Nucleotide sequence of the cDNA encoding the proenzyme of phenol oxidase A1 of Drosophila melanogaster

Cloning, identification, and functional characterization of novel prophenoloxidases (ShproPO) from the freshwater crab Sinopotamon henanense in response to cadmium exposure and Aeromonas hydrophila infection

Prophenoloxidase (proPO) is essential in the prophenoloxidase-activating system (proPO-AS) which is important for defense against foreign infection in crustaceans. However, most studies have focused on expression in the presence of a single pathogenic bacterium, and very few have addressed the presence of environmental contaminants simultaneously, such as cadmium (Cd) and Aeromonas hydrophila. Our study aimed to investigate the function of proPO in the freshwater crab Sinopotamon henanense and the changes in its expression by Cd and infection of A. hydrophila. A novel proPO from the hemocytes of S. henanense (ShproPO) was found in this research, the full-length cDNA of ShproPO was 2620 bp of encoding a protein of 678 amino acids containing three typical hemocyanin domains. The ShproPO protein could be found in both the granular (GHc) and the semi-granular hemocytes (SGHc). The ShproPO mRNA was found to be abundantly expressed in hemocytes and could be influenced by A. hydrophila infection. These results indicate that ShproPO could be involved in the antibacterial process. Further research found that low concentrations of Cd could promote its expression after infection with A. hydrophila. Therefore, it was hypothesized that Cd disrupted the response of crabs to A. hydrophila infection. Subsequently, PO enzyme activity was found to be significantly reduced through in vivo RNA interference with ShproPO, and the results suggested that ShproPO is likely to be a key enzyme in the melanization response. Finally, ShproPO was found to significantly enhance the phagocytosis of A. hydrophila-infected hemocytes by in vitro recombination, confirming that ShproPO is involved in hemocyte-mediated melanization and phagocytosis. Our findings reveal completely new insight into the immunotoxicity of Cd and the immune function of ShproPO in S. henanense.

Identification of a novel antibacterial protein from hemolymph of freshwater zooplankton Mesocyclops leuckarti

Bacterial infections are the most important problem of health care worldwide. The hemolymph antibacterial proteins of Mesocyclops leuckarti was isolated for the first time and its antibacterial efficacy was evaluated against four different human pathogenic microbes viz., Escherichia coli, Staphylococcus aureus, Klebsiella pneumonia and Shigella flexneri. The antibacterial potential of the antimicrobial proteins of hemolymph samples from plankton cultured in water enriched with Cow Urine Distillate (CUD) was compared with normal ones. The results indicated that the hemolymph proteins were more potential against Gram negative bacteria than Gram positive bacteria. Klebsiella pneumonia was more susceptible to the hemolymph proteins exhibiting a zone of inhibition measuring 27 mm. The supplement of CUD to the culture media further enriched the antibacterial activity of the hemolymph proteins (29 mm). The SDS-PAGE analysis indicated two different types of clear bands representing proteins of 53 kDa and 19 kDa. Overall, this investigation signified that the microcrustaceans have a defence mechanism hemolymph of Mesocyclops leuckarti have a potential agent for novel antibiotics.

Myriapod haemocyanin: the first three-dimensional reconstruction of Scolopendra subspinipes and preliminary structural analysis of S. viridicornis

Haemocyanins (Hcs) are copper-containing, respiratory proteins that occur in the haemolymph of many arthropod species. Here, we report the presence of Hcs in the chilopode Myriapoda, demonstrating that these proteins are more widespread among the Arthropoda than previously thought. The analysis of transcriptome of S. subspinipes subpinipes reveals the presence of two distinct subunits of Hc, where the signal peptide is present, and six of prophenoloxidase (PPO), where the signal peptide is absent, in the 75 kDa range. Size exclusion chromatography profiles indicate different quaternary organization for Hc of both species, which was corroborated by TEM analysis: S. viridicornis Hc is a 6 × 6-mer and S. subspinipes Hc is a 3 × 6-mer, which resembles the half-structure of the 6 × 6-mer but also includes the presence of phenoloxidases, since the 1 × 6-mer quaternary organization is commonly associated with hexamers of PPO. Studies with Chelicerata showed that PPO activity are exclusively associated with the Hcs. This study indicates that Scolopendra may have different proteins playing oxygen transport (Hc) and PO function, both following the hexameric oligomerization observed in Hcs.

Mini-review an insect-specific system for terrestrialization: Laccase-mediated cuticle formation

Insects are often regarded as the most successful group of animals in the terrestrial environment. Their success can be represented by their huge biomass and large impact on ecosystems. Among the factors suggested to be responsible for their success, we focus on the possibility that the cuticle might have affected the process of insects' evolution. The cuticle of insects, like that of other arthropods, is composed mainly of chitin and structural cuticle proteins. However, insects seem to have evolved a specific system for cuticle formation. Oxidation reaction of catecholamines catalyzed by a copper enzyme, laccase, is the key step in the metabolic pathway for hardening of the insect cuticle. Molecular phylogenetic analysis indicates that laccase functioning in cuticle sclerotization has evolved only in insects. In this review, we discuss a theory on how the insect-specific "laccase" function has been advantageous for establishing their current ecological position as terrestrial animals.

Activation of dioxygen by copper metalloproteins and insights from model complexes

Nature uses dioxygen as a key oxidant in the transformation of biomolecules. Among the enzymes that are utilized for these reactions are copper-containing metalloenzymes, which are responsible for important biological functions such as the regulation of neurotransmitters, dioxygen transport, and cellular respiration. Enzymatic and model system studies work in tandem in order to gain an understanding of the fundamental reductive activation of dioxygen by copper complexes. This review covers the most recent advancements in the structures, spectroscopy, and reaction mechanisms for dioxygen-activating copper proteins and relevant synthetic models thereof. An emphasis has also been placed on cofactor biogenesis, a fundamentally important process whereby biomolecules are post-translationally modified by the pro-enzyme active site to generate cofactors which are essential for the catalytic enzymatic reaction. Significant questions remaining in copper-ion-mediated O2-activation in copper proteins are addressed.

