Identification of a two-component regulatory system involved in antimicrobial peptide resistance in Streptococcus pneumoniae.

Two-component regulatory systems (TCS) are among the most widespread mechanisms that bacteria use to sense and respond to environmental changes. In the human pathogen Streptococcus pneumoniae, a total of 13 TCS have been identified and many of them have been linked to pathogenicity. Notably, TCS01 strongly contributes to pneumococcal virulence in several infection models. However, it remains one of the least studied TCS in pneumococci and its functional role is still unclear. In this study, we demonstrate that TCS01 cooperates with a BceAB-type ABC transporter to sense and induce resistance to structurally-unrelated antimicrobial peptides of bacterial origin that all target undecaprenyl-pyrophosphate or lipid II, which are essential precursors of cell wall biosynthesis. Even though tcs01 and bceAB genes do not locate in the same gene cluster, disruption of either of them equally sensitized the bacterium to the same set of antimicrobial peptides. We show that the key function of TCS01 is to upregulate the expression of the transporter, while the latter appears the main actor in resistance. Electrophoretic mobility shift assays further demonstrated that the response regulator of TCS01 binds to the promoter region of the bceAB genes, implying a direct control of these genes. The BceAB transporter was overexpressed and purified from E. coli. After reconstitution in liposomes, it displayed substantial ATPase and GTPase activities that were stimulated by antimicrobial peptides to which it confers resistance to, revealing new functional features of a BceAB-type transporter. Altogether, this inducible defense mechanism likely contributes to the survival of the opportunistic microorganism in the human host, in which competition among commensal microorganisms is a key determinant for effective host colonization and invasive path.

Spatio-temporal based deep learning for rapid detection and identification of bacterial colonies through lens-free microscopy time-lapses.

Detection and identification of pathogenic bacteria isolated from biological samples (blood, urine, sputum, etc.) are crucial steps in accelerated clinical diagnosis. However, accurate and rapid identification remain difficult to achieve due to the challenge of having to analyse complex and large samples. Current solutions (mass spectrometry, automated biochemical testing, etc.) propose a trade-off between time and accuracy, achieving satisfactory results at the expense of time-consuming processes, which can also be intrusive, destructive and costly. Moreover, those techniques tend to require an overnight subculture on solid agar medium delaying bacteria identification by 12-48 hours, thus preventing rapid prescription of appropriate treatment as it hinders antibiotic susceptibility testing. In this study, lens-free imaging is presented as a possible solution to achieve a quick and accurate wide range, non-destructive, label-free pathogenic bacteria detection and identification in real-time using micro colonies (10-500 µm) kinetic growth pattern combined with a two-stage deep learning architecture. Bacterial colonies growth time-lapses were acquired thanks to a live-cell lens-free imaging system and a thin-layer agar media made of 20 µl BHI (Brain Heart Infusion) to train our deep learning networks. Our architecture proposal achieved interesting results on a dataset constituted of seven different pathogenic bacteria-Staphylococcus aureus (S. aureus), Enterococcus faecium (E. faecium), Enterococcus faecalis (E. faecalis), Staphylococcus epidermidis (S. epidermidis), Streptococcus pneumoniae R6 (S. pneumoniae), Streptococcus pyogenes (S. pyogenes), Lactococcus Lactis (L. Lactis). At T = 8h, our detection network reached an average 96.0% detection rate while our classification network precision and sensitivity averaged around 93.1% and 94.0% respectively, both were tested on 1908 colonies. Our classification network even obtained a perfect score for E. faecalis (60 colonies) and very high score for S. epidermidis at 99.7% (647 colonies). Our method achieved those results thanks to a novel technique coupling convolutional and recurrent neural networks together to extract spatio-temporal patterns from unreconstructed lens-free microscopy time-lapses.

Nanoscale dynamics of peptidoglycan assembly during the cell cycle of Streptococcus pneumoniae.

