Protein secretion in gram-negative bacteria. The extracellular metalloprotease B from Erwinia chrysanthemi contains a C-terminal secretion signal analogous to that of Escherichia coli alpha-hemolysin

Scale-Up of the Fermentation Process for the Production and Purification of Serratiopeptidase Using Silkworm Pupae as a Substrate

Serratiopeptidase, a bacterial metalloprotease known for its pain-relieving and anti-inflammatory properties, can be produced through fermentation with S. marcescens. This study aimed to identify key factors related to nutrient composition and physicochemical conditions for production in Erlenmeyer flasks and to scale up the mixture to a bioreactor to obtain the maximum proteolytic activity. A Plackett-Burman design was used to determine whether the presence of silkworm pupae (at 1.5%) was a significant parameter for serratiopeptidase production. Along with the variables pH, temperature, and time, they were optimized using a Taguchi experimental design, resulting in values of 7, 25 °C, and 36 h, respectively. Scaling up with a kLa of 25.45 ± 3.12 h-1 showed the highest serratiopeptidase production at 24 h. A factorial design was used for ultrafiltration, resulting in an LMH (liters per square meter per hour) of 960 L/m2h, a TMP (transmembrane pressure) of 15 psi, and a concentration factor of five, with a specific activity of 24,325.81 ± 1515.69 U/mg. Afterward, the retentate was purified using strong anion exchange chromatography and ultrafiltration, yielding a 19.94 ± 3.07% recovery and a purification factor of 1.59 ± 0.31. In conclusion, waste from the sericulture industry can be used for serratiopeptidase production.

Redefining the bacterial Type I protein secretion system

Type I secretion systems (T1SS) are versatile molecular machines for protein transport across the Gram-negative cell envelope. The archetypal Type I system mediates secretion of the Escherichia coli hemolysin, HlyA. This system has remained the pre-eminent model of T1SS research since its discovery. The classic description of a T1SS is composed of three proteins: an inner membrane ABC transporter, a periplasmic adaptor protein and an outer membrane factor. According to this model, these components assemble to form a continuous channel across the cell envelope, an unfolded substrate molecule is then transported in a one-step mechanism, directly from the cytosol to the extracellular milieu. However, this model does not encapsulate the diversity of T1SS that have been characterized to date. In this review, we provide an updated definition of a T1SS, and propose the subdivision of this system into five subgroups. These subgroups are categorized as T1SSa for RTX proteins, T1SSb for non-RTX Ca²⁺-binding proteins, T1SSc for non-RTX proteins, T1SSd for class II microcins, and T1SSe for lipoprotein secretion. Although often overlooked in the literature, these alternative mechanisms of Type I protein secretion offer many avenues for biotechnological discovery and application.

Comparative Pathogenomic Analysis of Two Banana Pathogenic Dickeya Strains Isolated from China and the Philippines

Dickeya is a major and typical member of soft rot Pectobacteriaceae (SRP) with a wide range of plant hosts worldwide. Previous studies have identified D. zeae as the causal agent of banana soft rot disease in China. In 2017, we obtained banana soft rot pathogen strain FZ06 from the Philippines. Genome sequencing and analysis indicated that FZ06 can be classified as D. dadantii and represents a novel subspecies of D. dadantii, which we propose to name as subsp. paradisiaca. Compared with Chinese banana soft rot pathogenic strain D. zeae MS2, strain FZ06 has a similar host range but different virulence; FZ06 is significantly less virulent to banana and potato but more virulent to Chinese cabbage and onion. Characterization of virulence factors revealed obviously less production of pectate lyases (Pels), polygalacturonases (Pehs), proteases (Prts), and extrapolysaccharides (EPSs), as well as lower swimming and swarming motility and biofilm formation in strain FZ06. Genomic comparison of the two strains revealed five extra gene clusters in FZ06, including one Stt-type T2SS, three T4SSs, and one T4P. Expression of cell wall degrading enzyme (CWDE)-encoding genes is significantly lower in FZ06 than in MS2.

Genomic and Functional Dissections of Dickeya zeae Shed Light on the Role of Type III Secretion System and Cell Wall-Degrading Enzymes to Host Range and Virulence

Dickeya zeae is a worldwide destructive pathogen that causes soft rot diseases on various hosts such as rice, maize, banana, and potato. The strain JZL7 we recently isolated from clivia represents the first monocot-specific D. zeae and also has reduced pathogenicity compared to that of other D. zeae strains (e.g., EC1 and MS2). To elucidate the molecular mechanisms underlying its more restricted host range and weakened pathogenicity, we sequenced the complete genome of JZL7 and performed comparative genomic and functional analyses of JZL7 and other D. zeae strains. We found that, while having the largest genome among D. zeae strains, JZL7 lost almost the entire type III secretion system (T3SS), which is a key component of the virulence suite of many bacterial pathogens. Importantly, the deletion of T3SS in MS2 substantially diminished the expression of most type III secreted effectors (T3SEs) and MS2's pathogenicity on both dicots and monocots. Moreover, although JZL7 and MS2 share almost the same repertoire of cell wall-degrading enzymes (CWDEs), we found broad reduction in the production of CWDEs and expression levels of CWDE genes in JZL7. The lower expression of CWDEs, pectin lyases in particular, would probably make it difficult for JZL7 to break down the cell wall of dicots, which is rich in pectin. Together, our results suggest that the loss of T3SS and reduced CWDE activity together might have contributed to the host specificity and virulence of JZL7. Our findings also shed light on the pathogenic mechanism of Dickeya and other soft rot Pectobacteriaceae species in general. IMPORTANCE Dickeya zeae is an important, aggressive bacterial phytopathogen that can cause severe diseases in many crops and ornamental plants, thus leading to substantial economic losses. Strains from different sources showed significant diversity in their natural hosts, suggesting complicated evolution history and pathogenic mechanisms. However, molecular mechanisms that cause the differences in the host range of D. zeae strains remain poorly understood. This study carried out genomic and functional dissections of JZL7, a D. zeae strain with restricted host range, and revealed type III secretion system (T3SS) and cell wall-degrading enzymes (CWDEs) as two major factors contributing to the host range and virulence of D. zeae, which will provide a valuable reference for the exploration of pathogenic mechanisms in other bacteria and present new insights for the control of bacterial soft rot diseases on crops.

