MagC is a NplC/P60-like member of the α-2-macroglobulin Mag complex of Pseudomonas aeruginosa that interacts with peptidoglycan.

Bacterial α-2 macroglobulins (A2Ms) structurally resemble the large spectrum protease inhibitors of the eukaryotic immune system. In Pseudomonas aeruginosa, MagD acts as an A2M and is expressed within a six-gene operon encoding the MagA-F proteins. In this work, we employ isothermal calorimetry (ITC), analytical ultracentrifugation (AUC), and X-ray crystallography to investigate the function of MagC and show that MagC associates with the macroglobulin complex and with the peptidoglycan (PG). However, the catalytic residues of MagC display an inactive conformation that could suggest that it binds to PG but does not degrade it. We hypothesize that MagC could serve as an anchor between the MagD macroglobulin and the PG and could provide stabilization and/or regulation for the entire complex.

Architecture and regulation of an enterobacterial cellulose secretion system.

Many free-living and pathogenic enterobacteria secrete biofilm-promoting cellulose using a multicomponent, envelope-embedded Bcs secretion system under the control of intracellular second messenger c-di-GMP. The molecular understanding of system assembly and cellulose secretion has been largely limited to the crystallographic studies of a distantly homologous BcsAB synthase tandem and a low-resolution reconstruction of an assembled macrocomplex that encompasses most of the inner membrane and cytosolic subunits and features an atypical layered architecture. Here, we present cryo-EM structures of the assembled Bcs macrocomplex, as well as multiple crystallographic snapshots of regulatory Bcs subcomplexes. The structural and functional data uncover the mechanism of asymmetric secretion system assembly and periplasmic crown polymerization and reveal unexpected subunit stoichiometry, multisite c-di-GMP recognition, and ATP-dependent regulation.

Structure and Multitasking of the c-di-GMP-Sensing Cellulose Secretion Regulator BcsE.

Most bacteria respond to surfaces by biogenesis of intracellular c-di-GMP, which inhibits motility and induces secretion of biofilm-promoting adherence factors. Bacterial cellulose is a widespread biofilm component whose secretion in Gram-negative species requires an inner membrane, c-di-GMP-dependent synthase tandem (BcsAB), an outer membrane porin (BcsC), and various accessory subunits that regulate synthase assembly and function as well as the exopolysaccharide's chemical composition and mechanical properties. We recently showed that in Escherichia coli, most Bcs proteins form a megadalton-sized secretory nanomachine, but the role and structure of individual regulatory components remained enigmatic. Here, we demonstrate that essential-for-secretion BcsR and BcsQ regulate each other's folding and stability and are recruited to the inner membrane via c-di-GMP-sensing BcsE and its intraoperon partner BcsF. Crystallographic and solution-based data show that BcsE's predicted GIL domain is a degenerate receiver-GGDEF domain tandem (BcsEREC*-GGDEF*), where the divergent diguanylate cyclase module binds both dimeric c-di-GMP and BcsQ through mutually independent interfaces. In addition, we reveal that a third N-terminal domain (BcsENTD) determines the protein's homooligomerization and targeting of BcsERQ to the membrane as well as previously unreported interactions with transcription antitermination complex components. Together, the data suggest that BcsE acts on multiple levels to fine-tune bacterial cellulose secretion, from the early stages of secretion system assembly to the maintenance of a membrane-proximal pool of dimeric c-di-GMP for processive synthase activation.IMPORTANCE Bacterial cellulose is a widespread biofilm component that can modulate microbial fitness and virulence both in the environment and infected hosts. Whereas its secretion generally involves an inner membrane c-di-GMP-dependent synthase tandem (BcsAB) across the bacterial domain of life, enterobacteria feature sophisticated Escherichia coli-like Bcs secretion systems, where multiple additional subunits are either required for secretion or contribute to the maximal production of the polysaccharide in vivo. Here, we demonstrate that essential-for-secretion BcsR and BcsQ regulate each other's folding and stability and are recruited to the inner membrane via c-di-GMP-sensing BcsE and its intraoperon partner, BcsF. Crystallographic and functional data reveal that BcsE features unexpected domain architecture and likely acts on multiple levels to fine-tune bacterial cellulose production, from the early stages of secretion system assembly to the maintenence of a membrane-proximal pool of dimeric c-di-GMP for processive synthase activation.