Dynamics of cell elongation and septation are key determinants of bacterial morphogenesis. These processes are intimately linked to peptidoglycan synthesis performed by macromolecular complexes called the elongasome and the divisome. In rod-shaped bacteria, cell elongation and septation, which are dissociated in time and space, have been well described. By contrast, in ovoid-shaped bacteria, the dynamics and relationships between these processes remain poorly understood because they are concomitant and confined to a nanometer-scale annular region at midcell. Here, we set up a metabolic peptidoglycan labeling approach using click chemistry to image peptidoglycan synthesis by single-molecule localization microscopy in the ovoid bacterium Streptococcus pneumoniae. Our nanoscale-resolution data reveal spatiotemporal features of peptidoglycan assembly and fate along the cell cycle and provide geometrical parameters that we used to construct a morphogenesis model of the ovoid cell. These analyses show that septal and peripheral peptidoglycan syntheses first occur within a single annular region that later separates in two concentric regions and that elongation persists after septation is completed. In addition, our data reveal that freshly synthesized peptidoglycan is remodeled all along the cell cycle. Altogether, our work provides evidence that septal peptidoglycan is synthesized from the beginning of the cell cycle and is constantly remodeled through cleavage and insertion of material at its periphery. The ovoid-cell morphogenesis would thus rely on the relative dynamics between peptidoglycan synthesis and cleavage rather than on the existence of two distinct successive phases of peripheral and septal synthesis.

Deletion of the Zinc Transporter Lipoprotein AdcAII Causes Hyperencapsulation of Streptococcus pneumoniae Associated with Distinct Alleles of the Type I Restriction-Modification System.

The capsule is the dominant Streptococcus pneumoniae virulence factor, yet how variation in capsule thickness is regulated is poorly understood. Here, we describe an unexpected relationship between mutation of adcAII, which encodes a zinc uptake lipoprotein, and capsule thickness. Partial deletion of adcAII in three of five capsular serotypes frequently resulted in a mucoid phenotype that biochemical analysis and electron microscopy of the D39 adcAII mutants confirmed was caused by markedly increased capsule thickness. Compared to D39, the hyperencapsulated ΔadcAII mutant strain was more resistant to complement-mediated neutrophil killing and was hypervirulent in mouse models of invasive infection. Transcriptome analysis of D39 and the ΔadcAII mutant identified major differences in transcription of the Sp_0505-0508 locus, which encodes an SpnD39III (ST5556II) type I restriction-modification system and allelic variation of which correlates with capsule thickness. A PCR assay demonstrated close linkage of the SpnD39IIIC and F alleles with the hyperencapsulated ΔadcAII strains. However, transformation of ΔadcAII with fixed SpnD39III alleles associated with normal capsule thickness did not revert the hyperencapsulated phenotype. Half of hyperencapsulated ΔadcAII strains contained the same single nucleotide polymorphism in the capsule locus gene cps2E, which is required for the initiation of capsule synthesis. These results provide further evidence for the importance of the SpnD39III (ST5556II) type I restriction-modification system for modulating capsule thickness and identified an unexpected linkage between capsule thickness and mutation of ΔadcAII Further investigation will be needed to characterize how mutation of adcAII affects SpnD39III (ST5556II) allele dominance and results in the hyperencapsulated phenotype.IMPORTANCE The Streptococcus pneumoniae capsule affects multiple interactions with the host including contributing to colonization and immune evasion. During infection, the capsule thickness varies, but the mechanisms regulating this are poorly understood. We have identified an unsuspected relationship between mutation of adcAII, a gene that encodes a zinc uptake lipoprotein, and capsule thickness. Mutation of adcAII resulted in a striking hyperencapsulated phenotype, increased resistance to complement-mediated neutrophil killing, and increased S. pneumoniae virulence in mouse models of infection. Transcriptome and PCR analysis linked the hyperencapsulated phenotype of the ΔadcAII strain to specific alleles of the SpnD39III (ST5556II) type I restriction-modification system, a system which has previously been shown to affect capsule thickness. Our data provide further evidence for the importance of the SpnD39III (ST5556II) type I restriction-modification system for modulating capsule thickness and identify an unexpected link between capsule thickness and ΔadcAII, further investigation of which could further characterize mechanisms of capsule regulation.

Lipid Phases and Cell Geometry During the Cell Cycle of Streptococcus pneumoniae.

The coexistence of different lipid phases is well-known in vitro, but evidence for their presence and function in cellular membranes remains scarce. Using a combination of fluorescent lipid probes, we observe segregation of domains that suggests the coexistence of liquid and gel phases in the membrane of Streptococcus pneumoniae, where they are localized to minimize bending stress in the ellipsoid geometry defined by the cell wall. Gel phase lipids with high bending rigidity would be spontaneously organized at the equator where curvature is minimal, thus marking the future division site, while liquid phase membrane maps onto the oblong hemispheres. In addition, the membrane-bound cell wall precursor with its particular dynamic acyl chain localizes at the division site where the membrane is highly curved. We propose a complete "chicken-and-egg" model where cell geometry determines the localization of lipid phases that positions the cell division machinery, which in turn alters the localization of lamellar phases by assembling the cell wall with a specific geometry.

