The divisomal protein DivIB contains multiple epitopes that mediate its recruitment to incipient division sites

Comparative Genomic Reveals Clonal Heterogeneity in Persistent Staphylococcus aureus Infection

Persistent infections caused by Staphylococcus aureus remain a clinical challenge. Adaptational mechanisms of the pathogen influencing infection persistence, treatment success, and clinical outcome in these types of infections by S. aureus have not been fully elucidated so far. We applied a whole-genome sequencing approach on fifteen isolates retrieved from a persistent S. aureus infection to determine their genetic relatedness, virulome, and resistome. The analysis of the genomic data indicates that all isolates shared a common clonal origin but displayed a heterogenous composition of virulence factors and antimicrobial resistance. This heterogeneity was reflected by different mutations in the rpoB gene that were related to the phenotypic antimicrobial resistance towards rifampicin and different minimal inhibitory concentrations of oxacillin. In addition, one group of isolates had acquired the genes encoding for staphylokinase (sak) and staphylococcal complement inhibitor (scn), leading to the truncation of the hemolysin b (hlb) gene. These features are characteristic for temperate phages of S. aureus that carry genes of the immune evasion cluster and confer triple conversion by integration into the hlb gene. Modulation of immune evasion mechanisms was demonstrated by significant differences in biofilm formation capacity, while invasion and intracellular survival in neutrophils were not uniformly altered by the presence of the immune evasion cluster. Virulence factors carried by temperate phages of S. aureus may contribute to the course of infection at different stages and affect immune evasion and pathogen persistence. In conclusion, the application of comparative genomic demonstrated clonal heterogeneity in persistent S. aureus infection.

Bacteriophage SP01 Gene Product 56 Inhibits Bacillus subtilis Cell Division by Interacting with FtsL and Disrupting Pbp2B and FtsW Recruitment

Previous work identified gene product 56 (gp56), encoded by the lytic bacteriophage SP01, as being responsible for inhibition of Bacillus subtilis cell division during its infection. Assembly of the essential tubulin-like protein FtsZ into a ring-shaped structure at the nascent site of cytokinesis determines the timing and position of division in most bacteria. This FtsZ ring serves as a scaffold for recruitment of other proteins into a mature division-competent structure permitting membrane constriction and septal cell wall synthesis. Here, we show that expression of the predicted 9.3-kDa gp56 of SP01 inhibits later stages of B. subtilis cell division without altering FtsZ ring assembly. Green fluorescent protein-tagged gp56 localizes to the membrane at the site of division. While its localization does not interfere with recruitment of early division proteins, gp56 interferes with the recruitment of late division proteins, including Pbp2b and FtsW. Imaging of cells with specific division components deleted or depleted and two-hybrid analyses suggest that gp56 localization and activity depend on its interaction with FtsL. Together, these data support a model in which gp56 interacts with a central part of the division machinery to disrupt late recruitment of the division proteins involved in septal cell wall synthesis.IMPORTANCE Studies over the past decades have identified bacteriophage-encoded factors that interfere with host cell shape or cytokinesis during viral infection. The phage factors causing cell filamentation that have been investigated to date all act by targeting FtsZ, the conserved prokaryotic tubulin homolog that composes the cytokinetic ring in most bacteria and some groups of archaea. However, the mechanisms of several phage factors that inhibit cytokinesis, including gp56 of bacteriophage SP01 of Bacillus subtilis, remain unexplored. Here, we show that, unlike other published examples of phage inhibition of cytokinesis, gp56 blocks B. subtilis cell division without targeting FtsZ. Rather, it utilizes the assembled FtsZ cytokinetic ring to localize to the division machinery and to block recruitment of proteins needed for septal cell wall synthesis.

Staphylococcus aureus DivIB is a peptidoglycan-binding protein that is required for a morphological checkpoint in cell division

Bacterial cell division is a fundamental process that requires the coordinated actions of a number of proteins which form a complex macromolecular machine known as the divisome. The membrane-spanning proteins DivIB and its orthologue FtsQ are crucial divisome components in Gram-positive and Gram-negative bacteria respectively. However, the role of almost all of the integral division proteins, including DivIB, still remains largely unknown. Here we show that the extracellular domain of DivIB is able to bind peptidoglycan and have mapped the binding to its β subdomain. Conditional mutational studies show that divIB is essential for Staphylococcus aureus growth, while phenotypic analyses following depletion of DivIB results in a block in the completion, but not initiation, of septum formation. Localisation studies suggest that DivIB only transiently localises to the division site and may mark previous sites of septation. We propose that DivIB is required for a molecular checkpoint during division to ensure the correct assembly of the divisome at midcell and to prevent hydrolytic growth of the cell in the absence of a completed septum.