Purification and characterization of a phenoloxidase in the hemocytes of Ephestia kuehniella Zeller (Lepidoptera: Pyralidae): effects of insect growth regulators and endogenous inhibitors

A phenoloxidase was extracted and purified from hemocytes of Ephestia kuehniella by using ammonium sulfate, Sepharyl G-100 and DEAE-Cellulose fast flow chromatographies. At the final stage of purification, a protein was purified by molecular mass of 78.5 kDa, specific activity of 1.17 U/mg protein, recovery of 20.48% and purification fold of 16.71. The purified PO showed the highest activity at pH 4-5 and temperatures of 35-40 °C. Na(+), K(+), Mn(+), Zn(2+) and Mg(2+) decreased activity of the purified PO but Ca(2+) and Cu(2+) increased the enzymatic activity. EDTA (General chelating agent), DTC (Copper chelating agent) and EGTA (Calcium chelating agent) significantly decreased PO activity but TTHA (Magnesium chelating agent) showed no statistically significant effects. Kinetic parameters of the purified enzyme showed the highest Vmax when L-DOPA was used as substrate but no significant differences were observed in case of Km for used L-DOPA, pyrocatechol and hydroquinone. In vitro inhibition of the purified PO by using two insect growth regulators, Hexaflumuron and Pyriproxyfen, revealed IC50 of 96.41 and 38.59 µg/ml for these compounds, respectively. Kinetic studies using different concentrations of L-DOPA and IC50 concentrations of the two IGRs revealed the increase of Km value versus control and competitive inhibition. Finally, column chromatography of hemolymph revealed peak III showing endogenous inhibitors of phenoloxidase by molecular weight of 27.3 that showed competitive inhibition on the PO.

The crystal structure of a crustacean prophenoloxidase provides a clue to understanding the functionality of the type 3 copper proteins

Phenoloxidase (PO), which is classified as a type 3 copper protein, catalyzes the hydroxylation of monophenol to o-diphenol and subsequent oxidation to the corresponding o-quinone. The geometry and coordination environment of the active site of the arthropod PO are very similar to those of the arthropod hemocyanin (Hc). However, unlike the POs, Hc is an oxygen carrier in crustaceans, and does not possess PO activity in general. Recently, we identified a new type of proPO from a crustacean and designated it proPOβ. This enzyme has many characteristics that are rather similar to those of Hc, such as its maturation, localization, and oligomeric state. Here, we determined the crystal structure of proPOβ prepared from the hemolymph of kuruma prawns (Marsupenaeus japonicus) at 1.8-Å resolution. M. japonicus proPOβ forms a homohexamer rather similar to that of arthropod Hc. The geometry of the active copper site in proPOβ is nearly identical to that of arthropod Hc. Furthermore, the well-characterized 'place-holder' phenylalanine is present (Phe72). However, the accessibility to the active site differs in several ways. First, another phenylalanine, which shields the active site by interacting with a copper-coordinated histidine in crustacean Hc, is replaced by valine in the proPOβ structure. Second, two tyrosines, Tyr208 and Tyr209, both of which are absent in Hc, show the alternative conformations and form a pathway providing access to the reaction center. Thus, the present crystal structure clarifies the similarities and differences in the activity of two closely related proteins, PO and Hc.

DATABASE

Structural data are available in the RSCB protein data bank under the accession number 3WKY. ray crystallography (View interaction).

Gene cloning and expression patterns of two prophenoloxidases from Catantops pinguis (Orthoptera: Catantopidae)

In insect, fat body plays major roles in insect innate immunity. Phenoloxidase (PO) is an important component in insect innate immunity and is necessary for acclimatization. In our study, two prophenoloxidase (PPO) subunits were obtained from fat body of Catantops pinguis (Stål). The full-length cDNA sequence of one PPO (CpPPO1) consisted of 2347 bp with an open reading frame (ORF) of 2187 bp encoding 728 amino acids, while the other subunit (CpPPO2) had a full length of 2445 bp, encoding 691 amino acids. Both the PPO gene products are predicted to possess all the structural features of other PPO members, including two putative tyrosinase copper-binding motifs with six highly conserved histidine residues and a thiolester-like motif. Tissue distribution analysis showed that both PPO mRNAs were abundantly expressed in the fat body among 11 tissues examined, and they were transiently up-regulated after Escherichia coli infection, consistent with them being immune-responsive genes. Total levels of CpPPO1 and CpPPO2 mRNA transcripts were much higher in first instar larvae and adults. A much higher transcript level of CpPPO1 was detected in several months, while there were extremely high mRNA expression levels of CpPPO2 in January, July, October, and December. The above results suggested that PPO from fat body might also bring significant function during the processes of development and acclimatization for C. pinguis.

Origin, evolution and classification of type-3 copper proteins: lineage-specific gene expansions and losses across the Metazoa

Background

Tyrosinases, tyrosinase-related proteins, catechol oxidases and hemocyanins comprise the type-3 copper protein family and are involved in a variety of biological processes, including pigment formation, innate immunity and oxygen transport. Although this family is present in the three domains of life, its origin and early evolution are not well understood. Previous analyses of type-3 copper proteins largely have focussed on specific animal and plant phyla.

Results

Here, we combine genomic, phylogenetic and structural analyses to show that the original type-3 copper protein possessed a signal peptide and may have been secreted (we designate proteins of this type the α subclass). This ancestral type-3 copper protein gene underwent two duplication events, the first prior to the divergence of the unikont eukaryotic lineages and the second before the diversification of animals. The former duplication gave rise to a cytosolic form (β) and the latter to a membrane-bound form (γ). Structural comparisons reveal that the active site of α and γ forms are covered by aliphatic amino acids, and the β form has a highly conserved aromatic residue in this position. The subsequent evolution of this gene family in modern lineages of multicellular eukaryotes is typified by the loss of one or more of these three subclasses and the lineage-specific expansion of one or both of the remaining subclasses.

Conclusions

The diversity of type-3 copper proteins in animals and other eukaryotes is consistent with two ancient gene duplication events leading to α, β and γ subclasses, followed by the differential loss and expansion of one or more of these subclasses in specific kingdoms and phyla. This has led to many lineage-specific type-3 copper protein repertoires and in some cases the independent evolution of functionally-classified tyrosinases and hemocyanins. For example, the oxygen-carrying hemocyanins in arthropods evolved from a β-subclass tyrosinase, whilst hemocyanins in molluscs and urochordates evolved independently from an α-subclass tyrosinase. Minor conformational changes at the active site of α, β and γ forms can produce type-3 copper proteins with capacities to either carry oxygen (hemocyanins), oxidize diphenols (catechol oxidase) or o-hydroxylate monophenols (tyrosinases) and appear to underlie some functional convergences.

Drosophila melanogaster prophenoloxidases respond inconsistently to Cu2+ and have different activity in vitro

Dipteran insects, like mosquitoes, possess more than two prophenoloxidase (PPO) genes, but it is unclear whether their gene products differ in biochemical properties and physiological functions. Here, we used three Drosophila melanogaster PPOs as models to study their properties through expression in S2 cells. Our data revealed that the PPOs were expressed in the ethanol-activatable conformation: rPPO1 and rPPO2 needed additional Cu(2+) in the medium, but rPPO3 did not. rPPO1 bound Cu(2+) within minutes; rPPO2 did that in hours when Cu(2+) were present at a higher concentration. Thus, rPPO1 and rPPO2 were expressed as apo-rPPO and became holo-PPO upon Cu(2+) binding; rPPO3 was holo-PPO immediately after expression. Surprisingly, in the absence of ethanol, the apparently intact rPPO3 catalyzed dopamine oxidation and melanization. The successful method for rPPO expression in S2 cells described in this paper will provide us with an opportunity to study the properties of a specific PPO gene in a small insect like mosquitoes in the future.

