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

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.

Analysis of the potato calcium-dependent protein kinase family and characterization of StCDPK7, a member induced upon infection with Phytophthora infestans

We describe the potato CDPK family and place StCDPK7 as a player in potato response to Phytophthora infestans infection, identifying phenylalanine ammonia lyase as its specific phosphorylation target in vitro. Calcium-dependent protein kinases (CDPKs) decode calcium (Ca²⁺) signals and activate different signaling pathways involved in hormone signaling, plant growth, development, and both abiotic and biotic stress responses. In this study, we describe the potato CDPK/CRK multigene family; bioinformatic analysis allowed us to identify 20 new CDPK isoforms, three CDPK-related kinases (CRKs), and a CDPK-like kinase. Phylogenetic analysis indicated that 26 StCDPKs can be classified into four groups, whose members are predicted to undergo different acylation patterns and exhibited diverse expression levels in different tissues and in response to various stimuli. With the aim of characterizing those members that are particularly involved in plant-pathogen interaction, we focused on StCDPK7. Tissue expression profile revealed that StCDPK7 transcript levels are high in swollen stolons, roots, and mini tubers. Moreover, its expression is induced upon Phytophthora infestans infection in systemic leaves. Transient expression assays showed that StCDPK7 displays a cytosolic/nuclear localization in spite of having a predicted chloroplast transit peptide. The recombinant protein, StCDPK7:6xHis, is an active Ca²⁺-dependent protein kinase that can phosphorylate phenylalanine ammonia lyase, an enzyme involved in plant defense response. The analysis of the potato CDPK family provides the first step towards the identification of CDPK isoforms involved in biotic stress. StCDPK7 emerges as a relevant player that could be manipulated to deploy disease resistance in potato crops.

Expression profiling of a novel calcium-dependent protein kinase gene, LeCPK2, from tomato (Solanum lycopersicum) under heat and pathogen-related hormones

A full-length cDNA LeCPK2 (GenBank GQ205414) from tomato (Solanum lycopersicum) encoding a calcium-dependent protein kinase (CDPK) was cloned by in silico cloning using NtCPK5 (AY971376) as a virtual probe. The deduced amino acid sequence of LeCPK2 contained the kinase, autoinhibitory, and calmodulin-like domains typical of CDPKs. Expression profiling indicated that LeCPK2 expressed predominantly in flowers and responded divergently to heat and cold stress, in which obvious mRNA accumulation was detected at 4 h under 42 degrees C stress, but no change in LeCPK2 mRNA levels was observed in 6 h at 4 degrees C. Mechanical wounding and phytohormones including ethylene, methyl jasmonate, and salicylic acid were also observed to arouse the expression of LeCPK2 in a similar pattern. mRNA accumulation was enhanced at 30 min and reached a maximum at 3 h, followed by a decrease to the normal level. All the results suggest that LeCPK2 is a novel versatile isoform of tomato CDPKs.

Arabidopsis cytosolic glutamine synthetase AtGLN1;1 is a potential substrate of AtCRK3 involved in leaf senescence

While considerable progress has been achieved in plant CDPK studies in the past decade, there is relatively no information about the potential substrates of CRKs. In this report, a yeast two-hybrid screen was performed with truncated form of AtCRK3 as bait to identify its interacting proteins in an effort to dissect its physiological roles. One gene encoding cytosolic glutamine synthetase AtGLN1;1 was isolated. Further analyses indicated that AtGLN1;1 could interact specifically with AtCRK3 and the kinase domain of AtCRK3 and the catalytic domain of AtGLN1;1 were responsible for such interaction, respectively. Furthermore, in vitro and in vivo co-immunoprecipitation results strongly supported that they could physically interact with each other. Phosphorylation assays revealed that AtGLN1;1 could be specifically phosphorylated by AtCRK3 in vitro. All the results demonstrate that AtGLN1;1 may be a substrate of AtCRK3. In addition, both AtGLN1;1 and AtCRK3 could be induced by natural or artificially induced leaf senescence, implying that such interaction may be involved in the regulation of nitrogen remobilization during leaf senescence.

