Analysis of proteins from different phase variants of the entomopathogenic bacteria Photorhabdus luminescens by two-dimensional zymography

Transcriptional analysis of a Photorhabdus sp. variant reveals transcriptional control of phenotypic variation and multifactorial pathogenicity in insects

Photorhabdus luminescens lives in a mutualistic association with entomopathogenic nematodes and is pathogenic for insects. Variants of Photorhabdus frequently arise irreversibly and are studied because they have altered phenotypic traits that are potentially important for the host interaction. VAR* is a colonial and phenotypic variant displaying delayed pathogenicity when directly injected into the insect, Spodoptera littoralis. In this study, we evaluated the role of transcriptomic modulation in determining the phenotypic variation and delayed pathogenicity of VAR* with respect to the corresponding wild-type form, TT01α. A P. luminescens microarray identified 148 genes as differentially transcribed between VAR* and TT01α. The net regulator status of VAR* was found to be significantly modified. We also observed in VAR* a decrease in the transcription of genes supporting certain phenotypic traits, such as pigmentation, crystalline inclusion, antibiosis, and protease and lipase activities. Three genes encoding insecticidal toxins (pit and pirB) or putative insecticidal toxins (xnp2) were less transcribed in VAR* than in the TT01α. The overexpression of these genes was not sufficient to restore the virulence of VAR* to the levels of ΤΤ01α, which suggests that the lower virulence of VAR* does not result from impaired toxemia in insects. Three loci involved in oxidative stress responses (sodA, katE, and the hca operon) were found to be downregulated in VAR*. This is consistent with the greater sensitivity of VAR* to H(2)O(2) and may account for the impaired bacteremia in the hemolymph of S. littoralis larvae observed with VAR*. In conclusion, we demonstrate here that some phenotypic traits of VAR* are regulated transcriptionally and highlight the multifactorial nature of pathogenicity in insects.

Analysis of serine proteases from marine sponges by 2-D zymography

Proteolytic activities isolated from the marine demosponges Geodia cydonium and Suberites domuncula were analyzed by 2-D zymography, a technique that combines IEF and zymography. After purification, a 200 kDa proteolytically active protein band was obtained from G. cydonium when analyzed in gelatin copolymerized 1-D zymograms. The enzymatic activity was quantified using alpha-N-benzoyl-D-arginine p-nitroanilide (BAPNA) as a substrate and corresponded to a serine protease. The protease activity was resistant to urea and SDS. DTT and 2-mercaptoethanol (2-ME) did not significantly change the protease activity, but induced a shift in molecular mass of the proteolytic band to lower M(r) values as detected by zymography. Under mild denaturing conditions, lower M(r) bands (<200 kDa) were identified in 1-D zymograms, suggesting that the protease is composed of subunits which retain the catalytic activity. After 2-D zymography, the protease from G. cydonium revealed a pI of 8.0 and an M(r) shift from 200 to 66 kDa. To contrast these results, a cytosolic sample from S. domuncula was analyzed. The proteolytic activity of this sponge after 2-D zymography corresponded to an M(r) of 40 kDa and a pI of 4.0. The biological function of both sponge proteases is not yet known. This study demonstrates that mild denaturing conditions required for IEF may alter the interpretation of the 2-D zymography, and care must be taken during sample preparation.

Comparison of proteolytic activities produced by entomopathogenic Photorhabdus bacteria: strain- and phase-dependent heterogeneity in composition and activity of four enzymes

Twenty strains (including eight phase variant pairs) of nematode-symbiotic and insect-pathogenic Photorhabdus bacteria were examined for the production of proteolytic enzymes by using a combination of several methods, including gelatin liquefaction, zymography coupled to native and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and activity measurement with two chromogen substrate types. Four protease activities (approximately 74, approximately 55, approximately 54, and approximately 37 kDa) could be separated. The N-terminal sequences of three of the proteases were determined, and a comparison with sequences in databases allowed identification of these proteases as HEXXH metallopeptidases. Thus, the 74-kDa protease (described formerly as Php-B [J. Marokhazi, G. Koczan, F. Hudecz, L. Graf, A. Fodor, and I. Venekei, Biochem. J. 379:633-640, 2004) is an ortholog of OpdA, a member the thimet oligopeptidase family, and the 55-kDa protease is an ortholog of PrtA, a HEXXH+H peptidase in clan MB (metzincins), while the 37-kDa protease (Php-C) belongs to the HEXXH+E peptidases in clan MA. The 54-kDa protease (Php-D) is a nonmetalloenzyme. PrtA and Php-C were zymographically detected, and they occurred in several smaller forms as well. OpdA could not be detected by zymography. PrtA, Php-C, and Php-D were secreted proteases; OpdA, in contrast, was an intracellular enzyme. OpdA activity was found in every strain tested, while Php-D was detected only in the Brecon/1 strain. There was significant strain variation in the secretion of PrtA and Php-C activities, but reduced activity or a lack of activity was not specific to secondary-phase variants. The presence of PrtA, OpdA, and Php-C activities could be detected in the hemolymph of Galleria melonella larvae 20 to 40 h postinfection. These proteases appear not to be directly involved in the pathogenicity of Photorhabdus, since strains or phase variants lacking any of these proteases do not show reduced virulence when they are injected into G. melonella larvae.

