LptC from Anabaena sp. PCC 7120: Expression, purification and crystallization.

Lipopolysaccharides are central elements of the outer leaflet of the outer membrane of Gram-negative bacteria and as such, of cyanobacteria. In the past, the structural analysis of the system in proteobacteria like Escherichia coli has contributed to a deep understanding of the transport of lipopolysaccharides from plasma membrane to the outer membrane. While many components of the transport system are conserved between proteobacteria and cyanobacteria, the periplasmic LptC appears to be distinct. The cyanobacterial proteins are twice as long as the proteobacterial proteins or proteins from firmicutes. This prompted the question whether the structure of the cyanobacterial proteins is comparable the one of the proteobacterial proteins. To address this question, we expressed LptC from Anabaena sp. PCC 7120 in E. coli as truncated protein without the transmembrane segment. We purified the protein utilizing HIS-tag based affinity chromatography and polished the protein after removal of the tag by size exclusion chromatography. The purified recombinant protein was crystallized by the sitting-drop vapor diffusion technique and best crystals, despite being twinned, diffracted to a resolution of 2.6 Å.

Metaproteomics of marine viral concentrates reveals key viral populations and abundant periplasmic proteins in the oligotrophic deep chlorophyll maximum of the South China Sea.

Viral concentrates (VCs), containing bioinformative DNA and proteins, have been used to study viral diversity, viral metagenomics and virus-host interactions in natural ecosystems. Besides viruses, VCs also contain many noncellular biological components including diverse functional proteins. Here, we used a shotgun proteomic approach to characterize the proteins of VCs collected from the oligotrophic deep chlorophyll maximum (DCM) of the South China Sea. Proteins of viruses infecting picophytoplankton, that is, cyanobacteria and prasinophytes, and heterotrophic bacterioplankton, such as SAR11 and SAR116, dominated the viral proteome. Almost no proteins from RNA viruses or known gene transfer agents were detected, suggesting that they were not abundant at the sampling site. Remarkably, nonviral proteins made up about two thirds of VC proteins, including overwhelmingly abundant periplasmic transporters for nutrient acquisition and proteins for diverse cellular processes, that is, translation, energy metabolism and one carbon metabolism. Interestingly, three 56 kDa selenium-binding proteins putatively involved in peroxide reduction from gammaproteobacteria were abundant in the VCs, suggesting active removal of peroxide compounds at DCM. Our study demonstrated that metaproteomics provides a valuable avenue to explore the diversity and structure of the viral community and also the pivotal biological functions affiliated with microbes in the natural environment.

Rapid analysis of protein expression and solubility with the SpyTag-SpyCatcher system.

Successful isolation of well-folded and active protein often first requires the creation of many constructs. These are needed to assess the effects of truncations, insertions, mutations, and the presence and position of different affinity tags. Determining which constructs yield the highest expression and solubility requires the investigator to express and partially purify each construct, and, in the case of low-expressing proteins, to follow the protein using time-consuming Western blots. Even then, many proteins form soluble aggregates, which may only be apparent after more extensive purification via size exclusion chromatography. In this work, we have utilized a covalent bond-forming tag/domain pair, known as SpyTag/SpyCatcher, to rapidly and specifically attach a fluorescent label to proteins of interest in cellular lysates. Once labeled, tagged proteins can easily be followed via SDS-PAGE and fluorescence size exclusion chromatography (F-SEC) to assess expression levels, solubility, and monodispersity without the need for purification. These techniques enable rapid and facile analysis of proteins, which may greatly facilitate optimization of protein expression constructs.

Biophysical and physiological characterization of ZraP from Escherichia coli, the periplasmic accessory protein of the atypical ZraSR two-component system.