Assembly of an atypical α-macroglobulin complex from Pseudomonas aeruginosa.

Alpha-2-macroglobulins (A2Ms) are large spectrum protease inhibitors that are major components of the eukaryotic immune system. Pathogenic and colonizing bacteria, such as the opportunistic pathogen Pseudomonas aeruginosa, also carry structural homologs of eukaryotic A2Ms. Two types of bacterial A2Ms have been identified: Type I, much like the eukaryotic form, displays a conserved thioester that is essential for protease targeting, and Type II, which lacks the thioester and to date has been poorly studied despite its ubiquitous presence in Gram-negatives. Here we show that MagD, the Type II A2M from P. aeruginosa that is expressed within the six-gene mag operon, specifically traps a target protease despite the absence of the thioester motif, comforting its role in protease inhibition. In addition, analytical ultracentrifugation and small angle scattering show that MagD forms higher order complexes with proteins expressed in the same operon (MagA, MagB, and MagF), with MagB playing the key stabilization role. A P. aeruginosa strain lacking magB cannot stably maintain MagD in the bacterial periplasm, engendering complex disruption. This suggests a regulated mechanism of Mag complex formation and stabilization that is potentially common to numerous Gram-negative organisms, and that plays a role in periplasm protection from proteases during infection or colonization.

Structural Basis of Lipid Targeting and Destruction by the Type V Secretion System of Pseudomonas aeruginosa.

The type V secretion system is a macromolecular machine employed by a number of bacteria to secrete virulence factors into the environment. The human pathogen Pseudomonas aeruginosa employs the newly described type Vd secretion system to secrete a soluble variant of PlpD, a lipase of the patatin-like family synthesized as a single macromolecule that also carries a polypeptide transport-associated domain and a 16-stranded ß-barrel. Here we report the crystal structure of the secreted form of PlpD in its biologically active state. PlpD displays a classical lipase α/ß hydrolase fold with a catalytic site located within a highly hydrophobic channel that entraps a lipidic molecule. The active site is covered by a flexible lid, as in other lipases, indicating that this region in PlpD must modify its conformation in order for catalysis at the water-lipid interface to occur. PlpD displays phospholipase A1 activity and is able to recognize a number of phosphatidylinositols and other phosphatidyl analogs. PlpD is the first example of an active phospholipase secreted through the type V secretion system, for which there are more than 200 homologs, revealing details of the lipid destruction arsenal expressed by P. aeruginosa in order to establish infection.

NprR, a moonlighting quorum sensor shifting from a phosphatase activity to a transcriptional activator.

Regulation of biological functions requires factors (proteins, peptides or chemicals) able to sense and translate environmental conditions or any circumstances in order to modulate the transcription of a gene, the stability of a transcript or the activity of a protein. Quorum sensing is a regulation mechanism connecting cell density to the physiological state of a single cell. In bacteria, quorum sensing coordinates virulence, cell fate and commitment to sporulation and other adaptation properties. The critical role of such regulatory systems was demonstrated in pathogenicity and adaptation of bacteria from the Bacillus cereus group (i.e. B. cereus and Bacillus thuringiensis). Furthermore, using insects as a model of infection, it was shown that sequential activation of several quorum sensing systems allowed bacteria to switch from a virulence state to a necrotrophic lifestyle, allowing their survival in the host cadaver, and ultimately to the commitment into sporulation. The chronological development of these physiological states is directed by quorum sensors forming the RNPP family. Among them, NprR combines two distinct functions connecting sporulation to necrotrophism in B. thuringiensis. In the absence of its cognate signaling peptide (NprX), NprR negatively controls sporulation by acting as a phosphatase. In the presence of NprX, it acts as a transcription factor regulating a set of genes involved in the survival of the bacteria in the insect cadaver.

The structure of the Slrp-Trx1 complex sheds light on the autoinhibition mechanism of the type III secretion system effectors of the NEL family.

