Bacterial invasion of eukaryotic cells can be mediated by actin-hydrolysing metalloproteases grimelysin and protealysin

The Balance between Protealysin and Its Substrate, the Outer Membrane Protein OmpX, Regulates Serratia proteamaculans Invasion

Serratia are opportunistic bacteria, causing infections in plants, insects, animals and humans under certain conditions. The development of bacterial infection in the human body involves several stages of host-pathogen interaction, including entry into non-phagocytic cells to evade host immune cells. The facultative pathogen Serratia proteamaculans is capable of penetrating eukaryotic cells. These bacteria synthesize an actin-specific metalloprotease named protealysin. After transformation with a plasmid carrying the protealysin gene, noninvasive E. coli penetrate eukaryotic cells. This suggests that protealysin may play a key role in S. proteamaculans invasion. This review addresses the mechanisms underlying protealysin's involvement in bacterial invasion, highlighting the main findings as follows. Protealysin can be delivered into the eukaryotic cell by the type VI secretion system and/or by bacterial outer membrane vesicles. By cleaving actin in the host cell, protealysin can mediate the reversible actin rearrangements required for bacterial invasion. However, inactivation of the protealysin gene leads to an increase, rather than decrease, in the intensity of S. proteamaculans invasion. This indicates the presence of virulence factors among bacterial protealysin substrates. Indeed, protealysin cleaves the virulence factors, including the bacterial surface protein OmpX. OmpX increases the expression of the EGFR and β1 integrin, which are involved in S. proteamaculans invasion. It has been shown that an increase in the invasion of genetically modified S. proteamaculans may be the result of the accumulation of full-length OmpX on the bacterial surface, which is not cleaved by protealysin. Thus, the intensity of the S. proteamaculans invasion is determined by the balance between the active protealysin and its substrate OmpX.

Protease S of entomopathogenic bacterium Photorhabdus laumondii: expression, purification and effect on greater wax moth Galleria mellonella

BACKGROUND

Protease S (PrtS) from Photorhabdus laumondii belongs to the group of protealysin-like proteases (PLPs), which are understudied factors thought to play a role in the interaction of bacteria with other organisms. Since P. laumondii is an insect pathogen and a nematode symbiont, the analysis of the biological functions of PLPs using the PrtS model provides novel data on diverse types of interactions between bacteria and hosts.

METHODS AND RESULTS

Recombinant PrtS was produced in Escherichia coli. Efficient inhibition of PrtS activity by photorin, a recently discovered emfourin-like protein inhibitor from P. laumondii, was demonstrated. The Galleria mellonella was utilized to examine the insect toxicity of PrtS and the impact of PrtS on hemolymph proteins in vitro. The insect toxicity of PrtS is reduced compared to protease homologues from non-pathogenic bacteria and is likely not essential for the infection process. However, using proteomic analysis, potential PrtS targets have been identified in the hemolymph.

CONCLUSIONS

The spectrum of identified proteins indicates that the function of PrtS is to modulate the insect immune response. Further studies of PLPs' biological role in the PrtS and P. laumondii model must clarify the details of PrtS interaction with the insect immune system during bacterial infection.

Production and Characterization of Photorin, a Novel Proteinaceous Protease Inhibitor from the Entomopathogenic Bacteria Photorhabdus laumondii

Entomopathogenic bacteria of the genus Photorhabdus secrete protease S (PrtS), which is considered a virulence factor. We found that in the Photorhabdus genomes, immediately after the prtS genes, there are genes that encode small hypothetical proteins homologous to emfourin, a recently discovered protein inhibitor of metalloproteases. The gene of emfourin-like inhibitor from Photorhabdus laumondii subsp. laumondii TT01 was cloned and expressed in Escherichia coli cells. The recombinant protein, named photorin (Phin), was purified by metal-chelate affinity and gel permeation chromatography and characterized. It has been established that Phin is a monomer and inhibits activity of protealysin and thermolysin, which, similar to PrtS, belong to the M4 peptidase family. Inhibition constants were 1.0 ± 0.3 and 10 ± 2 µM, respectively. It was also demonstrated that Phin is able to suppress proteolytic activity of P. laumondii culture fluid (half-maximal inhibition concentration 3.9 ± 0.3 nM). Polyclonal antibodies to Phin were obtained, and it was shown by immunoblotting that P. laumondii cells produce Phin. Thus, the prtS genes in entomopathogenic bacteria of the genus Photorhabdus are colocalized with the genes of emfourin-like inhibitors, which probably regulate activity of the enzyme during infection. Strict regulation of the activity of proteolytic enzymes is essential for functioning of all living systems. At the same time, the principles of regulation of protease activity by protein inhibitors remain poorly understood. Bacterial protease-inhibitor pairs, such as the PrtS and Phin pair, are promising models for in vivo studies of these principles. Bacteria of the genus Photorhabdus have a complex life cycle with multiple hosts, being both nematode symbionts and powerful insect pathogens. This provides a unique opportunity to use the PrtS and Phin pair as a model for studying the principles of protease activity regulation by proteinaceous inhibitors in the context of bacterial interactions with different types of hosts.