The ZraSR system belongs to the family of TCSs (two-component signal transduction systems). In Escherichia coli, it was proposed to participate in zinc balance and to protect cytoplasmic zinc overload by sequestering this metal ion into the periplasm. This system controls the expression of the accessory protein ZraP that would be a periplasmic zinc scavenger. ZraPSR is functionally homologous with CpxPAR that integrates signals of envelope perturbation, including misfolded periplasmic proteins. The auxiliary periplasmic regulator CpxP inhibits the Cpx pathway by interacting with CpxA. Upon envelope stress sensing, the inhibitory function of CpxP is relieved, resulting in CpxR activation. Similarly to CpxPAR, ZraPSR probably plays a role in envelope stress response as a zinc-dependent chaperone activity was demonstrated for ZraP in Salmonella. We have purified ZraP from E. coli and shown that it is an octamer containing four interfacial metal-binding sites contributing to dimer stability. These sites are located close to the N-terminus, whereas the C-terminus is involved in polymerization of the protein to form a tetramer of dimers. In vitro, ZraP binds copper with a higher affinity than zinc and displays chaperone properties partially dependent on zinc binding. In vivo, zinc-bound ZraP is a repressor of the expression of the zraPSR operon. However, we have demonstrated that none of the Zra proteins are involved in zinc or copper resistance. We propose an integrated mechanism in which zinc is a marker of envelope stress perturbation and ZraPSR TCS is a sentinel sensing and responding to zinc entry into the periplasm.

Identification and characterization of a periplasmic trilactone esterase, Cee, revealed unique features of ferric enterobactin acquisition in Campylobacter.

Ferric enterobactin (FeEnt) acquisition is a highly efficient and conserved iron scavenging system in Gram-negative bacteria. Recently, we have characterized two FeEnt receptors (CfrA and CfrB) in Campylobacter jejuni and C. coli, the enteric human pathogens that do not produce any siderophores. In this study, whole-genome sequencing and comparative genomic analysis identified a unique Ent trilactone esterase Cee (Cj1376) in C. jejuni. Genomic analysis and biochemical assay strongly suggested that Cee is the sole trilactone esterase in C. jejuni. Thin-layer chromatography and HPLC analyses showed high efficiency of the purified Cee to hydrolyse Ent. Three Cee homologues previously characterized from other bacteria (IroE, IroD and Fes) were also purified and analysed together with Cee, indicating that Cee, Fes and IroD displayed similar hydrolysis dynamics for both apo and ferric forms of Ent while IroE catalysed Ent inefficiently. Unlike cytoplasmic Fes and IroD, Cee is localized in the periplasm as demonstrated by immunoblotting using Cee-specific antibodies. Genetic manipulation of diverse Campylobacter strains demonstrated that Cee is not only essential for CfrB-dependent FeEnt acquisition but also involved in CfrA-dependent pathway. Together, this study identified and characterized a novel periplasmic trilactone esterase and suggested a new model of FeEnt acquisition in Campylobacter.

Characterization of a periplasmic quinoprotein from Sphingomonas wittichii that functions as aldehyde dehydrogenase.

The α-proteobacterium Sphingomonas wittichii RW1 is known for its ability to degrade dioxins and related toxic substances. Bioinformatic analysis of the genome indicated that this organism may contain the largest number of pyrroloquinoline quinone-dependent dehydrogenases of any bacteria sequenced so far. Sequence analysis also showed that one of these genes (swit_4395) encodes an enzyme that belongs to the class of periplasmic glucose dehydrogenases. This gene was fused to a pelB signal sequence and a strep-tag coding region at the 5' and 3' ends, respectively. The fusion product was cloned into the broad-host range expression vector pBBR1p264-Streplong and the corresponding protein was heterologously produced in Escherichia coli, purified via Strep-Tactin affinity chromatography, and characterized. The protein Swit_4395 had a subunit mass of 39.3 kDa and formed active homooctamers and homododecamers. The enzyme showed the highest activities with short- and medium-chain aldehydes (chain length C1-C6) and ketoaldehydes, such as methylglyoxal and phenylglyoxal. Butyraldehyde was the best substrate, with V max and apparent K M values of 3,970 U/mg protein and 12.3 mM, respectively. Pyrroloquinoline quinone was detected using UV-Vis spectroscopy and was found to be a prosthetic group of the purified enzyme. Therefore, Swit_4395 was identified as a pyrroloquinoline quinone-dependent aldehyde dehydrogenase. The enzyme could be purified from the native host when the expression vector was introduced into S. wittichii RW1, indicating homologous protein production. Overproduction of Swit_4395 in S. wittichii RW1 dramatically increased the tolerance of the bacterium toward butyraldehyde and thus might contribute to the detoxification of toxic aldehydes.

