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

Genetic variation in ZmTIP1 contributes to root hair elongation and drought tolerance in maize

Drought is a major abiotic stress that threatens maize production globally. A previous genome-wide association study identified a significant association between the natural variation of ZmTIP1 and the drought tolerance of maize seedlings. Here, we report on comprehensive genetic and functional analysis, indicating that ZmTIP1, which encodes a functional S-acyltransferase, plays a positive role in regulating the length of root hairs and the level of drought tolerance in maize. We show that enhancing ZmTIP1 expression in transgenic Arabidopsis and maize increased root hair length, as well as plant tolerance to water deficit. In contrast, ZmTIP1 transposon-insertional mutants displayed the opposite phenotype. A calcium-dependent protein kinase, ZmCPK9, was identified as a substrate protein of ZmTIP1, and ZmTIP1-mediated palmitoylation of two cysteine residues facilitated the ZmCPK9 PM association. The results of this research enrich our knowledge about ZmTIP1-mediated protein S-acylation modifications in relation to the regulation of root hair elongation and drought tolerance. Additionally, the identification of a favourable allele of ZmTIP1 also provides a valuable genetic resource or selection target for the genetic improvement of maize.

A maize calcium-dependent protein kinase gene, ZmCPK4, positively regulated abscisic acid signaling and enhanced drought stress tolerance in transgenic Arabidopsis

Calcium-dependent protein kinases (CDPKs) play essential roles in calcium-mediated signal transductions in plant response to abiotic stress. Several members have been identified to be regulators for plants response to abscisic acid (ABA) signaling. Here, we isolated a subgroup I CDPK gene, ZmCPK4, from maize. Quantitative real time PCR (qRT-PCR) analysis revealed that the ZmCPK4 transcripts were induced by various stresses and signal molecules. Transient and stable expression of the ZmCPK4-GFP fusion proteins revealed ZmCPK4 localized to the membrane. Moreover, overexpression of ZmCPK4 in the transgenic Arabidopsis enhanced ABA sensitivity in seed germination, seedling growth and stomatal movement. The transgenic plants also enhanced drought stress tolerance. Taken together, the results suggest that ZmCPK4 might be involved in ABA-mediated regulation of stomatal closure in response to drought stress.

Genome-wide identification and expression analysis of calcium-dependent protein kinase in maize

BACKGROUND

Calcium-dependent protein kinases (CDPKs) have been shown to play important roles in various physiological processes, including plant growth and development, abiotic and biotic stress responses and plant hormone signaling in plants.

RESULTS

In this study, we performed a bioinformatics analysis of the entire maize genome and identified 40 CDPK genes. Phylogenetic analysis indicated that 40 ZmCPKs can be divided into four groups. Most maize CDPK genes exhibited different expression levels in different tissues and developmental stages. Twelve CDPK genes were selected to respond to various stimuli, including salt, drought and cold, as well as ABA and H2O2. Expression analyses suggested that maize CDPK genes are important components of maize development and multiple transduction pathways.

CONCLUSION

Here, we present a genome-wide analysis of the CDPK gene family in maize for the first time, and this genomic analysis of maize CDPK genes provides the first step towards a functional study of this gene family in maize.

Genome-wide identification of the maize calcium-dependent protein kinase gene family

In higher plants, calcium is a ubiquitous second messenger in eukaryotic signal transduction cascades. The plant-specific calcium-dependent protein kinases (CDPKs) play important roles regulating downstream components of calcium signaling. We conducted a genome-wide analysis of maize (Zea mays) CDPKs and identified 35 CDPK genes. Maize CDPKs were found to be similar to their counterparts in rice in gene structure, GC content and subgroup classification. Divergence time estimation suggested that maize-rice orthologs were largely consistent with the time when these two species diverged from the last common ancestor. Semiquantitative RT-PCR revealed that the 29 of total 35 maize CDPK genes were expressed in all tissues, including root, stem, leaf, tassel, ear, and kernel. Our genomic and bioinformatics analyses will provide an important foundation for further functional dissection of the maize CDPK gene family.