Involvement of Lipid Rafts in the Invasion of Opportunistic Bacteria Serratia into Eukaryotic Cells

Cell membrane rafts form signaling platforms on the cell surface, controlling numerous protein-protein and lipid-protein interactions. Bacteria invading eukaryotic cells trigger cell signaling to induce their own uptake by non-phagocytic cells. The aim of this work was to reveal the involvement of membrane rafts in the penetration of the bacteria Serratia grimesii and Serratia proteamaculans into eukaryotic cells. Our results show that the disruption of membrane rafts by MβCD in the three cell lines tested, M-HeLa, MCF-7 and Caco-2, resulted in a time-dependent decrease in the intensity of Serratia invasion. MβCD treatment produced a more rapid effect on the bacterial susceptibility of M-HeLa cells compared to other cell lines. This effect correlated with a faster assembly of the actin cytoskeleton upon treatment with MβCD in M-HeLa cells in contrast to that in Caco-2 cells. Moreover, the 30 min treatment of Caco-2 cells with MβCD produced an increase in the intensity of S. proteamaculans invasion. This effect correlated with an increase in EGFR expression. Together with the evidence that EGFR is involved in S. proteamaculans invasion but not in S. grimesii invasion, these results led to the conclusion that an increase in EGFR amount on the plasma membrane with the undisassembled rafts of Caco-2 cells after 30 min of treatment with MβCD may increase the intensity of S. proteamaculans but not of S. grimesii invasion. Thus, the MβCD-dependent degradation of lipid rafts, which enhances actin polymerization and disrupts signaling pathways from receptors on the host cell's surface, reduces Serratia invasion.

NMR structure of emfourin, a novel protein metalloprotease inhibitor: Insights into the mechanism of action

Emfourin (M4in) is a protein metalloprotease inhibitor recently discovered in the bacterium Serratia proteamaculans and the prototype of a new family of protein protease inhibitors with an unknown mechanism of action. Protealysin-like proteases (PLPs) of the thermolysin family are natural targets of emfourin-like inhibitors widespread in bacteria and known in archaea. The available data indicate the involvement of PLPs in interbacterial interaction as well as bacterial interaction with other organisms and likely in pathogenesis. Arguably, emfourin-like inhibitors participate in the regulation of bacterial pathogenesis by controlling PLP activity. Here, we determined the 3D structure of M4in using solution NMR spectroscopy. The obtained structure demonstrated no significant similarity to known protein structures. This structure was used to model the M4in-enzyme complex and the complex model was verified by small-angle X-ray scattering. Based on the model analysis, we propose a molecular mechanism for the inhibitor, which was confirmed by site-directed mutagenesis. We show that two spatially close flexible loop regions are critical for the inhibitor-protease interaction. One region includes aspartic acid forming a coordination bond with catalytic Zn2+ of the enzyme and the second region carries hydrophobic amino acids interacting with protease substrate binding sites. Such an active site structure corresponds to the noncanonical inhibition mechanism. This is the first demonstration of such a mechanism for protein inhibitors of thermolysin family metalloproteases, which puts forward M4in as a new basis for the development of antibacterial agents relying on selective inhibition of prominent factors of bacterial pathogenesis belonging to this family.

