Conservation of structure and protein-protein interactions mediated by the secreted mycobacterial proteins EsxA, EsxB, and EspA

EsxA, a type VII secretion system-dependent effector, reveals a novel function in the sporulation of Bacillus cereus ATCC14579

BACKGROUND

Bacillus cereus is a Gram-positive, spore-forming bacterium that produces a spectrum of effectors integral to bacterial niche adaptation and the development of various infections. Among those is EsxA, whose secretion depends on the EssC component of the type VII secretion system (T7SS). EsxA's roles within the bacterial cell are poorly understood, although postulations indicate that it may be involved in sporulation. However, the T7SS repertoire in B. cereus has not been reported, and its functions are unestablished.

METHODS

We used the type strain, B. cereus ATCC14579, to generate ΔessC mutant through homologous recombination using the homing endonuclease I-SceI mediated markerless gene replacement. Comparatively, we analyzed the culture supernatant of type strain and the ΔessC mutant through Liquid chromatography-tandem mass spectrometry (LC-MS/MS). We further generated T7SSb-specific gene mutations to explore the housekeeping roles of the T7SSb-dependent effectors. The sporulation process of B. cereus ATCC14579 and its mutants was observed microscopically through the classic Schaeffer-Fulton staining method. The spore viability of each strain in this study was established by enumerating the colony-forming units on LB agar.

RESULTS

Through LC-MS/MS, we identified a pair of nearly identical (94%) effector proteins named EsxA belonging to the sagEsxA-like subfamily of the WXG100 protein superfamily in the culture supernatant of the wild type and none in the ΔessC mutant. Homology analysis of the T7SSb gene cluster among B. cereus strains revealed diversity from the 3' end of essC, encoding additional substrates. Deletions in esxA1 and esxA2 neither altered cellular morphology nor growth rate, but the ΔesxA1ΔesxA2 deletion resulted in significantly fewer viable spores and an overall slower sporulation process. Within 24 h culture, more than 80% of wild-type cells formed endospores compared to less than 5% in the ΔesxA1ΔesxA2 mutant. The maximum spore ratios for the wild type and ΔesxA1ΔesxA2 were 0.96 and 0.72, respectively. Altogether, these results indicated that EsxA1 and EsxA2 work cooperatively and are required for sporulation in B. cereus ATCC14567.

CONCLUSION

B. cereus ATCC14579 possesses two nearly identical T7SSb-dependent effectors belonging to the sagEsxA-like proteins. Simultaneous deletion of genes encoding these effectors significantly delayed and reduced sporulation, a novel finding for EsxA.

Modulators targeting protein-protein interactions in Mycobacterium tuberculosis

Tuberculosis (TB) is a chronic infectious disease caused by Mycobacterium tuberculosis (M. tuberculosis), mainly transmitted through droplets to infect the lungs, and seriously affecting patients' health and quality of life. Clinically, anti-TB drugs often entail side effects and lack efficacy against resistant strains. Thus, the exploration and development of novel targeted anti-TB medications are imperative. Currently, protein-protein interactions (PPIs) offer novel avenues for anti-TB drug development, and the study of targeted modulators of PPIs in M. tuberculosis has become a prominent research focus. Furthermore, a comprehensive PPI network has been constructed using computational methods and bioinformatics tools. This network allows for a more in-depth analysis of the structural biology of PPIs and furnishes essential insights for the development of targeted small-molecule modulators. Furthermore, this article provides a detailed overview of the research progress and regulatory mechanisms of PPI modulators in M. tuberculosis, the causative agent of TB. Additionally, it summarizes potential targets for anti-TB drugs and discusses the prospects of existing PPI modulators.