The calcium-dependent protein kinase (PnCDPK1) is involved in Pharbitis nil flowering

Signaling pathways, and specifically the signaling pathway of calcium, have been widely implicated in the regulation of a variety of signals in plants. Calcium-dependent protein kinases (CDPKs) are essential sensor-transducers of calcium signaling pathways, the functional characterization of which is of great interest because they play important roles during growth and in response to a wide range of environmental and developmental stimuli. Here, we report the first evidence of transient and specific elevation of PnCDPK1 transcript level and enzyme activity following conversion of a leaf bud to a flower bud, as well as participation of PnCDPK1 in evocation and flower morphogenesis in Pharbitis nil. Fluorescence microscopy immunolocalization and biochemical analysis confirmed the presence of CDPK in shoot apexes. The protein level was low in leaves, vegetative apexes and increased significantly in apexes after a flowering long-induction night. In the vegetative apex, a very weak PnCDPK1 protein signal was accumulated prominently in the zone of the ground meristem and in external layers of tissues of the cortex. After the dark treatment, the signal in cells of the ground meristem was still present, but a significantly stronger signal appeared in epidermal cells, cortex tissue, and leaf primordium. At the onset of flower meristem development, the PnCDPK1 level diverged significantly. PnCDPK1 mRNA, protein level and enzyme activity were very low at the beginning of flower bud development and gradually increased in later stages, reaching the highest level in a fully open flower. Analysis of flower organs revealed that PnCDPK1 was accumulated mainly in petals and sepals rather than in pistils and stamens. Our results clearly indicate that PnCDPK1 is developmentally regulated and may be an important component in the signal transduction pathways for flower morphogenesis. Findings from this research are important for further dissecting mechanisms of flowering and functions of CDPKs in flowering plants.

Expression of the rice CDPK-7 in sorghum: molecular and phenotypic analyses

Sorghum (Sorghum bicolor (L.) Moench) is an important source for food, feed, and possesses many agronomic attributes attractive for a biofuels feedstock. A warm season crop originating from the semi-arid tropics, sorghum is relatively susceptible to both cold and freezing stress. Enhancing the ability of sorghum to tolerate cold and freezing offers a route to expand the acreage for production, and provides a potential drought avoidance strategy during flowering, an important parameter for protection of yield. Targeted perturbation of the signal transduction pathway, that is triggered by exposure to abiotic stress in plants, has been demonstrated in model systems as an avenue to augment tolerance. Calcium-dependent protein kinases (CDPKs) are key players in a plant's response to environmental assaults. To test the impact of modulating CDPK activity in sorghum as a means to enhanced abiotic stress tolerance, we introduced a constitutively expressed rice CDPK-7 (OsCDPK-7) gene construct. Sorghum transformants carrying this cassette, were not improved in cold or salt stress under the conditions tested. However, a lesion mimic phenotype and up-regulation of a number of pathogen related proteins, along with transcripts linked to photosynthesis were observed. These results demonstrate that modulating the Ca signaling cascade in planta via unregulated enhanced CDPK activity can lead to off-type effects likely due to the broadly integrated nature of these enzymes in signaling.

Expressional analysis and role of calcium regulated kinases in abiotic stress signaling

