Allosteric activation of cell wall synthesis during bacterial growth

Pbp4 provides transpeptidase activity to the FtsW-PbpB peptidoglycan synthase to drive cephalosporin resistance in Enterococcus faecalis

Enterococci exhibit intrinsic resistance to cephalosporins, mediated in part by the class B penicillin-binding protein (bPBP) Pbp4 that exhibits low reactivity toward cephalosporins and thus can continue crosslinking peptidoglycan despite exposure to cephalosporins. bPBPs partner with cognate SEDS (shape, elongation, division, and sporulation) glycosyltransferases to form the core catalytic complex of peptidoglycan synthases that synthesize peptidoglycan at discrete cellular locations, although the SEDS partner for Pbp4 is unknown. SEDS-bPBP peptidoglycan synthases of enterococci have not been studied, but some SEDS-bPBP pairs can be predicted based on sequence similarity. For example, FtsW (SEDS)-PbpB (bPBP) is predicted to form the catalytic core of the peptidoglycan synthase that functions at the division septum (the divisome). However, PbpB is readily inactivated by cephalosporins, raising the question-how could the FtsW-PbpB synthase continue functioning to enable growth in the presence of cephalosporins? In this work, we report that the FtsW-PbpB peptidoglycan synthase is required for cephalosporin resistance of Enterococcus faecalis, despite the fact that PbpB is inactivated by cephalosporins. Moreover, Pbp4 associates with the FtsW-PbpB synthase and the TPase activity of Pbp4 is required to enable growth in the presence of cephalosporins in an FtsW-PbpB-synthase-dependent manner. Overall, our results implicate a model in which Pbp4 directly interacts with the FtsW-PbpB peptidoglycan synthase to provide TPase activity during cephalosporin treatment, thereby maintaining the divisome SEDS-bPBP peptidoglycan synthase in a functional state competent to synthesize crosslinked peptidoglycan. These results suggest that two bPBPs coordinate within the FtsW-PbpB peptidoglycan synthase to drive cephalosporin resistance in E. faecalis.

Elongasome core proteins and class A PBP1a display zonal, processive movement at the midcell of Streptococcus pneumoniae

Ovoid-shaped bacteria, such as Streptococcus pneumoniae (pneumococcus), have two spatially separated peptidoglycan (PG) synthase nanomachines that locate zonally to the midcell of dividing cells. The septal PG synthase bPBP2x:FtsW closes the septum of dividing pneumococcal cells, whereas the elongasome located on the outer edge of the septal annulus synthesizes peripheral PG outward. We showed previously by sm-TIRFm that the septal PG synthase moves circumferentially at midcell, driven by PG synthesis and not by FtsZ treadmilling. The pneumococcal elongasome consists of the PG synthase bPBP2b:RodA, regulators MreC, MreD, and RodZ, but not MreB, and genetically associated proteins Class A aPBP1a and muramidase MpgA. Given its zonal location separate from FtsZ, it was of considerable interest to determine the dynamics of proteins in the pneumococcal elongasome. We found that bPBP2b, RodA, and MreC move circumferentially with the same velocities and durations at midcell, driven by PG synthesis. However, outside of the midcell zone, the majority of these elongasome proteins move diffusively over the entire surface of cells. Depletion of MreC resulted in loss of circumferential movement of bPBP2b, and bPBP2b and RodA require each other for localization and circumferential movement. Notably, a fraction of aPBP1a molecules also moved circumferentially at midcell with velocities similar to those of components of the core elongasome, but for shorter durations. Other aPBP1a molecules were static at midcell or diffusing over cell bodies. Last, MpgA displayed nonprocessive, subdiffusive motion that was largely confined to the midcell region and less frequently detected over the cell body.