Protealysin Targets the Bacterial Housekeeping Proteins FtsZ and RecA

Serratia proteamaculans synthesizes the intracellular metalloprotease protealysin. This work was aimed at searching for bacterial substrates of protealysin among the proteins responsible for replication and cell division. We have shown that protealysin unlimitedly cleaves the SOS response protein RecA. Even 20% of the cleaved RecA in solution appears to be incorporated into the polymer of uncleaved monomers, preventing further polymerization and inhibiting RecA ATPase activity. Transformation of Escherichia coli with a plasmid carrying the protealysin gene reduces the bacterial UV survival up to 10 times. In addition, the protealysin substrate is the FtsZ division protein, found in both E. coli and Acholeplasma laidlawii, which is only 51% identical to E. coli FtsZ. Protealysin cleaves FtsZ at the linker between the globular filament-forming domain and the C-terminal peptide that binds proteins on the bacterial membrane. Thus, cleavage of the C-terminal segment by protealysin can lead to the disruption of FtsZ’s attachment to the membrane, and thereby inhibit bacterial division. Since the protealysin operon encodes not only the protease, but also its inhibitor, which is typical for the system of interbacterial competition, we assume that in the case of penetration of protealysin into neighboring bacteria that do not synthesize a protealysin inhibitor, cleavage of FtsZ and RecA by protealysin may give S. proteamaculans an advantage in interbacterial competition.

Characterization of a strain of Serratia sp. with ixodicide activity against the cattle tick Rhipicephalus microplus

Cattle ticks are considered the most important ectoparasite in the livestock industry. Rhipicephalus microplus causes economic losses both through direct feeding on livestock and through disease transmission. Reports of the failure of chemical ixodicides to control this tick have led to a search for control alternatives, such as bacteria with ixodicide activity. The objective of this work was to select a bacterial strain with ixodicide activity against R. microplus. In total, 83 bacterial strains were isolated from soil and dead R. microplus specimens, and all strains were evaluated against larvae in a screening test. Bacteria with ixodicide activity were evaluated in larvae and engorged adult female ticks. The larvae were challenged using the larval immersion test (LIT) with 20 µg/mL total protein. The median lethal concentration (LC₅₀) for larvae was obtained by using nine total protein concentrations. Engorged adult female ticks were challenged using the adult immersion test (AIT) with six protein concentrations. We evaluated adult mortality on day 10, oviposition rate on day 14 and hatching rate on day 40 after challenge. Only one bacterial strain (EC-35) showed ixodicide activity against larvae and adult R. microplus. The highest larval mortality, 52.3%, occurred with a total protein concentration of 40 μg/mL, and the LC₅₀ was 13.9 µg/mL of protein. In adults, a total protein concentration of 10 µg/mL had the highest mortality (55%), oviposition inhibition (50.9%) and reproductive potential inhibition (52.5%). However, there was no significant effect on hatching. The 16S rRNA gene sequence showed 99% identity of EC-35 with Serratia sp.

Invasion of Serratia proteamaculans is regulated by the sprI gene encoding AHL synthase

Quorum Sensing (QS) system regulates gene expression in response to a change in the density of the bacterial population. Facultative pathogen Serratia proteamaculans 94 has a LuxI/LuxR type QS system consisting of regulatory protein SprR and AHL synthase SprI. Invasive activity of these bacteria appears at the stationary growth phase corresponding to a maximal density of the bacterial population in vitro. To evaluate the contribution of QS system of S. proteamaculans 94 to the regulation of invasive activity, in this work, S. proteamaculans SprI(-) mutant carrying the inactivated AHL synthase gene was used. Inactivation of the AHL synthase sprI gene resulted in a more than fourfold increase in the invasive activity of S. proteamaculans preceded by the increased adhesion of bacteria to the cell surface. This effect correlated with the increased expression of the outer membrane protein ompX gene and the decrease in the activity of intrabacterial protease protealysin, whose substrate is OmpX. The inverse correlation between activity of protealysin and bacterial invasion was also observed in the model experiments under the iron-limiting culture conditions. These results show that QS system regulates the S. proteamaculans invasion. This regulation can involve changes both in the protealysin activity and in the level of the ompX gene transcription.

NMR assignments and secondary structure distribution of emfourin, a novel proteinaceous protease inhibitor

Emfourin (M4in) from Serratia proteamaculans is a new proteinaceous inhibitor of protealysin-like proteases (PLPs), a subgroup of the well-known and widely represented metallopeptidase M4 family. Although the biological role of PLPs is debatable, data published indicate their involvement in pathogenesis, including bacterial invasion into eukaryotic cells, suppression of immune defense of some animals, and destruction of plant cell walls. Gene colocalization into a bicistronic operon observed for some PLPs and their inhibitors (as in the case of M4in) implies a mutually consistent functioning of both entities. The originality of the amino acid sequence of M4in suggests it belongs to a previously unknown protein family and this encourages structural studies. In this work, we report a near-complete assignment of 1H, 13C, and 15N resonances of recombinant M4in and its structural-dynamic properties derived from the chemical shifts. According the NMR data analysis, the M4in molecule comprises 3-5 helical elements and 4-6 β-strands, at least two of which are apparently antiparallel, ascribing this obviously globular protein to the α + β structural class. Besides, two disordered regions also exist in the central loops between the regular secondary structural elements. The obtained data provide the basis for determining the high-resolution structure as well as functioning mechanism of M4in that can be used for development of new antibacterial therapeutic strategies.