Genomic and Proteomic Study of Andreprevotia ripae Isolated from an Anthill Reveals an Extensive Repertoire of Chitinolytic Enzymes

Chitin is an abundant natural polysaccharide that is hard to degrade because of its crystalline nature and because it is embedded in robust co-polymeric materials containing other polysaccharides, proteins, and minerals. Thus, it is of interest to study the enzymatic machineries of specialized microbes found in chitin-rich environments. We describe a genomic and proteomic analysis of Andreprevotia ripae, a chitinolytic Gram-negative bacterium isolated from an anthill. The genome of A. ripae encodes four secreted family GH19 chitinases of which two were detected and upregulated during growth on chitin. In addition, the genome encodes as many as 25 secreted GH18 chitinases, of which 17 were detected and 12 were upregulated during growth on chitin. Finally, the single lytic polysaccharide monooxygenase (LPMO) was strongly upregulated during growth on chitin. Whereas 66% of the 29 secreted chitinases contained two carbohydrate-binding modules (CBMs), this fraction was 93% (13 out of 14) for the upregulated chitinases, suggesting an important role for these CBMs. Next to an unprecedented multiplicity of upregulated chitinases, this study reveals several chitin-induced proteins that contain chitin-binding CBMs but lack a known catalytic function. These proteins are interesting targets for discovery of enzymes used by nature to convert chitin-rich biomass. The MS proteomic data have been deposited in the PRIDE database with accession number PXD025087.

Structure, Assembly, and Function of Tripartite Efflux and Type 1 Secretion Systems in Gram-Negative Bacteria

Tripartite efflux pumps and the related type 1 secretion systems (T1SSs) in Gram-negative organisms are diverse in function, energization, and structural organization. They form continuous conduits spanning both the inner and the outer membrane and are composed of three principal components-the energized inner membrane transporters (belonging to ABC, RND, and MFS families), the outer membrane factor channel-like proteins, and linking the two, the periplasmic adaptor proteins (PAPs), also known as the membrane fusion proteins (MFPs). In this review we summarize the recent advances in understanding of structural biology, function, and regulation of these systems, highlighting the previously undescribed role of PAPs in providing a common architectural scaffold across diverse families of transporters. Despite being built from a limited number of basic structural domains, these complexes present a staggering variety of architectures. While key insights have been derived from the RND transporter systems, a closer inspection of the operation and structural organization of different tripartite systems reveals unexpected analogies between them, including those formed around MFS- and ATP-driven transporters, suggesting that they operate around basic common principles. Based on that we are proposing a new integrated model of PAP-mediated communication within the conformational cycling of tripartite systems, which could be expanded to other types of assemblies.

Structure-Function Relationships of the Repeat Domains of RTX Toxins

RTX proteins are a large family of polypeptides of mainly Gram-negative origin that are secreted into the extracellular medium by a type I secretion system featuring a non-cleavable C-terminal secretion signal, which is preceded by a variable number of nine-residue tandem repeats. The three-dimensional structure forms a parallel β-roll, where β-strands of two parallel sheets are connected by calcium-binding linkers in such a way that a right-handed spiral is built. The Ca²⁺ ions are an integral part of the structure, which cannot form without them. The structural determinants of this unique architecture will be reviewed with its conservations and variations together with the implication for secretion and folding of these proteins. The general purpose of the RTX domains appears to act as an internal chaperone that keeps the polypeptide unfolded in the calcium-deprived cytosol and triggers folding in the calcium-rich extracellular medium. A rather recent addition to the structural biology of the RTX toxin is a variant occurring in a large RTX adhesin, where this non-canonical β-roll binds to ice and diatoms.

Purification and characterization of a metalloprotease produced by the C8 isolate of Serratia marcescens using silkworm pupae or casein as a protein source

Serratiopeptidase, a metalloprotease produced by Serratia marcescens, is produced through a fermentation process using carbohydrates and proteins as carbon and nitrogen sources. However, some byproducts of the silk industry could be an alternative source for serratiopeptidase production. Therefore, the present work is focused on the purification and characterization of a serratiopeptidase produced from the C8 isolate of Serratia marcescens and obtained from a Colombian silkworm hybrid using casein or silkworm pupae. The protease was purified using ultrafiltration, anion-exchange, and size-exclusion chromatography. The purified enzyme showed a molecular weight of ~50 kDa with a purity above 96%, an isoelectric point of ~4.6, optimum pH and temperature of 6 and 50 °C, and stability at 4 °C for one month. The kinetic constants using azocasein as substrate were 0.63 mM (K_m), 2,016 μM/min (V_max), 41.41 s^-1 (K_cat), and 6.56 × 10⁷ M^-1 s^-1 (K_cat/K_m). Inhibition by 5 mM EDTA or 1,10-phenanthroline was recovered by adding Zn²⁺ at the same concentration. Mass spectrometry analysis indicated 94% homology with the sequence of serratiopeptidase produced by the E-15 strain. We purified and characterized a serratiopeptidase produced by the C8 isolate of S. marcescens in a culture medium based on a renewable source from the silk industry.

Structure and mechanism of bacterial tripartite efflux pumps

Efflux pumps are membrane proteins which contribute to multi-drug resistance. In Gram-negative bacteria, some of these pumps form complex tripartite assemblies in association with an outer membrane channel and a periplasmic membrane fusion protein. These tripartite machineries span both membranes and the periplasmic space, and they extrude from the bacterium chemically diverse toxic substrates. In this chapter, we summarise current understanding of the structural architecture, functionality, and regulation of tripartite multi-drug efflux assemblies.

Structure of a Type-1 Secretion System ABC Transporter

Type-1 secretion systems (T1SSs) represent a widespread mode of protein secretion across the cell envelope in Gram-negative bacteria. The T1SS is composed of an inner-membrane ABC transporter, a periplasmic membrane-fusion protein, and an outer-membrane porin. These three components assemble into a complex spanning both membranes and providing a conduit for the translocation of unfolded polypeptides. We show that ATP hydrolysis and assembly of the entire T1SS complex is necessary for protein secretion. Furthermore, we present a 3.15-Å crystal structure of AaPrtD, the ABC transporter found in the Aquifex aeolicus T1SS. The structure suggests a substrate entry window just above the transporter's nucleotide binding domains. In addition, highly kinked transmembrane helices, which frame a narrow channel not observed in canonical peptide transporters, are likely involved in substrate translocation. Overall, the AaPrtD structure supports a polypeptide transport mechanism distinct from alternating access.

Type I Protein Secretion-Deceptively Simple yet with a Wide Range of Mechanistic Variability across the Family

A very large type I polypeptide begins to reel out from a ribosome; minutes later, the still unidentifiable polypeptide, largely lacking secondary structure, is now in some cases a thousand or more residues longer. Synthesis of the final hundred C-terminal residues commences. This includes the identity code, the secretion signal within the last 50 amino acids, designed to dock with a waiting ATP binding cassette (ABC) transporter. What happens next is the subject of this review, with the main, but not the only focus on hemolysin HlyA, an RTX protein toxin secreted by the type I system. Transport substrates range from small peptides to giant proteins produced by many pathogens. These molecules, without detectable cellular chaperones, overcome enormous barriers, crossing two membranes before final folding on the cell surface, involving a unique autocatalytic process.Unfolded HlyA is extruded posttranslationally, C-terminal first. The transenvelope "tunnel" is formed by HlyB (ABC transporter), HlyD (membrane fusion protein) straddling the inner membrane and periplasm and TolC (outer membrane). We present a new evaluation of the C-terminal secretion code, and the structure function of HlyD and HlyB at the heart of this nanomachine. Surprisingly, key details of the secretion mechanism are remarkably variable in the many type I secretion system subtypes. These include alternative folding processes, an apparently distinctive secretion code for each type I subfamily, and alternative forms of the ABC transporter; most remarkably, the ABC protein probably transports peptides or polypeptides by quite different mechanisms. Finally, we suggest a putative structure for the Hly-translocon, HlyB, the multijointed HlyD, and the TolC exit.