Prophenoloxidase activating enzyme-III from giant freshwater prawn Macrobrachium rosenbergii: characterization, expression and specific enzyme activity

The prophenoloxidase activating system is an important innate immune response against microbial infections in invertebrates. The major enzyme, phenoloxidase, is synthesized as an inactive precursor and its activation to an active enzyme is mediated by a cascade of clip domain serine proteinases. In this study, a cDNA encoding a prophenoloxidase activating enzyme-III from the giant freshwater prawn Macrobrachium rosenbergii, designated as MrProAE-III, was identified and characterized. The full-length cDNA contains an open reading frame of 1110 base pair (bp) encoding a predicted protein of 370 amino acids including an 22 amino acid signal peptide. The MrProAE-III protein exhibits a characteristic sequence structure of a long serine proteases-trypsin domain and an N- and C-terminal serine proteases-trypsin family histidine active sites, respectively, which together are the characteristics of the clip-serin proteases. Sequence analysis showed that MrProAE-III exhibited the highest amino acid sequence similarity (63%) to a ProAE-III from Atlantic blue crab, Callinectes sapidus. MrProAE-III mRNA and enzyme activity of MrProAE-III were detectable in all examined tissues, including hepatopancreas, hemocytes, pleopods, walking legs, eye stalk, gill, stomach, intestine, brain and muscle with the highest level of both in hepatopancreas. This is regulated after systemic infectious hypodermal and hematopoietic necrosis virus infection supporting that it is an immune-responsive gene. These results indicate that MrProAE-III functions in the proPO system and is an important component in the prawn immune system.

Fly immunity: recognition of pathogens and induction of immune responses

Despite the lack of adaptive immunity based on gene rearrangement such as that in higher vertebrates, flies are able to defend themselves from a wide array of pathogens using multiple innate immune responses whose molecular mechanisms are strikingly similar to those of the innate immune responses of other multicellular organisms, including humans. Invading pathogens passing through the epithelial barriers, the first line of self-defense, are detected by pattern recognition receptors that identify pathogen-associated molecular patterns in the hemolymph or on the immune cell surface and are eliminated by humoral and cellular responses. Some pathogens escape recognition and elimination in the hemolymphby invading the host cell cytoplasm. Some of these intracellular pathogens, however, such as Listeria monocytogenes, are identified by pattern recognition receptors in the cytoplasm and are eliminated by intracellular responses, including autophagy, an intracellular degradation system. Although some of these pattern recognition receptors are encoded in the germ-line as protein families, another type of receptor in the immunoglobulin-superfamily is extensively diversified by alternative splicing in somatic immune cells in Drosophila.

Prophenoloxidase from Pieris rapae: gene cloning, activity, and transcription in response to venom/calyx fluid from the endoparasitoid wasp Cotesia glomerata

Prophenoloxidase (PPO) plays an important role in melanization, necessary for defense against intruding parasitoids. Parasitoids have evolved to inject maternal virulence factors into the host hemocoel to suppress hemolymph melanization for the successful development of their progeny. In this study, the full-length complementary DNA (cDNA) of a Pieris rapae PPO was cloned. Its cDNA contained a 2 076-base pair (bp) open reading frame (ORF) encoding 691 amino acids (aa). Two putative copper-binding sites, a proteolytic activation site, three conserved hemocyanin domains, and a thiol ester motif were found in the deduced amino acid sequence. According to both multiple alignment and phylogenetic analysis, P. rapae PPO gene cloned here is a member of the lepidopteran PPO-2 family. Injection of Cotesia glomerata venom or calyx fluid resulted in reduction of P. rapae hemolymph phenoloxidase activity, demonstrating the ability to inhibit the host's melanization. Real-time reverse transcriptase polymerase chain reaction (RT-PCR) showed that transcripts of P. rapae PPO-2 in the haemocytes from larvae had not significantly changed following venom injection, suggesting that the regulation of PPO messenger RNA (mRNA) expression by venom was not employed by C. glomerata to cause failure of melanization in parasitized host. While decreased P. rapae PPO-2 gene expression was observed in the haemocytes after calyx fluid injection, no detectable transcriptional change was induced by parasitization, indicating that transcriptional down-regulation of PPO by calyx fluid might play a minor role involved in inhibiting the host's melanization.

A systematic study on hemocyte identification and plasma prophenoloxidase from Culex pipiens quinquefasciatus at different developmental stages

Culexpipiens quinquefasciatus (C. quinquefasciatus) is an important vector that can transmit human diseases such as West Nile virus, lymphatic filariasis, Japanese encephalitis and St. Louis encephalitis. However, very limited research concerning the humoral and cellular immune defenses of C. quinquefasciatus has been done. Here we present the research on hemocyte identification and plasma including hemocyte prophenoloxidase from C. quinquefasciatus at all developmental stages in order to obtain a complete picture of C. quinquefasciatus innate immunity. We identified hemocytes into four types: prohemocytes, oenocytoids, plasmatocytes and granulocytes. Prophenoloxidase (PPO) is an essential enzyme to induce melanization after encapsulation. PPO-positive hemocytes and plasma PPO were observed at all developmental stages. As for specific hemocyte types, prophenoloxidase was found in the plasmatocytes at larval stage alone and in the smallest prohemocytes during almost all developmental stages. Moreover, the granulocytes were PPO-positive from blood-fed female mosquitoes and oenocytoids were observed PPO-positive in pupae and in adult females after blood-feeding. As for plasma, there were different patterns of PPO in C. quinquefasciatus at different developmental stages. These results are forming a basis for further studies on the function of C. quinquefasciatus hemocytes and prophenoloxidase as well as their involvement in fighting against mosquito-borne pathogens.

Identification of the gene encoding pro-phenoloxidase A(3) in the fruitfly, Drosophila melanogaster

The fruitfly, Drosophila melanogaster, has three proPO genes (DoxA1, CG8193 and CG2952). DoxA1 has been shown to encode proPO A(1), one of the two proPO isoforms (A(1) and A(3)). However, which of CG8193 or CG2952 encodes proPO A(3) has so far remained elusive. In Northern analysis, CG8193 expression was strong during the larval stage, yet expression of CG2952 was not detected at any stage. Immunoblot analyses with specific antibodies detected CG8193 in the larval hemolymph at the mobility of the endogenous proPOA(3), though no signal for CG2952. These results indicate that the expression of CG2952 is very low and that CG8193 is the gene that encodes proPO A3. Processing of A(1)and A(3) isoforms in adult homogenate and activity of recombinant proPOs were also investigated.