Perception of stimuli and activation of a signaling cascade is an intrinsic characteristic feature of all living organisms. Till date, several signaling pathways have been elucidated that are involved in multiple facets of growth and development of an organism. Exposure to unfavorable stimuli or stress condition activates different signaling cascades in both plants and animal. Being sessile, plants cannot move away from an unfavorable condition, and hence activate the molecular machinery to cope up or adjust against that particular stress condition. In plants, role of calcium as second messenger has been studied in detail in both abiotic and biotic stress signaling. Several calcium sensor proteins such as calmodulin (CaM), calcium dependent protein kinases (CDPK) and calcinuerin B-like (CBL) were discovered to play a crucial role in abiotic stress signaling in plants. Unlike CDPK, CBL and CaM are calcium-binding proteins, which do not have any protein kinase enzyme activity and interact with a target protein kinase termed as CBL-interacting protein kinase (CIPK) and CaM kinases respectively. Genome sequence analysis of Arabidopsis and rice has led to the identification of multigene familes of these calcium signaling protein kinases. Individual and global gene expression analysis of these protein kinase family members has been analyzed under several developmental and different abiotic stress conditions. In this review, we are trying to overview and emphasize the expressional analysis of calcium signaling protein kinases under different abiotic stress and developmental stages, and linking the expression to possible function for these kinases.

Expression profile of calcium-dependent protein kinase (CDPKs) genes during the whole lifespan and under phytohormone treatment conditions in rice (Oryza sativa L. ssp. indica)

Calcium-dependent protein kinases (CDPKs) control plant development and response to various stress environments through the important roles in the regulation of Ca(2+) signaling. Thirty-one CDPK genes have been identified in the rice genome by a complete search of the genome based upon HMM profiles. In this study, the expression of this gene family was analyzed using the Affymetrix rice genome array in three rice cultivars: Minghui 63, Zhenshan 97, and their hybrid Shanyou 63 independently. Twenty-seven tissues sampled throughout the entire rice life-span were studied, along with three hormone treatments (GA3, NAA and KT), applied to the seedling at the trefoil stage. All 31 genes were found to be expressed in at least one of the experimental stages studied and revealed diverse expression patterns. We identified differential expression of the OsCPK genes in the stamen (1 day before flowering), the panicle (at the heading stage), the endosperm (days after pollination) and also in callus, in all three cultivars. Eight genes, OsCPK2, OsCPK11, OsCPK14, OsCPK22, OsCPK25, OsCPK26, OsCPK27 and OsCPK29 were found dominantly expressed in the panicle and the stamen, and five genes, OsCPK6, OsCPK7, OsCPK12, OsCPK23 and OsCPK31 were up-regulated in the endosperm stage. The OsCPK genes were also found to be regulated in rice seedlings subjected to different hormone treatment conditions, however their expression were not the same for all varieties. These diverse expression profiles trigger the functional analysis of the CDPK family in rice.

Sucrose regulated expression of a Ca2+-dependent protein kinase (TaCDPK1) gene in excised leaves of wheat

Sucrose (Suc) can influence the expression of a large number of genes and thereby regulates many metabolic and developmental processes. However, the Suc sensing and the components of the ensuing signaling transduction pathway leading to the regulation of gene expression are not fully understood. We have shown that protein kinases and phosphatases are involved in the Suc induced expression of fructosyltransferase (FT) genes and fructan accumulation by an hexokinase independent pathway in wheat (Triticum aestivum). In the present study, using an RT-PCR based strategy, we have cloned a calcium-dependent protein kinase (TaCDPK1) cDNA that is upregulated during Suc treatment of excised wheat leaves. The deduced amino-acid sequence of CDPK1 has high sequence similarity (>70%) to known CDPKs from both monocots and dicots. Based on sequence homology, TaCDPK1 sequence shows a variable domain preceding a catalytic domain, an autoinhibitory function domain, and a C-terminal calmodulin-domain containing 4 EF-hand calcium-binding motifs, along with a N-myristoylation motif in the N-terminal variable domain. The recombinant Escherichia coli expressed TaCDPK1 was able to phosphorylate histone III-S in a calcium dependent manner in in vitro assays. The TaCDPK1 gene expression, as determined by quantitative RT-PCR, is induced by Suc and this effect is repressed by the inhibitors of the putative components of the Suc signal transduction pathway (calcium, Ser/Thr protein kinases and protein phosphatases). We propose that TaCDPK1 is involved in the Suc induced signaling pathway in wheat leaves.