Stay on track - revelations of bacterial cell wall synthesis enzymes and things that go by single-molecule imaging

In this review, we explore the regulation of septal peptidoglycan (sPG) synthesis in bacterial cell division, a critical process for cell viability and proper morphology. Recent single-molecule imaging studies have revealed the processive movement of the FtsW:bPBP synthase complex along the septum, shedding light on the spatiotemporal dynamics of sPG synthases and their regulators. In diderm bacteria (E. coli and C. crescentus), the movement occurs at two distinct speeds, reflecting active synthesis or inactivity driven by FtsZ-treadmilling. In monoderm bacteria (B. subtilis, S. pneumoniae, and S. aureus), however, these enzymes exhibit only the active sPG-track-coupled processive movement. By comparing the dynamics of sPG synthases in these organisms and that of class-A penicillin-binding proteins in vivo and in vitro, we propose a unifying model for septal cell wall synthesis regulation across species, highlighting the roles of the sPG- and Z-tracks in orchestrating a robust bacterial cell wall constriction process.

Structural and kinetic analysis of the monofunctional Staphylococcus aureus PBP1

Staphylococcus aureus, an ESKAPE pathogen, is a major clinical concern due to its pathogenicity and manifold antimicrobial resistance mechanisms. The commonly used β-lactam antibiotics target bacterial penicillin-binding proteins (PBPs) and inhibit crosslinking of peptidoglycan strands that comprise the bacterial cell wall mesh, initiating a cascade of effects leading to bacterial cell death. S. aureus PBP1 is involved in synthesis of the bacterial cell wall during division and its presence is essential for survival of both antibiotic susceptible and resistant S. aureus strains. Here, we present X-ray crystallographic data for S. aureus PBP1 in its apo form as well as acyl-enzyme structures with distinct classes of β-lactam antibiotics representing the penicillins, carbapenems, and cephalosporins, respectively: oxacillin, ertapenem and cephalexin. Our structural data suggest that the PBP1 active site is readily accessible for substrate, with little conformational change in key structural elements required for its covalent acylation of β-lactam inhibitors. Stopped-flow kinetic analysis and gel-based competition assays support the structural observations, with even the weakest performing β-lactams still having comparatively high acylation rates and affinities for PBP1. Our structural and kinetic analysis sheds insight into the ligand-PBP interactions that drive antibiotic efficacy against these historically useful antimicrobial targets and expands on current knowledge for future drug design and treatment of S. aureus infections.

Chemical biology tools to probe bacterial glycans

Bacterial cells are covered by a complex carbohydrate coat of armor that allows bacteria to thrive in a range of environments. As a testament to the importance of bacterial glycans, effective and heavily utilized antibiotics including penicillin and vancomycin target and disrupt the bacterial glycocalyx. Despite their importance, the study of bacterial glycans lags far behind their eukaryotic counterparts. Bacterial cells use a large palette of monosaccharides to craft glycans, leading to molecules that are significantly more complex than eukaryotic glycans and that are refractory to study. Fortunately, chemical tools designed to probe bacterial glycans have yielded insights into these molecules, their structures, their biosynthesis, and their functions.

Elongasome core proteins and class A PBP1a display zonal, processive movement at the midcell of Streptococcus pneumoniae

Ovoid-shaped bacteria, such as Streptococcus pneumoniae (pneumococcus), have two spatially separated peptidoglycan (PG) synthase nanomachines that locate zonally to the midcell of dividing cells. The septal PG synthase bPBP2x:FtsW closes the septum of dividing pneumococcal cells, whereas the elongasome located on the outer edge of the septal annulus synthesizes peripheral PG outward. We showed previously by sm-TIRFm that the septal PG synthase moves circumferentially at midcell, driven by PG synthesis and not by FtsZ treadmilling. The pneumococcal elongasome consists of the PG synthase bPBP2b:RodA, regulators MreC, MreD, and RodZ, but not MreB, and genetically associated proteins Class A aPBP1a and muramidase MpgA. Given its zonal location separate from FtsZ, it was of considerable interest to determine the dynamics of proteins in the pneumococcal elongasome. We found that bPBP2b, RodA, and MreC move circumferentially with the same velocities and durations at midcell, driven by PG synthesis. However, outside of the midcell zone, the majority of these elongasome proteins move diffusively over the entire surface of cells. Depletion of MreC resulted in loss of circumferential movement of bPBP2b, and bPBP2b and RodA require each other for localization and circumferential movement. Notably, a fraction of aPBP1a molecules also moved circumferentially at midcell with velocities similar to those of components of the core elongasome, but for shorter durations. Other aPBP1a molecules were static at midcell or diffusing over cell bodies. Last, MpgA displayed non-processive, subdiffusive motion that was largely confined to the midcell region and less frequently detected over the cell body.