Bacterial calpains and the evolution of the calpain (C2) family of peptidases

Homologues of calpain, often thought to be an essential, cytoplasmic, calcium-dependent cysteine endopeptidase found exclusively in eukaryotes, have been found in bacterial proteomes. The homologues lack calcium-binding sites, have differing domain architectures, and can be secreted or membrane-associated. Homologues are rare and occur in a minority of bacterial phyla and often in a minority of species in a genus. However, the differences in domain architecture argue against a recent, horizontal gene transfer from a eukaryote. From analysis of a phylogenetic tree and absence of homologues in archaea, calpains in eukaryotes may be derived from genes horizontally transferred from a bacterium.

Reviewers: This article was reviewed by L. Aravind and Frank Eisenhaber.

Solute and Ion Transport: Outer Membrane Pores and Receptors

Two membranes enclose Gram-negative bacteria-an inner membrane consisting of phospholipid and an outer membrane having an asymmetric structure in which the inner leaflet contains phospholipid and the outer leaflet consists primarily of lipopolysaccharide. The impermeable nature of the outer membrane imposes a need for numerous outer membrane pores and transporters to ferry substances in and out of the cell. These outer membrane proteins have structures distinct from their inner membrane counterparts and most often function without any discernable energy source. In this chapter, we review the structures and functions of four classes of outer membrane protein: general and specific porins, specific transporters, TonB-dependent transporters, and export channels. While not an exhaustive list, these classes exemplify small-molecule transport across the outer membrane and illustrate the diversity of structures and functions found in Gram-negative bacteria.

Inducible polymerization and two-dimensional assembly of the repeats-in-toxin (RTX) domain from the Pseudomonas aeruginosa alkaline protease

Self-assembling proteins represent potential scaffolds for the organization of enzymatic activities. The alkaline protease repeats-in-toxin (RTX) domain from Pseudomonas aeruginosa undergoes multiple structural transitions in the presence and absence of calcium, a native structural cofactor. In the absence of calcium, this domain is capable of spontaneous, ordered polymerization, producing amyloid-like fibrils and large two-dimensional protein sheets. This polymerization occurs under near-physiological conditions, is rapid, and can be controlled by regulating calcium in solution. Fusion of the RTX domain to a soluble protein results in the incorporation of engineered protein function into these macromolecular assemblies. Applications of this protein sequence in bacterial adherence and colonization and the generation of biomaterials are discussed.

Calcium-induced folding and stabilization of the Pseudomonas aeruginosa alkaline protease

Pseudomonas aeruginosa is an opportunistic pathogen that contributes to the mortality of immunocompromised individuals and patients with cystic fibrosis. Pseudomonas infection presents clinical challenges due to its ability to form biofilms and modulate host-pathogen interactions through the secretion of virulence factors. The calcium-regulated alkaline protease (AP), a member of the repeats in toxin (RTX) family of proteins, is implicated in multiple modes of infection. A series of full-length and truncation mutants were purified for structural and functional studies to evaluate the role of Ca(2+) in AP folding and activation. We find that Ca(2+) binding induces RTX folding, which serves to chaperone the folding of the protease domain. Subsequent association of the RTX domain with an N-terminal α-helix stabilizes AP. These results provide a basis for the Ca(2+)-mediated regulation of AP and suggest mechanisms by which Ca(2+) regulates the RTX family of virulence factors.

Characteristics of a novel secreted zinc-dependent endopeptidase of Bacillus intermedius

A novel zinc-dependent metalloendopeptidase of Bacillus intermedius (MprBi) was purified from the culture medium of a recombinant strain of Bacillus subtilis. The amino acid sequence of the homogeneous protein was determined using MALDI-TOF mass spectrometry. The sequence of the first ten residues from the N-terminus of the mature protein is ASTGSQKVTV. Physicochemical properties of the enzyme and its substrate specificity have been studied. The molecular weight of the metalloproteinase constitutes 19 kDa, the K(m) and k(cat) values are 0.06 mM and 1210 sec⁻¹, respectively, and the pI value is 5.4. The effect of different inhibitors and metal ions on the enzyme activity has been studied. Based on the analysis of the amino acid sequence of the active site motif and the Met-turn together with the enzyme characteristics, the novel bacterial metalloproteinase MprBi is identified as a metzincin clan adamalysin/reprolysin-like metalloprotease.

Export of recombinant proteins in Escherichia coli using ABC transporter with an attached lipase ABC transporter recognition domain (LARD)

Background

ATP binding cassette (ABC) transporter secretes the protein through inner and outer membranes simultaneously in gram negative bacteria. Thermostable lipase (TliA) of Pseudomonas fluorescens SIK W1 is secreted through the ABC transporter. TliA has four glycine-rich repeats (GGXGXD) in its C-terminus, which appear in many ABC transporter-secreted proteins. From a homology model of TliA derived from the structure of P. aeruginosa alkaline protease (AprA), lipase ABC transporter domains (LARDs) were designed for the secretion of fusion proteins.

Results

The LARDs included four glycine-rich repeats comprising a β-roll structure, and were added to the C-terminus of test proteins. Either Pro-Gly linker or Factor Xa site was added between fusion proteins and LARDs. We attached different length of LARDs such as LARD0, LARD1 or whole TliA (the longest LARD) to three types of proteins; green fluorescent protein (GFP), epidermal growth factor (EGF) and cytoplasmic transduction peptide (CTP). These fusion proteins were expressed in Escherichia coli together with ABC transporter of either P. fluorescens or Erwinia chrysanthemi. Export of fusion proteins with the whole TliA through the ABC transporter was evident on the basis of lipase enzymatic activity. Upon supplementation of E. coli with ABC transporter, GFP-LARDs and EGF-LARDs were excreted into the culture supernatant.

Conclusion

The LARDs or whole TliA were attached to C-termini of model proteins and enabled the export of the model proteins such as GFP and EGF in E. coli supplemented with ABC transporter. These results open the possibility for the extracellular production of recombinant proteins in Pseudomonas using LARDs or TliA as a C-terminal signal sequence.