Proteo-genetic analysis reveals clear hierarchy of ESX-1 secretion in Mycobacterium marinum

The ESX-1 (ESAT-6-system-1) system and the protein substrates it transports are essential for mycobacterial pathogenesis. The precise ways that ESX-1 substrates contribute to virulence remains unknown. Several known ESX-1 substrates are also required for the secretion of other proteins. We used a proteo-genetic approach to construct high-resolution dependency relationships for the roles of individual ESX-1 substrates in secretion and virulence in Mycobacterium marinum, a pathogen of humans and animals. Characterizing a collection of M. marinum strains with in-frame deletions in each of the known ESX-1 substrate genes and the corresponding complementation strains, we demonstrate that ESX-1 substrates are differentially required for ESX-1 activity and for virulence. Using isobaric-tagged proteomics, we quantified the degree of requirement of each substrate on protein secretion. We conclusively defined distinct contributions of ESX-1 substrates in protein secretion. Our data reveal a hierarchy of ESX-1 substrate secretion, which supports a model for the composition of the extracytoplasmic ESX-1 secretory machinery. Overall, our proteo-genetic analysis demonstrates discrete roles for ESX-1 substrates in ESX-1 function and secretion in M. marinum.

Post-translational knockdown and post-secretional modification of EsxA determine contribution of EsxA membrane permeabilizing activity for mycobacterial intracellular survival

Current genetic studies (e.g. gene knockout) have suggested that EsxA and EsxB function as secreted virulence factors that are essential for Mycobaterium tuberculosis (Mtb) intracellular survival, specifically in mediating phagosome rupture and translocation of Mtb to the cytosol of host cells, which further facilitates Mtb intracellular replicating and cell-to-cell spreading. The EsxA-mediated intracellular survival is presumably achieved by its pH-dependent membrane-permeabilizing activity (MPA). However, the data from other studies have generated a discrepancy regarding the role of EsxA MPA in mycobacterial intracellular survival, which has raised a concern that genetic manipulations, such as deletion of esxB-esxA operon or RD-1 locus, may affect other codependently secreted factors that could be also directly involved cytosolic translocation, or stimulate extended disturbance on other genes' expression. To avoid the drawbacks of gene knockout, we first engineered a Mycobacterium marinum (Mm) strain, in which a DAS4+ tag was fused to the C-terminus of EsxB to allow inducible knockdown of EsxB (also EsxA) at the post-translational level. We also engineered an Mm strain by fusing a SpyTag (ST) to the C-terminus of EsxA, which allowed inhibition of EsxA-ST MPA at the post-secretional level through a covalent linkage to SpyCatcher-GFP. Both post-translational knockdown and functional inhibition of EsxA resulted in attenuation of Mm intracellular survival in lung epithelial cells or macrophages, which unambiguously confirms the direct role of EsxA MPA in mycobacterial intracellular survival.

A Bridging Centrality Plugin for GEPHI and a Case Study for Mycobacterium Tuberculosis H37Rv

Bridging Centrality (BriCe) is a popular measure that combines the Betweenness centrality and Bridging coefficient metrics to characterize nodes acting as a bridge among clusters. However, there were no implementations of the BriCe plugin that can be readily used in the GEPHI software or any other software dedicated to graph-based studies. In this paper, we present the BriCe plugin for GEPHI. It is available as a third-party functionality from the native GEPHI interface as a handy plugin to add; hence, no additional download and installation process is necessary. The BriCe plugin for GEPHI is open-source, and one can access the code through the GEPHI GitHub repository. As a use case of the BriCe plugin, we analyzed the genome of Mycobacterium tuberculosis H37Rv to identify biological explanations on why some proteins were ranked with top BriCe values? For instance, we were able to formulate a new hypothesis combining the predicted sub cellular localization and high BriCe values concerning lipopolysaccharides (LPS) exportation. Our hypothesis provides a possible link among proteins of a glycosyltransferase group and the type VII Secretion System. The Bridging Centrality plugin for GEPHI is an easy to use tool for analyzing complex graphs and draw novel insights from graphical data.

