Studies of dynamic protein-protein interactions in bacteria using Renilla luciferase complementation are undermined by nonspecific enzyme inhibition

Split Proteins and Reassembly Modules for Biological Applications

Split systems, modular entities enabling controlled biological processes, have become instrumental in biological research. This review highlights their utility across applications like gene regulation, protein interaction identification, and biosensor development. Covering significant progress over the last decade, it revisits traditional split proteins such as GFP, luciferase, and inteins, and explores advancements in technologies like Cas proteins and base editors. We also examine reassembly modules and their applications in diverse fields, from gene regulation to therapeutic innovation. This review offers a comprehensive perspective on the recent evolution of split systems in biological research.

Mass spectrometry and split luciferase complementation assays reveal the MecA protein interactome of Streptococcus mutans

MecA is a highly conserved adaptor protein encoded by prokaryotes from the Bacillota phylum. MecA mutants exhibit similar pleiotropic defects in a variety of organisms, although most of these phenotypes currently lack a mechanistic basis. MecA mediates ClpCP-dependent proteolysis of its substrates, but only several such substrates have been reported in the literature and there are suggestions that proteolysis-independent regulatory mechanisms may also exist. Here, we provide the first comprehensive characterization of the MecA interactome and further assess its regulatory role in Clp-dependent proteolysis. Untargeted coimmunoprecipitation assays coupled with mass spectrometry revealed that the MecA ortholog from the oral pathobiont Streptococcus mutans likely serves as a major protein interaction network hub by potentially complexing with >100 distinct protein substrates, most of which function in highly conserved metabolic pathways. The interactome results were independently verified using a newly developed prokaryotic split luciferase complementation assay (SLCA) to detect MecA protein-protein interactions in vivo. In addition, we further develop a new application of SLCA to support in vivo measurements of MecA relative protein binding affinities. SLCA results were independently verified using targeted coimmunoprecipitation assays, suggesting the general utility of this approach for prokaryotic protein-protein interaction studies. Our results indicate that MecA indeed regulates its interactome through both Clp-dependent proteolysis as well as through an as-yet undefined proteolysis-independent mechanism that may affect more than half of its protein interactome. This suggests a significant aspect of the MecA regulatory function still has yet to be discovered.IMPORTANCEDespite multiple decades of study, the regulatory mechanism and function of MecA have remained largely a mystery. The current study provides the first detailed roadmap to investigate these functions in other medically significant bacteria. Furthermore, this study developed new genetic approaches to assay prokaryotic protein-protein interactions via the split luciferase complementation assay (SLCA). SLCA technology is commonly employed in eukaryotic genetic research but has not yet been established for studies of bacterial protein-protein interactions. The SLCA protein binding affinity assay described here is a new technological advance exclusive to the current study and has not been reported elsewhere.

High-throughput split-protein profiling by combining transposon mutagenesis and regulated protein-protein interactions with deep sequencing

Splitting a protein at a position may lead to self- or assisted-complementary fragments depending on whether two resulting fragments can reconstitute to maintain the native function spontaneously or require assistance from two interacting molecules. Assisted complementary fragments with high contrast are an important tool for probing biological interactions. However, only a small number of assisted-complementary split-variants have been identified due to manual, labour-intensive optimization of a candidate gene. Here, we introduce a technique for high-throughput split-protein profiling (HiTS) that allows fast identification of self- and assisted complementary positions by transposon mutagenesis, a rapamycin-regulated FRB-FKBP protein interaction pair, and deep sequencing. We test this technique by profiling three antibiotic-resistant genes (fosfomycin-resistant gene, fosA3, erythromycin-resistant gene, ermB, and chloramphenicol-resistant gene, catI). Self- and assisted complementary fragments discovered by the high-throughput technique were subsequently confirmed by low-throughput testing of individual split positions. Thus, the HiTS technique provides a quicker alternative for discovering the proteins with suitable self- and assisted-complementary split positions when combining with a readout such as fluorescence, bioluminescence, cell survival, gene transcription or genome editing.