Armigeres subalbatus prophenoloxidase III: Cloning, characterization and potential role in morphogenesis

It has long been suggested that phenoloxidases (POs) play key roles in various physiological functions in insects, e.g., cuticular sclerotization, wound healing, egg tanning and melanotic encapsulation of pathogens. Here we report that a mosquito PO, designated Armigeres subalbatus prophenoloxidase III (As-pro-PO III), is likely involved in the morphogenesis in mosquito. Expression profile analysis found that As-pro-PO III mRNA is persistently expressed in adult mosquitoes and is not significantly affected by blood feeding, microfilariae inoculation, or Escherichia coli inoculation, but expression levels of As-pro-PO III fluctuated in larval and pupal stages. Knockdown of As-pro-PO III expression in pupae using double-stranded RNA resulted in high pupal mortality and deformed adults that subsequently died following emergence. Promoter activity analyses by electrophoretic mobility-shift assays and transfection assays suggest that the As-pro-PO III gene is positively regulated by a putative Zeste motif, a developmental regulatory element. These results suggest that As-pro-PO III is associated with morphogenesis of mosquitoes.

Two prophenoloxidases are important for the survival of Vibrio harveyi challenged shrimp Penaeus monodon

Phenoloxidase (PO) plays an important role in arthropod melanization. Previously, a prophenoloxidase (PmproPO1) gene was cloned and characterized from the hemocytes of the black tiger shrimp, Penaeus monodon. In the present study, we report a novel proPO gene (PmproPO2) belonging to the proPO family identified from the P. monodon EST database (http://pmonodon.biotec.or.th). The full-length sequence of PmproPO2 consists of 2513bp encoding a predicted 689 amino acid residues with a calculated molecular mass and pI of 79.21kDa and 6.69, respectively. It is predicted to possess all the expected features of proPO members, including two putative tyrosinase copper-binding motifs with six histidine residues and a thiol ester-like motif, sharing 67% amino acid sequence identity with PmproPO1. Tissue distribution analyses revealed that the two proPO genes are primarily expressed in the hemocyte. Gene silencing of either PmproPO1 or PmproPO2 or both by RNA interference (RNAi) resulted in a significant decrease in the respective endogenous proPO mRNA level in hemocytes and a reduction of total PO enzyme activity by 75, 73 and 88%, respectively. Experimental infection of P. monodon with the pathogenic bacterium, Vibrio harveyi, revealed that PmproPO silenced shrimps were more susceptible to bacterial infection than the control GFP injected shrimps, and suggesting that the two proPOs are important components in the shrimp immune defense.

A novel prophenoloxidase 2 exists in shrimp hemocytes

The prophenoloxidase (proPO)-activating system in crustaceans and other arthropods is regarded as a constituent of the immune system and plays an important role in defense against pathogens. Hitherto in crustaceans, only one proPO gene per species has been identified. Here we report the identification of a novel proPO-2 (LvproPO-2) from the hemocytes of Litopenaeus vannamei, which shows 72% identity to proPO-1 (LvproPO-1) cloned previously. Northern blotting analysis and quantitative real-time PCR reveal that LvproPO-2 is mainly expressed in the hemocytes, and its expression is down-regulated in shrimp challenged with white spot syndrome virus (WSSV). Western blotting analysis shows that most LvproPO-2/LvPO-2 (L. vannamei phenoloxidase-2) exists in the hemocytes, but not in plasma of L. vannamei. LvproPO-2/LvPO-2 could be detected on the hemocyte surface and the nucleus of hemocytes by indirect immunofluorescence assay (IFA). These findings provide insight into the molecular biological basis for further studying on the defense mechanism of shrimp innate immunity, especially on the proPO-activating system and melanization cascade of shrimp.

A misexpression screen to identify regulators of Drosophila larval hemocyte development

In Drosophila, defense against foreign pathogens is mediated by an effective innate immune system, the cellular arm of which is composed of circulating hemocytes that engulf bacteria and encapsulate larger foreign particles. Three hemocyte types occur: plasmatocytes, crystal cells, and lamellocytes. The most abundant larval hemocyte type is the plasmatocyte, which is responsible for phagocytosis and is present either in circulation or in adherent sessile domains under the larval cuticle. The mechanisms controlling differentiation of plasmatocytes and their migration toward these sessile compartments are unclear. To address these questions we have conducted a misexpression screen using the plasmatocyte-expressed GAL4 driver Peroxidasin-GAL4 (Pxn-GAL4) and existing enhancer-promoter (EP) and EP yellow (EY) transposon libraries to systematically misexpress approximately 20% of Drosophila genes in larval hemocytes. The Pxn-GAL4 strain also contains a UAS-GFP reporter enabling hemocyte phenotypes to be visualized in the semitransparent larvae. Among 3412 insertions screened we uncovered 101 candidate hemocyte regulators. Some of these are known to control hemocyte development, but the majority either have no characterized function or are proteins of known function not previously implicated in hemocyte development. We have further analyzed three candidate genes for changes in hemocyte morphology, cell-cell adhesion properties, phagocytosis activity, and melanotic tumor formation.

Endonuclease activity of phenol oxidase from Musca domestica larvae

Phenol oxidase (PO), a copper-containing enzyme with oxygenase activity, can convert mono- or diphenol into quinone and plays an important role in the arthropod melanization reaction. Here, we report a new property of PO from Musca domestica larvae: a thermotolerant endonuclease activity, by which PO can degrade plasmid DNA even after being heated to 80 degrees C for 20 min. We cloned PO cDNA, constructed the expression vector pVAX1-PO, and expressed it in HeLa cells. The expression product showed the same properties as purified PO. Our data indicate that PO is a bifunctional enzyme, exhibiting both oxygenase and endonuclease activity, suggesting new roles for this important molecule in the innate responses of M. domestica.