Salmonella infections are a leading cause of bacterial foodborne illness in the U.S.A. and the European Union Antimicrobial therapy is often administered to treat the infection, but increasingly isolates are being detected that demonstrate resistance to multiple antibiotics. Salmonella enterica contains two virulence-related T3SS (type III secretion systems): one promotes invasion of the intestine and the other one mediates systemic disease. Both of them secrete the SlrP protein acting as E3 ubiquitin ligase in human host cells where it targets Trx1 (thioredoxin-1). SlrP belongs to the NEL family of bacterial E3 ubiquitin ligases that have been observed in two distinct autoinhibitory conformations. We solved the 3D structure of the SlrP-Trx1 complex and determined the Trx1 ubiquitination site. The description of the substrate-binding mode sheds light on the first step of the activation mechanism of SlrP. Comparison with the available structural data of other NEL effectors allowed us to gain new insights into their autoinhibitory mechanism. We propose a molecular mechanism for the regulation of SlrP in which structural constraints sequestrating the NEL domain would be sequentially released. This work thus constitutes a new milestone in the understanding of how these T3SS effectors influence pathogen virulence. It also provides the fundamental basis for future development of new antimicrobials.

Phosphorylation of Bacillus subtilis gene regulator AbrB modulates its DNA-binding properties.

AbrB is a global gene regulator involved in transition phase phenomena in Bacillus subtilis. It participates in a complex regulatory network governing the expression of stationary-phase functions. AbrB was previously found to be phosphorylated on serine 86 located close to its C-terminal oligomerization domain. Here we report that AbrB can be phosphorylated by three B. subtilis serine/threonine kinases expressed during the transition and stationary phase: PrkC, PrkD and YabT. Our in vitro findings suggest that AbrB phosphorylation impedes its DNA binding and abolishes binding cooperativity. In vivo we established that a phospho-mimetic mutation abrB S86D leads to a significant loss of AbrB control over several key target functions: exoprotease production, competence development and sporulation. A wider transcriptome analysis of abrB S86D and S86A mutant strains revealed deregulation of a large number of target genes. We therefore propose that AbrB phosphorylation serves as an additional input for fine-tuning the activity of this ambiactive gene regulator.

Peptide-binding dependent conformational changes regulate the transcriptional activity of the quorum-sensor NprR.

The transcriptional regulator NprR controls the expression of genes essential for the adaptative response of Bacillus cereus. NprR belongs to the RNPP family of directly regulated quorum sensors from Gram-positive bacteria. It is activated by the re-imported signaling peptide NprX. To elucidate the activation mechanism of this quorum-sensing system, we analyzed the conformation changes induced on binding of NprX. We solved the crystal structure of the NprR/NprX binary complex and characterized the apo form of NprR in solution. We demonstrated that apo NprR is a dimer that switches to a tetramer in the presence of NprX. Mutagenesis, and functional analysis allowed us to identify the protein and peptide residues directly involved in the NprR activation process. Based on the comparison with the Rap proteins, we propose a model for the peptide-induced conformational change allowing the apo dimer to switch to an active tetramer specifically recognizing target DNA sequences.

A cell-cell communication system regulates protease production during sporulation in bacteria of the Bacillus cereus group.

In sporulating Bacillus, major processes like virulence gene expression and sporulation are regulated by communication systems involving signalling peptides and regulators of the RNPP family. We investigated the role of one such regulator, NprR, in bacteria of the Bacillus cereus group. We show that NprR is a transcriptional regulator whose activity depends on the NprX signalling peptide. In association with NprX, NprR activates the transcription of an extracellular protease gene (nprA) during the first stage of the sporulation process. The transcription start site of the nprA gene has been identified and the minimal region necessary for full activation has been characterized by promoter mutagenesis. We demonstrate that the NprX peptide is secreted, processed and then reimported within the bacterial cell. Once inside the cell, the mature form of NprX, presumably the SKPDIVG heptapeptide, directly binds to NprR allowing nprA transcription. Alignment of available NprR sequences from different species of the B. cereus group defines seven NprR clusters associated with seven NprX heptapeptide classes. This cell-cell communication system was found to be strain-specific with a possible cross-talk between some pherotypes. The phylogenic relationship between NprR and NprX suggests a coevolution of the regulatory protein and its signalling peptide.