Localisation of DivIVA by targeting to negatively curved membranes

Cell wall synthesizing complexes in Mycobacteriales

Members of the order Mycobacteriales are distinguished by a characteristic diderm cell envelope, setting them apart from other Actinobacteria species. In addition to the conventional peptidoglycan cell wall, these organisms feature an extra polysaccharide polymer composed of arabinose and galactose, termed arabinogalactan. The nonreducing ends of arabinose are covalently linked to mycolic acids (MAs), forming the immobile inner leaflet of the highly hydrophobic MA membrane. The contiguous outer leaflet of the MA membrane comprises trehalose mycolates and various lipid species. Similar to all actinobacteria, Mycobacteriales exhibit apical growth, facilitated by a polar localized elongasome complex. A septal cell envelope synthesis machinery, the divisome, builds instead of the cell wall structures during cytokinesis. In recent years, a growing body of knowledge has emerged regarding the cell wall synthesizing complexes of Mycobacteriales., focusing particularly on three model species: Corynebacterium glutamicum, Mycobacterium smegmatis, and Mycobacterium tuberculosis.

Dynamic structure of E. coli cytoplasm: supramolecular complexes and cell aging impact spatial distribution and mobility of proteins

Protein diffusion is a critical factor governing the functioning and organization of a cell's cytoplasm. In this study, we investigate the influence of (poly)ribosome distribution, cell aging, protein aggregation, and biomolecular condensate formation on protein mobility within the E. coli cytoplasm. We employ nanoscale single-molecule displacement mapping (SMdM) to determine the spatial distribution of the proteins and to meticulously track their diffusion. We show that the distribution of polysomes does not impact the lateral diffusion coefficients of proteins. However, the degradation of mRNA induced by rifampicin treatment leads to an increase in protein mobility within the cytoplasm. Additionally, we establish a significant correlation between cell aging, the asymmetric localization of protein aggregates and reduced diffusion coefficients at the cell poles. Notably, we observe variations in the hindrance of diffusion at the poles and the central nucleoid region for small and large proteins, and we reveal differences between the old and new pole of the cell. Collectively, our research highlights cellular processes and mechanisms responsible for spatially organizing the bacterial cytoplasm into domains with different structural features and apparent viscosity.

Distinct dynamics and proximity networks of hub proteins at the prey-invading cell pole in a predatory bacterium

In bacteria, cell poles function as subcellular compartments where proteins localize during specific lifecycle stages, orchestrated by polar "hub" proteins. Whereas most described bacteria inherit an "old" pole from the mother cell and a "new" pole from cell division, generating cell asymmetry at birth, non-binary division poses challenges for establishing cell polarity, particularly for daughter cells inheriting only new poles. We investigated polarity dynamics in the obligate predatory bacterium Bdellovibrio bacteriovorus, proliferating through filamentous growth followed by non-binary division within prey bacteria. Monitoring the subcellular localization of two proteins known as polar hubs in other species, RomR and DivIVA, revealed RomR as an early polarity marker in B. bacteriovorus. RomR already marks the future anterior poles of the progeny during the predator's growth phase, during a precise period closely following the onset of divisome assembly and the end of chromosome segregation. In contrast to RomR's stable unipolar localization in the progeny, DivIVA exhibits a dynamic pole-to-pole localization. This behavior changes shortly before the division of the elongated predator cell, where DivIVA accumulates at all septa and both poles. In vivo protein interaction networks for DivIVA and RomR, mapped through endogenous miniTurbo-based proximity labeling, further underscore their distinct roles in cell polarization and reinforce the importance of the anterior "invasive" cell pole in prey-predator interactions. Our work also emphasizes the precise spatiotemporal order of cellular processes underlying B. bacteriovorus proliferation, offering insights into the subcellular organization of bacteria with filamentous growth and non-binary division.IMPORTANCEIn bacteria, cell poles are crucial areas where "hub" proteins orchestrate lifecycle events through interactions with multiple partners at specific times. While most bacteria exhibit one "old" and one "new" pole, inherited from the previous division event, setting polar identity poses challenges in bacteria with non-binary division. This study explores polar proteins in the predatory bacterium Bdellovibrio bacteriovorus, which undergoes filamentous growth followed by non-binary division inside another bacterium. Our research reveals distinct localization dynamics of the polar proteins RomR and DivIVA, highlighting RomR as an early "hub" marking polar identity in the filamentous mother cell. Using miniTurbo-based proximity labeling, we uncovered their unique protein networks. Overall, our work provides new insights into the cell polarity in non-binary dividing bacteria.

