Arginine kinase from the beetle Cissites cephalotes (Olivier). Molecular cloning, phylogenetic analysis and enzymatic properties

Metabolic pathways of methylmercury in rotifer Brachionus plicatilis

Methylmercury (MeHg) readily accumulates in aquatic organisms while transferring and amplifying in the aquatic food chains. This study firstly explores the in vivo accumulation sites and metabolic regulation of MeHg in the rotifer Brachionus plicatilis by aggregation-induced emission fluorogen (AIEgen) and metabolomics. Fluorescent image analysis by AIEgen showed that MeHg in B. plicatilis mainly occured in the ciliary corona, esophagus, mastax, stomach and intestine in the direct absorption group. In the other group, where B. plicatilis were indirectly supplied with MeHg via food intake, the accumulation of MeHg in the rotifer occurred in the ciliary corona, various digestive organs, and the pedal gland. However, the MeHg accumulated in the rotifer is difficult to metabolize outside the body. Metabolomics analysis showed that the significant enrichment of ABC transporters was induced by the direct exposure of rotifers to dissolved MeHg. In contrast, exposure of rotifers to MeHg via food intake appeared to influence carbon, galactose, alanine, aspartate and glutamate metabolisms. Besides, the disturbed biological pathways such as histidine metabolism, beta-alanine metabolism and pantothenate and CoA biosynthesis in rotifers may be associated with L-aspartic acid upregulation in the feeding group. The significant enrichment of ABC transporters and carbon metabolism in rotifers may be related to the accumulation of MeHg in the intestine of rotifers. In both pathways of MeHg exposure, the arginine biosynthesis and metabolism of rotifers were disturbed, which may support the hypothesis that rotifers produce more energy to resist MeHg toxicity. This study provides new insight into the accumulation and toxicity mechanisms of MeHg on marine invertebrates from the macro and micro perspectives.

RNAi-mediated knockdown of arginine kinase genes leads to high mortality and negatively affect reproduction and blood-feeding behavior of Culex pipiens pallens

BACKGROUND

Arginine kinase (AK) is one of the crucial enzymes involved in energy metabolism in invertebrates, and has been proposed as the target for RNA interference (RNAi)-based control of agricultural insect pests. While there is only one AK gene in most insects, two AK genes were identified in Culex pipiens pallens, the primary vector of lymphatic filariasis and epidemic encephalitis.

METHODS

The full-length cDNA sequences of CpAK1 and CpAK2 genes were obtained by reverse transcription PCR(RT-PCR) and rapid amplification of cDNA ends (RACE). The expression levels of CpAK1 and CpAK2 in different developmental stages and tissues were detected by reverse transcription quantitative PCR (RT-qPCR). The role of CpAK1 and CpAK2 in the reproduction and blood feeding behavior was analyzed using RNA interference (RNAi).

RESULTS

Full-length cDNAs of CpAK1 and CpAK2 were isolated from Cx. pipiens pallens. Analysis of the expression pattern revealed that the mRNA level of CpAK1 was significantly higher than CpAK2 in all development stages and tissues examined, and the expressions of both CpAK1 and CpAK2 were upregulated in response to blood feeding. The co-knockdown of CpAK1 and CpAK2 mediated by RNAi led to high mortality (74.3%) of adult female mosquitoes and decreased hatchability (59.9%). Remarkably, the blood feeding rate and the engorgement rate of the female mosquitoes were negatively affected by co-injection of dsRNAs targeting CpAK1 and CpAK2.

CONCLUSION

CpAK1 and CpAK2 were detected in all developmental stages and tissues, but showed divergence in expression level. RNAi-mediated knockdown of AK genes leads to high mortality and negatively affect blood-feeding behavior of Cx. pipiens pallens, suggesting that AK could be used for the target of RNAi-based mosquito control in the future.

The Arginine Kinase from the Tick Rhipicephalus sanguineus Is an Efficient Biocatalyst

Arginine kinase (AK) is a reversible enzyme that regulates invertebrates’ phosphagen arginine phosphate levels. AK also elicits an immune response in humans, and it is a major food allergen in crustacea and may be a target for novel antiparasitic drugs. Although AK has been primarily described in the shrimp, it is also present in other invertebrates, such as the brown tick Rhipicephalus sanguineus (Rs), the vector for Rocky Mountain Spotted Fever. Here we report the enzymatic activity and the crystal structure of AK from Rhipicephalus sanguineus (RsAK) in an open conformation without substrate or ligands and a theoretical structure of RsAK modeled bound with the substrate/product (Arg-ADP) in a closed conformation. The Michaelis-Menten kinetics confirmed that RsAK is an efficient biocatalyst due to its high kcat/Km parameter. The recombinant enzyme was expressed in bacteria and purified to a 20 mg/L culture yield. AK is an essential enzyme in invertebrates. Future work will be focused on the RsAK enzymatic inhibition that may lead to novel strategies to control this pest, a burden to animal and human health.

Differential white spot syndrome virus-binding proteins in two hemocyte subpopulations of Chinese shrimp (Fenneropenaeus chinensis)

A number of white spot syndrome virus (WSSV)-binding proteins have been identified previously in the hemocytes of Fenneropenaeus chinensis. In order to further investigate the differential WSSV-binding proteins in hemocyte subpopulations, granular hemocytes and hyalinocytes were sorted from WSSV-infected shrimp by immunomagnetic bead (IMB) method. The results of ELISA and immuno-dot blot assay showed that the WSSV-binding activity of granular hemocytes proteins was much stronger than that of hyalinocytes proteins. And the percentage of WSSV-positive granular hemocytes was significantly higher than that of hyalinocytes post WSSV infection, indicating that granular hemocytes were more susceptible to WSSV infection. Moreover, a total of 9 WSSV-binding proteins were successfully identified in granular hemocytes and hyalinocytes by two-dimensional virus overlay protein binding assay (2D-VOPBA) and MALDI-TOF MS analysis, of which 3 binding proteins (arginine kinase, protease 1 and transglutaminase) existing in both hyalinocytes and granular hemocytes and 6 proteins (F₁ATP synthase β-chain, hnRNPs, GAPDH, RACK1, β-actin and cellular retinoic acid) detected only in granular hemocytes. Among these identified WSSV-binding proteins, the transglutaminase (TG) was further recombinantly expressed, and the recombinant TG could be bound with WSSV. Subsequently, quantitative real-time PCR analysis showed that differential expression levels of WSSV-binding proteins were observed in granular hemocytes and hyalinocytes. The results of this study revealed that the WSSV-binding proteins were differentially expressed in granular hemocytes and hyalinocytes, which provided a deeper insight into the interaction between WSSV and hemocyte subpopulations.