The role of mass spectrometry in plant systems biology

Large-scale analyses of proteins and metabolites are intimately bound to advancements in MS technologies. The aim of these non-targeted "omic" technologies is to extend our understanding beyond the analysis of only parts of the system. Here, metabolomics and proteomics emerged in parallel with the development of novel mass analyzers and hyphenated techniques such as gas chromatography coupled to time-of-flight mass spectrometry (GC-TOF-MS) and multidimensional liquid chromatography coupled to mass spectrometry (LC-MS). The analysis of (i) proteins (ii) phosphoproteins, and (iii) metabolites is discussed in the context of plant physiology and environment and with a focus on novel method developments. Recently published studies measuring dynamic (quantitative) behavior at these levels are summarized; for these works, the completely sequenced plants Arabidopsis thaliana and Oryza sativa (rice) have been the primary models of choice. Particular emphasis is given to key physiological processes such as metabolism, development, stress, and defense. Moreover, attempts to combine spatial, tissue-specific resolution with systematic profiling are described. Finally, we summarize the initial steps to characterize the molecular plant phenotype as a corollary of environment and genotype.

Functional analysis of DNA sequences controlling the expression of the rice OsCDPK2 gene

Plant calcium-dependent protein kinases (CDPKs) are involved in calcium-mediated signal transduction pathways. Their expression is finely tuned in different tissues and in response to specific signals, but the mechanism of such a regulation is still largely unknown. OsCDPK2 gene expression is modulated in vivo during rice (Oryza sativa L.) flower development and is downregulated by white light in leaves. In order to identify OsCDPK2 regulatory sequences, we amplified and cloned both the 5' and 3'-flanking regions of the gene. Sequence analysis revealed that the leader sequence is interrupted by an intron, whose regulatory role was investigated. Different ss-gucuronidase (GUS) expression vectors, carrying combinations of the putative OsCDPK2 regulatory regions, were generated and GUS expression was analyzed both in transient assays and in transgenic rice plants. The whole 5'-flanking sequence was able to drive GUS expression in rice calli and leaves transiently transformed with the biolistic technique. Analysis of the GUS expression pattern in transgenic plants revealed strong activity in root tips, leaf veins and mesophyll cells, in flower reproductive organs and in mature pollen grains. Expression was also shown to be subject to an intron-mediated enhancement (IME) mechanism, since the deletion of the leader intron sequence from chimeric OsCDPK2::GUS plasmids almost completely abolished GUS activity. Furthermore, in transiently transformed leaves, GUS expression driven by the OsCDPK2 promoter-leader region was constitutively observed regardless of light or dark exposure. Light-regulated expression was restored by inserting the OsCDPK2 3' untranslated region (3'UTR) downstream of the chimeric OsCDPK2::GUS transcription unit, suggesting that light down-regulation is mediated by a mechanism driven by the 3'UTR.

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.

Molecular and biochemical characterization of the Capsicum annuum calcium-dependent protein kinase 3 (CaCDPK3) gene induced by abiotic and biotic stresses

The isolated full-length Capsicum annuum calcium-dependent protein kinase 3 (CaCDPK3) cDNA clone was selected from the chili pepper expressed sequence tag database (http://www.pdrc.re.kr/ks200201/pepper.html). Phylogenetic analysis based on the deduced amino acid sequence of CaCDPK3 cDNA revealed significant sequence similarity to the winter squash (Cucurbita maxima) CmCPK2 gene (81% identity). Genomic gel blot analysis disclosed that CaCDPK3 belongs to a multigene family in the pepper genome. CaCDPK3 expression was root tissue-specific, as shown by Northern blot data. The gene was rapidly induced in response to various osmotic stress factors and exogenous abscisic acid application in pepper leaves. Moreover, CaCDPK3 RNA expression was induced by an incompatible pathogen and by plant defense-related chemicals such as ethephon, salicylic acid and jasmonic acid. The biochemical properties of CaCDPK3 were investigated using a CaCDPK3 and glutathione S-transferase (GST) fusion protein. The recombinant proteins retained calcium-binding ability, and displayed autophosphorylation activity in vitro in a calcium-dependent manner. Further transient-expression studies showed that CaCDPK3 fused with soluble modified green fluorescent protein (smGFP) localized to the cytosol in chili pepper protoplasts. We propose that CaCDPK3 is implicated in biotic and abiotic stresses in pepper plants.