Cell division machinery drives cell-specific gene activation during differentiation in Bacillus subtilis

When faced with starvation, the bacterium Bacillus subtilis transforms itself into a dormant cell type called a "spore". Sporulation initiates with an asymmetric division event, which requires the relocation of the core divisome components FtsA and FtsZ, after which the sigma factor σF is exclusively activated in the smaller daughter cell. Compartment-specific activation of σF requires the SpoIIE phosphatase, which displays a biased localization on one side of the asymmetric division septum and associates with the structural protein DivIVA, but the mechanism by which this preferential localization is achieved is unclear. Here, we isolated a variant of DivIVA that indiscriminately activates σF in both daughter cells due to promiscuous localization of SpoIIE, which was corrected by overproduction of FtsA and FtsZ. We propose that the core components of the redeployed cell division machinery drive the asymmetric localization of DivIVA and SpoIIE to trigger the initiation of the sporulation program.

Identification of membrane curvature sensing motifs essential for VPS37A phagophore recruitment and autophagosome closure

VPS37A, an ESCRT-I complex component, is required for recruiting a subset of ESCRT proteins to the phagophore for autophagosome closure. However, the mechanism by which VPS37A is targeted to the phagophore remains obscure. Here, we demonstrate that the VPS37A N-terminal domain exhibits selective interactions with highly curved membranes, mediated by two membrane-interacting motifs within the disordered regions surrounding its Ubiquitin E2 variant-like (UEVL) domain. Site-directed mutations of residues in these motifs disrupt ESCRT-I localization to the phagophore and result in defective phagophore closure and compromised autophagic flux in vivo, highlighting their essential role during autophagy. In conjunction with the UEVL domain, we postulate that these motifs guide a functional assembly of the ESCRT machinery at the highly curved tip of the phagophore for autophagosome closure. These results advance the notion that the distinctive membrane architecture of the cup-shaped phagophore spatially regulates autophagosome biogenesis.

Clostridioides difficile Sporulation

Some members of the Firmicutes phylum, including many members of the human gut microbiota, are able to differentiate a dormant and highly resistant cell type, the endospore (hereinafter spore for simplicity). Spore-formers can colonize virtually any habitat and, because of their resistance to a wide variety of physical and chemical insults, spores can remain viable in the environment for long periods of time. In the anaerobic enteric pathogen Clostridioides difficile the aetiologic agent is the oxygen-resistant spore, while the toxins produced by actively growing cells are the main cause of the disease symptoms. Here, we review the regulatory circuits that govern entry into sporulation. We also cover the role of spores in the infectious cycle of C. difficile in relation to spore structure and function and the main control points along spore morphogenesis.

To let go or not to let go: how ParA can impact the release of the chromosomal anchoring in Caulobacter crescentus

Chromosomal maintenance is vital for the survival of bacteria. In Caulobacter crescentus, chromosome replication initiates at ori and segregation is delayed until the nearby centromere-like region parS is replicated. Our understanding of how this sequence of events is regulated remains limited. The segregation of parS has been shown to involve multiple steps including polar release from anchoring protein PopZ, slow movement and fast ParA-dependent movement to the opposite cell pole. In this study, we demonstrate that ParA's competing attractions from PopZ and from DNA are critical for segregation of parS. Interfering with this balance of attractions-by expressing a variant ParA-R195E unable to bind DNA and thus favoring interactions exclusively between ParA-PopZ-results in cell death. Our data revealed that ParA-R195E's sole interactions with PopZ obstruct PopZ's ability to release the polar anchoring of parS, resulting in cells with multiple parS loci fixed at one cell pole. We show that the inability to separate and segregate multiple parS loci from the pole is specifically dependent on the interaction between ParA and PopZ. Collectively, our results reveal that the initial steps in chromosome segregation are highly regulated.