Broad-complex transcription factor mediates opposing hormonal regulation of two phylogenetically distant arginine kinase genes in Tribolium castaneum

The phosphoarginine-arginine kinase shuttle system plays a critical role in maintaining insect cellular energy homeostasis. Insect molting and metamorphosis are coordinated by fluctuations of the ecdysteroid and juvenile hormone. However, the hormonal regulation of insect arginine kinases remain largely elusive. In this report, we comparatively characterized two arginine kinase genes, TcAK1 and TcAK2, in Tribolium castaneum. Functional analysis using RNAi showed that TcAK1 and TcAK2 play similar roles in adult fertility and stress response. TcAK1 was detected in cytoplasm including mitochondria, whereas TcAK2 was detected in cytoplasm excluding mitochondria. Interestingly, TcAK1 expression was negatively regulated by 20-hydroxyecdysone and positively by juvenile hormone, whereas TcAK2 was regulated by the opposite pattern. RNAi, dual-luciferase reporter assays and electrophoretic mobility shift assay further revealed that the opposite hormonal regulation of TcAK1 and TcAK2 was mediated by transcription factor Broad-Complex. Finally, relatively stable AK activities were observed during larval-pupal metamorphosis, which was generally consistent with the constant ATP levels. These results provide new insights into the mechanisms underlying the ATP homeostasis in insects by revealing opposite hormonal regulation of two phylogenetically distant arginine kinase genes.

Zhang et al. characterize the functions of two distinct arginine kinase genes in flour beetles. Using RNA interference and electophoretic mobility shift assays, they identify Broad-Complex transcription factor as the mediator of opposing hormonal regulation in these genes.

Evaluation of an Indirect ELISA Using Recombinant Arginine Kinase for Serodiagnosis of Psoroptes ovis var. cuniculi Infestation in Rabbits

Psoroptes ovis var. cuniculi is a common ectoparasite of the wild and domestic rabbits worldwide, which causes significant economic losses in commercial rabbit breeding. In China, the diagnosis of rabbits infested with P. ovis var. cuniculi currently relies on detection of clinical signs and Psoroptes mites in skin scrapings by microscopy examination. However, this method is not very efficient in detection of the low mite loads and/or sub-clinical infections. In the present study, we cloned and expressed an arginine kinase homolog gene from P. ovis var. cuniculi (Poc-AK) and used its recombinant protein rPoc-AK to develop an indirect enzyme-linked immunosorbent assay (iELISA) method for diagnosis of P. ovis var. cuniculi infestation in rabbits. The results showed that the rPoc-AK protein was ~61 kDa and had no signal peptide. The rPoc-AK-based iELISA achieved a 94.4% sensitivity and a 88.2% specificity, and was able to detect P. ovis var. cuniculi infection as early as the 1st week post-infection, prior to the appearance of clinical signs. Further field study showed 24.94% (66.33/266) clinically normal rabbits were seropositive with the highest and lowest seropositive rates for California (35.71%) and Belgian (15.14%), respectively. These results suggested that the rPoc-AK has potential as a diagnostic antigen for early P. ovis var. cuniculi infestation in rabbits.

Natural Products Containing 'Rare' Organophosphorus Functional Groups

Phosphorous-containing molecules are essential constituents of all living cells. While the phosphate functional group is very common in small molecule natural products, nucleic acids, and as chemical modification in protein and peptides, phosphorous can form P⁻N (phosphoramidate), P⁻S (phosphorothioate), and P⁻C (e.g., phosphonate and phosphinate) linkages. While rare, these moieties play critical roles in many processes and in all forms of life. In this review we thoroughly categorize P⁻N, P⁻S, and P⁻C natural organophosphorus compounds. Information on biological source, biological activity, and biosynthesis is included, if known. This review also summarizes the role of phosphorylation on unusual amino acids in proteins (N- and S-phosphorylation) and reviews the natural phosphorothioate (P⁻S) and phosphoramidate (P⁻N) modifications of DNA and nucleotides with an emphasis on their role in the metabolism of the cell. We challenge the commonly held notion that nonphosphate organophosphorus functional groups are an oddity of biochemistry, with no central role in the metabolism of the cell. We postulate that the extent of utilization of some phosphorus groups by life, especially those containing P⁻N bonds, is likely severely underestimated and has been largely overlooked, mainly due to the technological limitations in their detection and analysis.

Characterization and in vitro expression of arginine kinase gene in the invasive western flower thrips, Frankliniella occidentalis

Arginine kinase (AK) plays a critical role in insect energy metabolism and has been proposed to be a potential insecticide target for commercial exploitation. In this study, the full length cDNA encoding a typical group 1 insect AK (FoAK) was isolated from the western flower thrips (WFT), Frankliniella occidentalis (Pergande). Sequence analysis showed that FoAK contains an open reading frame of 1068 nucleotides, which encods a protein of 355 amino acid residues including the signature sequence pattern of ATP-guanidino kinases. Genomic structure analysis showed that the coding region of FoAK contains five exons connected by four introns. RT-qPCR analysis revealed that the mRNA expression of FoAK was developmentally regulated with the lowest level in prepupal stage. Enzymatic activity analysis of the recombinant enzymes expressed in Escherichia coli showed that FoAK is highly stereo specific for L-arginine versus D-arginine and the apparent Michaelis constant for L-arginine (Km^Arg) is comparable to that of AKs from a variety of species. This research should enable further investigation of the function as well as in vitro screening for inhibitors of FoAK.