Applications of Protein Fragment Complementation Assays for Analyzing Biomolecular Interactions and Biochemical Networks in Living Cells

Protein-protein interactions (PPIs) are indispensable for the dynamic assembly of multiprotein complexes that are central players of nearly all of the intracellular biological processes, such as signaling pathways, metabolic pathways, formation of intracellular organelles, establishment of cytoplasmic skeletons, etc. Numerous approaches have been invented to study PPIs both in vivo and in vitro, including the protein-fragment complementation assay (PCA), which is a widely applied technology to study PPIs and biomolecular interactions. PCA is a technology based on the expression of the bait and prey proteins in fusion with two complementary reporter protein fragments, respectively, that will reassemble when in close proximity. The reporter protein can be the enzymes or fluorescent proteins. Recovery of the enzymatic activity or fluorescent signal can be the indicator of PPI between the bait and prey proteins. Significant effort has been invested in developing many derivatives of PCA, along with various applications, in order to address specific questions. Therefore, a prompt review of these applications is important. In this review, we will categorize these applications according to the scenarios that the PCAs were applied and expect to provide a reference guideline for the future selection of PCA methods in solving a specific problem.

Functional characterization of a subtilisin-like serine protease from Vibrio cholerae

Vibrio cholerae, the causative agent of the human diarrheal disease cholera, exports numerous enzymes that facilitate its adaptation to both intestinal and aquatic niches. These secreted enzymes can mediate nutrient acquisition, biofilm assembly, and V. cholerae interactions with its host. We recently identified a V. cholerae-secreted serine protease, IvaP, that is active in V. cholerae-infected rabbits and human choleric stool. IvaP alters the activity of several host and pathogen enzymes in the gut and, along with other secreted V. cholerae proteases, decreases binding of intelectin, an intestinal carbohydrate-binding protein, to V. cholerae in vivo IvaP bears homology to subtilisin-like enzymes, a large family of serine proteases primarily comprised of secreted endopeptidases. Following secretion, IvaP is cleaved at least three times to yield a truncated enzyme with serine hydrolase activity, yet little is known about the mechanism of extracellular maturation. Here, we show that IvaP maturation requires a series of sequential N- and C-terminal cleavage events congruent with the enzyme's mosaic protein domain structure. Using a catalytically inactive reporter protein, we determined that IvaP can be partially processed in trans, but intramolecular proteolysis is most likely required to generate the mature enzyme. Unlike many other subtilisin-like enzymes, the IvaP cleavage pattern is consistent with stepwise processing of the N-terminal propeptide, which could temporarily inhibit, and be cleaved by, the purified enzyme. Furthermore, IvaP was able to cleave purified intelectin, which inhibited intelectin binding to V. cholerae These results suggest that IvaP plays a role in modulating intelectin-V. cholerae interactions.

Network Analyses in Plant Pathogens

Even in the age of big data in Biology, studying the connections between the biological processes and the molecular mechanisms behind them is a challenging task. Systems biology arose as a transversal discipline between biology, chemistry, computer science, mathematics, and physics to facilitate the elucidation of such connections. A scenario, where the application of systems biology constitutes a very powerful tool, is the study of interactions between hosts and pathogens using network approaches. Interactions between pathogenic bacteria and their hosts, both in agricultural and human health contexts are of great interest to researchers worldwide. Large amounts of data have been generated in the last few years within this area of research. However, studies have been relatively limited to simple interactions. This has left great amounts of data that remain to be utilized. Here, we review the main techniques in network analysis and their complementary experimental assays used to investigate bacterial-plant interactions. Other host-pathogen interactions are presented in those cases where few or no examples of plant pathogens exist. Furthermore, we present key results that have been obtained with these techniques and how these can help in the design of new strategies to control bacterial pathogens. The review comprises metabolic simulation, protein-protein interactions, regulatory control of gene expression, host-pathogen modeling, and genome evolution in bacteria. The aim of this review is to offer scientists working on plant-pathogen interactions basic concepts around network biology, as well as an array of techniques that will be useful for a better and more complete interpretation of their data.

Interaction of the cyclic-di-GMP binding protein FimX and the Type 4 pilus assembly ATPase promotes pilus assembly