Cellular localization and biochemical characterization of a novel calcium-dependent protein kinase from tobacco

By screening tobacco cDNA library with MCK1 as a probe, we isolated a cDNA clone NtCPK5 (accession number AY971376), which encodes a typical calcium-dependent protein kinase. Sequence analyses indicated that NtCPK5 is related to both CPKs and CRKs superfamilies and has all of the three conserved domains of CPKs. The biochemical activity of NtCPK5 was calcium-dependent. NtCPK5 had Vmax and Km of 526 nmol/min/mg and 210 microg/ml respectively with calf thymus histone (fraction III, abbreviated to histone IIIs) as substrate. For substrate syntide-2, NtCPK5 showed a higher Vmax of 2008 nmol/min/mg and a lower Km of 30 microM. The K0.5 of calcium activation was 0.04 microM or 0.06 microM for histone IIIs or syntide-2 respectively. The putative myristoylation and palmitoylation consensus sequence of NtCPK5 suggests that it could be a membrane-anchoring protein. Indeed, our transient expression experiments with wild type and mutant forms of NtCPK5/GFP fusion proteins showed that NtCPK5 was localized to the plasma membrane of onion epidermal cells and that the localization required the N-terminal acylation sites of NtCPK5/GFP. Taking together, our data have demonstrated the biochemical characteristics of a novel protein NtCPK5 and its subcellular localization as a membrane-anchoring protein.

Cloning and functional characterization of NtCPK4, a new tobacco calcium-dependent protein kinase

A cDNA clone, encoding calcium (Ca2+)-dependent protein kinase (CDPK or CPK), was isolated from tobacco (Nicotiana tabacum). The full-length cDNA of 2360 bp contains an open reading frame for NtCPK4 consisting of 572 amino acid residues. Sequence alignment indicated that NtCPK4 shared high similarities with other CPKs and some CPK-related protein kinases (CRKs). Biochemical analyses showed that NtCPK4 phosphorylated itself and calf thymus histones fraction III-S (histone III-S) in a calcium-dependent manner, and the K0.5 of calcium activation was 0.29 microM or 0.25 microM with histone III-S or syntide-2 as substrates, respectively. The Vmax and Km were 588 nmol min-1 mg-1 and 176 microg ml-1, respectively, when histone III-S was used as substrate, while they were 2415 nmol min-1 mg-1 and 58 microM, respectively, with syntide-2 as substrate. In addition, the phosphorylation of NtCPK4 occurred on threonine residue, as shown by capillary electrophoresis analyses. All of these data demonstrated that NtCPK4 was a serine/threonine protein kinase. NtCPK4 as a low copy gene was expressed in all tested organs including the root, leaf, stem, and flower of tobacco, while its expression was temporally and spatially modulated in both productive and vegetative tissues during tobacco growth and development. NtCPK4 expression was also increased in response to the treatment of gibberellin or NaCl. Our study suggested that NtCPK4 might play vital roles in plant development and responses to environmental stimuli.

Virus-induced gene silencing in tomato

We have previously demonstrated that a tobacco rattle virus (TRV)-based vector can be used in virus-induced gene silencing (VIGS) to study gene function in Nicotiana benthamiana. Here we show that recombinant TRV infects tomato plants and induces efficient gene silencing. Using this system, we suppressed the PDS, CTR1 and CTR2 genes in tomato. Suppression of CTR1 led to a constitutive ethylene response phenotype and up-regulation of an ethylene response gene, CHITINASE B. This phenotype is similar to Arabidopsis ctr1 mutant plants. We have constructed a modified TRV vector based on the GATEWAY recombination system, allowing restriction- and ligation-free cloning. Our results show that tomato expressed sequence tags (ESTs) can easily be cloned into this modified vector using a single set of primers. Using this vector, we have silenced RbcS and an endogenous gene homologous to the tomato EST cLED3L14. In the future, this modified vector system will facilitate large-scale functional analysis of tomato ESTs.