Reference genes selection for quantitative gene expression studies in tea green leafhoppers, Empoasca onukii Matsuda

Empoasca onukii Matsuda is one of the most devastating pests of the tea plant (Camellia sinensis). Still, the presumed expression stability of its reference genes (RGs) has not been analyzed. RGs are essential for accurate and reliable gene expression analysis, so this absence has hampered the study of the insect’s molecular biology. To find candidate RGs for normalizing gene expression data, we cloned ten common housekeeping genes from E. onukii. Using the ΔCt method, geNorm, NormFinder and BestKeeper, we screened the RGs that were appropriate for quantifying the mRNA transcription of cellular responses under five experimental conditions. We identified the combinations of α-TUB and G6PDH, α-TUB and UBC, two RGs (α-TUB and β-TUB1) or three RGs (α-TUB, RPL13 and GAPDH), AK and UBC, or RPL13 and α-TUB as the best for analyzing gene expression in E. onukii adults of both sexes in different tissues, nymphs at different developmental stages, nymphs exposed to different temperatures or nymphs exposed to photoperiod stress. Finally, the E. onukii cysteine proteinase (Eocyp) was chosen as the target gene to validate the rationality of the proposed RGs. In conclusion, our study suggests a series of RGs with which to study the gene expression profiles of E. onukii that have been manipulated (biotically or abiotically) using reverse transcription quantitative polymerase chain reaction. The results offer a solid foundation for further studies of the molecular biology of E. onukii.

Dynamic metabolic responses of brown planthoppers towards susceptible and resistant rice plants

Brown planthopper (Nilaparvata lugens Stål, BPH) causes huge economic losses in rice-growing regions, and new strategies for combating BPH are required. To understand how BPHs respond towards BPH-resistant plants, we systematically analysed the metabolic differences between BPHs feeding on the resistant and susceptible plants using NMR and GC-FID/MS. We also measured the expression of some related genes involving glycolysis and biosyntheses of trehalose, amino acids, chitin and fatty acids using real-time PCR. BPH metabonome was dominated by more than 60 metabolites including fatty acids, amino acids, carbohydrates, nucleosides/nucleotides and TCA cycle intermediates. After initial 12 h, BPHs feeding on the resistant plants had lower levels of amino acids, glucose, fatty acids and TCA cycle intermediates than on the susceptible ones. The levels of these metabolites recovered after 24 h feeding. This accompanied with increased level in trehalose, choline metabolites and nucleosides/nucleotides compared with BPH feeding on the susceptible plants. Decreased levels of BPH metabolites at the early feeding probably resulted from less BPH uptakes of sap from resistant plants and recovery of BPH metabolites at the later stage probably resulted from their adaptation to the adverse environment with their increased hopping frequency to ingest more sap together with contributions from yeast-like symbionts in BPHs. Throughout 96 h, BPH feeding on the resistant plants showed significant up-regulation of chitin synthase catalysing biosynthesis of chitin for insect exoskeleton, peritrophic membrane lining gut and tracheae. These findings provided useful metabolic information for understanding the BPH-rice interactions and perhaps for developing new BPH-combating strategies.

Biochemical and structural characterization of a novel arginine kinase from the spider Polybetes pythagoricus

Energy buffering systems are key for homeostasis during variations in energy supply. Spiders are the most important predators for insects and therefore key in terrestrial ecosystems. From biomedical interest, spiders are important for their venoms and as a source of potent allergens, such as arginine kinase (AK, EC 2.7.3.3). AK is an enzyme crucial for energy metabolism, keeping the pool of phosphagens in invertebrates, and also an allergen for humans. In this work, we studied AK from the Argentininan spider Polybetes pythagoricus (PpAK), from its complementary DNA to the crystal structure. The PpAK cDNA from muscle was cloned, and it is comprised of 1068 nucleotides that encode a 384-amino acids protein, similar to other invertebrate AKs. The apparent Michaelis-Menten kinetic constant (K_m) was 1.7 mM with a k_cat of 75 s⁻¹. Two crystal structures are presented, the apoPvAK and PpAK bound to arginine, both in the open conformation with the active site lid (residues 310–320) completely disordered. The guanidino group binding site in the apo structure appears to be organized to accept the arginine substrate. Finally, these results contribute to knowledge of mechanistic details of the function of arginine kinase.

Arginine kinase shows nucleoside diphosphate kinase-like activity toward deoxythymidine diphosphate

Arginine kinase (AK) (ATP: L-arginine phosphotransferase, E.C. 2.7.3.3) catalyzes the reversible transfer of ATP γ-phosphate group to L-arginine to synthetize phospho-arginine as a high-energy storage. Previous studies suggest additional roles for AK in cellular processes. Since AK is found only in invertebrates and it is homologous to creatine kinase from vertebrates, the objective of this work was to demonstrate nucleoside diphosphate kinase-like activity for shrimp AK. For this, AK from marine shrimp Litopenaeus vannamei (LvAK) was purified and its activity was assayed for phosphorylation of TDP using ATP as phosphate donor. Moreover, by using high-pressure liquid chromatography (HPLC) the phosphate transfer reaction was followed. Also, LvAK tryptophan fluorescence emission changes were detected by dTDP titration, suggesting that the hydrophobic environment of Trp 221, which is located in the top of the active site, is perturbed upon dTDP binding. The kinetic constants for both substrates Arg and dTDP were calculated by isothermal titration calorimetry (ITC). Besides, docking calculations suggested that dTDP could bind LvAK in the same cavity where ATP bind, and LvAK basic residues (Arg124, 126 and 309) stabilize the dTDP phosphate groups and the pyrimidine base interact with His284 and Ser122. These results suggest that LvAK bind and phosphorylate dTDP being ATP the phosphate donor, thus describing a novel alternate nucleoside diphosphate kinase-like activity for this enzyme.

PROTEOMICS ANALYSIS OF OVEREXPRESSED PLASMA PROTEINS IN RESPONSE TO COLD ACCLIMATION IN Ostrinia furnacalis

Many insects in temperate regions overwinter in diapause. In these insects, one of the metabolic adaptations to cold stress is the synthesis of responsive proteins. Using proteomic analysis, an investigation aimed to a better understanding of the molecular adaptation mechanisms to cold stress was carried out in Ostrinia furnacalis larva. Proteins were extracted from the larval hemolymph collected from both control and overwintering larva. By polyethylene glycol precipitation, approximately 560 protein spots were separated and visualized on two-dimensional (2D) gels after silver staining. Eighteen protein spots were found to be upregulated in overwinter larval plasma in different patterns. As an initial work, 13 of these proteins were identified using MALDI TOF/TOF MS. The differentially overexpressed proteins include heat shock 70 kDa cognate protein, small heat shock protein (sHSP), putative aliphatic nitrilase, arginine kinase, phosphoglyceromutase, triosephosphateisomerase, and glutathione transferase. Alterations in the levels of these proteins were further confirmed by qPCR. This study is the first analysis of differentially expressed plasma proteins in O. furnacalis diapause larvae under extremely low temperature conditions and gives new insights into the acclimation mechanisms responsive to cold stress. Our results also support the idea that energy metabolism, alanine and proline metabolism, and antioxidative reaction act in the cold acclimation of O. furnacalis diapause larvae.