Type IVa pili (T4P) are bacterial surface structures that enable motility, adhesion, biofilm formation and virulence. T4P are assembled by nanomachines that span the bacterial cell envelope. Cycles of T4P assembly and retraction, powered by the ATPases PilB and PilT, allow bacteria to attach to and pull themselves along surfaces, so-called “twitching motility”. These opposing ATPase activities must be coordinated and T4P assembly limited to one pole for bacteria to show directional movement. How this occurs is still incompletely understood. Herein, we show that the c-di-GMP binding protein FimX, which is required for T4P assembly in Pseudomonas aeruginosa, localizes to the leading pole of twitching bacteria. Polar FimX localization requires both the presence of T4P assembly machine proteins and the assembly ATPase PilB. PilB itself loses its polar localization pattern when FimX is absent. We use two different approaches to confirm that FimX and PilB interact in vivo and in vitro, and further show that point mutant alleles of FimX that do not bind c-di-GMP also do not interact with PilB. Lastly, we demonstrate that FimX positively regulates T4P assembly and twitching motility by promoting the activity of the PilB ATPase, and not by stabilizing assembled pili or by preventing PilT-mediated retraction. Mutated alleles of FimX that no longer bind c-di-GMP do not allow rapid T4P assembly in these assays. We propose that by virtue of its high-affinity for c-di-GMP, FimX can promote T4P assembly when intracellular levels of this cyclic nucleotide are low. As P. aeruginosa PilB is not itself a high-affinity c-di-GMP receptor, unlike many other assembly ATPases, FimX may play a key role in coupling T4P mediated motility and adhesion to levels of this second messenger.

Type IV pili (T4P) are assembled on the surfaces of many bacterial pathogens and commensals through the action of specialized assembly machines whose components and structures are the subject of intense study. Repeated cycles of T4P assembly, attachment and retraction allow bacteria to move or “twitch” along surfaces, efficiently colonize and intoxicate host tissues, and elaborate multicellular structures such as biofilms. Assembly and retraction are powered by specific ATPases, PilB and PilT respectively, but the manner in which their activity is coordinated is still poorly understood. In this work, we provide evidence that a high-affinity c-di-GMP binding protein of Pseudomonas aeruginosa, FimX, interacts with the ATPase PilB and promotes PilB-dependent assembly of T4P. Live cell imaging of twitching bacteria shows that FimX localizes to the leading pole of motile P. aeruginosa and that its recruitment requires both components of the T4P assembly machine and the PilB ATPase. Our work highlights a novel regulatory strategy employed by P. aeruginosa to control assembly of this broadly conserved virulence factor.

Chemoproteomic profiling of host and pathogen enzymes active in cholera

Activity-based protein profiling (ABPP) is a chemoproteomic tool for detecting active enzymes in complex biological systems. We used ABPP to identify secreted bacterial and host serine hydrolases that are active in animals infected with the cholera pathogen Vibrio cholerae. Four V. cholerae proteases were consistently active in infected rabbits, and one, VC0157 (renamed IvaP), was also active in human choleric stool. Inactivation of IvaP influenced the activity of other secreted V. cholerae and rabbit enzymes in vivo, and genetic disruption of all four proteases increased the abundance of intelectin, an intestinal lectin, and its binding to V. cholerae in infected rabbits. Intelectin also bound to other enteric bacterial pathogens, suggesting that it may constitute a previously unrecognized mechanism of bacterial surveillance in the intestine that is inhibited by pathogen-secreted proteases. Our work demonstrates the power of activity-based proteomics to reveal host-pathogen enzymatic dialog in an animal model of infection.

RpoS and quorum sensing control expression and polar localization of Vibrio cholerae chemotaxis cluster III proteins in vitro and in vivo

The diarrheal pathogen Vibrio cholerae contains three gene clusters that encode chemotaxis-related proteins, but only cluster II appears to be required for chemotaxis. Here, we present the first characterization of V. cholerae's 'cluster III' chemotaxis system. We found that cluster III proteins assemble into foci at bacterial poles, like those formed by cluster II proteins, but the two systems assemble independently and do not colocalize. Cluster III proteins are expressed in vitro during stationary phase and in conjunction with growth arrest linked to carbon starvation. This expression, as well as expression in vivo in suckling rabbits, is dependent upon RpoS. V. cholerae's CAI-1 quorum sensing (QS) system is also required for cluster III expression in stationary phase and modulates its expression in vivo, but is not required for cluster III expression in response to carbon starvation. Surprisingly, even though the CAI-1 and AI-2 QS systems are thought to feed into the same signaling pathway, the AI-2 system inhibited cluster III gene expression, revealing that the outputs of the two QS systems are not always the same. The distinctions between genetic determinants of cluster III expression in vitro and in vivo highlight the distinctive nature of the in vivo environment.