A new principle of oligomerization of plant DEG7 protease based on interactions of degenerated protease domains.

Deg/HtrA proteases are a large group of ATP-independent serine endoproteases found in almost every organism. Their usual domain arrangement comprises a trypsin-type protease domain and one or more PDZ domains. All Deg/HtrA proteases form homo-oligomers with trimers as the basic unit, where the active protease domain mediates the interaction between individual monomers. Among the members of the Deg/HtrA protease family, the plant protease DEG7 is unique since it contains two protease domains (one active and one degenerated) and four PDZ domains. In the present study, we investigated the oligomerization behaviour of this unusual protease using yeast two-hybrid analysis in vivo and with recombinant protein in vitro. We show that DEG7 forms trimeric complexes, but in contrast with other known Deg/HtrA proteases, it shows a new principle of oligomerization, where trimerization is based on the interactions between degenerated protease domains. We propose that, during evolution, a duplicated active protease domain degenerated and specialized in protein-protein interaction and complex formation.

Interactions between CusF and CusB identified by NMR spectroscopy and chemical cross-linking coupled to mass spectrometry.

The Escherichia coli periplasmic proteins CusF and CusB, as part of the CusCFBA efflux system, aid in the resistance of elevated levels of copper and silver by direct metal transfer between the metallochaperone CusF and the membrane fusion protein CusB before metal extrusion from the periplasm to the extracellular space. Although previous in vitro experiments have demonstrated highly specific interactions between CusF and CusB that are crucial for metal transfer to occur, the structural details of the interaction have not been determined. Here, the interactions between CusF and CusB are mapped through nuclear magnetic resonance (NMR) spectroscopy and chemical cross-linking coupled with high-resolution mass spectrometry to better understand how recognition and metal transfer occur between these proteins. The NMR (1)H-(15)N correlation spectra reveal that CusB interacts with the metal-binding face of CusF. In vitro chemical cross-linking with a 7.7 Å homobifunctional amine-reactive cross-linker, BS(2)G, was used to capture the CusF/CusB interaction site, and mass spectral data acquired on an LTQ-Orbitrap confirm the following two cross-links: CusF K31 to CusB K29 and CusF K58 to CusB K32, thus revealing that the N-terminal region of CusB interacts with the metal-binding face of CusF. The proteins transiently interact in a metal-dependent fashion, and contacts between CusF and CusB are localized to regions near their respective metal-binding sites.

Optimization of a lysis method to isolate periplasmic proteins from Gram-negative bacteria for clinical mass spectrometry.

PURPOSE: Clinical mass spectrometry requires a simple step process for sample preparation. This study aims to optimize the method for isolating periplasmic protein from Gram-negative bacteria and apply to clinical mass spectrometry. EXPERIMENTAL DESIGN: The Klebsiella pneumoniae carbapenemase (KPC)-producing E. coli standard cells were used for optimizing the osmotic shock (OS) lysis method. The supernatant from OS lysis was analysed by LC-MS/MS and MALDI-TOF MS. The effectiveness of the OS lysis method for KPC-2-producing Enterobacteriaceae clinical isolates were then confirmed by MALDI-TOF MS. RESULTS: The optimized OS lysis using KPC-2 producing E. coli standard cells showed a high yield of KPC-2 protein and enriches periplasmic proteins. Compared with other lysis methods, the detection sensitivity of KPC-2 protein significantly increased in MALDI-TOF MS analysis. Nineteen clinical isolates were validated by MALDI-TOF MS using the OS method, which also showed higher detection sensitivity compared to other lysis method (e.g., 1.5% n-octyl-ß-D-glucopyranoside) (p < 0.001). CONCLUSIONS AND CLINICAL RELEVANCE: This study provides a straightforward, rapid, affordable, and detergent-free method for the analysis of periplasmic proteins from Enterobacteriaceae clinical isolates. This approach may contribute to MS-based clinical diagnostics.