Arginine kinases from the marine feather star Tropiometra afra macrodiscus: The first finding of a prenylation signal sequence in metazoan phosphagen kinases

Two arginine kinase cDNAs (AK1 and AK2) were isolated from the marine feather star Tropiometra afra macrodiscus, and the gene structure (exon/intron organization) of AK1 was determined. The cDNA-derived amino acid sequences and the exon/intron organization of the Tropiometra AK1 gene were homologous to those of a human creatine kinase (CK) as well as the AK of the sea cucumber Stichopus. Phylogenetic analysis also supports the close relationship between human CKs and echinoderm AKs, indicating that the latter AKs evolved from an ancestral CK gene. We observed that the Tropiometra AK1 gene has a novel C-terminal extension (approximately 50 amino acid residues) encoded by a unique exon. Moreover, a typical prenylation signal sequence (CSLL) was found at the C-terminal end of this extension, suggesting that AK1 is anchored to a membrane. AK2 had no such C-terminal extension. This is the first finding of a prenylation signal in metazoan phosphagen kinases. Recombinant Tropiometra AK1 and AK2 enzymes were successfully expressed in Escherichia coli, and their kinetic constants were determined. Both enzymes showed activity comparable to that of typical invertebrate AKs.

Arginine kinase in the cladoceran Daphnia magna: cDNA sequencing and expression is associated with resistance to toxic Microcystis

Nutrient loading derived from anthropogenic activities into lakes have increased the frequency, severity and duration of toxic cyanobacterial blooms around the world. Although herbivorous zooplankton are generally considered to be unable to control toxic cyanobacteria, populations of some zooplankton, including Daphnia, have been shown to locally adapt to toxic cyanobacteria and suppress cyanobacterial bloom formation. However, little is known about the physiology of zooplankton behind this phenomenon. One possible explanation is that some zooplankton may induce more tolerance by elevating energy production, thereby adding more energy allocation to detoxification expenditure. It is assumed that arginine kinase (AK) serves as a core in temporal and spatial adenosine triphosphate (ATP) buffering in cells with high fluctuating energy requirements. To test this hypothesis, we studied the energetic response of a single Daphnia magna clone exposed to a toxic strain of Microcystis aeruginosa, PCC7806. Arginine kinase of D. magna (Dm-AK) was successfully cloned. An ATP-gua PtransN domain which was described as a guanidine substrate specificity domain and an ATP-gua Ptrans domain which was responsible for binding ATP were both identified in the Dm-AK. Phylogenetic analysis of AKs in a range of arthropod taxa suggested that Dm-AK was as dissimilar to other crustaceans as it was to insects. Dm-AK transcript level and ATP content in the presence of M. aeruginosa were significantly lower than those in the control diet containing only the nutritious chlorophyte, Scenedesmus obliquus, whereas the two parameters in the neonates whose mothers had been previously exposed to M. aeruginosa were significantly higher than those of mothers fed with pure S. obliquus. These findings suggest that Dm-AK might play an essential role in the coupling of energy production and utilization and the tolerance of D. magna to toxic cyanobacteria.

Arginine kinase of Litopenaeus vannamei involved in white spot syndrome virus infection

Virus-host interaction is important for virus infection. White spot syndrome virus VP14 contains transmembrane and signal peptides domain, which is considered to be important for virus infection. Until now, the function of this protein remains undefined. In this study, we explored the interaction of VP14 with host cell. A new shrimp protein (arginine kinase of Litopenaeus vannamei, LvAK) is selected and its localization in shrimp cells is also confirmed. Cellular localization of LvAK protein in shrimp hemocytes showed that LvAK was primarily located at the periphery of hemocytes and was scarcely detectable in the nucleus. Tissue distribution indicated that arginine kinase gene was spread commonly in the tissues and was highly present in shrimp muscle tissue. The expression of LvAK mRNA in muscle was significantly up-regulated after WSSV stimulation. Indirect immunofluorescence assay showed that LvAK interacted with VP14 in WSSV-infected shrimp. Injection of LvAK protein enhanced the mortality of shrimp infected with white spot syndrome virus (WSSV). These results showed that LvAK is involved in WSSV infection. Future research on this topic will help to reveal the molecular mechanism of WSSV infection.

T273 plays an important role in the activity and structural stability of arginine kinase

Arginine kinase (AK) is a key enzyme for cellular energy metabolism, catalyzing the reversible phosphoryl transfer from phosphoarginine to ADP in invertebrates. The amino acid residue C271 is involved in keeping AK's activity and constraining the orientation of the substrate arginine. However, the roles of the C271 interaction amino acid residues in AK's substrate synergism, activity and structural stability are still unclear. The crystal structure of AK implied that the amino acid residue T273 interacted with the residue C271 and might play vital roles in keeping AK's activity, substrate synergism and structural stability. The mutations T273G and T273A led to significantly loss of activity, obviously decreased of substrate synergism and structural stability. Furthermore, spectroscopic experiments indicated that mutations T273G and T273A impaired the structure of AK and led them to a partially unfolded state. The inability to fold to the functional state made the mutations prone to aggregate under environmental stresses. Moreover, the mutations T273S and T273D almost had no effects on AK's activity and structural stability. This study herein indicated that the residue T273 played key roles in AK's activity, substrate synergism and structural stability.

Identification of physicochemical properties of Scylla paramamosain allergen, arginin kinase

BACKGROUND

Arginine kinase (AK) is expressed in a wide variety of species, including human food sources (seafood) and pests (cockroaches and moths), and has been reported as a novel allergen. However, there has been little research on the allergenicity of AK in crustaceans. In this study the physicochemical properties of AK from mud crab (Scylla paramamosain) were investigated.