Eicosanoids mediate prophenoloxidase release from oenocytoids in the beet armyworm Spodoptera exigua

Phenoloxidase (PO) plays a critical role in insect immune reactions especially to form melanotic encapsulation and phagocytosis by hemocytes. PO is an active form of prophenoloxidase (PPO) after proteolytic cleavage by serine proteinase(s). It has been suggested that eicosanoids are implicated in PPO activation in the beet armyworm, Spodoptera exigua. However, it is not clear how eicosanoids mediate the reaction cascade of PPO activation. This study analyzed the PPO activation mediated by eicosanoids at both transcriptional and post-transcriptional levels. A cDNA encoding PPO (SePPO) was cloned from the hemocytes of S. exigua and its putative amino acid sequence shared homology with PPO-2 of other lepidopteran insects. Its expression was specific only to hemocytes and inducible in response to bacterial challenge. Eicosanoid biosynthesis inhibitors did not influence the gene expression of SePPO. Most SePPO proteins were located in a specific hemocyte type, oenocytoids, which were subjected to cell rupture to release the cellular contents in response to bacterial challenge. There was a significant negative correlation between PO activity and intact oenocytoid density. Interestingly, this cell rupture to release SePPO from oenocytoids was significantly inhibited in the larvae infected with the phospholipase A2-inhibiting bacterium, Xenorhabdus nematophila, which was resumed on addition of eicosanoid biosynthesis precursor, arachidonic acid. Furthermore, oenocytoids exposed to eicosanoid biosynthesis inhibitors such as dexamethasone and bromophenacyl bromide showed significant reduction in cell rupture. Prostaglandins, not lipoxygenase products appeared to be implicated in the cell rupture. These results indicate that eicosanoids mediate SePPO activation only at the post-transcriptional level by inducing release of PPO from oenocytoids through cell rupture.

Tolerance to Bacillus thuringiensis endotoxin in immune-suppressed larvae of the flour moth Ephestia kuehniella

Tolerance to Bacillus thuringiensis crystal endotoxins (Bt-toxins) is correlated with an elevated immune status in larvae of the flour moth Ephestia kuehniella. To gain more specific information about the effector pathways involved in the protection against the toxin, we studied the effects of Bt-toxin formulations in susceptible (non-induced) and tolerant (immune-induced) larvae after natural (parasitism-mediated) and chemical (tropolone-mediated) suppression of defence reactions. Although melanization in hemolymph was significantly reduced, there was no significant effect on susceptibility to the toxin in parasitised or tropolone-treated larvae. This suggests that melanization of hemolymph is correlated with an elevated immune status but not responsible for the observed tolerance to Bt-toxin. To examine whether hemolymph proteins exist in the gut lumen and function as pro-coagulants, we compared gut and plasma proteins of immune-induced with those of non-induced larvae. Here we show that the lipid carrier lipophorin represents a major component in the gut lumen and interacts with mature Bt-toxin to form a complex.

Pattern of nucleotide substitution and divergence of prophenoloxidase in decapods

Despite the unprecedented development in identification and characterization of prophenoloxidase (proPO) in commercially important decapods, little is known about the evolutionary relationship, rate of amino acid replacement and differential selection pressures operating on proPO of different species of decapods. Here we report the evolutionary relationship among these nine decapod species based on proPO gene and types of selective pressures operating on proPO codon sites. Our analyses revealed that all the nine decapod species shared a common ancestor. The mean percentage sequence divergence at proPO gene was 34.4+/-0.6%. Pairwise estimates of nonsynonymous to synonymous ratio (omega) for Homarus americanus-H. gammarus is greater than one, therefore indicating adaptive evolution (functional diversification) of proPO in these two species. In contrast, strong purifying selection (omega<1) was observed in all other species pairs. However, phylogenetically closely related decapods revealed relatively higher omega value (omega=0.15+/-0.3) than the distantly related species pairs (omega=0.0075+/-0.005). These discrepancies could be due to higher fixation probability of beneficial mutation in closely related species. Maximum likelihood-based codon substitution analyses revealed a strong purifying selection operating on most of the codon sites, therefore suggesting proPO is functionally constrained (purifying selection). Codon substitution analyses have also revealed the evidence of strong purifying selection in haemocyanin subunits of decapods.

Purification and primary structural characterization of prophenoloxidases from Aedes aegypti larvae

There are more prophenoloxidase (proPO) genes in mosquitoes than other model insect species studied to date. The high sequence similarity among mosquito proPOs makes it extremely difficult to use histochemical methods to determine the presence of individual proPOs in different stages of mosquito development or their tissue locations. As a consequence, there always are questions when attempting to assign any observed functions to a particular proPO. By following the PO fractions of Aedes aegypti larval proteins during chromatographic separations, we were able to isolate two proPO fractions. Each displayed a single protein band on SDS-PAGE gel. The two fractions showed relative molecular weights of 75 and 60k Da. In-gel trypsin-digestion of the two protein bands and subsequent mass spectrometry of their tryptic peptides confirmed their proPO identities. The 75 kDa protein was a new Aedes aegypti proPO that has not been described in databases, whereas the 60 kDa band contained three previously described Aedes aegypti proPO sequences, with the absence of approximately 125-128 residues at their carboxyl end as compared with their deduced sequences, which suggests that some proPOs might undergo specific proteolytic processing after synthesis. Comparison between the transcriptional profiles of different proPOs and the number of isolated proPO proteins in late-stage larvae indicates that individual proPOs might be transcribed during the earlier stages of larval development, and that resulting proPO proteins persist through all larval stages. Results of this study provide a basis for developing a comprehensive understanding of structure/function relationships of individual proPOs in mosquitoes.

Structural basis for the presence of a monoglucosylated oligosaccharide in mature glycoproteins

Arylphorin is an insect hexameric storage protein. The structures of the oligosaccharides attached to this protein have recently been determined. However, their precise functions remain to be established. Proteolysis and MALDI MS studies disclose that the amino acid residues Asn196 and Asn344 are N-glycosylated with Glc(1)Man(9)GlcNAc(2) and Man(5-6)GlcNAc(2) oligosaccharides, respectively. Interestingly, significant variations in the amounts of glycans involving Glc(1)Man(9)GlcNAc(2) are evident in arylphorins purified from larvae reared at different seasons. The data suggest that the metabolism of larvae and local protein structure contribute to glycan development. Three-dimensional model of the protein speculated that N-glycosidic linkage to Asn196 in the Glc(1)Man(9)GlcNAc(2) structure was buried inside the twofold axis of the hexamer, whereas oligosaccharide linkages to Asn344 were completely exposed to solvent. This finding is in agreement with previous biochemical data showing that limited Glc(1)Man(9)GlcNAc(2) was released by protein-N-glycosidase F under non-denaturing conditions, in contrast to Man(5-6)GlcNAc(2) oligosaccharides.