Bacterial membrane dynamics: Compartmentalization and repair

In every bacterial cell, the plasma membrane plays a key role in viability as it forms a selective barrier between the inside of the cell and its environment. This barrier function depends on the physical state of the lipid bilayer and the proteins embedded or associated with the bilayer. Over the past decade or so, it has become apparent that many membrane-organizing proteins and principles, which were described in eukaryote systems, are ubiquitous and play important roles in bacterial cells. In this minireview, we focus on the enigmatic roles of bacterial flotillins in membrane compartmentalization and bacterial dynamins and ESCRT-like systems in membrane repair and remodeling.

The C-terminal domain of MinC, a cell division regulation protein, is sufficient to form a copolymer with MinD

Assembly of cell division protein FtsZ into the Z-ring at the division site is a key step in bacterial cell division. The Min proteins can restrict the Z-ring to the middle of the cell. MinC is the main protein that obstructs Z-ring formation by inhibiting FtsZ assembly. Its N-terminal domain (MinCN ) regulates the localization of the Z-ring by inhibiting FtsZ polymerization, while its C-terminal domain (MinCC ) binds to MinD as well as to FtsZ. Previous studies have shown that MinC and MinD form copolymers in vitro. This copolymer may greatly enhance the binding of MinC to FtsZ, and/or prevent FtsZ filaments from diffusing to the ends of the cell. Here, we investigated the assembly properties of MinCC -MinD of Pseudomonas aeruginosa. We found that MinCC is sufficient to form the copolymers. Although MinCC -MinD assembles into larger bundles, most likely because MinCC is spatially more readily bound to MinD, its copolymerization has similar dynamic properties: the concentration of MinD dominates their copolymerization. The critical concentration of MinD is around 3 μm and when MinD concentration is high enough, a low concentration MinCC could still be copolymerized. We also found that MinCC -MinD can still rapidly bind to FtsZ protofilaments, providing direct evidence that MinCC also interacts directly with FtsZ. However, although the presence of minCC can slightly improve the division defect of minC-knockout strains and shorten the cell length from an average of 12.2 ± 6.7 to 6.6 ± 3.6 μm, it is still insufficient for the normal growth and division of bacteria.

Protein coopted from a phage restriction system dictates orthogonal cell division plane selection in Staphylococcus aureus

The spherical bacterium Staphylococcus aureus, a leading cause of nosocomial infections, undergoes binary fission by dividing in two alternating orthogonal planes, but the mechanism by which S. aureus correctly selects the next cell division plane is not known. To identify cell division placement factors, we performed a chemical genetic screen that revealed a gene which we termed pcdA. We show that PcdA is a member of the McrB family of AAA+ NTPases that has undergone structural changes and a concomitant functional shift from a restriction enzyme subunit to an early cell division protein. PcdA directly interacts with the tubulin-like central divisome component FtsZ and localizes to future cell division sites before membrane invagination initiates. This parallels the action of another McrB family protein, CTTNBP2, which stabilizes microtubules in animals. We show that PcdA also interacts with the structural protein DivIVA and propose that the DivIVA/PcdA complex recruits unpolymerized FtsZ to assemble along the proper cell division plane. Deletion of pcdA conferred abnormal, non-orthogonal division plane selection, increased sensitivity to cell wall-targeting antibiotics, and reduced virulence in a murine infection model. Targeting PcdA could therefore highlight a treatment strategy for combatting antibiotic-resistant strains of S. aureus.