One-Pot Two-Step Metabolic Labeling of Teichoic Acids and Direct Labeling of Peptidoglycan Reveals Tight Coordination of Both Polymers Inserted into Pneumococcus Cell Wall.

A method for labeling teichoic acids in the human pathogen Streptococcus pneumoniae has been developed using a one-pot two-step metabolic labeling approach. The essential nutriment choline modified with an azido-group was incorporated and exposed at the cell surface more rapidly than it reacted with the strain promoted azide alkyne cycloaddition (SPAAC) partner also present in the medium. Once at the cell surface on teichoic acids, coupling of the azido group could then occur within 5 min by the bio-orthogonal click reaction with a DIBO-linked fluorophore. This fast and easy method allowed pulse-chase experiments and was combined with another fluorescent labeling approach to compare the insertion of teichoic acids with peptidoglycan synthesis with unprecedented temporal resolution. It has revealed that teichoic acid and peptidoglycan processes are largely concomitant, but teichoic acid insertion persists later at the division site.

A multidrug ABC transporter with a taste for GTP.

During the evolution of cellular bioenergetics, many protein families have been fashioned to match the availability and replenishment in energy supply. Molecular motors and primary transporters essentially need ATP to function while proteins involved in cell signaling or translation consume GTP. ATP-Binding Cassette (ABC) transporters are one of the largest families of membrane proteins gathering several medically relevant members that are typically powered by ATP hydrolysis. Here, a Streptococcus pneumoniae ABC transporter responsible for fluoroquinolones resistance in clinical settings, PatA/PatB, is shown to challenge this concept. It clearly favors GTP as the energy supply to expel drugs. This preference is correlated to its ability to hydrolyze GTP more efficiently than ATP, as found with PatA/PatB reconstituted in proteoliposomes or nanodiscs. Importantly, the ATP and GTP concentrations are similar in S. pneumoniae supporting the physiological relevance of GTP as the energy source of this bacterial transporter.

Peptidoglycan O-acetylation is functionally related to cell wall biosynthesis and cell division in Streptococcus pneumoniae.

The peptidoglycan is a rigid matrix required to resist turgor pressure and to maintain the cellular shape. It is formed by linear glycan chains composed of N-acetylmuramic acid-(ß-1,4)-N-acetylglucosamine (MurNAc-GlcNAc) disaccharides associated through cross-linked peptide stems. The peptidoglycan is continually remodelled by synthetic and hydrolytic enzymes and by chemical modifications, including O-acetylation of MurNAc residues that occurs in most Gram-positive and Gram-negative bacteria. This modification is a powerful strategy developed by pathogens to resist to lysozyme degradation and thus to escape from the host innate immune system but little is known about its physiological function. In this study, we have investigated to what extend peptidoglycan O-acetylation is involved in cell wall biosynthesis and cell division of Streptococcus pneumoniae. We show that O-acetylation driven by Adr protects the peptidoglycan of dividing cells from cleavage by the major autolysin LytA and occurs at the septal site. Our results support a function for Adr in the formation of robust and mature MurNAc O-acetylated peptidoglycan and infer its role in the division of the pneumococcus.

New insights into histidine triad proteins: solution structure of a Streptococcus pneumoniae PhtD domain and zinc transfer to AdcAII.