A novel serralysin metalloprotease from Deinococcus radiodurans

A hypothetical protein (DR2310) from the radiation resistant organism Deinococcus radiodurans harbors highly conserved Zn+2-binding (HEXXH) domain and Met-turn (SVMSY), characteristic of the serralysin family of secreted metalloproteases from Gram negative bacteria. Deletion mutagenesis of DR2310 confirmed that the ORF is expressed in Deinococcus radiodurans as a secreted protease of 85 kDa. Biochemical analysis revealed DR2310 to be a Ca+2 and Zn+2-requiring metalloprotease. Unique features such as a long N-terminus, replacement of the highly conserved C-terminal glycine rich Ca+2-binding repeats with a single N-terminal aspartate rich eukaryotic thrombospondin type-3 Ca+2-binding repeat and absence of C-terminal secretion signals make it a novel member of serralysin family. This is the first report of a functional serralysin family metalloprotease from a Gram positive organism.

Gram-negative bacterial ATP-binding cassette protein exporter family and diverse secretory proteins

Protein translocation to the extracellular space is essential for the invasion, colonization, and survival of pathogenic gram-negative bacteria within a host organism. In addition to the N-terminal signal sequence-dependent secretion system, which is specific for protein transport to the periplasmic space, there are five major systems (type I, II, III, IV, and V) that are known to be involved in protein secretion into the extracellular space. Of the systems, the type I pathway, which is composed of three membrane components including an ATP-binding cassette (ABC) protein, translocates proteins into the extracellular space from the cytosol by directly using the energy generated from ATP hydrolysis, and therefore, the system is a member of the ABC transporter family and is also known as the ABC exporter. To date, ABC exporters have been discovered to be involved in the secretion of a wide variety of exoproteins including RTX (repeats-in-toxin) toxins, cell surface layer proteins, proteases, lipases, bacteriocins, heme-acquisition proteins, and nodulation-related proteins such as the exoglucanases of gram-negative bacteria. A secretory protein and its associated specific ABC exporter are encoded in the same gene cluster in most cases, and ABC exporters show substrate specificity for secretion. Consequently, ABC exporters are present based primarily on the number of secretory protein genes. A secretion signal is situated in the C-terminal region of secretory proteins, however, the characteristics of the secretion signal are not fully understood. Secretory substrates and their linked ABC exporters are reviewed in the following paper.

Importance of a repetitive nine-residue sequence motif for intracellular stability and functional structure of a family I.3 lipase

PML5 is a functional derivative of a family I.3 lipase from Pseudomonas sp. MIS38 and contains five repeats of a nine-residue sequence motif. Two aspartate residues within the second and third repetitive sequences of PML5 were replaced by Ala. The secretion level, intracellular accumulation level, and stability of the resultant mutant protein were greatly reduced as compared to those of PML5. In addition, this mutant protein was inactive and did not bind Ca2+ ion. We propose that the repetitive sequences of PML5 form a beta-roll structure in the cells and thereby contribute to the intracellular stability and secretion efficiency of the protein.

Structure and function of TolC: the bacterial exit duct for proteins and drugs

The bacterial TolC protein plays a common role in the expulsion of diverse molecules, which include protein toxins and antibacterial drugs, from the cell. TolC is a trimeric 12-stranded alpha/beta barrel, comprising an alpha-helical trans-periplasmic tunnel embedded in the outer membrane by a contiguous beta-barrel channel. This structure establishes a 140 A long single pore fundamentally different to other membrane proteins and presents an exit duct to substrates, large and small, engaged at specific inner membrane translocases. TolC is open to the outside medium but is closed at its periplasmic entrance. When TolC is recruited by a substrate-laden translocase, the entrance is opened to allow substrate passage through a contiguous machinery spanning the entire cell envelope, from the cytosol to the external environment. Transition to the transient open state is achieved by an iris-like mechanism in which entrance alpha-helices undergo an untwisting realignment, thought to be stabilized by interaction with periplasmic helices of the translocase. TolC family proteins are ubiquitous among gram-negative bacteria, and the conserved entrance aperture presents a possible cheomotherapeutic target in multidrug-resistant pathogens.

Two periplasmic disulfide oxidoreductases, DsbA and SrgA, target outer membrane protein SpiA, a component of the Salmonella pathogenicity island 2 type III secretion system

The formation of disulfide is essential for the folding, activity, and stability of many proteins secreted by Gram-negative bacteria. The disulfide oxidoreductase, DsbA, introduces disulfide bonds into proteins exported from the cytoplasm to periplasm. In pathogenic bacteria, DsbA is required to process virulence determinants for their folding and assembly. In this study, we examined the role of the Dsb enzymes in Salmonella pathogenesis, and we demonstrated that DsbA, but not DsbC, is required for the full expression of virulence in a mouse infection model of Salmonella enterica serovar Typhimurium. Salmonella strains carrying a dsbA mutation showed reduced function mediated by type III secretion systems (TTSSs) encoded on Salmonella pathogenicity islands 1 and 2 (SPI-1 and SPI-2). To obtain a more detailed understanding of the contribution of DsbA to both SPI-1 and SPI-2 TTSS function, we identified a protein component of the SPI-2 TTSS apparatus affected by DsbA. Although we found no substrate protein for DsbA in the SPI-1 TTSS apparatus, we identified SpiA (SsaC), an outer membrane protein of SPI-2 TTSS, as a DsbA substrate. Site-directed mutagenesis of the two cysteine residues present in the SpiA protein resulted in the loss of SPI-2 function in vitro and in vivo. Furthermore, we provided evidence that a second disulfide oxidoreductase, SrgA, also oxidizes SpiA. Analysis of in vivo mixed infections demonstrated that a Salmonella dsbA srgA double mutant strain was more attenuated than either single mutant, suggesting that DsbA acts in concert with SrgA in vivo.

TolC--the bacterial exit duct for proteins and drugs

The TolC structure has unveiled a common mechanism for the movement of molecules, large and small, from the bacterial cell cytosol, across two membranes and the intervening periplasm, into the environment. Trimeric TolC is a remarkable cell exit duct that differs radically from other membrane proteins, comprising a 100-A long alpha-barrel that projects across the periplasmic space, anchored by a 40-A long beta-barrel spanning the outer membrane. The periplasmic entrance of TolC is closed until recruitment by substrate-specific translocases in the inner membrane triggers its transition to the open state, achieved by an iris-like 'untwisting' of the tunnel alpha-helices. TolC-dependent machineries present ubiquitous exit routes for virulence proteins and antibacterial drugs, and their conserved structure, specifically the electronegative TolC entrance constriction, may present a target for inhibitors of multidrug-resistant pathogens.