The Type VII Secretion System of Staphylococcus

The type VII protein secretion system (T7SS) of Staphylococcus aureus is encoded at the ess locus. T7 substrate recognition and protein transport are mediated by EssC, a membrane-bound multidomain ATPase. Four EssC sequence variants have been identified across S. aureus strains, each accompanied by a specific suite of substrate proteins. The ess genes are upregulated during persistent infection, and the secretion system contributes to virulence in disease models. It also plays a key role in intraspecies competition, secreting nuclease and membrane-depolarizing toxins that inhibit the growth of strains lacking neutralizing immunity proteins. A genomic survey indicates that the T7SS is widely conserved across staphylococci and is encoded in clusters that contain diverse arrays of toxin and immunity genes. The presence of genomic islands encoding multiple immunity proteins in species such as Staphylococcus warneri that lack the T7SS points to a major role for the secretion system in bacterial antagonism. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

ESX-1-Independent Horizontal Gene Transfer by Mycobacterium tuberculosis Complex Strains

Current models of horizontal gene transfer (HGT) in mycobacteria are based on “distributive conjugal transfer” (DCT), an HGT type described in the fast-growing, saprophytic model organism Mycobacterium smegmatis, which creates genome mosaicism in resulting strains and depends on an ESX-1 type VII secretion system. In contrast, only few data on interstrain DNA transfer are available for tuberculosis-causing mycobacteria, for which chromosomal DNA transfer between two Mycobacterium canettii strains was reported, a process which, however, was not observed for Mycobacterium tuberculosis strains. Here, we have studied a wide range of human- and animal-adapted members of the Mycobacterium tuberculosis complex (MTBC) using an optimized filter-based mating assay together with three selected strains of M. canettii that acted as DNA recipients. Unlike in previous approaches, we obtained a high yield of thousands of recombinants containing transferred chromosomal DNA fragments from various MTBC donor strains, as confirmed by whole-genome sequence analysis of 38 randomly selected clones. While the genome organizations of the obtained recombinants showed mosaicisms of donor DNA fragments randomly integrated into a recipient genome backbone, reminiscent of those described as being the result of ESX-1-mediated DCT in M. smegmatis, we observed similar transfer efficiencies when ESX-1-deficient donor and/or recipient mutants were used, arguing that in tubercle bacilli, HGT is an ESX-1-independent process. These findings provide new insights into the genetic events driving the pathoevolution of M. tuberculosis and radically change our perception of HGT in mycobacteria, particularly for those species that show recombinogenic population structures despite the natural absence of ESX-1 secretion systems.

Principle and potential applications of the non-classical protein secretory pathway in bacteria

In addition to the extracellular proteins secreted by known secretory pathways, a number of cytoplasmic proteins without predicable or known signal sequences or secretory motifs have been found in the extracellular milieu, and were consequently classified as non-classically secreted proteins. Non-classical protein secretion is considered to be a general, conserved cellular phenomenon in both eukaryotes and prokaryotes. There are several research hotspots on the non-classical protein secretory pathway, and the most important two of them are the recognition principle of substrate proteins and possible secretory mechanisms. To date, researchers have made some progress in understanding the characteristics of these proteins. For example, it was discovered that many non-classically secreted proteins exist and are secreted in multimeric form. Some of these proteins prefer to be clustered and exported at the poles and the septum of the cell. The majority of these proteins play different functions when they are in the intra- and extracellular environments, and several of their functions are related to survival and pathogenicity. Furthermore, non-classically secreted proteins can be used as leading proteins to guide a POI (protein of interest) out of the cells, which provides a novel strategy for protein secretion with potential applications in the industry. Summarizing these findings, this review emphasizes the hot spots related to non-classically secreted proteins in bacteria, lists the most important hypotheses on the selection and secretion mechanisms of non-classically secreted proteins, and put forward their potential applications.

T CELL EPITOPES OF THE ESXA FULL GENE OF MYCOBACTERIUM TUBERCULOSIS FROM SPUTUM OF MDR-TB PATIENTS

Background:

In 2015, World Health Organization (WHO) discovered 10.4 million tuberculosis (TB) cases around the world. Multidrug-resistant tuberculosis (MDR-TB) became a threat because it has high mortality number. There were 480,000 new MDR-TB cases in 2015. Based on those problems, diagnostic development to detect M. tuberculosis rapidly and accurately is needed. The importance of detecting epitope expression of esxA full gene because there was a potential of complexity over the protein structure and might affect the protein concentration. By knowing epitope prediction, there’s an expectation that it can help the development of TB diagnostic. This research was aimed to determine the T cell epitope prediction of esxA full gene from MDR-TB patients

Material and Methods:

Total of 24 MDR-TB sputum isolate from TB patients at Dr. Soetomo Hospital were collected from September to December 2016. Samples were confirmed as MDR-TB using GeneXpert and Bactec MGIT 960. Those samples tested using PCR targeted 580 bp of esxA gene and sequencing. Gene sequence was aligned against wild type using Bioedit program version 7.2.5 and NCBI BLAST. T cell epitope prediction was analyzed by GENETYX version 10.

