK17 could prevent the nuclear localization of ZmCPK2 through phosphorylation of ZmCPK2T60. ZmCPK2 interacts with and phosphorylates and activates ZmYAB15, a negative transcriptional factor for drought resistance. Our results suggest that drought stress-induced Ca2+ can be decoded directly by ZmCPK17 that inhibits ZmCPK2, thereby promoting plant adaptation to water deficit.

Genome-wide association analysis of 101 accessions dissects the genetic basis of shell thickness for genetic improvement in Persian walnut (Juglans regia L.)

BACKGROUND

Understanding the underlying genetic mechanisms that drive phenotypic variations is essential for enhancing the efficacy of crop improvement. Persian walnut (Juglans regia L.), which is grown extensively worldwide, is an important economic tree fruit due to its horticultural, medicinal, and material value. The quality of the walnut fruit is related to the selection of traits such as thinner shells, larger filling rates, and better taste, which is very important for breeding in China. The complex quantitative fruit-related traits are influenced by a variety of physiological and environmental factors, which can vary widely between walnut genotypes.

RESULTS

For this study, a set of 101 Persian walnut accessions were re-sequenced, which generated a total of 906.2 Gb of Illumina sequence data with an average read depth of 13.8× for each accession. We performed the genome-wide association study (GWAS) using 10.9 Mb of high-quality single-nucleotide polymorphisms (SNPs) and 10 agronomic traits to explore the underlying genetic basis of the walnut fruit. Several candidate genes are proposed to be involved in walnut characteristics, including JrPXC1, JrWAKL8, JrGAMYB, and JrFRK1. Specifically, the JrPXC1 gene was confirmed to participate in the regulation of secondary wall cellulose thickening in the walnut shell.

CONCLUSION

In addition to providing considerable available genetic resources for walnut trees, this study revealed the underlying genetic basis involved in important walnut agronomic traits, particularly shell thickness, as well as providing clues for the improvement of genetic breeding and domestication in other perennial economic crops.

Proteins with calmodulin-like domains: structures and functional roles

The appearance of modular proteins is a widespread phenomenon during the evolution of proteins. The combinatorial arrangement of different functional and/or structural domains within a single polypeptide chain yields a wide variety of activities and regulatory properties to the modular proteins. In this review, we will discuss proteins, that in addition to their catalytic, transport, structure, localization or adaptor functions, also have segments resembling the helix-loop-helix EF-hand motifs found in Ca2+-binding proteins, such as calmodulin (CaM). These segments are denoted CaM-like domains (CaM-LDs) and play a regulatory role, making these CaM-like proteins sensitive to Ca2+ transients within the cell, and hence are able to transduce the Ca2+ signal leading to specific cellular responses. Importantly, this arrangement allows to this group of proteins direct regulation independent of other Ca2+-sensitive sensor/transducer proteins, such as CaM. In addition, this review also covers CaM-binding proteins, in which their CaM-binding site (CBS), in the absence of CaM, is proposed to interact with other segments of the same protein denoted CaM-like binding site (CLBS). CLBS are important regulatory motifs, acting either by keeping these CaM-binding proteins inactive in the absence of CaM, enhancing the stability of protein complexes and/or facilitating their dimerization via CBS/CLBS interaction. The existence of proteins containing CaM-LDs or CLBSs substantially adds to the enormous versatility and complexity of Ca2+/CaM signaling.

Autophosphorylation of Ser-6 via an intermolecular mechanism is important for the rapid reduction of NtCDPK1 kinase activity for substrate RSG

Tobacco (Nicotiana tabacum) Ca2+-dependent protein kinase 1 (NtCDPK1) is involved in feedback regulation of the plant hormone gibberellin through the phosphorylation of the transcription factor, REPRESSION OF SHOOT GROWTH (RSG). Previously, Ser-6 and Thr-21 were identified as autophosphorylation sites in NtCDPK1. Autophosphorylation of Ser-6 and Thr-21 not only decreases the binding affinity of NtCDPK1 for RSG, but also inhibits the homodimerization of NtCDPK1. Furthermore, autophosphorylation decreases the phosphorylation efficiency of RSG. We demonstrated that Ser-6 and Thr-21 of NtCDPK1 are phosphorylated in response to GAs in plants. The substitution of these autophosphorylation sites with Ala enhances the NtCDPK1 overexpression-induced sensitization of seeds to a GA biosynthetic inhibitor during germination. These findings suggested that autophosphorylation of Ser-6 and Thr-21 prevents excessive phosphorylation of RSG. In this study, we attempted to determine which autophosphorylation site is responsible for the functional regulation of NtCDPK1. Ser-6 was autophosphorylated within 1 min, whereas Thr-21 required over 5 min to be completely autophosphorylated. Furthermore, we found that Ser-6 and Thr-21 were autophosphorylated by inter- and intramolecular mechanisms, respectively, which may be reflected in the faster autophosphorylation of Ser-6. Although both autophosphorylation sites were involved in the reduction of the binding affinity of NtCDPK1 for RSG and the inhibition of NtCDPK1 homodimerization, autophosphorylation of Ser-6 alone was sufficient to decrease the kinase activity of NtCDPK1 for RSG. These results suggest that autophosphorylation of Ser-6 is important for the rapid reduction of NtCDPK1 kinase activity for RSG, whereas that of Thr-21 may play an auxiliary role.

DELLA-dependent and -independent gibberellin signaling

DELLA proteins act as negative regulators in gibberellin (GA) signal transduction. GA-induced DELLA degradation is a central regulatory system in GA signaling pathway. Intensive studies have revealed the degradation mechanism of DELLA and the functions of DELLA as a transcriptional regulator. Meanwhile, recent studies suggest the existence of a DELLA-independent GA signaling pathway. In this review, we summarized the DELLA-independent GA signaling pathway together with the well-analyzed DELLA-dependent pathway.

Biochemical characterization of a recombinant Swainsona canescens calcium-dependent protein kinase (ScCPK1)

Calcium-dependent protein kinases (CPKs) constitute a unique family of kinases involved in many physiological responses in plants. Biochemical and kinetic properties of a recombinant Swainsona canescens calcium-dependent protein kinase (ScCPK1) were examined in this study. The optimum pH and temperature for activity were pH 7.5 and 37 °C, respectively. Substrate phosphorylation activity of ScCPK1 was calmodulin (CaM) independent. Yet CaM antagonists, W7 [N-(6-aminohexyl)-5-chloro-1-naphthalene sulphonamide] and calmidazolium inhibited the activity with IC(50) values of 750 nM and 350 μM, respectively. Both serine and threonine residues were found to be phosphorylated in autophosphorylated ScCPK1 and in histone III-S phosphorylated by ScCPK1. The [Ca(2)(+)] for half maximal activity (K(0.5)) was found to be 0.4 μM for ScCPK1 with histone III-S as substrate. Kinetic analysis showed that K(M) of ScCPK1 for histone III-S was 4.8 μM. These data suggest that ScCPK1 is a functional Ser/Thr kinase, regulated by calcium, and may have a role in Ca(2)(+)-mediated signaling in S. canescens.