The protealysin operon encodes emfourin, a prototype of a novel family of protein metalloprotease inhibitors

Protealysin is a Serratia proteamaculans metalloproteinase of the M4 peptidase family and the prototype of a large group of protealysin-like proteases (PLPs). PLPs are likely involved in bacterial interaction with plants and animals as well as in bacterial pathogenesis. We demonstrated that the PLP genes in bacteria colocalize with the genes of putative conserved proteins. In S. proteamaculans, these two genes form a bicistronic operon. The putative S. proteamaculans protein that we called emfourin (M4in) was expressed in Escherichia coli and characterized. M4in forms a complex with protealysin with a 1:1 stoichiometry and is a potent slow-binding competitive inhibitor of protealysin (K_i = 52 ± 14 pM); besides, M4in is not secreted from S. proteamaculans constitutively. A comparison of amino acid sequences of M4in and its homologs with those of known inhibitors suggests that M4in is the prototype of a new family of protein inhibitors of proteases.

Salt-enhanced processing, proteolytic activity and stability of halophilic thermolysin-like proteinase, salilysin, isolated from a moderate halophile, Chromohalobacter salexigens DSM3043

Moderately halophilic bacterium, Chromohalobacter salexigens DSM3043, has a gene Csal_2537 encoding thermolysin-like M4 proteinase. This gene was cloned to pET expression vectors, resulting in high expression of recombinant proteinase, named as salilysin (salinity-dependent thermolysin-like proteinase), in Escherichia coli cytoplasm. This gene encodes precursor form of salilysin containing 348 amino acid residues (Pro-salilysin) consisting of 55 amino acids pro-sequence and following mature proteinase. Pro-sequence was cleaved three times to form intermediate 1, intermediate 2 and final mature salilysin. The processing rate was greatly accelerated in a salt concentration-dependent manner. Purified inactive mutant Pro-E167A-salilysin was correctly processed by purified mature salilysin, indicating that autolysis and inter-molecular processing occurred in its maturation processes. Proteolytic activity of mature salilysin against both peptide and protein substrates was also enhanced along with the addition of higher concentration of salt, 0-3.2 M NaCl, consistent with its halophilic origin. Mature salilysin was stabilized by ~8 °C in the presence of 1 M NaCl by thermal scanning using circular dichroism. One of the precursor form, intermediate 1, showed ~20 °C higher denaturation temperature than mature form, suggesting rigid and stable structure of this precursor form.

The Serratia grimesii outer membrane vesicles-associated grimelysin triggers bacterial invasion of eukaryotic cells

Serratia grimesii are facultative pathogenic bacteria that can penetrate a wide range of host cells and cause infection, especially in immunocompromised patients. Previously, we have found that bacterial metalloprotease grimelysin is a potential virulence determinant of S. grimesii invasion (E. S. Bozhokina et al., (2011). Cell Biology International, 35(2), 111-118). Protease is characterized as an actin-hydrolyzing enzyme with a narrow specificity toward other cell proteins. It is not known, however, whether grimelysin is transported into eukaryotic cells. Here, we show, for the first time, that S. grimesii can generate outer membrane vesicles (OMVs) displayed specific proteolytic activity against actin, characteristic of grimelysin. The presence of grimelysin was also confirmed by the Western blot analysis of S. grimesii OMVs lysate. Furthermore, confocal microscopy analysis revealed that the S. grimesii grimelysin-containing OMVs attached to the host cell membrane. Finally, pretreatment of HeLa cells with S. grimesii OMVs before the cells were infected with bacteria increased the bacterial penetration several times. These data strongly suggest that protease grimelysin promotes S. grimesii internalization by modifying bacterial and/or host molecule(s) when it is delivered as a component of OMVs.