The roles of GpsB and DivIVA in Staphylococcus aureus growth and division

The spheroid bacterium Staphylococcus aureus is often used as a model of morphogenesis due to its apparently simple cell cycle. S. aureus has many cell division proteins that are conserved across bacteria alluding to common functions. However, despite intensive study, we still do not know the roles of many of these components. Here, we have examined the functions of the paralogues DivIVA and GpsB in the S. aureus cell cycle. Cells lacking gpsB display a more spherical phenotype than the wild-type cells, which is associated with a decrease in peripheral cell wall peptidoglycan synthesis. This correlates with increased localization of penicillin-binding proteins at the developing septum, notably PBPs 2 and 3. Our results highlight the role of GpsB as an apparent regulator of cell morphogenesis in S. aureus.

Cell division machinery drives cell-specific gene activation during bacterial differentiation

When faced with starvation, the bacterium Bacillus subtilis transforms itself into a dormant cell type called a "spore". Sporulation initiates with an asymmetric division event, which requires the relocation of the core divisome components FtsA and FtsZ, after which the sigma factor σF is exclusively activated in the smaller daughter cell. Compartment specific activation of σF requires the SpoIIE phosphatase, which displays a biased localization on one side of the asymmetric division septum and associates with the structural protein DivIVA, but the mechanism by which this preferential localization is achieved is unclear. Here, we isolated a variant of DivIVA that indiscriminately activates σF in both daughter cells due to promiscuous localization of SpoIIE, which was corrected by overproduction of FtsA and FtsZ. We propose that a unique feature of the sporulation septum, defined by the cell division machinery, drives the asymmetric localization of DivIVA and SpoIIE to trigger the initiation of the sporulation program.

DivIVA Interacts with the Cell Wall Hydrolase MltG To Regulate Peptidoglycan Synthesis in Streptococcus suis

Bacterial morphology is largely determined by the spatial and temporal regulation of peptidoglycan (PG) biosynthesis. Ovococci possess a unique pattern of PG synthesis different from the well studied Bacillus, and the mechanism of the coordination of PG synthesis remains poorly understood. Several regulatory proteins have been identified to be involved in the regulation of ovococcal morphogenesis, among which DivIVA is an important one to regulate PG synthesis in streptococci, while its mechanism is largely unknown. Here, the zoonotic pathogen Streptococcus suis was used to investigate the regulation of DivIVA on PG synthesis. Fluorescent d-amino acid probing and 3D-structured illumination microscopy found that DivIVA deletion caused abortive peripheral PG synthesis, resulting in a decreased aspect ratio. The phosphorylation-depleted mutant (DivIVA3A) cells displayed a longer nascent PG and became longer, whereas the phosphorylation-mimicking mutant (DivIVA3E) cells showed a shorter nascent PG and became shorter, suggesting that DivIVA phosphorylation is involved in regulating peripheral PG synthesis. Several DivIVA-interacting proteins were identified, and the interaction was confirmed between DivIVA and MltG, a cell wall hydrolase essential for cell elongation. DivIVA did not affect the PG hydrolysis activity of MltG, while the phosphorylation state of DivIVA affected its interaction with MltG. MltG was mislocalized in the ΔdivIVA and DivIVA3E cells, and both ΔmltG and DivIVA3E cells formed significantly rounder cells, indicating an important role of DivIVA phosphorylation in regulating PG synthesis through MltG. These findings highlight the regulatory mechanism of PG synthesis and morphogenesis of ovococci. IMPORTANCE The peptidoglycan (PG) biosynthesis pathway provides a rich source of novel antimicrobial drug targets. However, bacterial PG synthesis and its regulation is a very complex process involving dozens of proteins. Moreover, unlike the well studied Bacillus, ovococci undergo unusual PG synthesis with unique mechanisms of coordination. DivIVA is an important regulator of PG synthesis in ovococci, while its exact role in regulating PG synthesis remains poorly understood. In this study, we determined the role of DivIVA in regulating lateral PG synthesis of Streptococcus suis and identified a critical interacting partner, MltG, in which DivIVA influenced the subcellular localizations of MltG through its phosphorylation. Our study characterizes the detailed role of DivIVA in regulating bacterial PG synthesis, which is very helpful for understanding the process of PG synthesis in streptococci.