Heterologous expression and biochemical characterization of two calcium-dependent protein kinase isoforms CaCPK1 and CaCPK2 from chickpea

In plants, calcium-dependent protein kinases (CPKs) constitute a unique family of enzymes consisting of a protein kinase catalytic domain fused to carboxy-terminal autoregulatory and calmodulin-like domains. We isolated two cDNAs encoding calcium-dependent protein kinase isoforms (CaCPK1 and CaCPK2) from chickpea. Both isoforms were expressed as fusion proteins in Escherichia coli. Biochemical analyses have identified CaCPK1 and CaCPK2 as Ca(2+)-dependent protein kinases since both enzymes phosphorylated themselves and histone III-S as substrate only in the presence of Ca(2+). The kinase activity of the recombinant enzymes was calmodulin independent and sensitive to CaM antagonists W7 [N-(6-aminohexyl)-5-chloro-1-naphthalene sulphonamide] and calmidazoilum. Phosphoamino acid analysis revealed that the isoforms transferred the gamma-phosphate of ATP only to serine residues of histone III-S and their autophosphorylation occurred on serine and threonine residues. These two isoforms showed considerable variations with respect to their biochemical and kinetic properties including Ca(2+) sensitivities. The recombinant CaCPK1 has a pH and temperature optimum of pH 6.8-8.6 and 35-42 degrees C, respectively, whereas CaCPK2 has a pH and temperature optimum of pH 7.2-9 and 35-42 degrees C, respectively. Taken together, our results suggest that CaCPK1 and CaCPK2 are functional serine/threonine kinases and may play different roles in Ca(2+)-mediated signaling in chickpea plants.

Anticoccidial kinase inhibitors: Identification of protein kinase targets secondary to cGMP-dependent protein kinase

Trisubstituted pyrrole inhibitors of the essential coccidian parasite cGMP dependent protein kinase (PKG) block parasite invasion and show in vivo efficacy against Eimeria in chickens and Toxoplasma in mice. An imidazopyridine inhibitor of PKG activity with greater potency in both parasite invasion assays and in vivo activity has recently been identified. Susceptibility experiments with a Toxoplasma knock-out strain expressing a complementing compound-refractory PKG allele ('T761Q-KO'), suggest a role for additional secondary protein kinase targets. Using extracts from this engineered T. gondii strain and a radiolabeled imidazopyridine ligand, a single peak of binding activity associated with calmodulin-like domain protein kinase (CDPK1) has been identified. Like PKG, CDPK1 has been implicated in host cell invasion and exhibits sub-nanomolar sensitivity to the compound. Amino acid sequence comparisons of coccidian CDPKs and a mutational analysis reveal that the binding of the ligand to PKG and CDPK1 (but not other CDPK isoforms) is mediated by similar contacts in a catalytic site hydrophobic binding pocket, and can be blocked by analogous amino acid substitutions. Transgenic strains over-expressing a biochemically active but compound-refractory CDPK1 mutant ('G128Q') fail to show reduced susceptibility to the compound in vivo, suggesting that selective inhibition of this enzyme is not responsible for the enhanced anti-parasitic potency of the imidazopyridine analog. An alternative secondary target candidate, the alpha-isoform of casein kinase 1 (CK1alpha), shows sensitivity to the compound in the low nanomolar range. These results provide an example of the utility of the Toxoplasma model system for investigating the mechanism of action of novel anticoccidial agents.

Structure of the regulatory apparatus of a calcium-dependent protein kinase (CDPK): a novel mode of calmodulin-target recognition

Calcium-dependent protein kinases (CDPKs) are a class of calcium-binding sensory proteins that are found in plants and certain protozoa, including the causative agent of malaria, Plasmodium falciparum. CDPKs have diverse regulatory functions, including involvement in the triggering of the lytic cycle of malarial infection. CDPKs contain an autoinhibitory junction (J) region whose calcium-dependent interaction with the tethered regulatory calmodulin-like domain (CaM-LD) activates the catalytic kinase domain. We report here the X-ray crystal structure of the J-CaM-LD region of CDPK from Arabidopsis thaliana (AtCPK1), determined to 2.0 A resolution using multiple-wavelength anomalous dispersion (MAD). The structure reveals a symmetric dimer of calcium-bound J-CaM-LD with domain-swap interactions, in which the J region of one protomer interacts extensively with the carboxy-terminal EF-hand domain (C-lobe) of the partner protomer. However, as the J-CaM-LD is monomeric in solution, the activated monomer was modelled to account for the intra-molecular recognition of the two domains. While the J-CaM-LD segment mimics certain aspects of target motif recognition by CaM other features are specific to CDPKs, in particular the combination of the strong interaction between the N and C-lobes of the CaM-LD and the exclusive use of only the C-lobe in the recognition of the covalently tethered target region. Combined with our previous observations showing that there is likely to be strong interactions between this tethered J region and the CaM-LD even at basal Ca(2+) concentrations, the new structural data indicate that the response to calcium of CDPKs is clearly unique among the CaM family.

Cloning and biochemical properties of CDPK gene OsCDPK14 from rice

A rice CDPK gene, OsCDPK14 (AY144497), was cloned from developing caryopses of rice (Oryza sativa cv. Zhonghua 15). Its cDNA sequence (1922 bp) contains an ORF encoding a 514 amino acids protein (56.7kD, pl 5.18). OsCDPK14 shows the typical structural features of the CDPK family, including a conserved catalytic Ser/Thr kinase domain, an autoinhibitory domain and a CaM-like domain with four putative Ca2+-binding EF hands. Subcellular targeting indicated that OsCDPK14 was located in the cytoplasm, probably due to the absence of myristoylation and palmitoylation motifs. OsCDPK14 was expressed in Escherichia coli and purified from bacterial extracts. The recombinant protein was shown to be a functional protein kinase using Syntide-2, a synthetic peptide. Kinase activity was shown to be Ca2+-dependent, and this activation was strongly enhanced by Mn2+ and inhibited by W7 in vitro. These results provide significant insights into the regulation and biochemical properties of OsCDPK14, suggesting OsCDPK14 may be a signal factor of cytoplasm in rice plant.