RESULTS

Analysis by sodium dodecyl sulfate polyacrylamide gel electrophoresis, immunoblotting and inhibition enzyme-linked immunosorbent assay revealed that purified AK was unstable in thermal processing and in acid buffer. Under simulated gastric fluid (SGF) and simulated intestinal fluid (SIF) conditions, purified AK was much more readily degraded by pepsin than by trypsin or chymotrypsin. The unpurified AK in crab myogen degraded more markedly than purified AK. In addition, in two-phase gastrointestinal digestion, AK was rapidly degraded by pepsin but resistant to trypsin and chymotrypsin digestion, while tropomyosin derived from mud crab was resistant to pepsin digestion but digested readily by trypsin or chymotrypsin. Further study of serum samples obtained from crab-allergic human patients indicated that the allergenicity of AK was markedly reduced by digestion with SGF but not SIF.

CONCLUSION

AK is an important food allergen despite its unstable physicochemical properties of digestibility.

The arginine kinase in Zhikong scallop Chlamys farreri is involved in immunomodulation

Arginine kinase (AK) catalyzes the reversible phosphorylation of l-arginine to form phosphoarginine, and plays a critical role in energy metabolism in invertebrates. In the present study, a scallop AK gene was identified from Chlamys farreri with an open reading frame (ORF) of 1101bp encoding for a protein of 366 amino acids (designed as CfAK). An ATP-gua PtransN domain which was described as a guanidine substrate specificity domain (GS domain) and an ATP-gua Ptrans domian which was responsible for binding ATP, were both identified in CfAK. The mRNA transcripts of CfAK were detectable in haemocytes, hepatopancreas, adductor muscle, mantle, gill, kidney and gonad, with the highest expression level in the muscle and the lowest level in the hemocytes. The expression level of CfAK mRNA increased from fertilized eggs to eyebot, and reached the highest in the trochophore stage. The relative expression level of CfAK mRNA in muscle was up-regulated significantly after LPS (0.5mg/mL) stimulation, and reached the peak at 6h (5.2-fold, P<0.05). The activity of inducible nitric oxide synthase (iNOS) in the supernatant of muscle homogenate increased significantly from 3.2U/mg at 0 h to 9.7 U/mg at 12h after LPS stimulation, while the concentration of nitric oxide (NO) in the supernatant of muscle homogenate began to increase at 3h (21.55 μmol/L), and reached the top concentration at 24h (42.27 μmol/L), then recovered to the normal level after 48 h. The recombinant protein of CfAK (rCfAK) expressed in Escherichia coli displayed Arginine kinase activity, and its apparent K(m) was 0.82 ± 0.11 and 1.24 ± 0.13 mM for L-arginine and ATP-Na, respectively. The results indicated that the CfAK was involved in energy production and utilization during the whole life process, and might refer to the immunomodulation process via altering the NO concentration and iNOS activity in scallop Chlamys farreri.

Metabolic and proteomic profiling of diapause in the aphid parasitoid Praon volucre

BACKGROUND

Diapause, a condition of developmental arrest and metabolic depression exhibited by a wide range of animals is accompanied by complex physiological and biochemical changes that generally enhance environmental stress tolerance and synchronize reproduction. Even though some aspects of diapause have been well characterized, very little is known about the full range of molecular and biochemical modifications underlying diapause in non-model organisms.

METHODOLOGY/PRINCIPAL FINDINGS

In this study we focused on the parasitic wasp, Praon volucre that exhibits a pupal diapause in response to environmental signals. System-wide metabolic changes occurring during diapause were investigated using GC-MS metabolic fingerprinting. Moreover, proteomic changes were studied in diapausing versus non-diapausing phenotypes using a combination of two-dimensional differential gel electrophoresis (2D-DIGE) and mass spectrometry. We found a reduction of Krebs cycle intermediates which most likely resulted from the metabolic depression. Glycolysis was galvanized, probably to favor polyols biosynthesis. Diapausing parasitoids accumulated high levels of cryoprotective polyols, especially sorbitol. A large set of proteins were modulated during diapause and these were involved in various functions such as remodeling of cytoskeleton and cuticle, stress tolerance, protein turnover, lipid metabolism and various metabolic enzymes.

CONCLUSIONS/SIGNIFICANCE

The results presented here provide some first clues about the molecular and biochemical events that characterize the diapause syndrome in aphid parasitoids. These data are useful for probing potential commonality of parasitoids diapause with other taxa and they will help creating a general understanding of diapause underpinnings and a background for future interpretations.

A diverse array of creatine kinase and arginine kinase isoform genes is present in the starlet sea anemone Nematostella vectensis, a cnidarian model system for studying developmental evolution

Phosphagen (guanidino) kinases (PK) constitute a family of homologous phosphotransferases catalyzing the reversible transfer of the high-energy phosphoryl group of ATP to naturally occurring guanidine compounds. Prior work has shown that PKs can be phylogenetically separated into two distinct groups- an arginine kinase (AK) subfamily and a creatine kinase (CK) subfamily. The latter includes three CK isoforms- cytoplasmic CK (CyCK), mitochondrial CK (MiCK) and three-domain flagellar CK (fCK). In the present study we identified six unique PK genes from the draft genome sequence of the starlet sea anemone Nematostella vectensis, a well-known model organism for understanding metazoan developmental evolution. Using reverse transcription polymerase chain reaction (RTPCR) methods, full length cDNAs were amplified for all of these PKs. These cDNAs were cloned and expressed in Escherichia coli as 6x His-tagged fusion proteins. The six PKs were identified as the three typical CK isoforms (CyCK, MiCK and fCK), two unusual AKs (a two-domain AK (2DAK) and a three-domain AK (3DAK)) and a PK which phosphorylated arginine. The latter enzyme had a very low AK activity (its apparent V(max) value being less than 0.2% that of 3DAK), lacks several key residues necessary for AK enzyme activity, and was tentatively designated as AK1. As far as we know, this constitutes the first report of an AK with the three fused AK domains. The Bayesian tree suggested that the third domain of 3DAK likely evolved from the gene for domain 2 of typical two-domain AK found widely in cnidarians. Construction of phylogenetic trees and comparison of exon-intron organizations of their respective genes indicated that the N. vectensis three-domain fCK and 3DAK evolved independently, and both enzymes are likely to be targeted to cell membranes since they have a myristoylation signal at their respective N-termini. These results complement prior work on other basal invertebrates showing that multiple CK and AK isoform genes were present at the dawn of the radiation of metazoans. The presence of isoform diversity in an organism lacking in structural complexity reflects an early imperative for targeting of PKs to particular cellular contexts such as muscle fibers, neurons, ciliated/flagellated epithelia and spermatozoa.