Results:

Epitope predictions that could be obtained were IEAAAS, ASAIQG, VTSIHS, TKLAAA, VTGMFA based IAd Pattern Position and EAAAS based Rothbard/Taylor Pattern Position. Those prediction epitopes can determine the severity of disease, therefore full gene of esxA could be used as diagnostic target.

Conclusion:

This research discovered five specific T cell epitope prediction based on IAd Pattern Position and one epitope prediction according to Rothbard/Taylor Pattern Position.

Interaction of Erp Protein of Mycobacterium tuberculosis with Rv2212 Enhances Intracellular Survival of Mycobacterium smegmatis

The Mycobacterium tuberculosis exported repetitive protein (RvErp) is a crucial virulence-associated factor as determined by its role in the survival and multiplication of mycobacteria in cultured macrophages and in vivo Although attempts have been made to understand the function of Erp protein, its exact role in Mycobacterium pathogenesis is still elusive. One way to determine this is by searching for novel interactions of RvErp. Using a yeast two-hybrid assay, an adenylyl cyclase (AC), Rv2212, was found to interact with RvErp. The interaction between RvErp and Rv2212 is direct and occurs at the endogenous level. The Erp protein of Mycobacterium smegmatis (MSMEG_6405, or MsErp) interacts neither with Rv2212 nor with Ms_4279, the M. smegmatis homologue of Rv2212. Deletion mutants of Rv2212 revealed its adenylyl cyclase domain to be responsible for the interaction. RvErp enhances Rv2212-mediated cyclic AMP (cAMP) production. Also, the biological significance of the interaction between RvErp and Rv2212 was demonstrated by the enhanced survival of M. smegmatis within THP-1 macrophages. Taken together, these studies address a novel mechanism by which Erp executes its function.

IMPORTANCE

RvErp is one of the important virulence factors of M. tuberculosis This study describes a novel function of RvErp protein of M. tuberculosis by identifying Rv2212 as its interacting protein. Rv2212 is an adenylyl cyclase (AC) and produces cAMP, one of the prime second messengers that regulate the intracellular survival of mycobacteria. Therefore, the significance of investigating novel interactions of RvErp is paramount in unraveling the mechanisms governing the intracellular survival of mycobacteria.

Separable roles for Mycobacterium tuberculosis ESX-3 effectors in iron acquisition and virulence

Mycobacterium tuberculosis (Mtb) encodes five type VII secretion systems (T7SS), designated ESX-1-ESX-5, that are critical for growth and pathogenesis. The best characterized is ESX-1, which profoundly impacts host cell interactions. In contrast, the ESX-3 T7SS is implicated in metal homeostasis, but efforts to define its function have been limited by an inability to recover deletion mutants. We overcame this impediment using medium supplemented with various iron complexes to recover mutants with deletions encompassing select genes within esx-3 or the entire operon. The esx-3 mutants were defective in uptake of siderophore-bound iron and dramatically accumulated cell-associated mycobactin siderophores. Proteomic analyses of culture filtrate revealed that secretion of EsxG and EsxH was codependent and that EsxG-EsxH also facilitated secretion of several members of the proline-glutamic acid (PE) and proline-proline-glutamic acid (PPE) protein families (named for conserved PE and PPE N-terminal motifs). Substrates that depended on EsxG-EsxH for secretion included PE5, encoded within the esx-3 locus, and the evolutionarily related PE15-PPE20 encoded outside the esx-3 locus. In vivo characterization of the mutants unexpectedly showed that the ESX-3 secretion system plays both iron-dependent and -independent roles in Mtb pathogenesis. PE5-PPE4 was found to be critical for the siderophore-mediated iron-acquisition functions of ESX-3. The importance of this iron-acquisition function was dependent upon host genotype, suggesting a role for ESX-3 secretion in counteracting host defense mechanisms that restrict iron availability. Further, we demonstrate that the ESX-3 T7SS secretes certain effectors that are important for iron uptake while additional secreted effectors modulate virulence in an iron-independent fashion.