Zinc (Zn(2+)) homeostasis is critical for pathogen host colonization and invasion. Polyhistidine triad (Pht) proteins, located at the surface of various streptococci, have been proposed to be involved in Zn(2+) homeostasis. The phtD gene, coding for a Zn(2+)-binding protein, is organized in an operon with adcAII coding for the extracellular part of a Zn(2+) transporter. In the present work, we investigate the relationship between PhtD and AdcAII using biochemical and structural biology approaches. Immuno-precipitation experiments on purified membranes of Streptococcus pneumoniae (S. pneumoniae) demonstrate that native PhtD and AdcAII interact in vivo confirming our previous in vitro observations. NMR was used to demonstrate Zn(2+) transfer from the Zn(2+)-bound form of a 137 amino acid N-terminal domain of PhtD (t-PhtD) to AdcAII. The high resolution NMR structure of t-PhtD shows that Zn(2+) is bound in a tetrahedral site by histidines 83, 86, and 88 as well as by glutamate 63. Comparison of the NMR parameters measured for apo- and Zn(2+)-t-PhtD shows that the loss of Zn(2+) leads to a diminished helical propensity at the C-terminus and increases the local dynamics and overall molecular volume. Structural comparison with the crystal structure of a 55-long fragment of PhtA suggests that Pht proteins are built from short repetitive units formed by three ß-strands containing the conserved HxxHxH motif. Taken together, these results support a role for S. pneumoniae PhtD as a Zn(2+) scavenger for later release to the surface transporter AdcAII, leading to Zn(2+) uptake.

On-chip microbial culture for the specific detection of very low levels of bacteria.

Microbial culture continues to be the most common protocol for bacterial detection and identification in medicine and agronomics. Using this process may take days to identify a specific pathogen for most bacterial strains. Surface Plasmon Resonance (SPR) detection is an emerging alternative technology that can be used for the detection of bacteria using protein microarrays although typical limits of detection are in the range of 10(3)-10(6) cfu mL(-1), which is not compatible with most Food Safety regulation requirements. In this work, we combine concomitant "on-chip" microbial culture with sensitive SPR detection of bacteria thus allowing rapid specific detection of bacteria pathogens - including Salmonella enterica serovar Enteritidis, Streptococcus pneumoniae and Escherichia coli O157:H7 - cultured on a protein microarray. This Culture-Capture-Measure (CCM) approach significantly decreases both the number of processing steps and the overall assay time for bacterial detection. Signal analysis of SPR responses allowed the fast and quantitative assessment of bacterial concentrations initially present in the sample as low as 2.8 ± 19.6 cfu per milliliter. Altogether, our results show how simple, easy-to-operate, fluidic-less and lo-tec microarrays can be used with unprocessed samples and yield - in a single assay - both qualitative and quantitative information regarding bacterial contamination.

PatA and PatB form a functional heterodimeric ABC multidrug efflux transporter responsible for the resistance of Streptococcus pneumoniae to fluoroquinolones.

All bacterial multidrug ABC transporters have been shown to work as either homodimers or heterodimers. Two possibly linked genes, patA and patB from Streptococcus pneumococcus, that encode half-ABC transporters have been shown previously to be involved in fluoroquinolone resistance. We showed that the ΔpatA, ΔpatB, or ΔpatA/ΔpatB mutant strains were more sensitive to unstructurally related compounds, i.e., ethidium bromide or fluoroquinolones, than the wild-type R6 strain. Inside-out vesicles prepared from Escherichia coli expressing PatA and/or PatB transported Hoechst 33342, a classical substrate of multidrug transporters, only when both PatA and PatB were coexpressed. This transport was inhibited either by orthovanadate or by reserpine, and mutation of the conserved Walker A lysine residue of either PatA or PatB fully abrogated Hoechst 33342 transport. PatA, PatB, and the PatA/PatB heterodimer were purified from detergent-solubilized E. coli membrane preparations. Protein dimers were identified in all cases, albeit in different proportions. In contrast to the PatA/PatB heterodimers, homodimers of PatA or PatB failed to show a vanadate-sensitive ATPase activity. Thus, PatA and PatB need to interact together to make a functional drug efflux transporter, and they work only as heterodimers.

Effects of deletion of the Streptococcus pneumoniae lipoprotein diacylglyceryl transferase gene lgt on ABC transporter function and on growth in vivo.

Lipoproteins are an important class of surface associated proteins that have diverse roles and frequently are involved in the virulence of bacterial pathogens. As prolipoproteins are attached to the cell membrane by a single enzyme, prolipoprotein diacylglyceryl transferase (Lgt), deletion of the corresponding gene potentially allows the characterisation of the overall importance of lipoproteins for specific bacterial functions. We have used a Δlgt mutant strain of Streptococcus pneumoniae to investigate the effects of loss of lipoprotein attachment on cation acquisition, growth in media containing specific carbon sources, and virulence in different infection models. Immunoblots of triton X-114 extracts, flow cytometry and immuno-fluorescence microscopy confirmed the Δlgt mutant had markedly reduced lipoprotein expression on the cell surface. The Δlgt mutant had reduced growth in cation depleted medium, increased sensitivity to oxidative stress, reduced zinc uptake, and reduced intracellular levels of several cations. Doubling time of the Δlgt mutant was also increased slightly when grown in medium with glucose, raffinose and maltotriose as sole carbon sources. These multiple defects in cation and sugar ABC transporter function for the Δlgt mutant were associated with only slightly delayed growth in complete medium. However the Δlgt mutant had significantly reduced growth in blood or bronchoalveolar lavage fluid and a marked impairment in virulence in mouse models of nasopharyngeal colonisation, sepsis and pneumonia. These data suggest that for S. pneumoniae loss of surface localisation of lipoproteins has widespread effects on ABC transporter functions that collectively prevent the Δlgt mutant from establishing invasive infection.