Channel-tunnels: outer membrane components of type I secretion systems and multidrug efflux pumps of Gram-negative bacteria

For translocation across the cell envelope of Gram-negative bacteria, substances have to overcome two permeability barriers, the inner and outer membrane. Channel-tunnels are outer membrane proteins, which are central to two distinct export systems: the type I secretion system exporting proteins such as toxins or proteases, and efflux pumps discharging antibiotics, dyes, or heavy metals and thus mediating drug resistance. Protein secretion is driven by an inner membrane ATP-binding cassette (ABC) transporter while drug efflux occurs via an inner membrane proton antiporter. Both inner membrane transporters are associated with a periplasmic accessory protein that recruits an outer membrane channel-tunnel to form a functional export complex. Prototypes of these export systems are the hemolysin secretion system and the AcrAB/TolC drug efflux pump of Escherichia coli, which both employ TolC as an outer membrane component. Its remarkable conduit-like structure, protruding 100 A into the periplasmic space, reveals how both systems are capable of transporting substrates across both membranes directly from the cytosol into the external environment. Proteins of the channel-tunnel family are widespread within Gram-negative bacteria. Their involvement in drug resistance and in secretion of pathogenic factors makes them an interesting system for further studies. Understanding the mechanism of the different export apparatus could help to develop new drugs, which block the efflux pumps or the secretion system.

Probing the role of divalent metal ions in a bacterial psychrophilic metalloprotease: binding studies of an enzyme in the crystalline state by x-ray crystallography

The psychrophilic alkaline metalloprotease (PAP) produced by a Pseudomonas bacterium isolated in Antarctica belongs to the clan of metzincins, for which a zinc ion is essential for catalytic activity. Binding studies in the crystalline state have been performed by X-ray crystallography in order to improve the understanding of the role of the zinc and calcium ions bound to this protease. Cocrystallization and soaking experiments with EDTA in a concentration range from 1 to 85 mM have resulted in five three-dimensional structures with a distinct number of metal ions occupying the ion-binding sites. Evolution of the structural changes observed in the vicinity of each cation-binding site has been studied as a function of the concentration of EDTA, as well as of time, in the presence of the chelator. Among others, we have found that the catalytic zinc ion was the first ion to be chelated, ahead of a weakly bound calcium ion (Ca 700) exclusive to the psychrophilic enzyme. Upon removal of the catalytic zinc ion, the side chains of the active-site residues His-173, His-179 and Tyr-209 shifted approximately 4, 1.0, and 1.6 A, respectively. Our studies confirm and also explain the sensitivity of PAP toward moderate EDTA concentrations and propose distinct roles for the calcium ions. A new crystal form of native PAP validates our previous predictions regarding the adaptation of this enzyme to cold environments as well as the proteolytic domain calcium ion being exclusive for PAP independent of crystallization conditions.

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.

Extracellular secretion of the virulence plasmid-encoded ADP-ribosyltransferase SpvB in Salmonella

Nontyphoid Salmonella enterica requires the plasmid-encoded spv genes to establish successful systemic infection in experimental animals. The SpvB virulence-associated protein has recently been shown to contain the ADP-ribosyltransferase domain. SpvB ADP-ribosilates actin and depolymerizes actin filaments when expressed in cultured epithelial cells. However, spontaneous secretion or release of SpvB has not been observed under in vitro growth conditions. In the present study we investigated the secretion of SpvB from Salmonella using in vitro and in vivo assay systems. We showed that SpvB is secreted into supernatant from Salmonella strains that contain the cloned spvB gene on a plasmid when they grew in intracellular salts medium (ISM), a minimal medium mimicing the intracellular iron concentrations of eukaryotic cells. A series of mutant SpvB proteins revealed that an N-terminal region of SpvB located at amino acids 1-229 was sufficient to promote secretion into extracellular milieu. Confocal immunofluorescence microscopy also demonstrated efficient localization of the N-terminal domain of SpvB(1-360) tagged with biotinylated peptide within infected host cell cytosol but not truncated SpvB(1-179) fusion protein. In addition, mutations that inactivate genes within Salmonella pathogenicity island 1 or Salmonella pathogenicity island 2 that encode type III secretion systems (TTSS) could secrete the SpvB protein into the culture medium. These results indicate that SpvB protein is transported from the bacteria and into the host cytoplasm independent of TTSS.

Soft rot erwiniae: from genes to genomes

SUMMARY The soft rot erwiniae, Erwinia carotovora ssp. atroseptica (Eca), E. carotovora ssp. carotovora (Ecc) and E. chrysanthemi (Ech) are major bacterial pathogens of potato and other crops world-wide. We currently understand much about how these bacteria attack plants and protect themselves against plant defences. However, the processes underlying the establishment of infection, differences in host range and their ability to survive when not causing disease, largely remain a mystery. This review will focus on our current knowledge of pathogenesis in these organisms and discuss how modern genomic approaches, including complete genome sequencing of Eca and Ech, may open the door to a new understanding of the potential subtlety and complexity of soft rot erwiniae and their interactions with plants.

TAXONOMY

The soft rot erwiniae are members of the Enterobacteriaceae, along with other plant pathogens such as Erwinia amylovora and human pathogens such as Escherichia coli, Salmonella spp. and Yersinia spp. Although the genus name Erwinia is most often used to describe the group, an alternative genus name Pectobacterium was recently proposed for the soft rot species.

HOST RANGE

Ech mainly affects crops and other plants in tropical and subtropical regions and has a wide host range that includes potato and the important model host African violet (Saintpaulia ionantha). Ecc affects crops and other plants in subtropical and temperate regions and has probably the widest host range, which also includes potato. Eca, on the other hand, has a host range limited almost exclusively to potato in temperate regions only. Disease symptoms: Soft rot erwiniae cause general tissue maceration, termed soft rot disease, through the production of plant cell wall degrading enzymes. Environmental factors such as temperature, low oxygen concentration and free water play an essential role in disease development. On potato, and possibly other plants, disease symptoms may differ, e.g. blackleg disease is associated more with Eca and Ech than with Ecc.

USEFUL WEBSITES

http://www.scri.sari.ac.uk/TiPP/Erwinia.htm, http://www.ahabs.wisc.edu:16080/ approximately pernalab/erwinia/index.htm, http://www.tigr.org/tdb/mdb/mdbinprogress.html, http://www.sanger.ac.uk/Projects/E_carotovora/.

Protease C of Erwinia chrysanthemi: the crystal structure and role of amino acids Y228 and E189

PrtC, a metallo-protease secreted by Erwinia chrysanthemi, is a member of the serralysin family and hence belongs to the metzincin superfamily. While the crystal structures of representatives of all metzincin subfamilies have been elucidated in the past, there is still some controversy about the reaction mechanism and the role of certain characteristic amino acids in the active centre. In this study, we probed the role of Tyr228 and Glu189 by site-directed mutagenesis and X-ray crystallography. There is evidence that these residues participate in catalysis, although conflicting hypotheses have been proposed. The crystal structures of wild-type and mutants have been refined to an R(free) of about 0.20 at resolutions of 2.0 A or better. Exchange of Glu189 versus alanine reduces the catalytic efficiency to less than 0.5 % using resorufin casein as substrate and to about 3 % using an assay utilising the thiol ester Ac-Pro-Leu-Gly-[(S)Leu]-Leu-Gly-OEt. The drop in activity is caused by a reduction in k(cat) while the K(M) values are virtually the same. In the resorufin casein assay, the mutant Y228F shows about 3 % of the wild-type activity and in the thiol ester assay this increases to about 56 %. In the latter case, the K(M) value of the mutant is increased from 5.3 mM to 9.0 mM with only little reduction in k(cat). The different behaviour of this mutant with respect to the two substrates can be explained by a switch in the rate-determining step during catalysis. The study presented here provides clear evidence that Glu189 of the HEXXHXXGXXH motif is the catalytic base, while Tyr228 is more likely involved in substrate binding and the stabilisation of the tetrahedral transition state.