Purification and characterization of two distinct metalloproteases secreted by the entomopathogenic bacterium Photorhabdus sp. strain Az29

Photorhabdus sp. strain Az29 is symbiotic with an Azorean nematode of the genus Heterorhabditis in a complex that is highly virulent to insects even at low temperatures. The virulence of the bacteria is mainly attributed to toxins and bacterial enzymes secreted during parasitism. The bacteria secrete proteases during growth, with a peak at the end of the exponential growth phase. Protease secretion was higher in cultures growing at lower temperatures. At 10 degrees C the activity was highest and remained constant for over 7 days, whereas at 23 and 28 degrees C it showed a steady decrease. Two proteases, PrtA and PrtS, that are produced in the growth medium were purified by liquid chromatography. PrtA was inhibited by 1,10-phenantroline and by EDTA and had a molecular mass of 56 kDa and an optimal activity at pH 9 and 50 degrees C. Sequences of three peptides of PrtA showed strong homologies with alkaline metalloproteases from Photorhabdus temperata K122 and Photorhabdus luminescens W14. Peptide PrtA-36 contained the residues characteristic of metzincins, known to be involved in bacterial virulence. In vitro, PrtA inhibited antibacterial factors of inoculated Lepidoptera and of cecropins A and B. PrtS had a molecular mass of 38 kDa and was inhibited by 1,10-phenanthroline but not by EDTA. Its activity ranged between 10 and 80 degrees C and was optimal at pH 7 and 50 degrees C. PrtS also destroyed insect antibacterial factors. Three fragments of PrtS showed homology with a putative metalloprotease of P. luminescens TTO1. Polyclonal antibody raised against PrtA did not recognize PrtS, showing they are distinct molecules.

Genetic and biochemical characterization of PrtA, an RTX-like metalloprotease from Photorhabdus

Proteases play a key role in the interaction between pathogens and their hosts. The bacterial entomopathogen Photorhabdus lives in symbiosis with nematodes that invade insects. Following entry into the insect, the bacteria are released from the nematode gut into the open blood system of the insect. Here they secrete factors which kill the host and also convert the host tissues into food for the replicating bacteria and nematodes. One of the secreted proteins is PrtA, which is shown here to be a repeats-in-toxin (RTX) alkaline zinc metalloprotease. PrtA has high affinity for artificial substrates such as casein and gelatin and can be inhibited by zinc metalloprotease inhibitors. The metalloprotease also shows a calcium- and temperature-dependent autolysis. The prtA gene carries the characteristic RTX repeated motifs and predicts high similarity to proteases from Erwinia chrysanthemi, Pseudomonas aeruginosa and Serratia marcescens. The prtA gene resides in a locus encoding both the protease ABC transporter (prtBCD) and an intervening ORF encoding a protease inhibitor (inh). PrtA activity is detectable 24 h after artificial bacterial infection of an insect, suggesting that the protease may play a key role in degrading insect tissues rather than in overcoming the insect immune system. Purified PrtA also shows cytotoxicity to mammalian cell cultures, supporting its proposed role in bioconversion of the insect cadaver into food for bacterial and nematode development.

A zinc metalloprotease inhibitor, Inh, from the insect pathogen Photorhabdus luminescens

The entomopathogen Photorhabdus luminescens secretes many proteins during the late stages of insect larvae infection and during in vitro laboratory culture. The authors have previously characterized and purified a 55 kDa zinc metalloprotease, PrtA, from culture supernatants of P. luminescens. PrtA is secreted via a classical type I secretory pathway and is encoded within the operon prtA-inh-prtBCD. The 405 bp inh gene encodes a 14.8 kDa pre-protein that is translocated to the periplasm by the classical signal-peptide-dependent sec pathway, yielding the mature 11.9 kDa inhibitor Inh. Inh is a specific inhibitor of the protease PrtA. This study describes the purification of Inh and the initial characterization of its in vitro protease inhibition properties.

A functional proteomic analysis of secreted fibrinolytic enzymes from Bacillus subtilis 168 using a combined method of two-dimensional gel electrophoresis and zymography

Here we describe a proteomic approach to detect fibrinolytic enzymes from the culture supernatant of Bacillus subtilis 168. Following isoelectric focusing without dithiothreitol, two gels, one for sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and the other for zymography, were run in parallel. After silver staining of SDS-PAGE and activity staining of zymography gel, the two gels were superimposed to detect protein spots that coincided with clear zones on the zymography gel. We identified four protein spots and characterized them with matrix-assisted laser desorption/ionization mass spectrometry. Database search revealed that four spots contained at least one of the extracellular serine proteases such as WprA and Vpr. This combined method of two-dimensional gel and zymography can be used as a powerful tool to detect proteases from various organisms.