Decoding Ca(2+) signals through plant protein kinases

Plants harbor four families of kinases that have been implicated in Ca(2+) signaling (CDPKs, CRKs, CCaMKs, and SnRK3s). Although each family appears to respond to Ca(2+) via different mechanisms, they all utilize Ca(2+) sensors that bind Ca(2+) through multiple EF-hands. The CDPK (Ca(2+)-dependent protein kinase) family is represented by the most genes, with 12 subfamilies comprised of 34 isoforms in Arabidopsis and 27 in rice. Some of the calcium-regulated kinases also show potential for regulation by lipid signals and kinase cascades. Thus, Ca(2+)-regulated kinases provide potential nodes of cross-talk for multiple signaling pathways that integrate Ca(2+) signals into all aspects of plant growth and development.

The Arabidopsis CDPK-SnRK superfamily of protein kinases

The CDPK-SnRK superfamily consists of seven types of serine-threonine protein kinases: calcium-dependent protein kinase (CDPKs), CDPK-related kinases (CRKs), phosphoenolpyruvate carboxylase kinases (PPCKs), PEP carboxylase kinase-related kinases (PEPRKs), calmodulin-dependent protein kinases (CaMKs), calcium and calmodulin-dependent protein kinases (CCaMKs), and SnRKs. Within this superfamily, individual isoforms and subfamilies contain distinct regulatory domains, subcellular targeting information, and substrate specificities. Our analysis of the Arabidopsis genome identified 34 CDPKs, eight CRKs, two PPCKs, two PEPRKs, and 38 SnRKs. No definitive examples were found for a CCaMK similar to those previously identified in lily (Lilium longiflorum) and tobacco (Nicotiana tabacum) or for a CaMK similar to those in animals or yeast. CDPKs are present in plants and a specific subgroup of protists, but CRKs, PPCKs, PEPRKs, and two of the SnRK subgroups have been found only in plants. CDPKs and at least one SnRK have been implicated in decoding calcium signals in Arabidopsis. Analysis of intron placements supports the hypothesis that CDPKs, CRKs, PPCKs and PEPRKs have a common evolutionary origin; however there are no conserved intron positions between these kinases and the SnRK subgroup. CDPKs and SnRKs are found on all five Arabidopsis chromosomes. The presence of closely related kinases in regions of the genome known to have arisen by genome duplication indicates that these kinases probably arose by divergence from common ancestors. The PlantsP database provides a resource of continuously updated information on protein kinases from Arabidopsis and other plants.

Characterization and expression analysis of genes encoding phosphoenolpyruvate carboxylase and phosphoenolpyruvate carboxylase kinase of Lotus japonicus, a model legume

Phosphoenolpyruvate carboxylases (PEPCs), one form of which in each legume species plays a central role in the carbon metabolism in symbiotic root nodules, are activated through phosphorylation of a conserved residue by a specific protein kinase (PEPC-PK). We characterized the cDNAs for two PEPC isoforms of Lotus japonicus, an amide-translocating legume that forms determinate nodules. One gene encodes a nodule-enhanced form, which is more closely related to the PEPCs in amide-type indeterminate nodules than those in ureide-type determinate nodules. The other gene is expressed in shoots and roots at a low level. Both forms have the putative phosphorylation site, Ser11. We also isolated a cDNA and the corresponding genomic DNA for PEPC-PK of L. japonicus. The recombinant PEPC-PK protein expressed in Escherichia coli phosphorylated recombinant maize C4-form PEPC efficiently in vitro. The level of mRNA for PEPC-PK was high in root nodules, and those in shoots and roots were also significant. In situ hybridization revealed that the expression patterns of the transcripts for PEPC and PEPC-PK were similar in mature root nodules, but were different in emerging nodules. When L. japonicus seedlings were subjected to prolonged darkness and subsequent illumination, the activity of PEPC-PK and the mRNA levels of both PEPC and PEPC-PK in nodules decreased and then recovered, suggesting that they are regulated according to the amounts of photosynthates transported from shoots.