Zinc uptake by Streptococcus pneumoniae depends on both AdcA and AdcAII and is essential for normal bacterial morphology and virulence.

Zinc is an essential trace metal for living cells. The ABC transporter AdcABC was previously shown to be required for zinc uptake by Streptococcus pneumoniae. As we have recently described AdcAII as another zinc-binding lipoprotein, we have investigated the role of both AdcA and AdcAII in S. pneumoniae zinc metabolism. Deletion of either adcA or adcAII but not phtD reduced S. pneumoniae zinc uptake, with dual mutation of both adcA and adcAII further decreasing zinc import. For the Δ(adcA/adcAII) mutant, growth and intracellular concentrations of zinc were both greatly reduced in low zinc concentration. When grown in zinc-deficient medium, the Δ(adcA/adcAII) mutant displayed morphological defects related to aberrant septation. Growth and morphology of the Δ(adcA/adcAII) mutant recovered after supplementation with zinc. Dual deletion of adcA and adcAII strongly impaired growth of the pneumococcus in bronchoalveolar lavage fluid and human serum, and prevented S. pneumoniae establishing infection in mouse models of nasopharyngeal colonization, pneumonia and sepsis without altering the capsule. Taken together, our results show that AdcA and AdcAII play an essential and redundant role in specifically importing zinc into the pneumococcus, and that both zinc transporters are required for proper cell division and for S. pneumoniae survival during infection.

Infection with conditionally virulent Streptococcus pneumoniae Δpab strains induces antibody to conserved protein antigens but does not protect against systemic infection with heterologous strains.

Avirulent strains of a bacterial pathogen could be useful tools for investigating immunological responses to infection and potentially effective vaccines. We have therefore constructed an auxotrophic TIGR4 Δpab strain of Streptococcus pneumoniae by deleting the pabB gene Sp_0665. The TIGR4 Δpab strain grew well in complete medium but was unable to grow in serum unless it was supplemented with para-aminobenzoic acid (PABA). The TIGR4 Δpab strain was markedly attenuated in virulence in mouse models of S. pneumoniae nasopharyngeal colonization, pneumonia, and sepsis. Supplementing mouse drinking water with PABA largely restored the virulence of TIGR4 Δpab. An additional Δpab strain constructed in the D39 capsular serotype 2 background was also avirulent in a sepsis model. Systemic inoculation of mice with TIGR4 Δpab induced antibody responses to S. pneumoniae protein antigens, including PpmA, PsaA, pneumolysin, and CbpD, but not capsular polysaccharide. Flow cytometry demonstrated that IgG in sera from TIGR4 Δpab-vaccinated mice bound to the surface of TIGR4 and D39 bacteria but not to a capsular serotype 3 strain, strain 0100993. Mice vaccinated with the TIGR4 Δpab or D39 Δpab strain by intraperitoneal inoculation were protected from developing septicemia when challenged with the homologous S. pneumoniae strain. Vaccination with the TIGR4 Δpab strain provided only weak or no protection against heterologous challenge with the D39 or 0100993 strain but did strongly protect against a TIGR4 capsular-switch strain expressing a serotype 2 capsule. The failure of cross-protection after systemic vaccination with Δpab bacteria suggests that parenteral administration of a live attenuated vaccine is not an attractive approach for preventing S. pneumoniae infection.

Biochemical characterization of the histidine triad protein PhtD as a cell surface zinc-binding protein of pneumococcus.