Comprehensive and cost-effective NMR spectroscopy of methyl groups in large proteins.

An NMR approach is described which yields the methyl resonance assignments of alanine, threonine, valine, leucine, and isoleucine residues in proteins with high sensitivity and excellent resolution. The method relies on protein samples produced by bacterial expression using [(1)H,(13)C]-D-glucose and approximately 100% D(2)O, which is cost-effective and ensures the isotopic enrichment of all possible methyl groups. Magnetization transfer throughout the methyl-containing side chains is possible with this labeling scheme due to the high level of deuteration along the amino acid side chain, coupled with the selection of the favorable CHD(2) methyl isotopomer for detection. In an application to the 34 kDa periplasmic binding protein FepB 164 out of 195 methyl groups (85%) were assigned sequence-specifically and stereospecifically. This percentage increases to 91% when taking into account that not all backbone assignments are available for this system. The remaining unassigned methyl groups belong to six leucine residues, caused by low cross-peak intensities, and four alanine residues due to degeneracy of the (13)C(alpha)/(13)C(beta) frequencies. Our results demonstrate that NMR spectroscopic investigations of protein structure, dynamics, and interactions can be extended to include all methyl-containing amino acids also for larger proteins.

Extraction of recombinant periplasmic proteins under industrially relevant process conditions: Selectivity and yield strongly depend on protein titer and methodology.

In this work, we attempted to identify a method for the selective extraction of periplasmic endogenously expressed proteins, which is applicable at an industrial scale. For this purpose, we used an expression model that allows coexpression of two fluorescent proteins, each of which is specifically targeted to either the cytoplasm or periplasm. We assessed a number of scalable lysis methods (high-pressure homogenization, osmotic shock procedures, extraction with ethylenediaminetetraacetic acid, and extraction with deoxycholate) for the ability to selectively extract periplasmic proteins rather than cytoplasmic proteins. Our main conclusion was that although we identified industrially scalable lysis conditions that significantly increased the starting purity for further purification, none of the tested conditions were selective for periplasmic protein over cytoplasmic protein. Furthermore, we demonstrated that efficient extraction of the expressed recombinant proteins was largely dependent on the overall protein concentration in the cell.

A simple high-throughput purification method for hit identification in protein screening.

Phage and ribosome display technologies have emerged as important tools in the high-throughput screening of protein pharmaceuticals. However, a challenge created by the implementation of such tools is the need to purify large numbers of proteins for screening. While some assays may be compatible with crude bacterial lysates or periplasmic extracts, many functional assays, particularly cell-based assays, require protein of high purity and concentration. Here we evaluate several methods for small-scale, high-throughput protein purification. From our initial assessment we identified the HIS-Select 96-well filter plate system as the method of choice for further evaluation. This method was optimized and used to produce scFvs that were tested in cell-based functional assays. The behavior of HIS-Select purified scFvs in these assays was found to be similar to scFvs purified using a traditional large-scale 2-step purification method. The HIS-Select method allows high-throughput purification of hundreds of scFvs with yields in the 50-100 microg range, and of sufficient purity to allow evaluation in a cell-based proliferation assay. In addition, the use of a similar 96-well-based method facilitates the purification and subsequent screening of large numbers of IgGs and Fc fusion proteins generated through reformatting of scFv fragments.

Identification and characterization of a novel periplasmic polygalacturonic acid binding protein from Yersinia enterolitica.

A number of bacteria in the family Enterobacteriaceae harbor the genes comprising well-developed pectinolytic pathways (e.g. Erwinia sp.) or abridged versions of this pathway (e.g. Yersinia sp.). One of the most enigmatic components present in some of these pathways is a small gene that encodes a predicted secreted protein of approximately 160 amino acid residues with unknown function. This protein shows distant amino acid sequence similarity over its entire length to galactose-specific family 32 carbohydrate-binding modules (CBMs). Here we demonstrate the ability of the Yersinia enterocolitica example, here called YeCBM32, to bind polygalacturonic acid containing components of pectin. This binding is selective for highly polymerized galacturonic acid and shows a complex mode of polysaccharide recognition. The high resolution X-ray crystal structure (1.35 A) shows YeCBM32s overall structural similarity to galactose specific CBMs and conserved binding site location but reveals a substantially different binding site topology, which likely reflects its unique polymeric and acidic ligand. The results suggest the possibility of a unique role for YeCBM32 in polygalacturonic acid transport.