Reconstitution of membrane protein complexes involved in pneumococcal septal cell wall assembly

The synthesis of peptidoglycan, the major component of the bacterial cell wall, is essential to cell survival, yet its mechanism remains poorly understood. In the present work, we have isolated several membrane protein complexes consisting of the late division proteins of Streptococcus pneumoniae: DivIB, DivIC, FtsL, PBP2x and FtsW, or subsets thereof. We have co-expressed membrane proteins from S. pneumoniae in Escherichia coli. By combining two successive affinity chromatography steps, we obtained membrane protein complexes with a very good purity. These complexes are functional, as indicated by the retained activity of PBP2x to bind a fluorescent derivative of penicillin and to hydrolyze the substrate analogue S2d. Moreover, we have evidenced the stabilizing role of protein-protein interactions within each complex. This work paves the way for a complete reconstitution of peptidoglycan synthesis in vitro, which will be critical to the elucidation of its intricate regulation mechanisms.

A role for the essential YycG sensor histidine kinase in sensing cell division

The YycG sensor histidine kinase co-ordinates cell wall remodelling with cell division in Gram-positive bacteria by controlling the transcription of genes for autolysins and their inhibitors. Bacillus subtilis YycG senses cell division and is enzymatically activated by associating with the divisome at the division septum. Here it is shown that the cytoplasmic PAS domain of this multi-domain transmembrane kinase is a determining factor translocating the kinase to the division septum. Furthermore, translocation to the division septum, per se, is insufficient to activate YycG, indicating that specific interactions and/or ligands produced there are required to stimulate kinase activity. N-terminal truncations of YycG lose negative regulation of their activity inferring that this regulation is accomplished through its transmembrane and extramembrane domains interacting with the membrane associated YycH and YycI proteins that do not localize to the divisome. The data indicate that YycG activity in non-dividing cells is suppressed by its interaction with YycH and YycI and its activation is co-ordinated to cell division in dividing cells by specific interactions that occur within the divisome.

Evidence from artificial septal targeting and site-directed mutagenesis that residues in the extracytoplasmic β domain of DivIB mediate its interaction with the divisomal transpeptidase PBP 2B

Bacterial cytokinesis is achieved through the coordinated action of a multiprotein complex known as the divisome. The Escherichia coli divisome is comprised of at least 10 essential proteins whose individual functions are mostly unknown. Most divisomal proteins have multiple binding partners, making it difficult to pinpoint epitopes that mediate pairwise interactions between these proteins. We recently introduced an artificial septal targeting approach that allows the interaction between pairs of proteins to be studied in vivo without the complications introduced by other interacting proteins (C. Robichon, G. F. King, N. W. Goehring, and J. Beckwith, J. Bacteriol. 190:6048-6059, 2008). We have used this approach to perform a molecular dissection of the interaction between Bacillus subtilis DivIB and the divisomal transpeptidase PBP 2B, and we demonstrate that this interaction is mediated exclusively through the extracytoplasmic domains of these proteins. Artificial septal targeting in combination with mutagenesis experiments revealed that the C-terminal region of the β domain of DivIB is critical for its interaction with PBP 2B. These findings are consistent with previously defined loss-of-function point mutations in DivIB as well as the recent demonstration that the β domain of DivIB mediates its interaction with the FtsL-DivIC heterodimer. These new results have allowed us to construct a model of the DivIB/PBP 2B/FtsL/DivIC quaternary complex that strongly implicates DivIB, FtsL, and DivIC in modulating the transpeptidase activity of PBP 2B.

FtsQ interaction mutants: a way to identify new antibacterial targets

FtsQ is a highly conserved component of the divisome that plays a central role in the assembly of early and late cell division proteins. The biological activity of this protein is still largely unknown, but its ability to interact with many components of the divisome was described by both two-hybrid assays and co-immunoprecipitation experiments. This paper describes the behaviour of ftsQ point mutants, created by random mutagenesis without regard to their phenotype, in which FtsQ is impaired in its ability to interact with its Escherichia coli division partners. Our results allow the identification of FtsQ residues involved in the interaction with other partner proteins and the determination of the biological significance of these interactions. The knowledge derived by this study could constitute not only the basis for understanding how these proteins assemble in the divisome, but also a starting point for the design of new antibacterial drugs that disrupt the bacterial division machinery.

YneA, an SOS-induced inhibitor of cell division in Bacillus subtilis, is regulated posttranslationally and requires the transmembrane region for activity

Cell viability depends on the stable transmission of genetic information to each successive generation. Therefore, in the event of intrinsic or extrinsic DNA damage, it is important that cell division be delayed until DNA repair has been completed. In Bacillus subtilis, this is accomplished in part by YneA, an inhibitor of division that is induced as part of the SOS response. We sought to gain insight into the mechanism by which YneA blocks cell division and the processes involved in shutting off YneA activity. Our data suggest that YneA is able to inhibit daughter cell separation as well as septum formation. YneA contains a LysM peptidoglycan binding domain and is predicted to be exported. We established that the YneA signal peptide is rapidly cleaved, resulting in secretion of YneA into the medium. Mutations within YneA affect both the rate of signal sequence cleavage and the activity of YneA. YneA does not stably associate with the cell wall and is rapidly degraded by extracellular proteases. Based on these results, we hypothesize that exported YneA is active prior to signal peptide cleavage and that proteolysis contributes to the inactivation of YneA. Finally, we identified mutations in the transmembrane segment of YneA that abolish the ability of YneA to inhibit cell division, while having little or no effect on YneA export or stability. These data suggest that protein-protein interactions mediated by the transmembrane region may be required for YneA activity.