Zinc homeostasis is critical for pathogen host colonization. Indeed, during invasion, Streptococcus pneumoniae has to finely regulate zinc transport to cope with a wide range of Zn(2+) concentrations within the various host niches. AdcAII was identified as a pneumococcal Zn(2+)-binding protein; its gene is present in an operon together with the phtD gene. PhtD belongs to the histidine triad protein family, but to date, its function has not been clarified. Using several complementary biochemical methods, we provide evidence that like AdcAII, PhtD is a metal-binding protein specific for zinc. When Zn(2+) binds (K(d) = 131 ± 10 nM), the protein displays substantial thermal stabilization. We also present the first direct evidence of a joint function of AdcAII and PhtD by demonstrating that their expression is corepressed by Zn(2+), that they interact directly in vitro, and that they are colocalized at the bacterial surface. These results suggest the common involvement of the AdcAII-PhtD system in pneumococcal zinc homeostasis.

The motor protein myosin-X transports VE-cadherin along filopodia to allow the formation of early endothelial cell-cell contacts.

Vascular endothelium (VE), the monolayer of endothelial cells that lines the vascular tree, undergoes damage at the basis of some vascular diseases. Its integrity is maintained by VE-cadherin, an adhesive receptor localized at cell-cell junctions. Here, we show that VE-cadherin is also located at the tip and along filopodia in sparse or subconfluent endothelial cells. We observed that VE-cadherin navigates along intrafilopodial actin filaments. We found that the actin motor protein myosin-X is colocalized and moves synchronously with filopodial VE-cadherin. Immunoprecipitation and pulldown assays confirmed that myosin-X is directly associated with the VE-cadherin complex. Furthermore, expression of a dominant-negative mutant of myosin-X revealed that myosin-X is required for VE-cadherin export to cell edges and filopodia. These features indicate that myosin-X establishes a link between the actin cytoskeleton and VE-cadherin, thereby allowing VE-cadherin transportation along intrafilopodial actin cables. In conclusion, we propose that VE-cadherin trafficking along filopodia using myosin-X motor protein is a prerequisite for cell-cell junction formation. This mechanism may have functional consequences for endothelium repair in pathological settings.

The interaction of Streptococcus pneumoniae with plasmin mediates transmigration across endothelial and epithelial monolayers by intercellular junction cleavage.

The precise mechanisms by which Streptococcus pneumoniae overcomes epithelial and endothelial barriers to access underlying human tissues remain to be determined. The plasminogen system is highly important for the tissue barrier degradation which allows cell migration. Plasminogen is known to interact with pneumococci via enolase, glyceraldehyde-3-phosphate dehydrogenase, and choline-binding protein E. These observations prompted us to evaluate the role of this proteolytic system in the pneumococcal invasion process. We observed that coating of S. pneumoniae R6 strain with plasminogen or inactivated plasmin increased adherence to pulmonary epithelial A549 and vascular endothelial EaHy cells in vitro. This indicates that plasminogen-mediated adherence is independent of the protease activity and involves plasminogen binding to receptors on eukaryotic cell surfaces. Conversely, decreased adherence of bacterial cells coated with active plasmin was observed, indicating that the protease activity limits bacterial attachment on the cell surface. We were then interested in investigating the role of the proteolytic plasmin activity in the traversal of tissue barriers. We observed that adherence of plasmin-coated D39 (encapsulated) or R6 (unencapsulated) pneumococci induced sporadic disruptions of EaHy and A549 monolayer cell junctions. This was not observed when plasmin was inhibited by aprotinin. Endothelial junction disorganization may proceed by proteolysis of the cell junction components. This is supported by our observation of the in vitro cleavage by plasmin bound to pneumococci of recombinant vascular endothelial cadherin, the main component of endothelial adherens junctions. Finally, junction damage induced by plasmin may be related to tissue barrier traversal, as we measured an increase of S. pneumoniae transmigration across epithelial A549 and endothelial EaHy layers when active plasmin was present on the bacterial surface. Our results highlight a novel function for the plasminogen recruitment at the bacterial surface in facilitating adherence of pneumococci to endothelial and epithelial cells, while active plasmin degrades intercellular junctions. This process promotes migration of pneumococci through cell barriers by a pericellular route, a prerequisite for dissemination of S. pneumoniae in the host organism.

AdcAII, a new pneumococcal Zn-binding protein homologous with ABC transporters: biochemical and structural analysis.