Plants, symbiosis and parasites: a calcium signalling connection

A unique family of protein kinases has evolved with regulatory domains containing sequences that are related to Ca(2+)-binding EF-hands. In this family, the archetypal Ca(2+)-dependent protein kinases (CDPKs) have been found in plants and some protists, including the malarial parasite, Plasmodium falciparum. Recent genetic evidence has revealed isoform-specific functions for a CDPK that is essential for Plasmodium berghei gametogenesis, and for a related chimeric Ca(2+) and calmodulin-dependent protein kinase (CCaMK) that is essential to the formation of symbiotic nitrogen-fixing nodules in plants. In Arabidopsis thaliana, the analysis of 42 isoforms of CDPK and related kinases is expected to delineate Ca(2+) signalling pathways in all aspects of plant biology.

Molecular cloning, characterization, tissue-specific and phytohormone-induced expression of calcium-dependent protein kinase gene in cucumber (Cucumis sativus L.)

A cucumber cDNA designated CsCPK5 and encoding a calcium-dependent protein kinase (CsCDPK5) was isolated and characterized. An open reading frame of 1542 bp was detected that could encode a protein of 514 amino acid residues with a calculated molecular mass of 56.5kDa. Comparison of the deduced amino acid sequence of CsCDPK5 with sequences of other CDPKs revealed the highest similarity (85%) to AtCDPK6. As described for other CDPKs, CsCDPK5 has a long variable domain preceding a catalytic domain, an autoinhibitory function domain, and a C-terminal calmodulin-domain containing 4 EF-hand calcium-binding motifs. The N-terminal long variable domain of CsCDPK5 does not contain the N-myristoylation motif, which is found in many CDPKs. The relative expression level of the CsCPK genes in various organs of cucumber plants and seedlings and in etiolated, excised cotyledons and hypocotyls following treatments with light and/or benzyladenine (BA), abscisic acid (ABA), gibberellic acid (GA) or indole acetic acid (IAA) was determined by northern analysis using the CsCPK5 cDNA probe. The CsCPK transcripts are most abundant in cucumber plant Leaves with less accumulation in cucumber seedling roots and hypocotyls and lowest Levels in cucumber plant flowers and seedling hooks and cotyledons. All phytohormones tested enhanced the accumulation of the transcripts 2-3-fold in etiolated cotyledons. On the other hand, levels of the transcripts increased to a lesser extent in both light and BA- or IAA-treated cotyledons and no effect was noted in response to light treatment with GA. In hypocotyls, no major changes in the relative levels of CsCPK transcripts were observed in the phytohormone-treated etiolated and light-exposed tissues, except an up-regulatory effect with IAA treatment in the etiolated and IAA, ABA and GA treatments in light-exposed hypocotyls. These observations suggest that exogenous phytohormones can up-regulate the CsCPK transcript levels in tissue-specific, and light-dependent and independent manners.

Evidence for differing roles for each lobe of the calmodulin-like domain in a calcium-dependent protein kinase

Calcium-dependent protein kinases (CDPKs) are structurally unique Ser/Thr kinases found in plants and certain protozoa. They are distinguished by a calmodulin-like regulatory apparatus (calmodulin-like domain (CaM-LD)) that is joined via a junction (J) region to the C-terminal end of the kinase catalytic domain. Like CaM, the CaM-LD is composed of two globular EF structural domains (N-lobe, C-lobe), each containing a pair of Ca(2+) binding sites. Spectroscopic analysis shows that the CaM-LD is comprised of helical elements, but the isolated CaM-LD does not form a conformationally homogeneous tertiary structure in the absence of Ca(2+). The addition of substoichiometric amounts of Ca(2+) is sufficient to stabilize the C-terminal lobe in a construct containing J and CaM-LD (JC) but not in the CaM-LD alone. Moreover, as J is titrated into Ca(2+)-saturated CaM-LD, interactions are stronger with the C-lobe than the N-lobe of the CaM-LD. Measurements of Ca(2+) affinity for JC reveal two cooperatively interacting high affinity binding sites (K(d)(,mean) = 5.6 nm at 20 mm KCl) in the C-lobe and two weaker sites in the N-lobe (K(d,mean) = 110 nm at 20 mm KCl). The corresponding Ca(2+) binding constants in the isolated CaM-LD are lower by more than 2 orders of magnitude, which indicates that the J region has an essential role in stabilizing the structure of the CDPK regulatory apparatus. The large differential affinity between the two domains together with previous studies on a plasmodium CDPK (Zhao, Y., Pokutta, S., Maurer, P., Lindt, M., Franklin, R. M., and Kappes, B. (1994) Biochemistry 33, 3714-3721) suggests a model whereby even at normally low cytosolic levels of Ca(2+), the C-lobe interacts with the junction, but the kinase remains in an autoinhibited state. Activation then occurs when Ca(2+) levels rise to fill the two weaker affinity binding sites in the N-lobe, thereby triggering a conformational change that leads to release of the autoinhibitory region.

Subcellular targeting of nine calcium-dependent protein kinase isoforms from Arabidopsis

Calcium-dependent protein kinases (CDPKs) are specific to plants and some protists. Their activation by calcium makes them important switches for the transduction of intracellular calcium signals. Here, we identify the subcellular targeting potentials for nine CDPK isoforms from Arabidopsis, as determined by expression of green fluorescent protein (GFP) fusions in transgenic plants. Subcellular locations were determined by fluorescence microscopy in cells near the root tip. Isoforms AtCPK3-GFP and AtCPK4-GFP showed a nuclear and cytosolic distribution similar to that of free GFP. Membrane fractionation experiments confirmed that these isoforms were primarily soluble. A membrane association was observed for AtCPKs 1, 7, 8, 9, 16, 21, and 28, based on imaging and membrane fractionation experiments. This correlates with the presence of potential N-terminal acylation sites, consistent with acylation as an important factor in membrane association. All but one of the membrane-associated isoforms targeted exclusively to the plasma membrane. The exception was AtCPK1-GFP, which targeted to peroxisomes, as determined by covisualization with a peroxisome marker. Peroxisome targeting of AtCPK1-GFP was disrupted by a deletion of two potential N-terminal acylation sites. The observation of a peroxisome-located CDPK suggests a mechanism for calcium regulation of peroxisomal functions involved in oxidative stress and lipid metabolism.

Calcium signaling through protein kinases. The Arabidopsis calcium-dependent protein kinase gene family

In plants, numerous Ca(2+)-stimulated protein kinase activities occur through calcium-dependent protein kinases (CDPKs). These novel calcium sensors are likely to be crucial mediators of responses to diverse endogenous and environmental cues. However, the precise biological function(s) of most CDPKs remains elusive. The Arabidopsis genome is predicted to encode 34 different CDPKs. In this Update, we analyze the Arabidopsis CDPK gene family and review the expression, regulation, and possible functions of plant CDPKs. By combining emerging cellular and genomic technologies with genetic and biochemical approaches, the characterization of Arabidopsis CDPKs provides a valuable opportunity to understand the plant calcium-signaling network.