Bacillus subtilis, a Swiss Army Knife in Science and Biotechnology

Next to Escherichia coli, Bacillus subtilis is the most studied and best understood organism that also serves as a model for many important pathogens. Due to its ability to form heat-resistant spores that can germinate even after very long periods of time, B. subtilis has attracted much scientific interest. Another feature of B. subtilis is its genetic competence, a developmental state in which B. subtilis actively takes up exogenous DNA. This makes B. subtilis amenable to genetic manipulation and investigation. The bacterium was one of the first with a fully sequenced genome, and it has been subject to a wide variety of genome- and proteome-wide studies that give important insights into many aspects of the biology of B. subtilis. Due to its ability to secrete large amounts of proteins and to produce a wide range of commercially interesting compounds, B. subtilis has become a major workhorse in biotechnology. Here, we review the development of important aspects of the research on B. subtilis with a specific focus on its cell biology and biotechnological and practical applications from vitamin production to concrete healing. The intriguing complexity of the developmental programs of B. subtilis, paired with the availability of sophisticated tools for genetic manipulation, positions it at the leading edge for discovering new biological concepts and deepening our understanding of the organization of bacterial cells.

To let go or not to let go: how ParA can impact the release of the chromosomal anchoring in Caulobacter crescentus

Chromosomal maintenance is vital for the survival of bacteria. In Caulobacter crescentus, chromosome replication initiates at ori and segregation is delayed until the nearby centromere-like region parS is replicated. Our understanding of how this sequence of events is regulated remains limited. The segregation of parS has been shown to involve multiple steps including polar release from anchoring protein PopZ, slow movement, and fast ParA-dependent movement to opposite cell pole. In this study, we demonstrate that ParA's competing attractions from PopZ and from DNA are critical for segregation of parS. Interfering with this balance of attractions - by expressing a variant ParA-R195E unable to bind DNA and thus favoring interactions exclusively between ParA-PopZ - results in cell death. Our data revealed that ParA-R195E's sole interactions with PopZ obstruct PopZ's ability to release the polar anchoring of parS resulting in cells with multiple parS loci fixed at one cell pole. We show that the inability to separate and segregate multiple parS loci from the pole is specifically dependent on the interaction between ParA and PopZ. Interfering with interactions between PopZ and the partitioning protein ParB, which is the interaction that anchors parS at the cell pole, does not rescue the ability of cells to separate the fixed parS loci when expressing parA-R195E. Thus, ParA and PopZ appear to have a distinct conversation from ParB yet can impact the release of ParB-parS from the anchoring at the cell pole. Collectively, our results reveal that the initial steps in chromosome segregation are highly regulated.

Preferential localization of a single spot in reaction-diffusion systems on non-spherical surfaces

The present work systematically examines the effect of breaking the rotational symmetry of a surface on the spot positioning in reaction-diffusion (RD) systems. In particular, we study analytically and numerically the steady-state positioning of a single spot in RD systems on a prolate and an oblate ellipsoid. We adapt perturbative techniques to perform a linear stability analysis of the RD system on both ellipsoids. Furthermore, the spot positionings in the steady states of non-linear RD equations are obtained numerically on both ellipsoids. Our analysis suggests that preferential spot positioning can be observed on non-spherical surfaces. The present work may provide useful insights into the role of cell geometry on various symmetry-breaking mechanisms in cellular processes.