Dissecting capture and twisting of aureolysin and pseudolysin: functional amino acids of the Dispase autolysis-inducing protein

The Dispase autolysis-inducing protein (DAIP) from Streptomyces mobaraensis attracts M4 metalloproteases, which results in inhibition and autolysis of bacillolysin (BL) and thermolysin (TL). The present study shows that aureolysin (AL) from Staphylococcus aureus and pseudolysin (LasB) from Pseudomonas aeruginosa are likewise impaired by DAIP. Complete inhibition occurred when DAIP significantly exceeded the amount of the target protease. At low DAIP concentrations, AL and BL performed autolysis, while LasB and TL degradation required reductants or detergents that break intramolecular disulfide bonds or change the protein structure. Site directed mutagenesis of DAIP and removal of an exposed protein loop either influenced binding or inhibition of AL and TL but had no effect on LasB and BL. The Y170A and Δ239-248 variants had completely lost affinity for TL and AL. The exchange of Asn-275 also impaired the interaction of DAIP with AL. In contrast, DAIP Phe-297 substitution abolished inhibition and autolysis of both target proteases but still allowed complex formation. Our results give rise to the conclusion that other, yet unknown DAIP amino acids inactivate LasB and BL. Obviously, various bacteria in the same habitat caused Streptomyces mobaraensis to continuously optimize DAIP in inactivating the tackling metalloproteases.

Bacterial Actin-Specific Endoproteases Grimelysin and Protealysin as Virulence Factors Contributing to the Invasive Activities of Serratia

The article reviews the discovery, properties and functional activities of new bacterial enzymes, proteases grimelysin (ECP 32) of Serratia grimesii and protealysin of Serratia proteamaculans, characterized by both a highly specific "actinase" activity and their ability to stimulate bacterial invasion. Grimelysin cleaves the only polypeptide bond Gly42-Val43 in actin. This bond is not cleaved by any other proteases and leads to a reversible loss of actin polymerization. Similar properties were characteristic for another bacterial protease, protealysin. These properties made grimelysin and protealysin a unique tool to study the functional properties of actin. Furthermore, bacteria Serratia grimesii and Serratia proteamaculans, producing grimelysin and protealysin, invade eukaryotic cells, and the recombinant Escherichia coli expressing the grimelysin or protealysins gene become invasive. Participation of the cellular c-Src and RhoA/ROCK signaling pathways in the invasion of eukaryotic cells by S. grimesii was shown, and involvement of E-cadherin in the invasion has been suggested. Moreover, membrane vesicles produced by S. grimesii were found to contain grimelysin, penetrate into eukaryotic cells and increase the invasion of bacteria into eukaryotic cells. These data indicate that the protease is a virulence factor, and actin can be a target for the protease upon its translocation into the host cell.

An Internally Quenched Fluorescent Peptide Substrate for Protealysin

Protealysin, a metalloprotease of Serratia proteamaculans, is the prototype of a subgroup of the M4 peptidase family. Protealysin-like proteases (PLPs) are widely spread in bacteria but also occur in fungi and certain archaea. The interest in PLPs is primarily due to their putative involvement in the bacterial pathogenesis in animals and plants. Studying PLPs requires an efficient quantitative assay for their activity; however, no such assay has been reported so far. Here, we used the autoprocessing site sequence of the protealysin precursor to construct an internally quenched fluorescent peptide substrate 2-aminobenzoyl-L-arginyl-L-seryl-L-valyl-L-isoleucyl-L-(ε-2,4-dinitrophenyl)lysine. Protealysin and thermolysin, the prototype of the M4 family, proved to hydrolyze only the Ser-Val bond of the substrate. The substrate exhibited a K_M = 35 ± 4 μM and k_cat = 21 ± 1 s⁻¹ for protealysin as well as a K_M = 33 ± 8 μM and k_cat = 7 ± 1 s⁻¹ for thermolysin at 37 °C. Comparison of the effect of different enzymes (thermolysin, trypsin, chymotrypsin, savinase, and pronase E) on the substrate has demonstrated that it is not strictly specific for protealysin; however, this enzyme has higher molar activity even compared to the closely related thermolysin. Thus, the proposed substrate can be advantageous for quantitative studies of protealysin as well as for activity assays of other M4 peptidases.

Guardians of the Cell: Effector-Triggered Immunity Steers Mammalian Immune Defense

The mammalian innate immune system deals with invading pathogens and stress by activating pattern-recognition receptors (PRRs) in the host. Initially proposed to be triggered by the discrimination of defined molecular signatures from pathogens rather than from self, it is now clear that PRRs can also be activated by endogenous ligands, bacterial metabolites and, following pathogen-induced alterations of cellular processes, changes in the F-actin cytoskeleton. These processes are collectively referred to as effector-triggered immunity (ETI). Here, we summarize the molecular and conceptual advances in our understanding of cell autonomous innate immune responses against bacterial pathogens, and discuss how classical activation of PRRs and ETI interplay to drive inflammatory responses.