Auxin and heat shock activation of a novel member of the calmodulin like domain protein kinase gene family in cultured alfalfa cells

A calmodulin like domain protein kinase (CPK) homologue was identified in alfalfa and termed MsCPK3. The full-length sequence of cDNA encoded a 535 amino acid polypeptide with a molecular weight of 60.2 kDa. The deduced amino acid sequence showed all the conserved motifs that define other members of this kinase family, such as serine-threonine kinase domain, a junction region and four potential Ca2+ -binding EF sites. The recombinant MsCPK3 protein purified from E. coli was activated by Ca2+ and inhibited by calmodulin antagonist (W-7) in in vitro phosphorylation assays. The expression of MsCPK3 gene increased in the early phase of the 2,4-D induced alfalfa somatic embryogenesis. Heat shock also activated this gene while kinetin, ABA and NaCl treatment did not result in MsCPK3 mRNA accumulation. The data presented suggest that the new alfalfa CPK differs in stress responses from the previously described homologues and in its potential involvement in hormone and stress-activated reprogramming of developmental pathways during somatic embryogenesis.

Comparative modeling studies of the calmodulin-like domain of calcium-dependent protein kinase from soybean

Calmodulin-like domain protein kinases (CDPKs) represent a new class of calcium-dependent protein-phosphorylating enzymes that are not activated by calmodulin or phospholipid compounds. They have been found exclusively in plant and protozoal tissues. CDPKs are typified by four distinct domains: an N-terminal leader sequence, a protein kinase (PK) domain, a calmodulin-like domain (CLD), and a junction domain (JD) between the PK domain and CLD. Structural characterization of the CLD of CDPKalpha from soybean was undertaken based on the amino acid sequence homology of CLD to the structurally well-characterized calmodulin (CaM) family of structures. Tertiary models of apo-CLD, Ca(2+)-CLD complex, and intermolecularly bound Ca(2+)-CLD-JD complexes were obtained via automated and non-automated homology building methods. The resulting structures were compared and validated based on energy differences, phi-psi angle distribution, solvent accessibility, and hydrophobic potential. Circular dichroism, one-dimensional, and two-dimensional nuclear magnetic resonance spectroscopy studies of the CLD and peptides encompassing the JD provide experimental support to the models. The results suggest that there is a possible interaction between the CLD and JD domain similar to that of the CaM/calmodulin-dependent protein kinase II system. At low Ca(2+) levels, the JD may act as an autoinhibitory domain for kinase activity, and during calcium activation an intramolecular CLD-JD complex may form, relieving inhibition of the PK domain. Interactions between the JD and the C terminus of the CLD appear to be particularly important. The outcome of this study supports an intramolecular binding model for calcium activation of CDPK, although not exclusively.

Lipid-linked proteins of plants

Increasing numbers of plant proteins are being shown to have posttranslationally-attached lipids. The modifications include N-myristoylation, S-palmitoylation, prenylation by farnesyl or geranylgeranyl moieties, or attachment of glycosylphosphatidylinositol anchors. This report summarizes recent findings regarding the structure, metabolism and physiological functions of these important protein-linked lipids.

Calcium and phospholipid activation of a recombinant calcium-dependent protein kinase (DcCPK1) from carrot (Daucus carota L.)

A calmodulin-like domain protein kinase (DcCPK1, previously designated CDPK431) cloned from carrot (Daucus carota L.) was expressed at high levels in Escherichia coli and partially purified. Ca(2+)-induced gel mobility shift and (45)Ca(2+) ligand binding assays confirmed that recombinant DcCPK1 binds Ca(2+) through its calmodulin-like domain and undergoes a significant conformational change. Ca(2+) activated the kinase activity of recombinant DcCPK1 (K(0.5)=1.7 microM) up to 20-fold. Ca(2+) combined with certain lipids, including phosphatidic acid, phosphatidylserine and phosphatidylinositol, but not diolein or lysophosphatidylcholine, provided even greater Ca(2+)-dependent protein kinase activity. DcCPK1 phosphorylated casein and histone III-S, and a variety of peptide substrates containing a hydrophobic and a basic residue situated P-5 and P-3 amino acids N-terminal to a Ser or Thr residue. The calmodulin and protein kinase inhibitors, W-7 and staurosporine, inhibited CDPK activity. The similarities between DcCPK1 and mammalian protein kinase C (PKC) in substrate specificity, sensitivity to inhibitors, and activation by Ca(2+) and phospholipid suggest that various CDPK isoforms may be responsible for some PKC-like activities in plant cells.

Potato protein kinase StCPK1: a putative evolutionary link between CDPKs and CRKs

Calcium-dependent protein kinases (CDPKs) in plants are characterized by a four-domain structure including conserved sequences in the catalytic domain, and in the C-terminal calmodulin-like domain. Based on this conservation we have PCR-amplified and isolated a potato cDNA clone (StCPK1) from a library representing an early stage of tuber development. DNA sequence analysis revealed that in the catalytic domain, StCPK1 shares more homology with CDPK-related kinases than with CDPKs; however, like CDPKs, it possesses canonical EF-hands at the calmodulin-like 3' end. StCPK1 exists in a few copies in the potato genome and is abundantly expressed in the sepals of mature flowers. Floral expression of genes homologous to StCPK1 appears to be widespread in the family Solanaceae.

[Protein phosphatases and protein kinases in higher plants]

The recent gain in knowledge concerning enzymes involved in signal transduction pathways is a direct consequence of the considerable advances made in molecular biology. Protein kinases and protein phosphatases, the two major enzymes implicated in post-translational modifications, have been studied in particular. The number of characterized plant genes and/or cDNAs encoding these enzymes is increasing everyday. Since 1991, 26 genes and cDNAs coding for plant protein phosphatases have been isolated and characterized. The huge number of protein kinases (estimated at several thousands) makes it impossible to give an exhaustive list of the genes already identified, but a classification of these enzymes, based on phylogenetic criteria, allows us to appreciate the range of functions this protein family may play in plants.

cDNA cloning and prokaryotic expression of maize calcium-dependent protein kinases

Using degenerate oligonucleotide primers corresponding to conserved regions of the calcium-dependent protein kinase (CDPK) family, we carried out a polymerase chain reaction and obtained four distinct partial-length cDNAs from a maize leaf library. We then used these clones as probes for conventional screening and isolated 19 longer clones from another cDNA library of maize seedlings. These clones were classified into four groups based on their DNA cross-hybridization. Two full-length cDNAs, designated as ZmCDPK9 and ZmCDPK7, were sequenced and characterized. The predicted protein of each clone was a typical CDPK with eleven canonical subdomains of protein kinases, and four EF-hand calcium-binding motifs in its N-terminal and C-terminal halves, respectively. The catalytic and regulatory domains were linked by a well-conserved junction domain. The N-terminus of the protein also contained a consensus sequence for an N-myristoylation signal. Northern blot analysis showed that the transcription level of each gene was higher in roots and etiolated leaves than in green leaves. To confirm the calcium dependency of the maize enzymes, the entire coding region of ZmCDPK9 was subcloned into an expression vector so that it was in frame with the vector-encoded peptide tags. A cell-free extract of Escherichia coli transformed with the recombinant plasmid exhibited calcium-dependent phosphorylation activity, using casein as a substrate.