Substrate-triggered recruitment of the TolC channel-tunnel during type I export of hemolysin by Escherichia coli

A defining event in type I export of hemolysin by Escherichia coli is the substrate-triggered recruitment of the TolC channel-tunnel by an inner membrane complex. This complex comprises a traffic ATPase (HlyB) and the 478 residue adaptor protein (HlyD), which contacts TolC during recruitment. HlyD has a large periplasmic domain (amino acid residues 81-478) linked by a single transmembrane helix to a small N-terminal cytosolic domain (1-59). Export was disabled by deletion of the ca 60 amino acid residue cytosolic domain of HlyD, even though the truncated HlyD (HlyDDelta45) was, like the wild-type, able to trimerise in the cytosolic membrane, and interact with the traffic ATPase. The mutant HlyB/HlyDDelta45 inner membrane complex engaged the hemolysin substrate, but this substrate-engaged complex failed to trigger recruitment of TolC. Further analyses showed that HlyDDelta45 was specifically unable to bind the substrate. The result suggests that substrate engagement by the traffic ATPase alone is insufficient to trigger TolC recruitment, and that substrate binding to the HlyD cytosolic domain is essential. Analysis of three further N-terminal deletion variants, HlyDDelta26, HlyDDelta26-45 and HlyDDelta34-38, indicated that an extreme N-terminal amphipathic helix and a cytosolic cluster of charged residues are central to the cytosolic domain function. The cytosolic amphipathic helix was not essential for substrate engagement or TolC recruitment, but export was impaired without it. In contrast, when the charged amino acid residues were deleted, the substrate was still engaged by HlyD but engagement was unproductive, i.e. TolC recruitment was not triggered. Our results are compatible with the HlyD cytosolic domain mediating transduction of the substrate binding signal directly, presumably to the HlyD periplasmic domain, to trigger recruitment of TolC and assemble the type I export complex.

Crystal structure of a complex between Pseudomonas aeruginosa alkaline protease and its cognate inhibitor: inhibition by a zinc-NH2 coordinative bond

Serralysins are a family of metalloproteases secreted by Gram-negative bacteria into the medium in the form of inactive zymogens. Usually, all serralysin secretors have on the same operon a gene coding for a periplasmic 10-kDa protein, which is an inhibitor of the secreted protease. The recent characterization of the inhibitor of the alkaline protease from Pseudomonas aeruginosa revealed a surprisingly low dissociation constant of 4 pm, contrary to earlier studies on homologous systems, where inhibition constants in the microm range were reported. To approach a more accurate understanding, the crystal structure of the complex between inhibitor and protease from P. aeruginosa was determined at 1.74 A resolution and refined to R(free) = 0.204. The structure reported here shows clearly that the N terminus of the inhibitor forms a coordinative bond to the catalytic Zn(2+) ion with a nitrogen-zinc distance of 2.17 A. We conclude that this interaction adds substantially to the complex stability and show also that similar interactions are found in other metzincin-inhibitor complexes.

Antimicrobial peptides of lactic acid bacteria: mode of action, genetics and biosynthesis

A survey is given of the main classes of bacteriocins, produced by lactic acid bacteria: I. lantibiotics II. small heat-stable non-lanthionine containing membrane-active peptides and III. large heat-labile proteins. First, their mode of action is detailed, with emphasis on pore formation in the cytoplasmatic membrane. Subsequently, the molecular genetics of several classes of bacteriocins are described in detail, with special attention to nisin as the most prominent example of the lantibiotic-class. Of the small non-lanthionine bacteriocin class, the Lactococcus lactococcins, and the Lactobacillus sakacin A and plantaricin A-bacteriocins are discussed. The principles and mechanisms of immunity and resistance towards bacteriocins are also briefly reported. The biosynthesis of bacteriocins is treated in depth with emphasis on response regulation, post-translational modification, secretion and proteolytic activation of bacteriocin precursors. To conclude, the role of the leader peptides is outlined and a conceptual model for bacteriocin maturation is proposed.

Subset of hybrid eukaryotic proteins is exported by the type I secretion system of Erwinia chrysanthemi

Erwinia chrysanthemi exports degradative enzymes by using a type I protein secretion system. The proteases secreted by this system lack an N-terminal signal peptide but contain a C-terminal secretion signal. To explore the substrate specificity of this system, we have expressed the E. chrysanthemi transporter system (prtDEF genes) in Escherichia coli and tested the ability of this ABC transporter to export hybrid proteins carrying C-terminal fragments of E. chrysanthemi protease B. The C terminus contains six glycine-rich repeated motifs, followed by two repeats of the sequences DFLV and DIIV. Two types of hybrid proteins were assayed for transport, proteins with the 93-residue-protease-B C terminus containing one glycine-rich repeat and both hydrophobic terminal repeats and proteins with the 181-residue C terminus containing all repeat motifs. Although the shorter C terminus is unable to export the hybrids, the longer C terminus can promote the secretion of hybrid proteins with N termini as large as 424 amino acids, showing that the glycine-rich motifs are required for the efficient secretion of these hybrids. However, the secretion of hybrids occurs only if these proteins do not carry disulfide bonds in their mature structures. These latter results suggest that disulfide bond formation can occur prior to or during the secretion. Disulfide bonds may prevent type I secretion of hybrids. One simple hypothesis to explain these results is that the type I channel is too narrow to permit the export of proteins with secondary structures stabilized by disulfide bonds.

The Sinorhizobium meliloti ExpE1 protein secreted by a type I secretion system involving ExpD1 and ExpD2 is required for biosynthesis or secretion of the exopolysaccharide galactoglucan

In Sinorhizobium meliloti the biosynthesis of the exopolysaccharide galactoglucan (EPS II) is directed by the exp genes. The expD1 and expD2 gene products are homologous to components of type I secretion systems. ExpE1, the gene of which is located adjacent to expD1 and expD2, was detected in S. meliloti cells and culture supernatants. ExpD1 and ExpD2 were required for the secretion of ExpE1, indicating that ExpE1 is secreted by a type I secretion system involving ExpD1 and ExpD2. ExpE1 contains 15 aspartate- and glycine-rich nonapeptide repeats that were suggested to bind Ca(2+). The ability to bind Ca(2+) was demonstrated for a recombinant ExpE1 protein. Extracellular EPS II was not detected in cultures of non-polar expD1, expD2 and expE1 deletion mutants implying that these three genes are required for biosynthesis or secretion of galactoglucan.