MAP kinases mediate phagocytosis and melanization via prophenoloxidase activation in medfly hemocytes

E. coli phagocytosis by medfly hemocytes, in contrast to mammalian macrophages, associates with E. coli-challenged hemocyte secretion by mitogen activating protein (MAP) kinases. In the present work, we examined whether this system links with the proteolytic activation of prophenoloxidase (proPO). ProPO and prophenoloxidase-activating proteinases (PAPs) were initially identified within freshly isolated medfly hemocytes. Moreover, flow cytometry and immunocytochemical analysis revealed the constitutive expression of proPO and its stable association with hemocyte surface. The expression level of hemocyte surface proPO is not affected by E. coli infection. In addition, flow cytometry analysis in freshly isolated hemocytes showed that E. coli phagocytosis is markedly blocked by antibodies against proPO or PAPs, as well as by several serine protease inhibitors, strongly supporting the involvement of proPO cascade in the phagocytosis process. Similarly, it was shown that melanization process depends on proPO activation. MAP kinases appeared to control both phagocytosis and melanization, since they regulate PAPs secretion, a prerequisite for the conversion of proPO to active PO. From this and previous studies, hemocytes appear to be central to immune response in medfly.

Molecular characterization of a cDNA encoding prophenoloxidase and its expression in Apis mellifera

Phenoloxidase (PO), a melanin-synthesizing enzyme known to play an important role in insect defense, is found as a zymogen (ProPO) in hemolymph and cuticle, where it is activated by proteolysis. We characterized the first proPO cDNA in an eusocial insect, the Apis mellifera honey bee. The AmproPO cDNA contains an ORF of 2079 bp encoding 693 amino acids, and is composed of 9 exons and 8 introns. Southern blot of digested genomic DNA suggested that only one copy of the proPO gene is present in A. mellifera. The molecular mass of the deduced ProPO and the active enzyme was predicted to be 80.1 and 74.4 kDa, respectively. The calculated pI was 6.28. BLASTp search of the deduced amino acid sequence, and neighbor-joining analysis, showed similarity with ProPOs from other insects, ranging from 47% to 63%. Protein signature analyses revealed four conserved regions, including the two copper binding sites characteristic of arthropod ProPOs. RT-PCR and Southern blot showed the highest amount of AmproPO transcripts in workers whole body, followed by queens and drones. Expression was also detected in hemocytes and integument. Real time RT-PCR showed higher amounts of AmproPO transcripts in adults and older pupae than in younger pupae and larvae, suggesting a function of AmproPO in adult exoskeleton melanization and differentiation.

Molecular cloning and characterisation of prophenoloxidase from haemocytes of the white shrimp, Litopenaeus vannamei

cDNA encoding prophenoloxidase (proPO) of the white shrimp Litopenaeus vannamei was obtained from haemocytes by a reverse-transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA using oligonucleotide primers based on the proPO sequence of tiger shrimp Penaeus monodon, freshwater crayfish Pacifastacus leniusculus, green tiger shrimp Penaeus semisulcatus (accession no.: AF521949) and kuruma shrimp Marsupenaeus japonicus (accession no.: AB0733223). proPO of L. vannamei was constitutively expressed. The 2471-bp cDNA contained an open reading frame (ORF) of 2058 bp, a 96-bp 5'-untranslated region, and a 317-bp 3'-untranslated region containing the poly A tail. The molecular mass of the deduced amino acid sequence (686 amino acids) was 78.1 kDa with an estimated pI of 6.02. It contained putative copper binding sites, a complement-like motif (GCGWPQHM), a proteolytic activation site, and a conserved C-terminal region common to all known proPOs. However, no signal peptide sequence was detected in white shrimp proPO. Comparison of amino acid sequences showed that white shrimp proPO is more closely related to the proPO of another penaeid than to that of a freshwater crayfish. White shrimp proPO mRNA was synthesized in haemocytes and not in the hepatopancreas or muscle. The activation responses of the proPO of the white shrimp to an exogenous protease (trypsin), a detergent (sodium dodecyl sulphate), and algal and microbial cell wall components (laminarin, sodium alginate, zymosan, and lipopolysaccharide), and its susceptibility to protease inhibitors in vitro resemble the proPO activation system of other crustaceans. These facts suggest that the proPO system in haemocytes of the white shrimp Litopenaeus vannamei serves an important function in non-self recognition and host immune reactions.

Identification and molecular characterization of a prophenoloxidase involved in Aedes aegypti chorion melanization

Chorion melanization is a vital biochemical event for the survival of mosquito eggs in the environment. This study describes the identification and molecular characterization of a prophenoloxidase (proPO) involved in chorion melanization in Aedes aegypti by various biochemical and molecular techniques. Results revealed that transcription of the chorion proPO occurs only in adults, blood feeding greatly stimulated its transcription and haemocytes are responsible for its transcription. Our study provides a solid basis for suggesting an essential role of the isolated proPO in chorion melanization during chorion hardening and also raises fundamental questions regarding its transportation and distribution in the chorion. This study should serve as a useful reference towards functional elucidation of other proPOs in mosquitoes.

New insights into Drosophila larval haemocyte functions through genome-wide analysis

Drosophila blood cells or haemocytes comprise three cell lineages, plasmatocytes, crystal cells and lamellocytes, involved in immune functions such as phagocytosis, melanisation and encapsulation. Transcriptional profiling of activities of distinct haemocyte populations and from naive or infected larvae, was performed to find genes contributing to haemocyte functions. Of the 13 000 genes represented on the microarray, over 2500 exhibited significantly enriched transcription in haemocytes. Among these were genes encoding integrins, peptidoglycan recognition proteins (PGRPs), scavenger receptors, lectins, cell adhesion molecules and serine proteases. One relevant outcome of this analysis was the gain of new insights into the lamellocyte encapsulation process. We showed that lamellocytes require betaPS integrin for encapsulation and that they transcribe one prophenoloxidase gene enabling them to produce the enzyme necessary for melanisation of the capsule. A second compelling observation was that following infection, the gene encoding the cytokine Spatzle was uniquely upregulated in haemocytes and not the fat body. This shows that Drosophila haemocytes produce a signal molecule ready to be activated through cleavage after pathogen recognition, informing distant tissues of infection.

Phenoloxidase activity in Apis mellifera honey bee pupae, and ecdysteroid-dependent expression of the prophenoloxidase mRNA

Phenoloxidase (monophenol, l-dopa: oxygen oxidoreductase, EC 1.14.18.1) is a multicopper oxidase, which plays an important role in melanin synthesis, necessary for defense against intruding microorganisms and parasites, wound healing and cuticle pigmentation. A phenoloxidase from the hemolymph of honey bee pupae exhibited an apparent molecular mass of 70 kDa, as estimated by gel filtration and SDS-PAGE. Optimal pH and temperature were 6.5 and 20 degrees C, respectively. Activity was fully stable for 30 min at 50 degrees C. Like phenoloxidases from the hemolymph of other insects, the honey bee enzyme was activated by trypsin and inhibited by protease inhibitors and phenylthiourea. Only high concentrations of sodium azide effectively inhibited the detected activity. A low concentration (5 microM) of Ca2+, Mg2+, and Mn2+ had a stimulatory effect on the activity. Single Michaelis-Menten curves were observed for l-dopa and dopamine oxidation, but the affinity of the enzyme for dopamine was greater than for L-dopa. Semiquantitative RT-PCR and Southern blot analysis using a 359 bp labeled probe, and quantification of the prophenoloxidase mRNA levels by real-time PCR showed increased amounts of transcripts in hemocytes and integument from young pupae injected with 20-hydroxyecdysone.