Ca2+-dependent protein kinases and stress signal transduction in plants

Stress responses in plants involve changes in the transcription of specific genes. The constitutively active mutants of two related Ca2+-dependent protein kinases (CDPK1 and CDPK1a) activate a stress-inducible promoter, bypassing stress signals. Six other plant protein kinases, including two distinct CDPKs, fail to mimic this stress signaling. The activation is abolished by a CDPK1 mutation in the kinase domain and diminished by a constitutively active protein phosphatase 2C that is capable of blocking responses to the stress hormone abscisic acid. A variety of functions are mediated by different CDPKs. CDPK1 and CDPK1a may be positive regulators controlling stress signal transduction in plants.

Characterization of eight new members of the calmodulin-like domain protein kinase gene family from Arabidopsis thaliana

A family of calcium-responsive protein kinases is abundant in plant cell extracts but has not been identified in animals and fungi. These enzymes have a unique structure consisting of a protein kinase catalytic domain fused to carboxy-terminal autoregulatory and calmodulin-like domains. In this report, we present the amino acid sequences for eight new Arabidopsis cDNA clones encoding isoforms of this enzyme. Three isoforms were expressed as fusion proteins in Escherichia coli and exhibited calcium-stimulated protein kinase activity. We propose CPK as the gene designation for this family of enzymes and describe a phylogenetic analysis for all known isoforms.

Calcium-dependent protein kinase gene expression in response to physical and chemical stimuli in mungbean (Vigna radiata)

Protein kinases are important in eukaryotic signal transduction pathways. In this study we designed degenerate oligonucleotides corresponding to two conserved regions of protein kinases and using the polymerase chain reaction (PCR) have amplified a 141 bp fragment of DNA from mungbeans (Vigna radiata Rwilcz cv. Berken). Sequence analysis of the PCR products indicates that they encode several putative protein kinases with respect to their identity with other known plant protein kinases. Using one of the six fragments (CPK3-8), we isolated a 2022 bp cDNA (VrCDPK-1) from a Vigna radiata lambda gt11 library. VrCDPK-1 has a 96 bp 5'-untranslated region and a 465 bp 3'-untranslated region and an open reading frame of 1461 bp. VrCDPK-1 contains all of the conserved regions commonly found in calcium dependent protein kinases (CDPK). VrCDPK-1 shares 24 to 89% sequence identity with previously reported sequences for plant CDPKs at the protein level. Southern analysis revealed the presence of several copies of the CDPK gene. VrCDPK-1 expression was stimulated when mungbean cuttings were treated with CaCl2, while treatment with MgCl2 had no effect. We are reporting for the first time a CDPK gene in mungbean which is inducible by mechanical strain. Cuttings treated with indole-3-acetic acid (IAA) or subjected to salt stress showed an increase in VrCDPK-1 expression. There was a dramatic stimulation in VrCDPK-1 expression 6 h after cuttings were treated with cycloheximide.

Expression of three members of the calcium-dependent protein kinase gene family in Arabidopsis thaliana

Calcium-dependent protein kinases (CDPKs) belong to a unique family of enzymes containing a single polypeptide chain with a kinase domain at the amino terminus and a putative calcium-binding EF hands structure at the carboxyl terminus. From Arabidopsis thaliana, we have cloned three distinct cDNA sequences encoding CDPKs, which were designated as atcdpk6, atcdpk9 and atcdpk19. The full-length cDNA sequences for atcdpk6, atcdpk9 and atcdpk19 encode proteins with a molecular weight of 59343, 55376 and 59947, respectively. Recombinant atCDPK6 and atCDPK9 proteins were fully active as kinases whose activities were induced by Ca2+. Biochemical studies suggested the presence of an autoinhibitory domain in the junction between the kinase domain and the EF hands structure. Serial deletion of the four EF hands of atCDPK6 demonstrated that the integrity of the four EF hands was crucial to the Ca2+ response. All the three atcdpk genes were ubiquitiously expressed in the plant as demonstrated by RNA gel blot experiments. Comparison of the genomic sequences suggested that the three cdpk genes have evolved differently. Using antibodies against atCDPK6 and atCDPK9 for immunohistochemical experiments, CDPKs were found to be expressed in specific cell types in a temporally and developmentally regulated manner.

Calcium-dependent protein kinase genes in corn roots

Two cDNAs encoding Ca(2+)-dependent protein kinases (CDPKs), CRPK1 and CRPK2 (corn root protein kinase 1 and 2), were isolated from the root tip library of corn (Zea mays L., cv. Merit) and their nucleotide sequences were determined. Deduced amino acid sequences of both the clones have features characteristic of plant CDPKs, including all 11 conserved serine/threonine kinase subdomains, a junction domain and a calmodulin-like domain with four Ca(2+)-binding sites. Northern analysis revealed that CRPK1 mRNA is preferentially expressed in roots, especially in the root tip; whereas, the expression of CRPK2 mRNA was very low in all the tissues tested. In situ hybridization experiments revealed that CRPK1 mRNA is highly expressed in the root apex, as compared to other parts of the root. Partially purified CDPK from the root tip phosphorylates syntide-2, a common peptide substrate for plant CDPKs, and the phosphorylation was stimulated 7-fold by the addition of Ca2+. Our results show that two CDPK isoforms are expressed in corn roots and they may be involved in the Ca(2+)-dependent signal transduction process.

Differential accumulation of the transcripts of 22 novel protein kinase genes in Arabidopsis thaliana

22 novel members of the Arabidopsis thaliana protein kinase family (AKs) were identified by using degenerate oligonucleotide primers directed to highly conserved amino acid sequences of the protein kinase (PK) catalytic domain. Of these 22 genes, 16 turned out to carry intron sequences. Homologies of AK sequences were detected to S-locus receptor protein kinases (SRKs) from Brassica spp., to SRK-like PKs from maize and A. thaliana and to several other receptor PKs from A. thaliana. Sequence similarity was also detected to Ca(2+)-dependent PKs (CDPKs) from rape and soybean, to SNF1 and to CDC2 homologues. The genomic organization and the accumulation of the mRNAs from these 22 AK genes were investigated.