Eukaryotic-like gephyrin and cognate membrane receptor coordinate corynebacterial cell division and polar elongation

The order Corynebacteriales includes major industrial and pathogenic actinobacteria such as Corynebacterium glutamicum or Mycobacterium tuberculosis . Their elaborate multi-layered cell wall, composed primarily of the mycolyl-arabinogalactan-peptidoglycan complex, and their polar growth mode impose a stringent coordination between the septal divisome, organized around the tubulin-like protein FtsZ, and the polar elongasome, assembled around the tropomyosin-like protein Wag31. Here, we report the identification of two new divisome members, a gephyrin-like repurposed molybdotransferase (GLP) and its membrane receptor (GLPR). We show that the interplay between the GLPR/GLP module, FtsZ and Wag31 is crucial for orchestrating cell cycle progression. Our results provide a detailed molecular understanding of the crosstalk between two essential machineries, the divisome and elongasome, and reveal that Corynebacteriales have evolved a protein scaffold to control cell division and morphogenesis similar to the gephyrin/GlyR system that in higher eukaryotes mediates synaptic signaling through network organization of membrane receptors and the microtubule cytoskeleton.

Interferon-induced transmembrane protein 3 (IFITM3) and its antiviral activity

Infections caused by enveloped viruses require fusion with cellular membranes for viral genome entry. Viral entry occurs following an interaction of viral and cellular membranes allowing the formation of fusion pores, by which the virus accesses the cytoplasm. Here, we focus on interferon-induced transmembrane protein 3 (IFITM3) and its antiviral activity. IFITM3 is predicted to block or stall viral fusion at an intermediate state, causing viral propagation to fail. After introducing IFITM3, we describe the generalized lipid membrane fusion pathway and how it can be stalled, particularly with respect to IFITM3, and current questions regarding IFITM3's topology, with specific emphasis on IFITM3's amphipathic α-helix (AAH) ₅₉V-₆₈M, which is necessary for the antiviral activity. We report new hydrophobicity and hydrophobic moment calculations for this peptide and a variety of active site peptides from known membrane-remodeling proteins. Finally, we discuss the effects of posttranslational modifications and localization, how IFITM3's AAH may block viral fusion, and possible ramifications of membrane composition.

FrzS acts as a polar beacon to recruit SgmX, a central activator of type IV pili during Myxococcus xanthus motility

In rod-shaped bacteria, type IV pili (Tfp) promote twitching motility by assembling and retracting at the cell pole. In Myxococcus xanthus, a bacterium that moves in highly coordinated cell groups, Tfp are activated by a polar activator protein, SgmX. However, while it is known that the Ras-like protein MglA is required for unipolar targeting, how SgmX accesses the cell pole to activate Tfp is unknown. Here, we demonstrate that a polar beacon protein, FrzS, recruits SgmX at the cell pole. We identified two main functional domains, including a Tfp-activating domain and a polar-binding domain. Within the latter, we show that the direct binding of MglA-GTP unveils a hidden motif that binds directly to the FrzS N-terminal response regulator (CheY). Structural analyses reveal that this binding occurs through a novel binding interface for response regulator domains. In conclusion, the findings unveil the protein interaction network leading to the spatial activation of Tfp at the cell pole. This tripartite system is at the root of complex collective behaviours in this predatory bacterium.

Chromosome remodelling by SMC/Condensin in B. subtilis is regulated by monomeric Soj/ParA during growth and sporulation

SMC complexes, loaded at ParB-parS sites, are key mediators of chromosome organization in bacteria. ParA/Soj proteins interact with ParB/Spo0J in a pathway involving adenosine triphosphate (ATP)-dependent dimerization and DNA binding, facilitating chromosome segregation in bacteria. In Bacillus subtilis, ParA/Soj also regulates DNA replication initiation and along with ParB/Spo0J is involved in cell cycle changes during endospore formation. The first morphological stage in sporulation is the formation of an elongated chromosome structure called an axial filament. Here, we show that a major redistribution of SMC complexes drives axial filament formation in a process regulated by ParA/Soj. Furthermore, and unexpectedly, this regulation is dependent on monomeric forms of ParA/Soj that cannot bind DNA or hydrolyze ATP. These results reveal additional roles for ParA/Soj proteins in the regulation of SMC dynamics in bacteria and yet further complexity in the web of interactions involving chromosome replication, segregation and organization, controlled by ParAB and SMC.