Xenorhabdus nematophilus as a model for host-bacterium interactions: rpoS is necessary for mutualism with nematodes

Xenorhabdus nematophilus, a gram-negative bacterium, is a mutualist of Steinernema carpocapsae nematodes and a pathogen of larval-stage insects. We use this organism as a model of host-microbe interactions to identify the functions bacteria require for mutualism, pathogenesis, or both. In many gram-negative bacteria, the transcription factor sigma(S) controls regulons that can mediate stress resistance, survival, or host interactions. Therefore, we examined the role of sigma(S) in the ability of X. nematophilus to interact with its hosts. We cloned, sequenced, and disrupted the X. nematophilus rpoS gene that encodes sigma(S). The X. nematophilus rpoS mutant pathogenized insects as well as its wild-type parent. However, the rpoS mutant could not mutualistically colonize nematode intestines. To our knowledge, this is the first report of a specific allele that affects the ability of X. nematophilus to exist within nematode intestines, an important step in understanding the molecular mechanisms of this association.

Purification and characterization of two serine protease inhibitors from the hemolymph of Mythimna unipuncta

Two serine protease inhibitors, trypsin inhibitor and alpha-chymotrypsin inhibitor, were isolated from the hemolymph of Mythimna unipuncta. Mythimna trypsin and alpha-chymotrypsin inhibitors were purified by gel filtration and anion-exchange chromatography. They displayed molecular masses of 52 kDa and 43 kDa, respectively, as determined by electrophoresis under reducing and non-reducing conditions on denaturing polyacrylamide gels. Their isoelectric points were evaluated by isoelectric focusing and two-dimensional electrophoresis. Their N-terminal sequences have been analyzed as APSDTTIAETLTITEEFFPD and FDESFGFQGPSTYEKTPLGEP, respectively. The role of these inhibitors in the regulation of the defense reaction of the insect is discussed.

A simple method for assigning multiple immunogens to their protein on a two-dimensional blot and its application to asthma-causing allergens

A "one-step" procedure, that not only removes the color and blocking proteins used in the colorimetric immunodetection step but also stains the proteins originally on the blot, is presented. Following immunostaining and recording of immunoreactive spots, the blot was allowed to air-dry overnight (or longer) at room temperature and then counterstained with a colloidal gold solution. This "air-drying" process apparently altered the affinity of the blocking proteins (and possibly other proteins added subsequently to the blotting step) towards the nitrocellulose membrane causing them to be removed by the acidic colloidal gold solution while the "blotted" proteins were being stained. The sensitivity of this counterstained blot was comparable to that of the blot without going through the immunodetection process. Since both immunodetection and protein staining were carried out on the same blot, this allowed easy identification of many immunoreactive spots to their corresponding proteins when the two profiles were superimposed. Using this procedure, we have detected 25 immunoreactive spots (or allergens) from the whole body extract of the German cockroach (Blattella germanica) that contribute to asthma and assigned them to their corresponding proteins on a two-dimensional (2-D) protein map. The apparent Mr and pI for each of the allergens were determined. We have also located one of the major cockroach allergens, Bla g 5 (glutathione S-transferase). Two-dimensional zymography revealed the presence of ten gelatinase-type proteolytic enzymes. Only one of the ten proteases comigrated with the immunoreactive proteins indicating that unlike other allergen-producing systems, most of the cockroach allergens do not possess protease activity.

A new broad-spectrum protease inhibitor from the entomopathogenic bacterium Photorhabdus luminescens

A new protease inhibitor was purified to apparent homogeneity from a culture medium of Photorhabdus luminescens by ammonium sulfate precipitation and preparative isoelectric focusing followed by affinity chromatography. Ph. luminescens, a bacterium symbiotically associated with the insect-parasitic nematode Heterorhabditis bacteriophora, exists in two morphologically distinguishable phases (primary and secondary). It appears that only the secondary-phase bacterium produces this protease inhibitor. The protease inhibitor has an M:(r) of approximately 12000 as determined by SDS-PAGE. Its activity is stable over a pH range of 3.5-11 and at temperatures below 50 degrees C. The N-terminal 16 amino acids of the protease inhibitor were determined as STGIVTFKND(X)GEDIV and have a very high sequence homology with the N-terminal region of an endogenous inhibitor (IA-1) from the fruiting bodies of an edible mushroom, Pleurotus ostreatus. The purified protease inhibitor inactivated the homologous protease with an almost 1:1 stoichiometry. It also inhibited proteases from a related insect-nematode-symbiotic bacterium, Xenorhabdus nematophila. Interestingly, when present at a molar ratio of 5 to 1, this new protease inhibitor completely inactivated the activity of both trypsin and elastase. The activity of proteinase A and cathepsin G was partially inhibited by this bacterial protease inhibitor, but it had no effect on chymotrypsin, subtilisin, thermolysin and cathepsins B and D. The newly isolated protease inhibitor from the secondary-phase bacteria and its specific inhibition of its own protease provides an explanation as to why previous investigators failed to detect the presence of protease activity in the secondary-phase bacteria. The functional implications of the protease inhibitor are also discussed in relation to the physiology of nematode-symbiotic bacteria.