A carrot cDNA encoding an atypical protein kinase homologous to plant calcium-dependent protein kinases

Calcium-dependent protein kinases (CDPKs) in plants typically contain a C-terminal calmodulin-like domain with four EF-hand calcium-binding motifs. We have isolated a carrot somatic embryo cDNA clone which encodes a new, divergent isoform of this family, designated CRK (CDPK-related kinase). The catalytic domain of CRK shares a high degree of homology with the catalytic domains of plant CDPKs (53.5% average identity with its two closest phylogenetic relatives, CDPK431 (carrot) and AK1 (Arabidopsis). However, the C-terminal domain of CRK bears significantly less homology to calmodulin (22.0% identity to barley calmodulin) than other plant CDPKs (38.0% average identity between barley calmodulin and the C-terminal domains of CDPK431 and AK1). This degeneracy also involves the EF-hand motifs of CRK, which have diverged to varying extents. The predicted structure of CRK also contains an extended N-terminal domain 145 amino acids in length possessing a consensus N-myristoylation signal. CRK transcripts are most abundant in somatic embryos, with lesser accumulations in flowers and leaves and lowest levels in roots. Homologous genomic DNA sequences that hybridize with CRK cDNA but not with a carrot CDPK probe have been detected in a variety of higher plant taxa, including monocotyledonous species, suggesting that this CDPK-related kinase is widely conserved among angiosperms.

Molecular cloning of two novel rice cDNA sequences encoding putative calcium-dependent protein kinases

We have isolated, from a cDNA library constructed from rice coleoptiles, two sequences, OSCPK2 and OSCPK11, that encode for putative calcium-dependent protein kinase (CDPK) proteins. OSCPK2 and OSCPK11 cDNAs are related to SPK, another gene encoding a rice CDPK that is specifically expressed in developing seeds [20]. OSCPK2 and OSCPK11-predicted protein sequences are 533 and 542 amino acids (aa) long with a corresponding molecular mass of 59436 and 61079 Da respectively. Within their polypeptide chain, they all contain those conserved features that define a plant CDPK; kinase catalytic sequences are linked to a calmodulin-like regulatory domain through a junction region. The calmodulin-like regulatory domain of the predicted OSCPK2 protein contains 4 EF-hand calcium-binding sites while OSCPK11 has conserved just one canonical EF-hand motif. In addition, OSCPK2- and OSCPK11-predicted proteins contain, at their N-terminal region preceding the catalytic domain, a stretch of 80 or 74 residues highly rich in hydrophilic amino acids. Comparison of the NH2-terminal sequence of all three rice CDPKs so far identified (OSCPK2, OSCPK11 and SPK) indicates the presence of a conserved MGxxC(S/Q)xxT motif that may define a consensus signal for N-myristoylation. OSCPK2 and OSCPK11 proteins are both encoded by a single-copy gene and their polyadenylated transcripts are 2.4 and 3.5 kb long respectively. OSCPK2 and OSCPK11 mRNAs are equally abundant in rice roots and coleoptiles. A 12 h white light treatment of the coleoptiles reduces the amount of OSCPK2 mRNA with only a slight effect on the level of OSCPK11 transcript. With anoxic treatments, OSCPK2 mRNA level declined significantly and promptly while the amount of OSCPK11 transcript remained constant.

Cloning and characterization of a maize pollen-specific calcium-dependent calmodulin-independent protein kinase

A calcium-dependent calmodulin-independent protein kinase (CDPK) has been cloned from maize (Zea mays). The sequence predicts a 550-amino acid (predicted molecular mass is 60 kDa) protein with two major functional domains: an N-terminal catalytic domain highly homologous to protein kinases and a C-terminal domain resembling calmodulins. Northern analysis shows that the expression of the maize CDPK gene is pollen specific and that its transcription is restricted to late stages of pollen development. Western blots reveal a major abundance of CDPK protein at the stage of pollen germination. In vitro germination and pollen tube growth are impaired upon addition of a calmodulin antagonist (calmidazolium), CDPK inhibitors (W-7), and antisense oligonucleotides directed against CDPK mRNA. These observations indicate that the function of the pollen-specific maize CDPK protein is required for germination and pollen tube growth.

Two genes that encode Ca(2+)-dependent protein kinases are induced by drought and high-salt stresses in Arabidopsis thaliana

Two cDNA clones, cATCDPK1 and cATCDPK2, encoding Ca(2+)-dependent, calmodulin-independent protein kinases (CDPK) were cloned from Arabidopsis thaliana and their nucleotide sequences were determined. Northern blot analysis indicated that the mRNAs corresponding to the ATCDPK1 and ATCDPK2 genes are rapidly induced by drought and high-salt stress but not by low-temperature stress or heat stress. Treatment of Arabidopsis plants with exogenous abscisic acid (ABA) had no effect on the induction of ATCDPK1 or ATCDPK2. These findings suggest that a change in the osmotic potential of the environment can serve as a trigger for the induction of ATCDPK1 and ATCDPK2. Putative proteins encoded by ATCDPK1 and ATCDPK2 which contain open reading frames of 1479 and 1488 bp, respectively, are designated ATCDPK1 and ATCDPK2 and show 52% identity at the amino acid sequence level. ATCDPK1 and ATCDPK2 exhibit significant similarity to a soybean CDPK (51% and 73%, respectively). Both proteins contain a catalytic domain that is typical of serine/threonine protein kinases and a regulatory domain that is homologous to the Ca(2+)-binding sites of calmodulin. Genomic Southern blot analysis suggests the existence of a few additional genes that are related to ATCDPK1 and ATCDPK2 in the Arabidopsis genome. The ATCDPK2 protein expressed in Escherichia coli was found to phosphorylate casein and myelin basic protein preferentially, relative to a histone substrate, and required Ca2+ for activation.

Inhibition of wheat embryo calcium-dependent protein kinase by acridines and azaacridines

The inhibition of wheat Ca(2+)-dependent protein kinase (CDPK) by substituted acridines and substituted 5-, 6-, 7- and 8-azaacridines (5-AA, 6-AA, 7-AA and 8-AA) was examined. Of a total of 71 substituted acridines and azaacridines examined, only 20 have IC50 values for wheat CDPK of less than 200 microM. Of these, effective compounds all have neutral or basic 4-substituents, except for 2,7-dibromo-4-carboxy-5-AAO (IC50 73 microM), the carboxymethyl ester of which is a much better inhibitor (IC50 20 microM). There is a large aza position effect so that various 4-substituted azaacridines can be either very active or very poor inhibitors depending upon the azaacridine nucleus substituted. One of the most potent inhibitors found is the 8-AA 4-N-(2-dimethylaminoethyl)carboxamide (4-P) derivative (IC50 1.5 microM), but the corresponding 4-substituted acridine analogue is a very poor inhibitor. Other potent inhibitors found include 1-nitro-4-P-8-AA (IC50 4 microM) and 7-bromo-4-methyl-5-AA (IC50 0.7 microM). These potent and relatively specific CDPK inhibitors may be useful in obtaining evidence for CDPK involvement in plant cell responses to specific signals.