Molecular and catalytic properties of an arginine kinase from the nematode Ascaris suum

We amplified the cDNA coding for arginine kinase (AK) from the parasitic nematode Ascaris suum, cloned it in pMAL plasmid and expressed the enzyme as a fusion protein with the maltose-binding protein. The whole cDNA was 1260 bp, encoding 400 amino acids, and the recombinant protein had a molecular mass of 45,341 Da. Ascaris suum recombinant AK showed significant activity and strong affinity ( K(m)(Arg) = 0.126 mM) for the substrate L-arginine. It also exhibited high catalytic efficiency ( k(ca)/K(m)(Arg) = 352) comparable with AKs from other organisms. Sequence analysis revealed high amino acid sequence identity between A. suum AK and other nematode AKs, all of which cluster in a phylogenetic tree. However, comparison of gene structures showed that A. suum AK gene intron/exon organization is quite distinct from that of other nematode AKs. Phosphagen kinases (PKs) from certain parasites have been shown to be potential novel drug targets or tools for detection of infection. The characterization of A. suum AK will be useful in the development of strategies for control not only of A. suum but also of related species infecting humans.

Impact of inter-subunit interactions on the dimeric arginine kinase activity and structural stability

Arginine kinase (AK) is a key enzyme for cellular energy metabolism, catalyzing the reversible phosphoryl transfer from phosphoarginine to ADP in invertebrates. In this study, the inter-subunit hydrogen bonds between the Q53 and D200 and between D57 and D200 were disrupted to explore their roles in the activity and structural stability of Stichopus japonicus (S. japonicus) AK. Mutating Q53 and/or D57 to alanine (A) can cause pronounced loss of activity and substrate synergism, and cause distinct conformational changes. Spectroscopic experiments indicated that mutations destroying the inter-subunit hydrogen bonds impaired the structure of dimer AK, and resulted in a partially unfolded state. The inability to fold to the functional compact state made the mutants prone to be inactivated and aggregate under environmental stresses. Restoring hydrogen bonds in Q53E and D57E mutants could rescue the loss of activity and substrate synergism, and conformational changes. All those results suggested that the inter-subunit interactions played a key role in keeping the activity, substrate synergism and structural stability of dimer AK. The result herein may provide a clue in understanding the folding and self-assembly processes of oligomeric proteins.

Proteomic analysis of the venom from the endoparasitoid wasp Pteromalus puparum (Hymenoptera: Pteromalidae)

Parasitoid venom is a complex mixture of active substances with diversified biological functions. Because of its range of activities, venom is an important resource with respect to potential application in agriculture and medicine. Only a limited number of peptides, proteins, and enzymes have been identified and characterized from parasitoid venom. Here we describe a proteomic analysis of the venom from the endoparasitoid wasp Pteromalus puparum (Hymenoptera: Pteromalidae). Venom resolved by two-dimensional electrophoresis yielded 56 protein spots with major proteins in the pI range 4-7 and molecular mass range of 25-66.2 kDa. The amino acid sequences of the proteins were identified by mass spectrometry. Several venom proteins such as calreticulin, venom acid phosphatase, serine protease, arginine kinase, serine protease homolog, aminotransferase-like venom protein, and heat shock protein 70, were identified in silico based on their amino acid sequences. The full-length cDNAs of calreticulin and arginine kinase were cloned. Calreticulin showed 62% identity with calreticulin in the venom of Cotesia rubecula. Arginine kinase showed a high level of sequence identity (92%) with its counterpart in the venom of Cyphononyx dorsalis. RT-PCR analysis revealed that the transcript levels of calreticulin and arginine kinase were developmentally changed, suggesting a possible correlation with the oviposition process. This study contributes to our appreciation of a parasitoid wasp venom composition.

Embryonic diapause highlighted by differential expression of mRNAs for ecdysteroidogenesis, transcription and lipid sparing in the cricket Allonemobius socius

Embryos of the ground cricket, Allonemobius socius, enter diapause 4-5 days post-oviposition and overwinter in this dormant state that is characterized by developmental arrest. Suppressive subtractive hybridization and quantitative real-time PCR reveal eight candidate genes in pre-diapause embryos that show promise as regulators of diapause entry, when compared with embryos not destined for diapause. Identifications are based both on the magnitude/consistency of differential mRNA abundances and the predicted functions of their products when placed in context of the physiological and biochemical events of diapause characterized in our companion paper. The proteins CYP450, AKR and RACK1 (associated with ecdysteroid synthesis and signaling) are consistently upregulated in pre-diapause, followed by major downregulation later in diapause. The pattern suggests that elevated ecdysone may facilitate onset of diapause in A. socius. Upregulation seen for the transcription factors Reptin and TFDp2 may serve to depress transcription and cell cycle progression. Cathpesin B-like protease, ACLY and MSP are three downregulated genes associated with yolk mobilization and/or metabolism that we predict may promote lipid sparing. Finally, embryos that have been in diapause for 10 days show a substantially different pattern of mRNA expression compared with either pre-diapause or embryos not destined for diapause, with the majority of mRNAs examined being downregulated. These transcript levels in later diapause suggest that a number of upregulated genes in pre-diapause are transiently expressed and are less essential as diapause progresses.

Identification and characterization of two arginine kinases from the parasitic insect Ctenocephalides felis

Arginine kinase (ATP:l-arginine omega-N-phosphotransferase, EC2.7.3.3.; AK) is an enzyme crucial for the energy metabolism of insects and other invertebrates, that has known allergenic potential in humans and that has been proposed as a pesticidal drug target. Here we report the identification, cDNA cloning, genomic gene structure and functional expression of AK genes from Ctenocephalides (C.) felis (cat flea). In contrast to other insect species investigated so far, C. felis possesses two AK genes, cfak1 and cfak2, encoding the functional enzymes CfAK1 and CfAK2 that can be distinguished by their guanidino substrate specificity and the kinetic parameters for their natural substrates. Molecular modelling on CfAK1 and CfAK2 based on the Limulus polyphemus AK X-ray structure (Zhou et al., 1998) and substrate docking studies provide a potential rational for the observed specificities. Evidence is provided that adult fleas express predominantly CfAK1 as an abundant soluble protein, and that in vivo in C. felis, the AK metabolites are present in concentration ranges relevant for this enzyme.