Secretion of nucleoside diphosphate kinase by mucoid Pseudomonas aeruginosa 8821: involvement of a carboxy-terminal motif in secretion

Nucleoside diphosphate kinase (Ndk) is a ubiquitous enzyme which functions in balancing the nucleotide pool of the cell. We have recently reported that in addition to being intracellular in both mucoid and nonmucoid Pseudomonas aeruginosa, Ndk is also secreted into the extracellular environment by mucoid P. aeruginosa cells. This secreted Ndk has biochemical activity similar to the intracellular Ndk and is 16 kDa in size. To demonstrate that Ndk is indeed secreted and to localize the secretion motif, we constructed an ndk knockout mutant, which lacks both intracellular and extracellular forms of Ndk. In this study, we report the construction of deletion derivatives made from the carboxy-terminal region of Ndk. These deletion derivatives were introduced into the ndk::Cm knockout mutant and were examined for the intracellular and extracellular presence of Ndk. It was observed that the carboxy-terminal 8-amino-acid region is required for the secretion of Ndk into the extracellular region. This region has the sequence DXXX, where X is a predominantly hydrophobic residue. Such sequences represent a conserved motif in proteins secreted by the type I secretory pathway in gram-negative microorganisms. To investigate the significance of this motif in the secretion of Ndk, we constructed a fusion protein of Ndk and the blue fluorescent protein (BFP) as well as a fusion protein of mutated Ndk (whose DTEV motif has been changed to AAAA) and the BFP. The presence of extracellular Ndk was detected only in the ndk::Cm knockout mutant harboring the wild-type BFP-Ndk protein fusion. We could not detect the presence of extracellular Ndk in the ndk::Cm knockout mutant containing the mutated BFP-Ndk protein fusion. In addition, we have also used immunofluorescence microscopy to localize the wild-type and mutated BFP-Ndk proteins in the cell. The significance of these observations is discussed.

Molecular characterization of a protease secreted by Erwinia amylovora

A protease with a molecular mass of 48 kDa is secreted by the fire blight pathogen Erwinia amylovora in minimal medium. We characterized this activity as a metalloprotease, since the enzyme was inhibited by EDTA and o -phenanthroline. A gene cluster was determined to encode four genes connected to protease expression, including a structural gene (prtA) and three genes (prtD, prtE, prtF) for secretion of the protease, which are transcribed in the same direction. The organization of the protease gene cluster in E. amylovora is different from that in other Gram-negative bacteria, such as Erwinia chrysanthemi, Pseudomonas aeruginosa and Serratia marcescens. On the basis of the conservative motif of metalloproteases, PrtA was identified to be a member of the metzincin subfamily of zinc-binding metalloproteases, and was confirmed to be the 48 kDa protease on gels by sequencing of tryptic peptide fragments derived from the protein. The protease is apparently secreted into the external medium through the type I secretion pathway via PrtD, PrtE and PrtF which share more than 90% identity with the secretion apparatus for lipase of S. marcescens. A protease mutant was created by Tn 5 -insertions, and the mutation localized in the prtD gene. The lack of protease reduced colonization of an E. amylovora secretion mutant labelled with the gene for the green fluorescent protein (gfp) in the parenchyma of apple leaves.

ZapA, the IgA-degrading metalloprotease of Proteus mirabilis, is a virulence factor expressed specifically in swarmer cells

The IgA-degrading metalloprotease, ZapA, of the urinary tract pathogen Proteus mirabilis is co-ordinately expressed along with other proteins and virulence factors during swarmer cell differentiation. In this communication, we have used zapA to monitor IgA protease expression during the differentiation of vegetative swimmer cells to fully differentiated swarmer cells. Northern blot analysis of wild-type cells and beta-galactosidase measurements using a zapA:lacZ fusion strain indicate that zapA is fully expressed only in differentiated swarmer cells. Moreover, the expression of zapA on nutrient agar medium is co-ordinately regulated in concert with the cycles of cellular differentiation, swarm migration and consolidation that produce the bull's-eye colonies typically associated with P. mirabilis. ZapA activity is not required for swarmer cell differentiation or swarming behaviour, as ZapA- strains produce wild-type colony patterns. ZapA- strains fail to degrade IgA and show decreased survival compared with the wild-type cells during infection in a mouse model of ascending urinary tract infection (UTI). These data underscore the importance of the P. mirabilis IgA-degrading metalloprotease in UTI. Analysis of the nucleotide sequences adjacent to zapA reveals four additional genes, zapE, zapB, zapC and zapD, which appear to possess functions required for ZapA activity and IgA proteolysis. Based on homology to other known proteins, these genes encode a second metalloprotease, ZapE, as well as a ZapA-specific ABC transporter system (ZapB, ZapC and ZapD). A model describing the function and interaction of each of these five proteins in the degradation of host IgA during UTI is presented.

NMR studies of the C-terminal secretion signal of the haem-binding protein, HasA

HasA is a haem-binding protein which is secreted under iron-deficiency conditions by the gram-negative bacterium Serratia marcescens. It is a monomer of 19 kDa (187 residues) able to bind free haem as well as to capture it from haemoglobin. HasA delivers haem to a specific outer-membrane receptor HasR and allows the bacteria to grow in the absence of any other source of iron. It is secreted by a signal peptide-independent pathway which involves a C-terminal secretion signal and an ABC (ATP-binding cassette) transporter. The C-terminal region of the secretion signal containing the essential secretion motif is cleaved during or after the secretion process by proteases secreted by the bacteria. In this work, we study by 1H NMR the conformation of the C-terminal extremity of HasA in the whole protein and that of the isolated secretion signal peptide in a zwitterionic micelle complex that mimicks the membrane environment. We identify a helical region followed by a random-coil C-terminus in the peptide-micelle complex and we show that in both the whole protein and the complex, the last 15 residues containing the motif essential for secretion are highly flexible and unstructured. This flexibility may be a prerequisite to the recognition of HasA by its ABC transporter. We determine the cleavage site of the C-terminal extremity of the protein and analyse the effect of the cleavage on the haem acquisition process.