The gene encoding a calcium-dependent protein kinase located near the sbe1 gene encoding starch branching enzyme I is specifically expressed in developing rice seeds

A gene (spk) encoding a Ca(2+)-dependent protein kinase (SPK) is located in the region immediately upstream of the sbe1 gene encoding a starch branching enzyme. The spk gene is specifically expressed in developing seeds and its expression pattern is very similar to those of genes encoding starch-synthesizing enzymes such as sbe1 and waxy, seed lipid-synthesizing enzymes, as well as genes encoding seed storage proteins. A full-length spk cDNA was isolated from a cDNA library constructed from developing seeds. The deduced amino acid sequence showed that SPK has a high degree of homology to soybean and carrot Ca(2+)-dependent protein kinase, both of which contain calmodulin domains. The calmodulin domain, as well as the catalytic subdomain consensus regions of protein kinases are highly conserved in SPK. These results suggest that a tissue- and stage-specific protein kinase, SPK, is involved in the synthesis of seed storage compounds during seed development. They also strongly suggest that Ca2+ is required for seed development.

Phosphorylation of tobacco mosaic virus cell-to-cell movement protein by a developmentally regulated plant cell wall-associated protein kinase

In host plants, cell-to-cell spread of tobacco mosaic virus (TMV) presumably occurs through intercellular connections, the plasmodesmata. TMV movement is mediated by a specific virus-encoded single-strand nucleic acid-binding protein, P30. The mechanism by which P30 operates is largely unknown. Here, we demonstrate that P30 expressed in transgenic plants is a phosphoprotein. We have developed an assay for in vitro phosphorylation of purified P30 by plant cell wall fractions and have localized the phosphorylation sites to amino acid residues Ser-258, Thr-261, and Ser-265. Interestingly, the P30 phosphorylation sites do not correspond to any known consensus phosphorylation sites for protein kinases. While P30 binding to single-stranded DNA (ssDNA) was shown to involve Thr-261, phosphorylation of this residue does not appear to play a role in binding activity. The protein kinase activity contained in the cell wall fractions was developmentally regulated, expressed predominantly in leaves. Within a leaf, this protein kinase activity increased with leaf maturation and correlated with the reported development of secondary plasmodesmata, sites of P30 accumulation. We suggest that phosphorylation may represent a mechanism for the host plant to sequester P30 following its localization to cell walls.

Isolation of an Arabidopsis cDNA sequence encoding a 22 kDa calcium-binding protein (CaBP-22) related to calmodulin

Complementary DNA sequences were isolated from a library of cloned Arabidopsis leaf mRNA sequences in lambda gt10 that encoded a 21.7 kDa polypeptide (CaBP-22), which shared 66% amino acid sequence identity with Arabidopsis calmodulin. The putative Ca(2+)-binding domains of CaBP-22 and calmodulin, however, were more conserved and shared 79% sequence identity. Ca2+ binding by CaBP-22, which was inferred from its amino acid sequence similarity with calmodulin, was demonstrated indirectly by Ca(2+)-induced mobility shifting of in vitro translated CaBP-22 during SDS-polyacrylamide gel electrophoresis. CaBP-22 is encoded by a ca. 0.9 kb mRNA that was detected by northern blotting of leaf poly(A)+ RNA; this mRNA was slightly larger than the 809 bp CaBP-22 cDNA insert, indicating that the deduced amino acid sequence of CaBP-22 is near full-length. CaBP-22 mRNA was detected in RNA fractions isolated from leaves of both soil-grown and hydroponically grown Arabidopsis, but below the limits of detection in RNA isolated from roots, and developing siliques. Thus, CaBP-22 represents a new member of the EF-hand family of Ca(2+)-binding proteins with no known animal homologue and may participate in transducing Ca2+ signals to a specific subset of response elements.

Study of phosphorylation of translation elongation factor 2 (EF-2) from wheat germ

Phosphorylation of elongation factor 2 (EF-2) by specific Ca2+/calmodulin-dependent kinase is considered as a possible mechanism of regulation of protein biosynthesis in animal cells at the level of polypeptide chain elongation. In this report we show that wheat germ EF-2 can be intensively phosphorylated by the rabbit reticulocyte EF-2 kinase. Phosphorylation results in inhibition of the activity of plant EF-2 in poly(U)-dependent cell-free translation system. Thus, the activity of EF-2 in plant cells can be potentially regulated by phosphorylation. However, we could not detect endogenous EF-2 kinase activity in wheat germ either in vitro or in vivo. Furthermore, EF-2 kinase activity is not displayed in different organs of wheat and other higher plants.

Calcium and lipid regulation of an Arabidopsis protein kinase expressed in Escherichia coli

Calcium-dependent protein kinases (CDPKs) represent a new family of protein kinases which are proposed to contain, in a single polypeptide, both a kinase domain and an adjoining calmodulin-like domain with four calcium-binding EF-hand motifs [Harper, J.F., Sussman, M.R., Schaller, G.E., Putnam-Evans, C., Charbonneau, H., & Harmon, A.C. (1991) Science 252, 951-954]. DNA cloning and Western blot analysis indicate that multiple CDPK isoforms are present in the model plant system Arabidopsis thaliana. One CDPK gene called AK1 was isolated from Arabidopsis as a full-length cDNA. The predicted AK1 protein has a M(r) of 72,645 and is 116 amino acid residues longer at the amino terminus than the prototype CDPK alpha gene previously identified in soybean. The most highly conserved region between these two CDPKs is a region of 31 amino acids that joins the kinase and calmodulin-like domains. To verify the kinase activity of the enzyme encoded by AK1, a fusion of an amino-terminally truncated AK1 to the C-terminus of glutathione S-transferase was expressed in Escherichia coli. The fusion protein was purified and displayed a maximum kinase activity of 40 nmol of phosphate/(min.mg), using histone IIIs as a substrate. The enzyme activity was stimulated 3-6-fold by calcium and 2-5-fold by crude lipid. However, a synergistic stimulation of 16-30-fold was observed by the addition of both calcium and crude lipid. Lipid stimulation was specific for lysophosphatidylcholine and phosphatidylinositol and did not occur with the addition of phosphatidylserine or phosphatidylcholine.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein kinases with calmodulin-like domains: novel targets of calcium signals in plants

Recently, a novel calcium-dependent protein kinase has been identified that is structurally distinguished by the localization of a calcium-binding regulatory domain fused to a serine/threonine catalytic domain. The regulatory domain is homologous to calmodulin and contains four helix-loop-helix calcium-binding sites. As a result, the kinase is directly activated by calcium without a requirement for other effector molecules.