Deficiency of current methods in assaying endochitinase activity

Since chitin is degraded by a combination of both endo- and exochitinases, it is likely that both enzymes will be present in a crude extract. Currently used substrates for detecting endochitinase activity suffer from the fact that they could easily be digested by the contaminating exochitinase, thus giving either a false-positive or an inaccurate reading of the endochitinase activity. Using Photorhabdus luminescens, a bacterium symbiotically associated with insect-parasitic nematode Heterorhabditis bacteriophora as an exemplary system, we have identified these two chitinases by a simple "fluorimetric zymography" procedure. The exochitinase is a metalloenzyme and its activity is inhibited by 1,10-phenanthroline. Once the exochitinase activity is detected in a crude extract, its contribution must be eliminated before accurate determination of the endochitinase activity can be carried out. Specific properties of these enzymes including the pH activity profile, the requirement of metal ions for activity, and the molecular weight of the enzymes are among the factors to be considered in developing assaying procedures for endochitinase.

Secreted proteases from Photorhabdus luminescens: separation of the extracellular proteases from the insecticidal Tc toxin complexes

Photorhabdus luminescens secretes both high molecular weight insecticidal toxin complexes and also a range of extracellular proteases into culture broth. Previous studies by others have suggested that insecticidal activity of the broth is associated with these proteases. However, by gene cloning and targeted knock-out, we have previously shown that oral insecticidal activity is associated with high molecular weight 'toxin complexes' (Tc) encoded by toxin complex or tc genes. Here we further clarify this distinction by biochemically separating the protease fractions away from the oral insecticidal activity of the Tc proteins. We purified three distinct protease fractions from the broth: one consisting of a single species of 55 kDa and two of several putatively related species of approximately 40 kDa. All of these clearly separate from the oral insecticidal activity associated with the high molecular weight Tc proteins and also show no effect on insect weight gain following injection into the haemocoel. Here we examine the substrate preferences and inhibitor profiles of these protease fractions and discuss their relationship with those previously described from other P. luminescens strains and phase variants.

Photorhabdus toxins: novel biological insecticides

Following concerns over the potential for insect resistance to insecticidal Bacillus thuringiensis toxins expressed in transgenic plants, there has been recent interest in novel biological insecticides. Over the past year there has been considerable progress in the cloning of several alternative toxin genes from the bacteria Photorhabdus luminescens and Xenorhabdus nematophilus. These genes encode large insecticidal toxin complexes with little homology to other known toxins.

Two-dimensional zymography for analysis of proteolytic enzymes in human pure pancreatic juice

Proteolytic enzymes in human pure pancreatic juice (PPJ), which was collected by cannulating the main pancreatic duct using endoscopy, were investigated by two-dimensional zymography (2-DZ). 2-DZ was carried out by combining isoelectric focusing (IEF) in the first dimension with sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) in the second dimension, using gels containing casein or gelatin as a substrate for the proteolytic enzymes. After electrophoresis, the gels were incubated in Triton X-100 followed by incubation at 37 degrees C in Tris buffer (pH 8.5) containing CaCl2. By staining the gels with Coomassie Brilliant Blue (CBB R-250), proteolytic enzymes were detected as clear spots and zones against a blue background. Proteinase inhibitors, such as a cysteine proteinase inhibitor (E-64), a metalloproteinase inhibitor (EDTA), and a serine proteinase inhibitor (Pefabloc SC), were added to PPJ in order to determine the types of proteinases. In patients with pancreatic cancer, spots of molecular weight (Mr) 70,000 and isoelectric points (pI) 5.3-5.5 were clearly detected on the gels containing casein and gelatin, while these spots were not detected in the PPJ from healthy subjects. The proteolytic activities of these spots were strongly inhibited by EDTA and Pefabloc SC but not E-64. These results suggest that the spots of Mr 70,000 and pI 5.3-5.5 in PPJ of pancreatic cancer might be matrix metalloproteinase 2, which is a candidate for tumor-associated proteinase. 2-DZ proved to be a tool for analysis of proteolytic enzymes in PPJ and for the clinical diagnosis of pancreatic cancer.