Circular dichroism of prophenol oxidase in relation to the structural stability in Drosophila melanogaster

Circular dichroism (CD) of purified Drosophila melanogaster prophenol oxidase has been measured in the range of 195-245 nm. So far, few investigations about the interaction on higher-order structures have been performed. CD spectra of Drosophila prophenol oxidase with 2-propanol activator showed fluctuation of alpha-helices. At a high temperature of 80 degrees C, prophenol oxidase was partially denatured. However, it showed reversible recovery by renaturation after returning to low temperature at 30 degrees C. The conformational changes and reversible denaturation-renaturation interaction of the prophenol oxidase protein are discussed.

The prophenoloxidase-activating system in invertebrates

A major innate defense system in invertebrates is the melanization of pathogens and damaged tissues. This important process is controlled by the enzyme phenoloxidase (PO) that in turn is regulated in a highly elaborate manner for avoiding unnecessary production of highly toxic and reactive compounds. Recent progress, especially in arthropods, in the elucidation of mechanisms controlling the activation of zymogenic proPO into active PO by a cascade of serine proteinases and other factors is reviewed. The proPO-activating system (proPO system) is triggered by the presence of minute amounts of compounds of microbial origins, such as beta-1,3-glucans, lipopolysaccharides, and peptidoglycans, which ensures that the system will become active in the presence of potential pathogens. The presence of specific proteinase inhibitors prevents superfluous activation. Concomitant with proPO activation, many other immune reactions will be produced, such as the generation of factors with anti-microbial, cytotoxic, opsonic, or encapsulation-promoting activities.

Blood cells of Drosophila: cell lineages and role in host defence

Drosophila haemopoiesis gives rise to three independent cell lineages: plasmatocytes, crystal cells and lamellocytes. The regulation of Drosophila stem cell proliferation and lineage specification involves transactivators and signalling pathways, many of which have mammalian counterparts that control haemopoietic processes. Drosophila plasmatocytes are professional phagocytes that resemble the monocyte/macrophage lineage, crystal cells play a critical role in defence-related melanisation, and lamellocytes encapsulate large invaders. Crystal cells and lamellocytes have no clear mammalian homologues. Research into the molecular mechanisms that underlie the various immune functions of Drosophila blood cells, such as non-self recognition, is now taking wing.

Purification, characterization and molecular cloning of tyrosinase from the cephalopod mollusk,Illex argentinus

Tyrosinase (monophenol, L-DOPA:oxygen oxidoreductase) was isolated from the ink of the squid, Illex argentinus. Squid tyrosinase, termed ST94, was found to occur as a covalently linked homodimeric protein with a molecular mass of 140.2 kDa containing two copper atoms per a subunit. The tyrosinase activity of ST94 was enhanced by proteolysis with trypsin to form a protein, termed ST94t, with a molecular mass of 127.6 kDa. The amino acid sequence of the subunit was deduced from N-terminal amino acid sequencing and cDNA cloning, indicating that the subunit of ST94 is synthesized as a premature protein with 625 amino acid residues and an 18-residue signal sequence region is eliminated to form the mature subunit comprised of 607 amino acid residues with a deduced molecular mass of 68,993 Da. ST94 was revealed to contain two putative copper-binding sites per a subunit, that showed sequence similarities with those of hemocyanins from mollusks, tyrosinases from microorganisms and vertebrates and the hypothetical tyrosinase-related protein of Caenorhabditis elegans. The squid tyrosinase was shown to catalyze the oxidation of monophenols as well as o-diphenols and to exhibit temperature-dependency of o-diphenolase activity like a psychrophilic enzyme.

Drosophila blood cells

Drosophila blood cells or haemocytes belong to three lineages: plasmatocytes, crystal cells and lamellocytes. There is no equivalent of a lymphoid lineage in insects which have no adaptive immunity. Haematopoiesis is under the control of a number of transcription factors and signalling pathways (such as GATA factors, JAK/STAT or Notch pathways) most of which have homologues which participate in the control of mammalian haematopoiesis. Drosophila plasmatocytes are professional phagocytes reminiscent of the cells from the mammalian monocyte/macrophage lineage. Several receptors responsible for recognition of microorganisms or apoptotic corpses have been identified, which include a Scavenger Receptor, a CD36 homologue and a peptidoglycan recognition protein. Crystal cells contain the enzymes necessary for humoral melanization that accompanies a number of immune reactions. The production of melanin generates, as by-products, cytotoxic free radicals that are believed to participate in the killing of pathogens. Finally, lamellocytes represent a cell type that specifically differentiates after parasitism of Drosophila larvae and forms a capsule around the invader. Encapsulation together with melanization eventually kill the parasite within the capsule.

Overexpression of a pattern-recognition receptor, peptidoglycan-recognition protein-LE, activates imd/relish-mediated antibacterial defense and the prophenoloxidase cascade in Drosophila larvae

In Drosophila, microbial infection activates an antimicrobial defense system involving the activation of proteolytic cascades in the hemolymph and intracellular signaling pathways, the immune deficiency (imd) and Toll pathways, in immune-responsive tissues. The mechanisms for microbial recognition are largely unknown. We report that, in larvae, the imd-mediated antibacterial defense is activated by peptidoglycan-recognition protein (PGRP)-LE, a PGRP-family member in Drosophila. Consistent with this, PGRP-LE binds to the diaminopimelic acid-type peptidoglycan, a cell-wall component of the bacteria capable of activating the imd pathway, but not to the lysine-type peptidoglycan. Moreover, PGRP-LE activates the prophenoloxidase cascade, a proteolytic cascade in the hemolymph. Therefore, PGRP-LE acts as a pattern-recognition receptor to the diaminopimelic acid-type peptidoglycan and activates both the proteolytic cascade and intracellular signaling in Drosophila immunity.