Cloning and characterization of a plant gene encoding a protein kinase

The cloning and sequence analysis of a gene that encodes a homologue of protein kinase (PK) from Arabidopsis thaliana is reported. We screened a genomic DNA library of A. thaliana using as probes oligodeoxyribonucleotides or fragments from the polymerase chain reaction that correspond to conserved regions in the catalytic domains of various PKs. One genomic clone, named Atpk5, was sequenced and analyzed. Transcripts of the corresponding gene, Atpk5, were detected in root, leaf and flower tissues by Northern blot analysis. The deduced amino acid sequence of the putative product of Atpk5 resembles those of kinases that phosphorylate ribosomal protein S6, cAMP-dependent PKs and protein kinase C. From the results of sequence comparisons, the ATPK5 protein appears to be a member of a subfamily of Ser/Thr-PKs specific to plants.

Calcium and signal transduction in plants

Environmental and hormonal signals control diverse physiological processes in plants. The mechanisms by which plant cells perceive and transduce these signals are poorly understood. Understanding biochemical and molecular events involved in signal transduction pathways has become one of the most active areas of plant research. Research during the last 15 years has established that Ca2+ acts as a messenger in transducing external signals. The evidence in support of Ca2+ as a messenger is unequivocal and fulfills all the requirements of a messenger. The role of Ca2+ becomes even more important because it is the only messenger known so far in plants. Since our last review on the Ca2+ messenger system in 1987, there has been tremendous progress in elucidating various aspects of Ca(2+) -signaling pathways in plants. These include demonstration of signal-induced changes in cytosolic Ca2+, calmodulin and calmodulin-like proteins, identification of different Ca2+ channels, characterization of Ca(2+) -dependent protein kinases (CDPKs) both at the biochemical and molecular levels, evidence for the presence of calmodulin-dependent protein kinases, and increased evidence in support of the role of inositol phospholipids in the Ca(2+) -signaling system. Despite the progress in Ca2+ research in plants, it is still in its infancy and much more needs to be done to understand the precise mechanisms by which Ca2+ regulates a wide variety of physiological processes. The purpose of this review is to summarize some of these recent developments in Ca2+ research as it relates to signal transduction in plants.

Cloning and characterization of two cDNAs encoding casein kinase II catalytic subunits in Arabidopsis thaliana

Two cDNA clones, ATCKA1 and ATCKA2, encoding casein kinase II (CKII) catalytic subunits, were cloned from Arabidopsis thaliana and their nucleotide sequences were determined. Both cDNAs contain 999 bp open reading frames and are 94% identical on the amino acid sequence level. The deduced amino acid sequences of ATCKA1 and ATCKA2 are very similar to that of the human CKII catalytic alpha subunit (72% homology). Northern blot analysis indicates that the ATCKA1 and ATCKA2 mRNAs are present in all plant organs, but that ATCKA1 transcript levels are quite low compared to those of ATCKA2. Genomic Southern blot analysis suggests that there are at least three CKII genes in the A. thaliana genome. We expressed the ATCKA1 and ATCKA2 cDNAs in Escherichia coli using a pET vector derivative and analyzed the expressed protein in vitro. The expressed ATCKA1 protein phosphorylated casein using either ATP or GTP. This activity was inhibited by heparin, indicating that the expressed protein has activity similar to those reported for animal and yeast CKII.

A novel calcium-binding protein from Euglena gracilis. Characterisation of a cDNA encoding a 74-kDa acidic-repeat protein targeted across the endoplasmic reticulum

We have isolated a novel cDNA from Euglena gracilis that encodes a protein composed of 24.9% aspartate with an estimated pI of 3.56, and a deduced molecular mass of 73,542 Da. The first 20 or so amino acids are hydrophobic and resemble a signal sequence. The rest of the polypeptide is composed of a 23-amino-acid repeat. There are 30 repeats, of which 23 are full length. Part of the consensus sequence derived from the repeats has some similarity to the loop of the EF-hand type calcium-binding motif. Evidence is presented that a fusion protein of this novel protein with beta-galactosidase can bind calcium. Northern blotting indicates a single transcript of 2.3 kb (the same size as the cDNA). In-vitro translation of the cDNA gives a protein that migrates on SDS/PAGE with an apparent molecular mass of 120-125 kDa. The protein is processed into a smaller, protease-protected form (110-120 kDa) when translated in the presence of canine pancreatic microsomal vesicles. This suggests that the protein is targeted across the endoplasmic reticulum membrane in vivo, and is the first report of a signal sequence from E. gracilis. We propose that the cDNA obtained encodes a novel calcium-binding protein that is either secreted or resident in the endomembrane system of E. gracilis, and call it the acidic-repeat protein.

Characterization of a gene that encodes a homologue of protein kinase in Arabidopsis thaliana

Cloning and analysis of a gene that encodes a homologue of protein kinase (PK) from Arabidopsis thaliana is reported. Oligodeoxyribonucleotides (oligos) corresponding to conserved regions in catalytic domains of various PKs were used for polymerase chain reaction (PCR) amplification with genomic DNA from A. thaliana as template, in an attempt to identify genes encoding PK in plants. We obtained several amplified DNA fragments that encoded part of a PK. We screened a genomic DNA library of A. thaliana with these oligos or PCR fragments as probes. Three genomic clones were obtained and one of them, named Atpk7, was sequenced and analyzed. Atpk7 was demonstrated by PCR to contain an intron. The mRNA transcribed from Atpk7 was detected predominantly in root tissue by Northern blot analysis. The transcription start point was determined by primer extension. The deduced amino acid (aa) sequence of the putative product of Atpk7 resembles those of S6 kinases, cyclic nucleotide-dependent PKs and calcium-dependent PKs. From this comparison of aa sequences, the ATPK7 protein is considered to be a member of a novel subfamily of Ser/Thr PKs in plants.

Isolation of a wheat cDNA clone for an abscisic acid-inducible transcript with homology to protein kinases

Increases in the plant hormone abscisic acid (ABA) initiate water-stress responses in plants. We present evidence that a transcript with homology to protein kinases is induced by ABA and dehydration in wheat. A 1.2-kilobase cDNA clone (PKABA1) was isolated from an ABA-treated wheat embryo cDNA library by screening the library with a probe developed by polymerase chain reaction amplification of serine/threonine protein kinase subdomains VIb to VIII. The deduced amino acid sequence of the PKABA1 clone contains the features of serine/threonine protein kinases, including homology with all 12 conserved regions of the catalytic domain. PKABA1 transcript levels are barely detectable in growing seedlings but are induced dramatically when plants are subjected to dehydration stress. The PKABA1 transcript can also be induced by supplying low concentrations of ABA, and coordinate increases in ABA levels and PKABA1 mRNA occur when seedlings are water-stressed. Identification of this ABA-inducible transcript with homology to protein kinases provides a basis for examining the role of protein phosphorylation in plant responses to dehydration.