Mycobacterium tuberculosis type VII secreted effector EsxH targets host ESCRT to impair trafficking

Mycobacterium tuberculosis (Mtb) disrupts anti-microbial pathways of macrophages, cells that normally kill bacteria. Over 40 years ago, D'Arcy Hart showed that Mtb avoids delivery to lysosomes, but the molecular mechanisms that allow Mtb to elude lysosomal degradation are poorly understood. Specialized secretion systems are often used by bacterial pathogens to translocate effectors that target the host, and Mtb encodes type VII secretion systems (TSSSs) that enable mycobacteria to secrete proteins across their complex cell envelope; however, their cellular targets are unknown. Here, we describe a systematic strategy to identify bacterial virulence factors by looking for interactions between the Mtb secretome and host proteins using a high throughput, high stringency, yeast two-hybrid (Y2H) platform. Using this approach we identified an interaction between EsxH, which is secreted by the Esx-3 TSSS, and human hepatocyte growth factor-regulated tyrosine kinase substrate (Hgs/Hrs), a component of the endosomal sorting complex required for transport (ESCRT). ESCRT has a well-described role in directing proteins destined for lysosomal degradation into intraluminal vesicles (ILVs) of multivesicular bodies (MVBs), ensuring degradation of the sorted cargo upon MVB-lysosome fusion. Here, we show that ESCRT is required to deliver Mtb to the lysosome and to restrict intracellular bacterial growth. Further, EsxH, in complex with EsxG, disrupts ESCRT function and impairs phagosome maturation. Thus, we demonstrate a role for a TSSS and the host ESCRT machinery in one of the central features of tuberculosis pathogenesis.

Mycobacterium tuberculosis (Mtb) causes the disease tuberculosis, one of the world's most deadly infections. The host immune system can't eradicate Mtb because it grows within macrophages, cells that normally kill bacteria. One of the intracellular survival strategies of Mtb is to avoid delivery to lysosomes, a phenomenon described over 40 years ago, but for which the mechanism and molecular details remain incomplete. Mtb possess specialized secretion systems (Type VII secretion systems; TSSS) that transfer particular proteins out of the bacteria, but how these proteins promote infection is not well understood. In this study, we used a high stringency yeast two-hybrid system to identify interactions between secreted effectors from Mtb and human host factors. We identified ninety-nine such interactions and focused our attention on the interaction between EsxH, secreted by Esx-3, a TSSS of Mtb, and Hrs, a component of the host ESCRT machinery. We provide evidence that Mtb EsxH directly targets host Hrs to disrupt delivery of bacteria to lysosomes. Thus, this study demonstrates the role of a TSSS effector and the ESCRT machinery in what is one of the central features of tuberculosis pathogenesis, thereby providing molecular insight into why humans can't clear Mtb infection.

Phenotypic profiling of Mycobacterium tuberculosis EspA point mutants reveals that blockage of ESAT-6 and CFP-10 secretion in vitro does not always correlate with attenuation of virulence

The EspA protein of Mycobacterium tuberculosis is essential for the type VII ESX-1 protein secretion apparatus, which delivers the principal virulence factors ESAT-6 and CFP-10. In this study, site-directed mutagenesis of EspA was performed to elucidate its influence on the ESX-1 system. Replacing Trp(55) (W55) or Gly(57) (G57) residues in the putative W-X-G motif of EspA with arginines impaired ESAT-6 and CFP-10 secretion in vitro and attenuated M. tuberculosis. Replacing the Phe(50) (F50) and Lys(62) (K62) residues, which flank the W-X-G motif, with arginine and alanine, respectively, destabilized EspA, abolished ESAT-6 and CFP-10 secretion in vitro, and attenuated M. tuberculosis. Likewise, replacing the Phe(5) (F5) and Lys(41) (K41) residues with arginine and alanine, respectively, also destabilized EspA and blocked ESAT-6 and CFP-10 secretion in vitro. However, these two particular mutations did not attenuate M. tuberculosis in cellular models of infection or during acute infection in mice. We have thus identified amino acid residues in EspA that are important for facilitating ESAT-6 and CFP-10 secretion and virulence. However, our data also indicate for the first time that blockage of M. tuberculosis ESAT-6 and CFP-10 secretion in vitro and attenuation are mutually exclusive.