Protealysin is not Secreted Constitutively

Background:

Protealysin, a zinc metalloprotease of Serratia proteamaculans, is the prototype of a new group within the peptidase family M4. Protealysin-like proteases (PLPs) are widely spread in bacteria but are also found in fungi and archaea. The biological functions of PLPs have not been well studied, but published data showed the involvement of enzymes of this group in the interaction of bacteria with higher organisms, and most likely in the pathogenesis. Such functionality requires the release of the proteases from bacterial cells; however, the data on the cellular localization of PLPs are contradictory and no direct data of this kind have been published. </P><P> Objective: Here, the protealysin cellular localization was studied for the first time using immunochemical methods. </P><P> Methods and Results: We have produced polyclonal rabbit antibodies against the protealysin precursor. The enzyme was evaluated in cells and medium of periodic culture of S. proteamaculans 94 using Western blotting as well as the enzyme localization was analysed by immunoelectron microscopy. It was shown that more than 99% of the enzyme is in a cell-associated form. Protealysin is accumulated in cells as an inactive precursor. It matures only after the release from cells (after their lysis). Immunoelectron microscopy analysis of bacterial cells has revealed no specific localization of protealysin; it was evenly distributed in the cytoplasm.

Conclusion:

The data obtained suggest that S. proteamaculans protealysin and supposedly other protealysin-like proteases are not secreted constitutively and their release from bacteria is likely induced by a certain stimulus such as a contact with a eukaryotic cell. This finding is critical for further studies of the involvement of these enzymes in pathogenesis.

Molecular mechanisms driving Streptococcus mitis entry into human gingival fibroblasts in presence of chitlac-nAg and saliva

The molecular mechanisms leading to Streptococcus mitis capability of entering oral cells were investigated in a co-culture of S. mitis and Human Gingival Fibroblasts (HGFs) in the presence of saliva. An innovative colloidal solution based on silver nanoparticles (Chitlac-nAg), a promising device for daily oral care, was added to the experimental system in order to study the effects of silver on the bacterial overgrowth and ability to enter non-phagocytic eukaryotic cells. The entry of bacteria into the eukaryotic cells is mediated by a signalling pathway involving FAK, integrin β1, and the two cytoskeleton proteins vinculin and F-actin, and down-regulated by the presence of saliva both at 3 and 48 h of culture, whereas Chitlac-n Ag exposure seems to influence, by incrementing it, the number of bacteria entering the fibroblasts only at 48 h. The formation of fibrillary extrusion from HGFs and the co-localization of bacteria and silver nanoparticles within the fibroblast vacuoles were also recorded. After longer experimental times (72 and 96 h), the number of S. mitis chains inside gingival cells is reduced, mainly in presence of saliva. The results suggest an escape of bacteria from fibroblasts to restore the microbial balance of the oral cavity.

A Legionella Effector Disrupts Host Cytoskeletal Structure by Cleaving Actin

Legionella pneumophila, the etiological agent of Legionnaires' disease, replicates intracellularly in protozoan and human hosts. Successful colonization and replication of this pathogen in host cells requires the Dot/Icm type IVB secretion system, which translocates approximately 300 effector proteins into the host cell to modulate various cellular processes. In this study, we identified RavK as a Dot/Icm substrate that targets the host cytoskeleton and reduces actin filament abundance in mammalian cells upon ectopic expression. RavK harbors an H95EXXH99 motif associated with diverse metalloproteases, which is essential for the inhibition of yeast growth and for the induction of cell rounding in HEK293T cells. We demonstrate that the actin protein itself is the cellular target of RavK and that this effector cleaves actin at a site between residues Thr351 and Phe352. Importantly, RavK-mediated actin cleavage also occurs during L. pneumophila infection. Cleavage by RavK abolishes the ability of actin to form polymers. Furthermore, an F352A mutation renders actin resistant to RavK-mediated cleavage; expression of the mutant in mammalian cells suppresses the cell rounding phenotype caused by RavK, further establishing that actin is the physiological substrate of RavK. Thus, L. pneumophila exploits components of the host cytoskeleton by multiple effectors with distinct mechanisms, highlighting the importance of modulating cellular processes governed by the actin cytoskeleton in the intracellular life cycle of this pathogen.

[New metalloendopeptidase of Morganella morganii ZM]

Proteolytic activity which is inhibited in the presence of o-phenanthroline was found in M. morganii ZM. Intracellular proteases of M. morganii ZM unlimited split musculoskeletal actin in contrast to grimelysin. Several proteolitic proteins of M. morganii ZM cells were identified by zymography with gelatin. Metalloproteinase of M. morganii ZM cell lysate was purified by hydrophobic chromatography fractionation. The molecular weight of the protein was 35 kDa.