Substrate-dependent activation of the Vibrio cholerae vexAB RND efflux system requires vexR

Vibrio cholerae encodes six resistance-nodulation-division (RND) efflux systems which function in antimicrobial resistance, virulence factor production, and intestinal colonization. Among the six RND efflux systems, VexAB exhibited broad substrate specificity and played a predominant role in intrinsic antimicrobial resistance. The VexAB system was encoded in an apparent three gene operon that included vexR; which encodes an uncharacterized TetR family regulator. In this work we examined the role of vexR in vexRAB expression. We found that VexR bound to the vexRAB promoter and vexR deletion resulted in decreased vexRAB expression and increased susceptibility to VexAB antimicrobial substrates. Substrate-dependent induction of vexRAB was dependent on vexR and episomal vexR expression provided a growth advantage in the presence of the VexAB substrate deoxycholate. The expression of vexRAB increased, in a vexR-dependent manner, in response to the loss of RND efflux activity. This suggested that VexAB may function to export intracellular metabolites. Support for this hypothesis was provided by data showing that vexRAB was upregulated in several metabolic mutants including tryptophan biosynthetic mutants that were predicted to accumulate indole. In addition, vexRAB was found to be upregulated in response to exogenous indole and to contribute to indole resistance. The collective results indicate that vexR is required for vexRAB expression in response to VexAB substrates and that the VexAB RND efflux system modulates the intracellular levels of metabolites that could otherwise accumulate to toxic levels.

Insights into Vibrio cholerae intestinal colonization from monitoring fluorescently labeled bacteria

Vibrio cholerae, the agent of cholera, is a motile non-invasive pathogen that colonizes the small intestine (SI). Most of our knowledge of the processes required for V. cholerae intestinal colonization is derived from enumeration of wt and mutant V. cholerae recovered from orogastrically infected infant mice. There is limited knowledge of the distribution of V. cholerae within the SI, particularly its localization along the villous axis, or of the bacterial and host factors that account for this distribution. Here, using confocal and intravital two-photon microscopy to monitor the localization of fluorescently tagged V. cholerae strains, we uncovered unexpected and previously unrecognized features of V. cholerae intestinal colonization. Direct visualization of the pathogen within the intestine revealed that the majority of V. cholerae microcolonies attached to the intestinal epithelium arise from single cells, and that there are notable regiospecific aspects to V. cholerae localization and factors required for colonization. In the proximal SI, V. cholerae reside exclusively within the developing intestinal crypts, but they are not restricted to the crypts in the more distal SI. Unexpectedly, V. cholerae motility proved to be a regiospecific colonization factor that is critical for colonization of the proximal, but not the distal, SI. Furthermore, neither motility nor chemotaxis were required for proper V. cholerae distribution along the villous axis or in crypts, suggesting that yet undefined processes enable the pathogen to find its niches outside the intestinal lumen. Finally, our observations suggest that host mucins are a key factor limiting V. cholerae intestinal colonization, particularly in the proximal SI where there appears to be a more abundant mucus layer. Collectively, our findings demonstrate the potent capacity of direct pathogen visualization during infection to deepen our understanding of host pathogen interactions.

Bioluminescence resonance energy transfer system for measuring dynamic protein-protein interactions in bacteria

Protein-protein interactions are important for virtually every biological process, and a number of elegant approaches have been designed to detect and evaluate such interactions. However, few of these methods allow the detection of dynamic and real-time protein-protein interactions in bacteria. Here we describe a bioluminescence resonance energy transfer (BRET) system based on the bacterial luciferase LuxAB. We found that enhanced yellow fluorescent protein (eYFP) accepts the emission from LuxAB and emits yellow fluorescence. Importantly, BRET occurred when LuxAB and eYFP were fused, respectively, to the interacting protein pair FlgM and FliA. Furthermore, we observed sirolimus (i.e., rapamycin)-inducible interactions between FRB and FKBP12 and a dose-dependent abolishment of such interactions by FK506, the ligand of FKBP12. Using this system, we showed that osmotic stress or low pH efficiently induced multimerization of the regulatory protein OmpR and that the multimerization induced by low pH can be reversed by a neutralizing agent, further indicating the usefulness of this system in the measurement of dynamic interactions. This method can be adapted to analyze dynamic protein-protein interactions and the importance of such interactions in bacterial processes such as development and pathogenicity.

Real-time measurement of protein-protein interactions in prokaryotes is highly desirable for determining the roles of protein complex in the development or virulence of bacteria, but methods that allow such measurement are not available. Here we describe the development of a bioluminescence resonance energy transfer (BRET) technology that meets this need. The use of endogenous excitation light in this strategy circumvents the requirement for the sophisticated instrument demanded by standard fluorescence resonance energy transfer (FRET). Furthermore, because the LuxAB substrate decanal is membrane permeable, the assay can be performed without lysing the bacterial cells, thus allowing the detection of protein-protein interactions in live bacterial cells. This BRET system added another useful tool to address important questions in microbiological studies.