Processing of the pectate lyase PelI by extracellular proteases of Erwinia chrysanthemi 3937

Erwinia chrysanthemi causes soft rot on various plants. The maceration of plant tissues is mainly due to the action of endopectate lyases. The E. chrysanthemi strain 3937 produces eight endopectate lyases (PelA, PelB, PelC, PelD, PelE, PelI, PelL and PelZ) that are secreted by the Out pathway. The necrotic response elicited by the wild-type E. chrysanthemi strain on tobacco leaves is due to an extracellular protein secreted by the Out machinery. Purification of the active factor revealed that it corresponds to a pectate lyase presenting immunological cross-reaction with PelI. Analysis of pelI and out mutants indicated that the necrosis-inducing pectate lyase results from a post-translational modification of PelI occurring extracellularly both in culture media and in planta. This modification consists of the cleavage of 97 N-terminal amino acids by the extracellular proteases of E. chrysanthemi. The enzymatic properties of the maturated form, PelI-3, are not, or only weakly, modified. However, this maturation gives rise to a small size and basic form that is active as a defence elicitor in plants.

E. coli alpha-hemolysin: a membrane-active protein toxin

alpha-Hemolysin is synthesized as a 1024-amino acid polypeptide, then intracellularly activated by specific fatty acylation. A second activation step takes place in the extracellular medium through binding of Ca2+ ions. Even in the absence of fatty acids and Ca2+ HlyA is an amphipathic protein, with a tendency to self-aggregation. However, Ca(2+)-binding appears to expose hydrophobic patches on the protein surface, facilitating both self-aggregation and irreversible insertion into membranes. The protein may somehow bind membranes in the absence of divalent cations, but only when Ca2+ (or Sr2+, or Ba2+) is bound to the toxin in aqueous suspensions, i.e., prior to its interaction with bilayers, can alpha-hemolysin bind irreversibly model or cell membranes in such a way that the integrity of the membrane barrier is lost, and cell or vesicle leakage ensues. Leakage is not due to the formation of proteinaceous pores, but rather to the transient disruption of the bilayer, due to the protein insertion into the outer membrane monolayer, and subsequent perturbations in the bilayer lateral tension. Protein or glycoprotein receptors for alpha-hemolysin may exist on the cell surface, but the toxin is also active on pure lipid bilayers.

The Caulobacter crescentus paracrystalline S-layer protein is secreted by an ABC transporter (type I) secretion apparatus

Caulobacter crescentus is a gram-negative bacterium that produces a two-dimensional crystalline array on its surface composed of a single 98-kDa protein, RsaA. Secretion of RsaA to the cell surface relies on an uncleaved C-terminal secretion signal. In this report, we identify two genes encoding components of the RsaA secretion apparatus. These components are part of a type I secretion system involving an ABC transporter protein. These genes, lying immediately 3' of rsaA, were found by screening a Tn5 transposon library for the loss of RsaA transport and characterizing the transposon-interrupted genes. The two proteins presumably encoded by these genes were found to have significant sequence similarity to ABC transporter and membrane fusion proteins of other type I secretion systems. The greatest sequence similarity was found to the alkaline protease (AprA) transport system of Pseudomonas aeruginosa and the metalloprotease (PrtB) transport system of Erwinia chrysanthemi. The prtB and aprA genes were introduced into C. crescentus, and their products were secreted by the RsaA transport system. Further, defects in the S-layer protein transport system led to the loss of this heterologous secretion. This is the first report of an S-layer protein secreted by a type I secretion apparatus. Unlike other type I secretion systems, the RsaA transport system secretes large amounts of its substrate protein (it is estimated that RsaA accounts for 10 to 12% of the total cell protein). Such levels are expected for bacterial S-layer proteins but are higher than for any other known type I secretion system.

Secretion of proteins and assembly of bacterial surface organelles: shared pathways of extracellular protein targeting

Extracellular or surface localization of virulence determinants is an important attribute of pathogenic microorganisms. The past decade has seen significant research advances in defining the steps and identifying the necessary machinery for protein secretion from bacterial cells. In Gram-negative pathogens, four distinct classes of secretion pathways have been identified that deliver virulence factors to their sites of action. These pathways are responsible for the delivery of soluble extracellular enzymes into the surrounding medium, or for specifically targeting proteins to the host cell. In several instances protein secretion pathways are similar to those involved in assembly of bacterial appendages. Combination of biochemical and genetic analyses has recently revealed that the pathways of protein secretion and surface localization of various organelles are mechanistically similar which was not apparent simply by comparing amino acid sequences of related proteins. The choice of the pathway that a protein will utilize may not be dictated only by the specific requirement of the secreted protein to traverse the cell envelope in the functional form, but also by the need to assure its delivery to the correct site of action outside the bacterial cell.

Detection of RTX toxin genes in gram-negative bacteria with a set of specific probes

The family of RTX (RTX representing repeats in the structural toxin) toxins is composed of several protein toxins with a characteristic nonapeptide glycine-rich repeat motif. Most of its members were shown to have cytolytic activity. By comparing the genetic relationships of the RTX toxin genes we established a set of 10 gene probes to be used for screening as-yet-unknown RTX toxin genes in bacterial species. The probes include parts of apxIA, apxIIA, and apxIIIA from Actinobacillus pleuropneumoniae, cyaA from Bordetella pertusis, frpA from Neisseria meningitidis, prtC from Erwinia chrysanthemi, hlyA and elyA from Escherichia coli, aaltA from Actinobacillus actinomycetemcomitans and lktA from Pasteurella haemolytica. A panel of pathogenic and nonpathogenic gram-negative bacteria were investigated for the presence of RTX toxin genes. The probes detected all known genes for RTX toxins. Moreover, we found potential RTX toxin genes in several pathogenic bacterial species for which no such toxins are known yet. This indicates that RTX or RTX-like toxins are widely distributed among pathogenic gram-negative bacteria. The probes generated by PCR and the hybridization method were optimized to allow broad-range screening for RTX toxin genes in one step. This included the binding of unlabelled probes to a nylon filter and subsequent hybridization of the filter with labelled genomic DNA of the strain to be tested. The method constitutes a powerful tool for the assessment of the potential pathogenicity of poorly characterized strains intended to be used in biotechnological applications. Moreover, it is useful for the detection of already-known or new RTX toxin genes in bacteria of medical importance.

Protein secretion by gram-negative bacterial ABC exporters

One of the strategies used by Gram-negative bacteria to secrete proteins across the two membranes which delimit the cells, is sec independent and dedicated to proteins lacking an N-terminal signal peptide. It depends on ABC protein-mediated exporters, which consist of three cell envelope proteins: two inner membrane proteins: an ATPase (the ABC protein), a membrane fusion protein (MFP) and an outer membrane polypeptide. Erwinia chrysanthemi metalloproteinases B and C, and Serratia marcescens hemoprotein HasA are secreted by such homologous pathways and interact with the ABC protein. Interaction between the ABC protein and its substrate has also been evidenced by studies on proteinase and HasA hybrid transporters obtained by combining components from each system. Association between hemoprotein HasA and the three exporter/secretion proteins was demonstrated by affinity chromatography on hemin agarose on which the substrate remained bound with the three secretion proteins. The three component association was ordered and substrate binding was required for the formation of this multiprotein complex.