Identification of SlpB, a Cytotoxic Protease from Serratia marcescens

The Gram-negative bacterium and opportunistic pathogen Serratia marcescens causes ocular infections in healthy individuals. Secreted protease activity was characterized from 44 ocular clinical isolates, and a higher frequency of protease-positive strains was observed among keratitis isolates than among conjunctivitis isolates. A positive correlation between protease activity and cytotoxicity to human corneal epithelial cells in vitro was determined. Deletion of prtS in clinical keratitis isolate K904 reduced, but did not eliminate, cytotoxicity and secreted protease production. This indicated that PrtS is necessary for full cytotoxicity to ocular cells and implied the existence of another secreted protease(s) and cytotoxic factors. Bioinformatic analysis of the S. marcescens Db11 genome revealed three additional open reading frames predicted to code for serralysin-like proteases noted here as slpB, slpC, and slpD. Induced expression of prtS and slpB, but not slpC and slpD, in strain PIC3611 rendered the strain cytotoxic to a lung carcinoma cell line; however, only prtS induction was sufficient for cytotoxicity to a corneal cell line. Strain K904 with deletion of both prtS and slpB genes was defective in secreted protease activity and cytotoxicity to human cell lines. PAGE analysis suggests that SlpB is produced at lower levels than PrtS. Purified SlpB demonstrated calcium-dependent and AprI-inhibited protease activity and cytotoxicity to airway and ocular cell lines in vitro. Lastly, genetic analysis indicated that the type I secretion system gene, lipD, is required for SlpB secretion. These genetic data introduce SlpB as a new cytotoxic protease from S. marcescens.

Dihydrolipoic but not alpha-lipoic acid affects susceptibility of eukaryotic cells to bacterial invasion

Sensitivity of eukaryotic cells to facultative pathogens can depend on physiological state of host cells. Previously we have shown that pretreatment of HeLa cells with N-acetylcysteine (NAC) makes the cells 2-3-fold more sensitive to invasion by the wild-type Serratia grimesii and recombinant Escherichia coli expressing gene of actin-specific metalloprotease grimelysin [1]. To evaluate the impact of chemically different antioxidants, in the present work we studied the effects of α-Lipoic acid (LA) and dihydrolipoic acid (DHLA) on efficiency of S. grimesii and recombinant E. coli expressing grimelysin gene to penetrate into HeLa and CaCo cells. Similarly to the effect of NAC, pretreatment of HeLa and CaCo cells with 0.6 or 1.25 mM DHLA increased the entry of grimelysin producing bacteria by a factor of 2.5 and 3 for the wild-type S. grimesii and recombinant E. coli, respectively. In contrast, pretreatment of the cells with 0.6 or 1.25 mM LA did not affect the bacteria uptake. The increased invasion of HeLa and CaCo cells correlated with the enhanced expression of E-cadherin and β-catenin genes, whereas expression of these genes in the LA-treated cells was not changed. Comparison of these results suggests that it is sulfhydryl group of DHLA that promotes efficient modification of cell properties assisting bacterial uptake. We assume that the NAC- and DHLA-induced stimulation of the E-cadherin-catenin pathway contributes to the increased internalization of the grimelysin producing bacteria within transformed cells.

Molecular characterization of protease activity in Serratia sp. strain SCBI and its importance in cytotoxicity and virulence

A newly recognized Serratia species, termed South African Caenorhabditis briggsae isolate (SCBI), is both a mutualist of the nematode Caenorhabditis briggsae KT0001 and a pathogen of lepidopteran insects. Serratia sp. strain SCBI displays high proteolytic activity, and because secreted proteases are known virulence factors for many pathogens, the purpose of this study was to identify genes essential for extracellular protease activity in Serratia sp. strain SCBI and to determine what role proteases play in insect pathogenesis and cytotoxicity. A bank of 2,100 transposon mutants was generated, and six SCBI mutants with defective proteolytic activity were identified. These mutants were also defective in cytotoxicity. The mutants were found defective in genes encoding the following proteins: alkaline metalloprotease secretion protein AprE, a BglB family transcriptional antiterminator, an inosine/xanthosine triphosphatase, GidA, a methyl-accepting chemotaxis protein, and a PIN domain protein. Gene expression analysis on these six mutants showed significant downregulation in mRNA levels of several different types of predicted protease genes. In addition, transcriptome sequencing (RNA-seq) analysis provided insight into how inactivation of AprE, GidA, and a PIN domain protein influences motility and virulence, as well as protease activity. Using quantitative reverse transcription-PCR (qRT-PCR) to further characterize expression of predicted protease genes in wild-type Serratia sp. SCBI, the highest mRNA levels for the alkaline metalloprotease genes (termed prtA1 to prtA4) occurred following the death of an insect host, while two serine protease and two metalloprotease genes had their highest mRNA levels during active infection. Overall, these results indicate that proteolytic activity is essential for cytotoxicity in Serratia sp. SCBI and that its regulation appears to be highly complex.