Mycobacterium tuberculosis ESAT-6 exhibits a unique membrane-interacting activity that is not found in its ortholog from non-pathogenic Mycobacterium smegmatis

Mycobacterium tuberculosis ESAT-6 (MtbESAT-6) reportedly shows membrane/cell-lysis activity, and recently its biological roles in pathogenesis have been implicated in rupture of the phagosomes for bacterial cytosolic translocation. However, molecular mechanism of MtbESAT-6-mediated membrane interaction, particularly in relation with its biological functions in pathogenesis, is poorly understood. In this study, we investigated the pH-dependent membrane interaction of MtbESAT-6, MtbCFP-10, and the MtbESAT-6/CFP-10 heterodimer, by using liposomal model membranes that mimic phagosomal compartments. MtbESAT-6, but neither MtbCFP-10 nor the heterodimer, interacted with the liposomal membranes at acidic conditions, which was evidenced by release of K(+) ions from the liposomes. Most importantly, the orthologous ESAT-6 from non-pathogenic Mycobacterium smegmatis (MsESAT-6) was essentially inactive in release of K(+). The differential membrane interactions between MtbESAT-6 and MsESAT-6 were further confirmed in an independent membrane leakage assay using the dye/quencher pair, 8-aminonapthalene-1,3,6 trisulfonic acid (ANTS)/p-xylene-bis-pyridinium bromide (DPX). Finally, using intrinsic and extrinsic fluorescence approaches, we probed the pH-dependent conformational changes of MtbESAT-6 and MsESAT-6. At acidic pH conditions, MtbESAT-6 underwent a significant conformational change, which was featured by an increased solvent-exposed hydrophobicity, while MsESAT-6 showed little conformational change in response to acidification. In conclusion, we have demonstrated that MtbESAT-6 possesses a unique membrane-interacting activity that is not found in MsESAT-6 and established the utility of rigorous biochemical approaches in dissecting the virulence of M. tuberculosis.

Protein secretion and surface display in Gram-positive bacteria

The cell wall peptidoglycan of Gram-positive bacteria functions as a surface organelle for the transport and assembly of proteins that interact with the environment, in particular, the tissues of an infected host. Signal peptide-bearing precursor proteins are secreted across the plasma membrane of Gram-positive bacteria. Some precursors carry C-terminal sorting signals with unique sequence motifs that are cleaved by sortase enzymes and linked to the cell wall peptidoglycan of vegetative forms or spores. The sorting signals of pilin precursors are cleaved by pilus-specific sortases, which generate covalent bonds between proteins leading to the assembly of fimbrial structures. Other precursors harbour surface (S)-layer homology domains (SLH), which fold into a three-pronged spindle structure and bind secondary cell wall polysaccharides, thereby associating with the surface of specific Gram-positive microbes. Type VII secretion is a non-canonical secretion pathway for WXG100 family proteins in mycobacteria. Gram-positive bacteria also secrete WXG100 proteins and carry unique genes that either contribute to discrete steps in secretion or represent distinctive substrates for protein transport reactions.

A three-hybrid system to probe in vivo protein-protein interactions: application to the essential proteins of the RD1 complex of M. tuberculosis

Background

Protein-protein interactions play a crucial role in enabling a pathogen to survive within a host. In many cases the interactions involve a complex of proteins rather than just two given proteins. This is especially true for pathogens like M. tuberculosis that are able to successfully survive the inhospitable environment of the macrophage. Studying such interactions in detail may help in developing small molecules that either disrupt or augment the interactions. Here, we describe the development of an E. coli based bacterial three-hybrid system that can be used effectively to study ternary protein complexes.