Regulation of metal homeostasis is vital for pathogenic bacteria facing drastic metal concentration changes in various locations within the host during invasion. Metal-binding receptors (MBRs), one of the extracellular components of ATP-binding cassette transporters, have been shown to be essential in this process. Streptococcus pneumoniae expresses two characterized MBRs: PsaA and AdcA, two extracellular lipoproteins encoded by the psaABCD and adcRCBA operons, respectively. The Mn- and Zn-uptake functions of PsaA and AdcA, respectively, have been well established. Here we describe AdcAII as a third putative S. pneumoniae MBR. The analysis of a phylogenetic tree built from the sequence alignment of 68 proteins reveals a subgroup of members displaying an unusual genetic operon organisation. The adcAII gene belongs to a 6670-nucleotide-long transcript spanning the spr0903 to spr0907 loci encoding for the CcdA, thioredoxine, YfnA, AdcAII and PhtD proteins. Two adjacent repeats of imperfect AdcR-binding consensus sequence were identified upstream of the adcAII gene, suggesting a transcriptional co-regulation of adcAII and phtD genes. Biophysical and structural studies of recombinant AdcAII were performed to identify the metal specificity of the protein. Using electrospray mass spectrometry in native conditions, we found that Zn was bound to recombinant AdcAII. Screening of the effect of 10 cationic ions on the thermal stability of AdcAII revealed that Zn had the most pronounced stabilizing effect. The crystal structure of AdcAII has been solved to 2.4 A resolution. One Zn ion is bound to each AdcAII molecule in a symmetrical active site composed of three His and one Glu. The structure almost perfectly superimposed on the known MBR structures. The presence of a flexible 15-residue-long loop close to the metal-binding site is specific to those specialized in Zn transport. Taken together, these functional and structural data provide new perspectives related to the physiological role of AdcAII in pneumococcus Zn homeostasis.

Structure of artificial and natural VE-cadherin-based adherens junctions.

In vascular endothelium, adherens junctions between endothelial cells are composed of VE-cadherin (vascular endothelial cadherin), an adhesive receptor that is crucial for the proper assembly of vascular structures and the maintenance of vascular integrity. As a classical cadherin, VE-cadherin links endothelial cells together by homophilic interactions mediated by its extracellular part and associates intracellularly with the actin cytoskeleton via catenins. Although, from structural crystallographic data, a dimeric structure arranged in a trans orientation has emerged as a potential mechanism of cell-cell adhesion, the cadherin organization within adherens junctions remains controversial. Concerning VE-cadherin, its extracellular part possesses the capacity to self-associate in solution as hexamers consisting of three antiparallel cadherin dimers. VE-cadherin-based adherens junctions were reconstituted in vitro by assembly of a VE-cadherin EC (extracellular repeat) 1-EC4 hexamer at the surfaces of liposomes. The artificial adherens junctions revealed by cryoelectron microscopy appear as a two-dimensional self-assembly of hexameric structures. This cadherin organization is reminiscent of that found in native desmosomal junctions. Further structural studies performed on native VE-cadherin junctions would provide a better understanding of the cadherin organization within adherens junctions. Homophilic interactions between cadherins are strengthened intracellularly by connection to the actin cytoskeleton. Recently, we have discovered that annexin 2, an actin-binding protein connects the VE-cadherin-catenin complex to the actin cytoskeleton. This novel link is labile and promotes the endothelial cell switch from a quiescent to an angiogenic state.

Contribution of annexin 2 to the architecture of mature endothelial adherens junctions.

The vascular endothelial cadherin (VE-cad)-based complex is involved in the maintenance of vascular endothelium integrity. Using immunoprecipitation experiments, we have demonstrated that, in confluent human umbilical vein endothelial cells, the VE-cad-based complex interacts with annexin 2 and that annexin 2 translocates from the cytoplasm to the cell-cell contact sites as cell confluence is established. Annexin 2, located in cholesterol rafts, binds to both the actin cytoskeleton and the VE-cad-based complex so the complex is docked to cholesterol rafts. These multiple connections prevent the lateral diffusion of the VE-cad-based complex, thus strengthening adherens junctions in the ultimate steps of maturation. Moreover, we observed that the down-regulation of annexin 2 by small interfering RNA induces a delocalization of VE-cad from adherens junctions and consequently a destabilization of these junctions. Furthermore, our data indicate that the decoupling of the annexin 2/p11 complex from the VE-cad-based junction, triggered by vascular endothelial growth factor treatment, facilitates the switch from a quiescent to an immature state.