Central domain of DivIB caps the C-terminal regions of the FtsL/DivIC coiled-coil rod

DivIB(FtsQ), FtsL, and DivIC(FtsB) are enigmatic membrane proteins that are central to the process of bacterial cell division. DivIB(FtsQ) is dispensable in specific conditions in some species, and appears to be absent in other bacterial species. The presence of FtsL and DivIC(FtsB) appears to be conserved despite very low sequence conservation. The three proteins form a complex at the division site, FtsL and DivIC(FtsB) being associated through their extracellular coiled-coil region. We report here structural investigations by NMR, small-angle neutron and x-ray scattering, and interaction studies by surface plasmon resonance, of the complex of DivIB, FtsL, and DivIC from Streptococcus pneumoniae, using soluble truncated forms of the proteins. We found that one side of the "bean"-shaped central beta-domain of DivIB interacts with the C-terminal regions of the dimer of FtsL and DivIC. This finding is corroborated by sequence comparisons across bacterial genomes. Indeed, DivIB is absent from species with shorter FtsL and DivIC proteins that have an extracellular domain consisting only of the coiled-coil segment without C-terminal conserved regions (Campylobacterales). We propose that the main role of the interaction of DivIB with FtsL and DivIC is to help the formation, or to stabilize, the coiled-coil of the latter proteins. The coiled-coil of FtsL and DivIC, itself or with transmembrane regions, could be free to interact with other partners.

Artificial septal targeting of Bacillus subtilis cell division proteins in Escherichia coli: an interspecies approach to the study of protein-protein interactions in multiprotein complexes

Bacterial cell division is mediated by a set of proteins that assemble to form a large multiprotein complex called the divisome. Recent studies in Bacillus subtilis and Escherichia coli indicate that cell division proteins are involved in multiple cooperative binding interactions, thus presenting a technical challenge to the analysis of these interactions. We report here the use of an E. coli artificial septal targeting system for examining the interactions between the B. subtilis cell division proteins DivIB, FtsL, DivIC, and PBP 2B. This technique involves the fusion of one of the proteins (the "bait") to ZapA, an E. coli protein targeted to mid-cell, and the fusion of a second potentially interacting partner (the "prey") to green fluorescent protein (GFP). A positive interaction between two test proteins in E. coli leads to septal localization of the GFP fusion construct, which can be detected by fluorescence microscopy. Using this system, we present evidence for two sets of strong protein-protein interactions between B. subtilis divisomal proteins in E. coli, namely, DivIC with FtsL and DivIB with PBP 2B, that are independent of other B. subtilis cell division proteins and that do not disturb the cytokinesis process in the host cell. Our studies based on the coexpression of three or four of these B. subtilis cell division proteins suggest that interactions among these four proteins are not strong enough to allow the formation of a stable four-protein complex in E. coli in contrast to previous suggestions. Finally, our results demonstrate that E. coli artificial septal targeting is an efficient and alternative approach for detecting and characterizing stable protein-protein interactions within multiprotein complexes from other microorganisms. A salient feature of our approach is that it probably only detects the strongest interactions, thus giving an indication of whether some interactions suggested by other techniques may either be considerably weaker or due to false positives.

Roles of pneumococcal DivIB in cell division

DivIB, also known as FtsQ in gram-negative organisms, is a division protein that is conserved in most eubacteria. DivIB is localized at the division site and forms a complex with two other division proteins, FtsL and DivIC/FtsB. The precise function of these three bitopic membrane proteins, which are central to the division process, remains unknown. We report here the characterization of a divIB deletion mutant of Streptococcus pneumoniae, which is a coccus that divides with parallel planes. Unlike its homologue FtsQ in Escherichia coli, pneumococcal DivIB is not required for growth in rich medium, but the Delta divIB mutant forms chains of diplococci and a small fraction of enlarged cells with defective septa. However, the deletion mutant does not grow in a chemically defined medium. In the absence of DivIB and protein synthesis, the partner FtsL is rapidly degraded, whereas other division proteins are not affected, pointing to a role of DivIB in stabilizing FtsL. This is further supported by the finding that an additional copy of ftsL restores growth of the Delta divIB mutant in defined medium. Functional mapping of the three distinct alpha, beta, and gamma domains of the extracellular region of DivIB revealed that a complete beta domain is required to fully rescue the deletion mutant. DivIB with a truncated beta domain reverts only the chaining phenotype, indicating that DivIB has distinct roles early and late in the division process. Most importantly, the deletion of divIB increases the susceptibility to beta-lactams, more evidently in a resistant strain, suggesting a function in cell wall synthesis.