Immunolocalization of prophenoloxidase in the process of wound healing in the mosquito Armigeres subalbatus (Diptera: Culicidae)

Hemolymph coagulation began almost immediately after wounding in mosquito, Armigeres subalbatus, (Coquillett) larvae. Immunocytochemical localization showed that prophenoloxidase (pro-PO) was distributed in the wound site. In the initial wounding, coagulation and wound plug formation occurred with granulocyte migration. The hemocytes lysed and released granular materials around the wound site, prophenoloxidase being mostly localized in granules and cuticle. In the second phase of wound healing, melanin accumulation occurred at the wound site along the margin of the cuticle and rapidly increased in thickness. Immunogold-labeled pro-PO was localized in vacuoles, melanins, and cuticle, with the gold particles labeled intensely on the undarkened cuticle and weakly on the darkened cuticle. It is believed that pro-PO is activated upon wound initiation to produce melanin product and deposited on the cuticle. In the final phase of healing, scab melanization and pro-PO immunogold localization were reduced and accompanied by epithelial cell regeneration. This proenzyme was localized in vesicles and flocculent materials, but was absent in the melanized scab. Our results further indicate that pro-PO was present in granules, cuticles, epithelial cells, vacuoles, and flocculent materials but not in melanized scab and coagulated clot. The pro-PO immunogold particles labeled intensely in the initial wounding but weakly in the final phase. Our observations also suggest that pro-PO is released from granulocytes by cell rupture, synthesized or stored in granulocytes, and then is released into the wound site via the cytoplasmic granules. This study indicates that the pro-PO is involved in numerous physiological roles in the process of wound healing in this mosquito.

Comparative biochemistry of eumelanogenesis and the protective roles of phenoloxidase and melanin in insects

The phenolic biopolymer eumelanin is an important skin pigment found throughout the animal kingdom. The enzyme, tyrosinase, initiates melanogenesis in mammals. The biogenesis is assisted by a number of mammalian protein factors including dopachrome tautomerase and 5,6-dihydroxyindole-2-carboxylate oxidase. Invertebrates, such as insects, employ phenoloxidase and dopachrome (decarboxylating) isomerase for melanin biosynthesis. Recently generated molecular biological and biochemical data indicate that tyrosinase and phenoloxidase are distinctly different enzymes in spite of possessing both monophenol monooxygenase activity as well as o-diphenoloxidase activity. Similarly, insect dopachrome isomerase also differs significantly from its mammalian counterpart in several of its properties including the nature of the enzymatic reaction. In addition, there are considerable differences in the eumelanogenic pathways of these two animal groups that include the utility of substrates, use of dihydroxyindoles and the nature of eumelanin pigment. Thus, the biochemistry and molecular biology of melanogenesis in mammals and insects are significantly different. The advantages of generating different eumelanin pigments and intermediates by the insects are discussed.

Functional Conversion of Hemocyanin to Phenoloxidase by Horseshoe Crab Antimicrobial Peptides

Arthropod hemocyanins and phenoloxidases serve different physiological functions as oxygen transporters and enzymes involved in defense reactions, respectively. However, they are equipped with a structurally similar oxygen-binding center. We have shown that the clotting enzyme of the horseshoe crab, Tachypleus tridentatus, functionally converts hemocyanin to phenoloxidase by forming a complex without proteolytic cleavage (Nagai, T., and Kawabata, S. (2000) J. Biol. Chem. 275, 35297-35301). Here we show that chitin-binding antimicrobial peptides of the horseshoe crab induce the intrinsic phenoloxidase activity of hemocyanin. Tachyplesin, a major Tachypleus antimicrobial peptide with an amphiphilic structure, converted the hemocyanin to phenoloxidase. Surface plasmon resonance analysis revealed the specific interaction of tachyplesin with hemocyanin at K(d) = 3.4 x 10(-)6 m. The chemical modification of Trp or Tyr in tachyplesin, but not Lys or Arg, dramatically reduced the affinity to hemocyanin, suggesting that the binding site is located in the hydrophobic face of tachyplesin. Hemocyanin has no affinity with chitin, but it significantly binds to tachyplesin-coated chitin, leading to the expression of phenoloxidase activity. The chitin coated with antimicrobial peptides may serve as a scaffold for the binding of hemocyanin, and the resulting phenoloxidase activity appears to function as a trigger of exoskeleton wound healing.

SDS-induced phenoloxidase activity of hemocyanins from Limulus polyphemus, Eurypelma californicum, and Cancer magister

Phenoloxidase, widely distributed among animals, plants, and fungi, is involved in many biologically essential functions including sclerotization and host defense. In chelicerates, the oxygen carrier hemocyanin seems to function as the phenoloxidase. Here, we show that hemocyanins from two ancient chelicerates, the horseshoe crab Limulus polyphemus and the tarantula Eurypelma californicum, exhibit O-diphenoloxidase activity induced by submicellar concentrations of SDS, a reagent frequently used to identify phenoloxidase activity. The enzymatic activity seems to be restricted to only a few of the heterogeneous subunits. These active subunit types share similar topological positions in the quaternary structures as linkers of the two tightly connected 2 x 6-mers. Because no other phenoloxidase activity was found in the hemolymph of these animals, their hemocyanins may act as a phenoloxidase and thus be involved in the primary immune response and sclerotization of the cuticle. In contrast, hemolymph of a more recent arthropod, the crab Cancer magister, contains both hemocyanin with weak phenoloxidase activity and another hemolymph protein with relatively strong phenoloxidase activity. The chelicerate hemocyanin subunits showing phenoloxidase activity may have evolved into a separate phenoloxidase in crustaceans.

Cuticular pro-phenoloxidase of the silkworm, Bombyx mori. Purification and demonstration of its transport from hemolymph

Pro-phenoloxidase (proPO) in insects is implicated in the defense against microbes and wounding. The presence of proPO in the cuticle was suggested more than 30 years ago, but it has not been purified. The extract of cuticles of the silkworm, Bombyx mori, was shown to contain two proPO isoforms (F-type and S-type proPOs, which have slightly different mobilities in polyacrylamide gel electrophoresis under nondenaturing conditions). The two isoforms were purified to homogeneity. From hemolymph of the same insect, two types of proPO with the same electrophoretic mobilities as those of cuticular isoforms were separated and were shown to be different at five amino acid residues in one of their subunits. The isoforms in the hemolymph and cuticle were activated by a specific activating enzyme. The resulting active phenoloxidases exhibited almost the same substrate specificities and specific activities toward o-diphenols. The substrate specificities and the susceptibilities to inhibitors, including carbon monoxide, indicated that the purified proPO isoforms were not zymogens of laccase-type phenoloxidase. The proPO in hemolymph was shown to be transported to the cuticle. This demonstration was corroborated by the failure to detect proPO transcripts by Northern analysis of total RNA from epidermal cells. In reversed-phase column chromatography, cuticular and hemolymph proPOs gave distinct elution profiles, indicating that some yet to be identified modification occurs in hemolymph proPO and results in the formation of cuticular proPO. There was little transportation of cuticular proPO to the cuticle when it was injected into the hemocoel. The nature of the modification is described in the accompanying paper (Asano, T., and Ashida, M. (2001) J. Biol. Chem. 276, 11113-11125).