Thioredoxin-mediated reductive activation of a protein kinase for the regulatory phosphorylation of C4-form phosphoenolpyruvate carboxylase from maize

The activity of phosphoenolpyruvate carboxylase (PEPC, EC4.1.1.31) for the C4 photosynthesis is known to be regulated mainly in response to light/dark transitions through reversible phosphorylation by a specific protein kinase (PK). PEPC-PK with an M(r) of 30 kDa was purified about 1.4 million-fold to homogeneity from maize leaves and characterized. The purified PEPC-PK was readily inactivated under mild oxidative conditions, but the activity could be recovered by dithiothreitol (DTT). The recovery by DTT was strongly accelerated by thioredoxin (Trx) from E. coli. Trxs of plant origin such as Trx-m from spinach chloroplast and Trx-h from rice cytoplasm were also effective. These results suggest the possibility of PEPC-PK being redox-regulated via Trx in vivo.

Overexpression of the calcium-dependent protein kinase OsCDPK2 in transgenic rice is repressed by light in leaves and disrupts seed development

Independent transgenic rice lines overexpressing the rice CDPK isoform OsCDPK2 were generated by particle bombardment. High levels of OsCDPK2 were detected in leaves removed from etiolated plants, as well as in stems and flowers. However, there was no overexpression in green leaves that had been exposed to light, confirming that OsCDPK2 protein stability was subject to light regulation. The morphological phenotype of transgenic plants producing high levels of recombinant OsCDPK2 was normal until the onset of seed development. Flowers developed normally, producing well-shaped ovaries and stigmas, and mature anthers filled with pollen grains. However, seed formation in these plants was strongly inhibited, with only 3-7% of the flowers producing seeds. Seed development was arrested at an early stage. We discuss these data with respect to the possible requirement for specific CDPK isoforms during rice seed development.

Over-expression of a single Ca2+-dependent protein kinase confers both cold and salt/drought tolerance on rice plants

A rice gene encoding a calcium-dependent protein kinase (CDPK), OsCDPK7, was induced by cold and salt stresses. To elucidate the physiological function of OsCDPK7, we generated transgenic rice plants with altered levels of the protein. The extent of tolerance to cold and salt/drought stresses of these plants correlated well with the level of OsCDPK7 expression. Therefore, OsCDPK7 was shown to be a positive regulator commonly involved in the tolerance to both stresses in rice. Over-expression of OsCDPK7 enhanced induction of some stress-responsive genes in response to salinity/drought, but not to cold. Thus, it was suggested that the downstream pathways leading to the cold and salt/drought tolerance are different from each other. It seems likely that at least two distinct pathways commonly use a single CDPK, maintaining the signalling specificity through unknown post-translational regulation mechanisms. These results demonstrate that simple manipulation of CDPK activity has great potential with regard to plant improvement.

Molecular cloning, genomic organization, and biochemical characterization of myristoyl-CoA:protein N-myristoyltransferase from Arabidopsis thaliana