Draft Genome Sequence of Serratia grimesii Strain A2

We report the first draft genome assembly of Serratia grimesii strain A2, previously identified as Escherichia coli strain A2, which produces protease ECP32 with a high specificity toward actin. S. grimesii strain A2 has multidrug resistance associated with a number of efflux pump genes.

Friend or foe? A review of the mechanisms that driveSerratiatowards diverse lifestyles

Found widespread around the globe, Serratia are Gram-negative bacteria capable of thriving in a diverse number of environments that include water, soil, and the digestive tracts of various animals. Known for their ability to produce a myriad of extracellular enzymes, these bacteria also produce various secondary metabolites that directly contribute to their survival. While the effects Serratia species have on other organisms range from parasitic to symbiotic, what these bacteria have in common is their ability to resist attack, respond appropriately to environmental conditions, and outcompete other microorganisms when colonizing their respective niche. This review highlights the mechanisms utilized by Serratia species that drive their ubiquitous nature, with emphasis on the latest findings. Also discussed is how secreted compounds drive these bacteria towards pathogenic, mutualistic, and antagonistic associations.

Serratia infections: from military experiments to current practice

Serratia species, in particular Serratia marcescens, are significant human pathogens. S. marcescens has a long and interesting taxonomic, medical experimentation, military experimentation, and human clinical infection history. The organisms in this genus, particularly S. marcescens, were long thought to be nonpathogenic. Because S. marcescens was thought to be a nonpathogen and is usually red pigmented, the U.S. military conducted experiments that attempted to ascertain the spread of this organism released over large areas. In the process, members of both the public and the military were exposed to S. marcescens, and this was uncovered by the press in the 1970s, leading to U.S. congressional hearings. S. marcescens was found to be a certain human pathogen by the mid-1960s. S. marcescens and S. liquefaciens have been isolated as causative agents of numerous outbreaks and opportunistic infections, and the association of these organisms with point sources such as medical devices and various solutions given to hospitalized patients is striking. Serratia species appear to be common environmental organisms, and this helps to explain the large number of nosocomial infections due to these bacteria. Since many nosocomial infections are caused by multiply antibiotic-resistant strains of S. marcescens, this increases the danger to hospitalized patients, and hospital personnel should be vigilant in preventing nosocomial outbreaks due to this organism. S. marcescens, and probably other species in the genus, carries several antibiotic resistance determinants and is also capable of acquiring resistance genes. S. marcescens and S. liquefaciens are usually identified well in the clinical laboratory, but the other species are rare enough that laboratory technologists may not recognize them. 16S rRNA gene sequencing may enable better identification of some of the less common Serratia species.

Porphyromonas gingivalis influences actin degradation within epithelial cells during invasion and apoptosis

Porphyromonas gingivalis, a Gram-negative oral pathogen, has been shown to induce apoptosis in human gingival epithelial cells, yet the underlining cellular mechanisms controlling this process are poorly understood. We have previously shown that the P. gingivalis proteases arginine and lysine gingipains, are necessary and sufficient to induce host cell apoptosis. In the present study, we demonstrate that 'P. gingivalis-induced apoptosis' is mediated through degradation of actin leading to cytoskeleton collapse. Stimulation of human gingival epithelial cells with P. gingivalis strains 33277 and W50 at moi:100 induced β-actin cleavage as early as 1 h and human serum inhibited this effect. By using gingipain-deficient mutants of P. gingivalis and purified gingipains, we demonstrate that lysine gingipain is involved in actin hydrolysis in a dose and time-dependent manner. Use of Jasplakinolide and cytochalasin D revealed that P. gingivalis internalization is necessary for actin cleavage. Further, we also show that lysine gingipain from P. gingivalis can cleave active caspase 3. Taken together, we have identified actin as a substrate for lysine gingipain and demonstrated a novel mechanism involved in microbial host cell invasion and apoptosis.