Methodology/Principal Findings

The protein-protein interactions involved in M. tuberculosis pathogenesis have been used as a model for the validation of the three-hybrid system. Using the M. tuberculosis RD1 encoded proteins CFP10, ESAT6 and Rv3871 for our proof-of-concept studies, we show that the interaction between the proteins CFP10 and Rv3871 is strengthened and stabilized in the presence of ESAT6, the known heterodimeric partner of CFP10. Isolating peptide candidates that can disrupt crucial protein-protein interactions is another application that the system offers. We demonstrate this by using CFP10 protein as a disruptor of a previously established interaction between ESAT6 and a small peptide HCL1; at the same time we also show that CFP10 is not able to disrupt the strong interaction between ESAT6 and another peptide SL3.

Conclusions/Significance

The validation of the three-hybrid system paves the way for finding new peptides that are stronger binders of ESAT6 compared even to its natural partner CFP10. Additionally, we believe that the system offers an opportunity to study tri-protein complexes and also perform a screening of protein/peptide binders to known interacting proteins so as to elucidate novel tri-protein complexes.

A screen to identify small molecule inhibitors of protein-protein interactions in mycobacteria

Despite extensive efforts in tuberculosis (TB) drug research, very few novel inhibitors have been discovered. This issue emphasizes the need for innovative methods to discover new anti-TB drugs. In this study, we established a new high-throughput screen (HTS) platform technology that differs from traditional TB drug screens because it utilizes Mycobacterial-Protein Fragment Complementation (M-PFC) to identify small molecule inhibitors of protein-protein interactions in mycobacteria. Several examples of protein-protein interactions were tested with M-PFC to highlight the diversity of selectable drug targets that could be used for screening. These included interactions of essential regulators (IdeR dimerization), enzymatic complexes (LeuCD), secretory antigens (Cfp10-Esat6), and signaling pathways (DevR dimerization). The feasibility of M-PFC in a HTS platform setting was tested by performing a proof-of-concept quantitative HTS of 3,600 small molecule compounds on DevR-DevR interaction, which was chosen because of its strong implications in Mycobacterium tuberculosis persistence and the need for effective drugs against latent TB. The calculated Z'-factor was consistently ≥0.8, indicating a robust and reproducible assay. Completion of the proof-of-concept screen allowed for the identification of advantages and disadvantages in the current assay design, where improvements made will further pioneer M-PFC-based applications in a large-scale HTS format.

The terminal immunoglobulin-like repeats of LigA and LigB of Leptospira enhance their binding to gelatin binding domain of fibronectin and host cells

Leptospira spp. are pathogenic spirochetes that cause the zoonotic disease leptospirosis. Leptospiral immunoglobulin (Ig)-like protein B (LigB) contributes to the binding of Leptospira to extracellular matrix proteins such as fibronectin, fibrinogen, laminin, elastin, tropoelastin and collagen. A high-affinity Fn-binding region of LigB has been localized to LigBCen2, which contains the partial 11th and full 12th Ig-like repeats (LigBCen2R) and 47 amino acids of the non-repeat region (LigBCen2NR) of LigB. In this study, the gelatin binding domain of fibronectin was shown to interact with LigBCen2R (K(D) = 1.91+/-0.40 microM). Not only LigBCen2R but also other Ig-like domains of Lig proteins including LigAVar7'-8, LigAVar10, LigAVar11, LigAVar12, LigAVar13, LigBCen7'-8, and LigBCen9 bind to GBD. Interestingly, a large gain in affinity was achieved through an avidity effect, with the terminal domains, 13th (LigA) or 12th (LigB) Ig-like repeat of Lig protein (LigAVar7'-13 and LigBCen7'-12) enhancing binding affinity approximately 51 and 28 fold, respectively, compared to recombinant proteins without this terminal repeat. In addition, the inhibited effect on MDCKs cells can also be promoted by Lig proteins with terminal domains, but these two domains are not required for gelatin binding domain binding and cell adhesion. Interestingly, Lig proteins with the terminal domains could form compact structures with a round shape mediated by multidomain interaction. This is the first report about the interaction of gelatin binding domain of Fn and Lig proteins and provides an example of Lig-gelatin binding domain binding mediating bacterial-host interaction.