Surviving the cold: molecular analyses of insect cryoprotective dehydration in the Arctic springtail Megaphorura arctica (Tullberg)

Background

Insects provide tractable models for enhancing our understanding of the physiological and cellular processes that enable survival at extreme low temperatures. They possess three main strategies to survive the cold: freeze tolerance, freeze avoidance or cryoprotective dehydration, of which the latter method is exploited by our model species, the Arctic springtail Megaphorura arctica, formerly Onychiurus arcticus (Tullberg 1876). The physiological mechanisms underlying cryoprotective dehydration have been well characterised in M. arctica and to date this process has been described in only a few other species: the Antarctic nematode Panagrolaimus davidi, an enchytraied worm, the larvae of the Antarctic midge Belgica antarctica and the cocoons of the earthworm Dendrobaena octaedra. There are no in-depth molecular studies on the underlying cold survival mechanisms in any species.

Results

A cDNA microarray was generated using 6,912 M. arctica clones printed in duplicate. Analysis of clones up-regulated during dehydration procedures (using both cold- and salt-induced dehydration) has identified a number of significant cellular processes, namely the production and mobilisation of trehalose, protection of cellular systems via small heat shock proteins and tissue/cellular remodelling during the dehydration process. Energy production, initiation of protein translation and cell division, plus potential tissue repair processes dominate genes identified during recovery. Heat map analysis identified a duplication of the trehalose-6-phosphate synthase (TPS) gene in M. arctica and also 53 clones co-regulated with TPS, including a number of membrane associated and cell signalling proteins. Q-PCR on selected candidate genes has also contributed to our understanding with glutathione-S-transferase identified as the major antioxdidant enzyme protecting the cells during these stressful procedures, and a number of protein kinase signalling molecules involved in recovery.

Conclusion

Microarray analysis has proved to be a powerful technique for understanding the processes and genes involved in cryoprotective dehydration, beyond the few candidate genes identified in the current literature. Dehydration is associated with the mobilisation of trehalose, cell protection and tissue remodelling. Energy production, leading to protein production, and cell division characterise the recovery process. Novel membrane proteins, along with aquaporins and desaturases, have been identified as promising candidates for future functional analyses to better understand membrane remodelling during cellular dehydration.

A novel arginine kinase from the shrimp Neocaridina denticulata: the fourth arginine kinase gene lineage

Arginine kinases (AK) evolved independently at least three times: first at an early stage of phosphagen kinase evolution (typical AK), second from the cytoplasmic creatine kinase (CK) gene later in metazoan evolution (Stichopus AK) and third from the mitochondrial CK gene in the course of annelid radiation (Sabellastarte AK). Here, we present a possible fourth AK lineage. We amplified cDNA encoding three AKs (AK1, AK2 and AK3) from the shrimp Neocaridina denticulata, and determined their amino acid sequences (355-356 residues). Each cDNA sequence was cloned in a pET plasmid and the corresponding recombinant kinase was expressed in E. coli. The AKs showed monomeric nature similar to typical AKs on an analytical gel filtration column. While the amino acid sequence of AK2 corresponded to that of typical AK, containing the conserved key residues established in Limulus AK for the substrate binding site, those of AK1 and AK3 lacked some of these key residues, indicating a similar evolution to Stichopus and Sabellastarte AKs. Moreover, phylogenetic analysis of phosphagen kinases indicated that Neocaridina AK1 and AK3 diverged at the deepest branching point close to the root of the tree and formed a distinct cluster outside the typical AK cluster, which included Neocaridina AK2. Kinetic constants of Neocaridina AKs were similar to those of other AKs. However, activation energy (E(a)) for the transition state of AK1 and AK3 was about 1.5-fold larger than that of AK2. The DeltaH(o++) values for AK1 and AK3 were also about 1.5-fold larger than that for AK2, but all three DeltaG(o++) values were very similar (71-72 kJ/mol); this results in similar reaction velocities for the three AK reactions due to a lower decrease in entropy in the activated complexes of AK1 and AK3 reactions compared to that of AK2. These findings suggest that Neocaridina AK1 and AK3 are unique from the known three AK gene lineages and therefore comprises a fourth AK lineage.

Proteomic analysis of parasitized Plutella xylostella larvae plasma

Insects use their innate immunity to defend themselves against foreign invaders, such as microorganisms, nematodes and parasites. Cotesia plutellae, an endoparasitoid wasp that parasitizes the diamondback moth Plutella xylostella, uses several strategies to attack the host immune system, such as injection of viruses, venom, and serosal membrane-derived cells denoted teratocytes. However, the proteome profiles related to these immune deficiency systems have yet to be clearly defined. In this study, we investigate differences in protein expression patterns in parasitized P. xylostella larvae, with a view to identifying parasitism-specific factors. Using 2D polyacrylamide gel electrophoresis, proteins in the host plasma were assessed every 48 h after parasitism by C. plutellae. A large number of protein spots (350 in total) were detected, and approximately 50 spots were differentially expressed in the parasitized P. xylostella larvae every 48 h. In total, 26 potential candidates, including P. xylostella Serpin 2 (pxSerpin 2), translationally controlled tumor protein, signal transduction histidine kinase, apolipophorin-III, and fatty-acid binding protein were identified through quadrupole time-of-flight tandem mass spectrometry and sequence homology analysis. These proteins were classified into the following functional groups: immunity, signaling, lipid metabolism, energy metabolism, amino acid/nucleotide metabolism, and others. The pxSerpin 2 gene was cloned, and its expression profile investigated during the course of parasitism. Real-time PCR analysis of pxSerpin 2 revealed a poor correlation between the mRNA level and protein abundance. Our results clearly suggest that parasitism-specific proteins participate in suppression of the host immune response.