Expression and purification of the 26 kDa periplasmic protein of Brucella abortus: a reagent for the diagnosis of bovine brucellosis.

Development of a single diagnostic test for brucellosis in animals is the top priority of present-day research in the field. There is currently a battery of serological tests relying mainly on the use of LPS (lipopolysaccharide) as an antigen, culminating in false positives due to serological cross-reactivity. Other problems include difficulties in antigen production and the associated biohazard risk. This has prompted the need to develop an alternative antigen to replace LPS. In the present study, we cloned and expressed a BP26 (26 kDa periplasmic protein) antigen gene (bp26) of Brucella abortus. The recombinant periplasmic protein [rBP26 (recombinant BP26)] was expressed to high levels in Escherichia coli and purified in a single step. The purified rBP26 was examined for its binding activity with antibodies in a serum derived from a rabbit immunized intramuscularly with whole-cell lysate of B. abortus, as well as with commercial Brucella antibody (Difco). The purified rBP26 was used to develop an in-house plate ELISA and was further tested with a panel of 75 bovine brucellosis sera samples characterized previously by conventional serological tests. The results of both were in excellent agreement. The results show that rBP26 has potential use in the diagnosis of brucellosis, both in the laboratory and in field-based conditions with high levels of sensitivity and specificity.

Recombinant Purification of the Periplasmic Portion of the LINC Complex.

Recombinant expression of proteins and their complexes is the routine laboratory procedure to generate pure reagents for biochemical and structural studies. Here we present the standard procedure developed in our lab for the production of milligram quantities of stoichiometric SUN-KASH complexes. The protocol was specifically developed for the purification of the periplasmic portion of LINC complexes.

A method for the detection of antibiotic resistance markers in clinical strains of Escherichia coli using MALDI mass spectrometry.

Matrix-assisted laser-desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) is one of the most widely used mass spectrometry based approaches for bacterial identification and classification. The relatively simple sample preparation requirements and the speed of analysis which can usually be completed within a few minutes have resulted in the adoption and assimilation of MALDI-TOF MS into the routine diagnostic workflow of Clinical microbiology laboratories worldwide. This study describes the facilitation of bacterial discrimination based on antibiotic resistance markers through the implementation of MALDI-TOF MS. The periplasmic compartment of whole bacterial cells contains several proteins which confer antibiotic resistance in the Enterobacteriaceae. In order to reduce the complexity of the sample to be analysed via MALDI-TOF MS, the periplasm was extracted and subjected to in solution tryptic digestion followed by nano-LC separation. This method, established that peptide sequence biomarkers from several classes of antibiotic resistance proteins could be predicted using protein/peptide database tools such as Mascot. Biomarkers for a CTX-M-1 group extended spectrum ß-lactamase, CMY-2 an Amp-C ß-lactamase, VIM a metallo-ß-lactamase, TEM a ß-lactamase and KanR an aminoglycoside modifying enzyme were detected. This allowed for discrimination at a species level and at an almost identical strain level where the only difference between strains was the carriage of a modified antibiotic resistance carrying plasmid. This method also was able to detect some of these biomarkers in clinical strains where multiple resistance mechanisms were present.

The periplasmic protein TolB as a potential drug target in Pseudomonas aeruginosa.

The Gram-negative bacterium Pseudomonas aeruginosa is one of the most dreaded pathogens in the hospital setting, and represents a prototype of multi-drug resistant "superbug" for which effective therapeutic options are very limited. The identification and characterization of new cellular functions that are essential for P. aeruginosa viability and/or virulence could drive the development of anti-Pseudomonas compounds with novel mechanisms of action. In this study we investigated whether TolB, the periplasmic component of the Tol-Pal trans-envelope protein complex of Gram-negative bacteria, represents a potential drug target in P. aeruginosa. By combining conditional mutagenesis with the analysis of specific pathogenicity-related phenotypes, we demonstrated that TolB is essential for P. aeruginosa growth, both in laboratory and clinical strains, and that TolB-depleted P. aeruginosa cells are strongly defective in cell-envelope integrity, resistance to human serum and several antibiotics, as well as in the ability to cause infection and persist in an insect model of P. aeruginosa infection. The essentiality of TolB for P. aeruginosa growth, resistance and pathogenicity highlights the potential of TolB as a novel molecular target for anti-P. aeruginosa drug discovery.