Structural insights into the FtsEX-EnvC complex regulation on septal peptidoglycan hydrolysis in Vibrio cholerae

During bacterial cell division, hydrolysis of septal peptidoglycan (sPG) is crucial for cell separation. This sPG hydrolysis is performed by the enzyme amidases whose activity is regulated by the integral membrane protein complex FtsEX-EnvC. FtsEX is an ATP-binding cassette transporter, and EnvC is a long coiled-coil protein that interacts with and activates the amidases. The molecular mechanism by which the FtsEX-EnvC complex activates amidases remains largely unclear. We present the cryo-electron microscopy structure of the FtsEX-EnvC complex from the pathogenic bacteria V. cholerae (FtsEX-EnvCVC). FtsEX-EnvCVC in the presence of ADP adopts a distinct conformation where EnvC is "horizontally extended" rather than "vertically extended". Subsequent structural studies suggest that EnvC can swing between these conformations in space in a nucleotide-dependent manner. Our structural analysis and functional studies suggest that FtsEX-EnvCVC employs spatial control of EnvC for amidase activation, providing mechanistic insights into the FtsEX-EnvC regulation on septal peptidoglycan hydrolysis.

Evidence of two differentially regulated elongasomes in Salmonella

Cell shape is genetically inherited by all forms of life. Some unicellular microbes increase niche adaptation altering shape whereas most show invariant morphology. A universal system of peptidoglycan synthases guided by cytoskeletal scaffolds defines bacterial shape. In rod-shaped bacteria, this system consists of two supramolecular complexes, the elongasome and divisome, which insert cell wall material along major and minor axes. Microbes with invariant shape are thought to use a single morphogenetic system irrespective of the occupied niche. Here, we provide evidence for two elongasomes that generate (rod) shape in the same bacterium. This phenomenon was unveiled in Salmonella, a pathogen that switches between extra- and intracellular lifestyles. The two elongasomes can be purified independently, respond to different environmental cues, and are directed by distinct peptidoglycan synthases: the canonical PBP2 and the pathogen-specific homologue PBP2SAL. The PBP2-elongasome responds to neutral pH whereas that directed by PBP2SAL assembles in acidic conditions. Moreover, the PBP2SAL-elongasome moves at a lower speed. Besides Salmonella, other human, animal, and plant pathogens encode alternative PBPs with predicted morphogenetic functions. Therefore, contrasting the view of morphological plasticity facilitating niche adaptation, some pathogens may have acquired alternative systems to preserve their shape in the host.

Structural basis of peptidoglycan synthesis by E. coli RodA-PBP2 complex

Peptidoglycan (PG) is an essential structural component of the bacterial cell wall that is synthetized during cell division and elongation. PG forms an extracellular polymer crucial for cellular viability, the synthesis of which is the target of many antibiotics. PG assembly requires a glycosyltransferase (GT) to generate a glycan polymer using a Lipid II substrate, which is then crosslinked to the existing PG via a transpeptidase (TP) reaction. A Shape, Elongation, Division and Sporulation (SEDS) GT enzyme and a Class B Penicillin Binding Protein (PBP) form the core of the multi-protein complex required for PG assembly. Here we used single particle cryo-electron microscopy to determine the structure of a cell elongation-specific E. coli RodA-PBP2 complex. We combine this information with biochemical, genetic, spectroscopic, and computational analyses to identify the Lipid II binding sites and propose a mechanism for Lipid II polymerization. Our data suggest a hypothesis for the movement of the glycan strand from the Lipid II polymerization site of RodA towards the TP site of PBP2, functionally linking these two central enzymatic activities required for cell wall peptidoglycan biosynthesis.