Global and comparative protein profiles of the pronotum of the southern pine beetle, Dendroctonus frontalis

The southern pine beetle (Dendroctonus frontalis Zimmermann) kills all pines within its range and is among the most important forest pest species in the US. Using a specialized mycangium surrounded by gland cells in the pronotum, adult females culture, transport, and inoculate two fungi into beetle galleries during oviposition. These fungal symbionts, to varying degrees, exclude antagonistic fungi and provide nutrients to larvae. However, the mechanisms (e.g. secreted antibiotic chemicals or nutrients, proteins or pathways) by which this relationship is maintained are not known. Here we present the first global and differential proteome profile of the southern pine beetle pronotum. Two-dimensional polyacrylamide electrophoresis, tandem mass spectrometry, and database searches revealed that the majority of pronotal proteins were related to energy-yielding metabolism, contractile apparati, cell structure, and defence. The identified proteins provide important insights into the molecular and biochemical processes of, and candidates for functional genomics to understand mycangia and pronotum functions in, the southern pine beetle.

Evaluating nuclear protein-coding genes for phylogenetic utility in beetles

Although nuclear protein-coding genes have proven broadly useful for phylogenetic inference, relatively few such genes are regularly employed in studies of Coleoptera, the most diverse insect order. We increase the number of loci available for beetle systematics by developing protocols for three genes previously unused in beetles (alpha-spectrin, RNA polymerase II and topoisomerase I) and by refining protocols for five genes already in use (arginine kinase, CAD, enolase, PEPCK and wingless). We evaluate the phylogenetic performance of each gene in a Bayesian framework against a presumably known test phylogeny. The test phylogeny covers 31 beetle specimens and two outgroup taxa of varying age, including three of the four extant beetle suborders and a denser sampling in Adephaga and in the carabid genus Bembidion. All eight genes perform well for Cenozoic divergences and accurately separate closely related species within Bembidion, but individual genes differ markedly in accuracy over the older Mesozoic and Permian divergences. The concatenated data reconstruct the test phylogeny with high support in both Bayesian and parsimony analyses, indicating that combining data from multiple nuclear loci will be a fruitful approach for assembling the beetle tree of life.

Characterization of a novel bacterial arginine kinase from Desulfotalea psychrophila

Phosphagen kinases are found throughout the animal kingdom and catalyze the transfer of a high-energy gamma phosphoryl-group from ATP to a guanidino group on a suitable acceptor molecule such as creatine or arginine. Recent genome sequencing efforts in several proteobacteria, including Desulfotalea psychrophila LSv54, Myxococcus xanthus, Sulfurovum sp. NBC37-1, and Moritella sp. PE36 have revealed what appears to be a phosphagen kinase homolog present in their genomes. Based on sequence comparisons these putative homologs bear a strong resemblance to arginine kinases found in many invertebrates and some protozoa. We describe here a biochemical characterization of one of these homologs from D. psychrophila expressed in E. coli that confirms its ability to reversibly catalyze phosphoryl transfer from ATP to arginine. A phylogenetic analysis suggests that these bacteria homologs are not widely distributed in proteobacteria species. They appear more related to protozoan arginine kinases than to similar proteins seen in some Gram-positive bacteria that share key catalytic residues but encode protein tyrosine kinases. This raises the possibility of horizontal gene transfer as a likely origin of the bacterial arginine kinases.

Evidence that amino-acid residues are responsible for substrate synergism of locust arginine kinase

A series of mutants were constructed to investigate the amino-acid residues responsible for the synergism in substrate binding of arginine kinase (AK). AK contains a pair of highly conserved amino acids (Y75 and P272) that form a hydrogen bond. In the locust (Locusta migratoria manilensis) AK, mutants in two highly conserved sites can cause pronounced loss of activity, conformational changes and distinct substrate synergism alteration. The Y75F and Y75D mutants showed strong synergism (Kd/Km=6.2-13.4), while in single mutants, P272G and P272R, and a double mutant, Y75F/P272G, the synergism was almost completely lost (Kd/Km=1.1-1.4). Another double mutant, Y75D/P272R, had characteristics similar to those of the wild-type enzyme. All these results suggest that the amino-acid residues 75 and 272 play an important role in regulating the synergism in substrate binding of AK. Fluorescence spectra showed that all mutants except Y75D/P272R displayed a red shift to different degrees. All the results provided direct evidence that there is a subtle relationship between the synergism in substrate binding and the conformational change.

Proteomic profiling of a parasitic wasp exposed to constant and fluctuating cold exposure

When insects are exposed to fluctuating thermal regimes (FTRs) (i.e., cold exposure alternating with periodic short pulses to high temperature), in contrast to constant low temperature (CLT), mortality due to accumulation of chill injuries is markedly reduced. To investigate the physiological processes behind the positive impact of FTR, based on a holistic approach, two-dimensional electrophoresis (2-DE) analysis were performed with the parasitic wasp Aphidius colemani. Parasitoid proteomes revealed 369 well-distinguishable protein spots, where the overall response to cold exposure was clearly specific to treatments (CLT versus FTR). The reduced mortality under FTR was associated with up-regulation of several proteins playing key roles in energy metabolism (glycolysis, TCA cycle, synthesis and conversion of ATP), protein chaperoning (Hsp70/Hsp90), and protein degradation (proteasome). Our results also support the idea that cytoskeleton components, particularly actin arrangement, could play a role in the higher survival rates of insects under FTR.

Cloning, expression, purification, and characterization of arginine kinase from Locusta migratoria manilensis

Arginine kinase (AK) is a phosphotransferase that plays a critical role in energy metabolism in invertebrates. The gene encoding Locusta migratoria manilensis AK was cloned and expressed in Escherichia coli by two prokaryotic expression plasmids, pET-30a and pET-28a. The recombinant protein was expressed as inclusion bodies using pET-30a. After denaturation, the recombinant AK was successfully renatured and confirmed to be enzymatically active. Addition of Tween-20 and SDS to the dilution system led to higher renaturation efficiency. Using another expression plasmid, pET-28a, and changing the expression conditions resulted in a soluble and functional form of AK, which was purified by an improved method using Sephadex G-75 chromotography to a final yield of 358 mg L(-1) of LB medium. Some parameters for the renatured and soluble forms of AK, including Km, Kd, specific activity, electrophoretic mobility and isoelectric focusing, were identical with those of AK obtained directly from L. migratoria manilensis leg muscle. Comparison of kinetic constants with those of AKs from other sources indicated that L. migratoria manilensis AKs have the highest kcat and stronger synergistic substrate binding. The first report of a concise purification method enables the enzyme to be prepared in large quantities. This research should enable further detailed investigations of the enzymatic mechanism by site directed mutagenesis techniques.