Myristoyl-CoA:protein N-myristoyltransferase (NMT, EC 2.3.1.97) catalyzes the co-translational addition of myristic acid to the amino-terminal glycine residue of a number of important proteins of diverse functions. We have isolated a full-length Arabidopsis thaliana cDNA encoding NMT (AtNMT1), the first described from a higher plant. This AtNMT1 cDNA clone has an open reading frame of 434 amino acids and a predicted molecular mass of 48,706 Da. The primary structure is 50% identical to the mammalian NMTs. Analyses of Southern blots, genomic clones, and database sequences suggested that the A. thaliana genome contains two copies of NMT gene, which are present on different chromosomes and have distinct genomic organizations. The recombinant AtNMT1 expressed in Escherichia coli exhibited a high catalytic efficiency for the peptides derived from putative plant myristoylated proteins AtCDPK6 and Fen kinase. The AtNMT was similar to the mammalian NMTs with respect to a relative specificity for myristoyl CoA among the acyl CoA donors and also inhibition by the bovine brain NMT inhibitor NIP(71). The AtNMT1 expression profile indicated ubiquity in roots, stem, leaves, flowers, and siliques (approximately 1.7 kb transcript and approximately 50 kDa immunoreactive polypeptide) but a greater level in the younger tissue, which are developmentally very active. NMT activity was also evident in all these tissues. Subcellular distribution studies indicated that, in leaf extracts, approximately 60% of AtNMT activity was associated with the ribosomal fractions, whereas approximately 30% of the activity was observed in the cytosolic fractions. The NMT is biologically important to plants, as noted from the stunted development when the AtNMT1 was down-regulated in transgenic Arabidopsis under the control of an enhanced CaMV 35S promoter. The results presented in this study provide the first direct molecular evidence for plant protein N-myristoylation and a mechanistic basis for understanding the role of this protein modification in plants.

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.

Peptide phosphorylation by calcium-dependent protein kinase from maize seedlings

Ca2+-dependent protein kinase (CDPK-1) was purified from maize seedlings, and its substrate specificity studied using a set of synthetic peptides derived from the phosphorylatable sequence RVLSRLHS15VRER of maize sucrose synthase 2. The decapeptide LARLHSVRER was found to be efficiently phosphorylated as a minimal substrate. The same set of peptides were found to be phosphorylated by mammalian protein kinase Cbeta (PKC), but showed low reactivity with protein kinase A (PKA). Proceeding from the sequence LARLHSVRER, a series of cellulose-membrane-attached peptides of systematically modified structure was synthesised. These peptides had hydrophobic (Ala, Leu) and ionic (Arg, Glu) amino acids substituted in each position. The phosphorylation of these substrates by CDPK-1 was measured and the substrate specificity of the maize protein kinase characterised by the consensus sequence motif A/L-5X-4R-3X-2X-1SX+1R+2Z+3R+4, where X denotes a position with no strict amino acid requirements and Z a position strictly not tolerating arginine compared with the other three varied amino acids. This motif had a characteristic sequence element RZR at positions +2 to +4 and closely resembled the primary structure of the sucrose synthase phosphorylation site. The sequence surrounding the phosphorylatable serine in this consensus motif was similar to the analogous sequence K/RXXS/TXK/R proposed for mammalian PKC, but different from the consensus motif RRXS/TX for PKA.

N-myristoyltransferase

Myristoylation refers to the co-translational addition of a myristoyl group to an amino-terminal glycine residue of a protein by an ubiquitously distributed enzyme myristoyl-CoA:protein N-myristoyltransferase (NMT, EC 2.3.1.97). This review describes the basic enzymology, molecular cloning and regulation of NMT activity in various pathophysiological processes such as colon cancer and diabetes.

The myristylation motif of Pto is not required for disease resistance

The tomato Pto kinase confers resistance to bacterial speck disease caused by strains of Pseudomonas syringae pv. tomato that express the avirulence gene avrPto. Pto contains a putative myristylation site at its amino terminus that was hypothesized to play a role in localizing Pto in the plant cell. Site-directed mutagenesis was used to change the invariant glycine residue in the myristylation motif to an alanine. Transgenes encoding the mutant Pto(G2A) and wild-type Pto were placed behind the cauliflower mosaic virus 35S promoter and transformed into tomato plants that are susceptible to bacterial speck disease. Both the mutant and wild-type forms of Pto conferred resistance to a strain of P. syringae pv. tomato expressing avrPto. These results indicate that the myristylation motif of Pto is not required for bacterial speck disease resistance.

[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.