Structural proteins of the primary cell wall: extraction, purification, and analysis.

Structural proteins of the primary cell wall present unusual but interesting problems for structural biologists in particular and plant biologists in general. As structure is the key to function; then the biochemical isolation of these glycoproteins for further study is paramount. Here, we detail the "classical" method for isolating soluble extensin monomers by elution of monomeric precursors to network extensin from tissue cultures. We also outline an additional approach involving genetic engineering that can potentially yield the complete genomic range of extensins and other hydroxyproline-rich glycoprotein (HRGPs) currently underutilized for biotechnology.

Bacterial sulfite dehydrogenases in organotrophic metabolism: separation and identification in Cupriavidus necator H16 and in Delftia acidovorans SPH-1.

The utilization of organosulfonates as carbon sources by aerobic or nitrate-reducing bacteria usually involves a measurable, uncharacterized sulfite dehydrogenase. This is tacitly assumed to be sulfite : ferricytochrome-c oxidoreductase [EC 1.8.2.1], despite negligible interaction with (eukaryotic) cytochrome c: the enzyme is assayed at high specific activity with ferricyanide as electron acceptor. Purified periplasmic sulfite dehydrogenases (SorAB, SoxCD) are known from chemoautotrophic growth and are termed 'sulfite oxidases' by bioinformatic services. The catalytic unit (SorA, SoxC; termed 'sulfite oxidases' cd02114 and cd02113, respectively) binds a molybdenum-cofactor (Moco), and involves a cytochrome c (SorB, SoxD) as electron acceptor. The genomes of several bacteria that express a sulfite dehydrogenase during heterotrophic growth contain neither sorAB nor soxCD genes; others contain at least four paralogues, for example Cupriavidus necator H16, which is known to express an inducible sulfite dehydrogenase during growth with taurine (2-aminoethanesulfonate). This soluble enzyme was enriched 320-fold in four steps. The 40 kDa protein (denatured) had an N-terminal amino acid sequence which started at position 42 of the deduced sequence of H16_B0860 (termed 'sulfite oxidase' cd02114), which we named SorA. The neighbouring gene is an orthologue of sorB, and the sorAB genes were co-transcribed. Cell fractionation showed SorA to be periplasmic. The corresponding enzyme in Delftia acidovorans SPH-1 was enriched 270-fold, identified as Daci_0055 (termed 'sulfite oxidase' cd02110) and has a cytochrome c encoded downstream. We presume, from genomic data for bacteria and archaea, that there are several subgroups of sulfite dehydrogenases, which all contain a Moco, and transfer electrons to a specific cytochrome c.

Biosynthesis of osmoregulated periplasmic glucans in Escherichia coli: the membrane-bound and the soluble periplasmic phosphoglycerol transferases are encoded by the same gene.

In Escherichia coli, osmoregulated periplasmic glucans (OPGs) are highly substituted by phosphoglycerol, phosphoethanolamine and succinyl residues. A two-step model was proposed to account for phosphoglycerol substitution: first, the membrane-bound phosphoglycerol transferase I transfers residues from membrane phosphatidylglycerol to nascent OPG molecules; second, the periplasmic phosphoglycerol transferase II swaps residues from one OPG molecule to another. Gene opgB was reported to encode phosphoglycerol transferase I. In this study, we demonstrate that the periplasmic enzyme II is a soluble form of the membrane-bound enzyme I. In addition, timing of OPG substitution was investigated. OPG substitution by succinyl residues occurs rapidly, probably during the backbone polymerization, whereas phosphoglycerol addition is a very progressive process. Thus, both phosphoglycerol transferase activities appear biologically necessary for complete OPG substitution.