Endocytosis-like DNA uptake by cell wall-deficient bacteria

Horizontal gene transfer in bacteria is widely believed to occur via conjugation, transduction and transformation. These mechanisms facilitate the passage of DNA across the protective cell wall using sophisticated machinery. Here, we report that cell wall-deficient bacteria can engulf DNA and other extracellular material via an endocytosis-like process. Specifically, we show that L-forms of the filamentous actinomycete Kitasatospora viridifaciens can take up plasmid DNA, polysaccharides (dextran) and 150-nm lipid nanoparticles. The process involves invagination of the cytoplasmic membrane, leading to formation of intracellular vesicles that encapsulate extracellular material. DNA uptake is not affected by deletion of genes homologous to comEC and comEA, which are required for natural transformation in other species. However, uptake is inhibited by sodium azide or incubation at 4 °C, suggesting the process is energy-dependent. The encapsulated materials are released into the cytoplasm upon degradation of the vesicle membrane. Given that cell wall-deficient bacteria are considered a model for early life forms, our work reveals a possible mechanism for primordial cells to acquire food or genetic material before invention of the bacterial cell wall.

Horizontal gene transfer in bacteria can occur through mechanisms such as conjugation, transduction and transformation, which facilitate the passage of DNA across the cell wall. Here, Kapteijn et al. show that cell wall-deficient bacteria can take up DNA and other extracellular materials via an endocytosis-like process.

MraZ Transcriptionally Controls the Critical Level of FtsL Required for Focusing Z-Rings and Kickstarting Septation in Bacillus subtilis

The bacterial division and cell wall (dcw) cluster is a highly conserved region of the genome which encodes several essential cell division factors, including the central divisome protein FtsZ. Understanding the regulation of this region is key to our overall understanding of the division process. mraZ is found at the 5' end of the dcw cluster, and previous studies have described MraZ as a sequence-specific DNA binding protein. In this article, we investigate MraZ to elucidate its role in Bacillus subtilis. Through our investigation, we demonstrate that increased levels of MraZ result in lethal filamentation due to repression of its own operon (mraZ-mraW-ftsL-pbpB). We observed rescue of filamentation upon decoupling ftsL expression, but not other genes in the operon, from MraZ control. Our data suggest that regulation of the mra operon may be an alternative way for cells to quickly arrest cytokinesis, potentially during entry into the stationary phase and in the event of DNA replication arrest. Furthermore, through time-lapse microscopy, we were able to identify that overexpression of mraZ or depletion of FtsL results in decondensation of the FtsZ ring (Z-ring). Using fluorescent d-amino acid labeling, we also observed that coordinated peptidoglycan insertion at the division site is dysregulated in the absence of FtsL. Thus, we reveal that the precise role of FtsL is in Z-ring maturation and focusing septal peptidoglycan synthesis. IMPORTANCE MraZ is a highly conserved protein found in a diverse range of bacteria, including genome-reduced Mycoplasma. We investigated the role of MraZ in Bacillus subtilis and found that overproduction of MraZ is toxic due to cell division inhibition. Upon further analysis, we observed that MraZ is a repressor of its own operon, which includes genes that encode the essential cell division factors FtsL and PBP2B. We noted that decoupling of ftsL alone was sufficient to abolish MraZ-mediated cell division inhibition. Using time-lapse microscopy, we showed that under conditions where the FtsL level is depleted, the cell division machinery is unable to initiate cytokinesis. Thus, our results pinpoint that the precise role of FtsL is in concentrating septal cell wall synthesis to facilitate cell division.

Catching a Walker in the Act-DNA Partitioning by ParA Family of Proteins

Partitioning the replicated genetic material is a crucial process in the cell cycle program of any life form. In bacteria, many plasmids utilize cytoskeletal proteins that include ParM and TubZ, the ancestors of the eukaryotic actin and tubulin, respectively, to segregate the plasmids into the daughter cells. Another distinct class of cytoskeletal proteins, known as the Walker A type Cytoskeletal ATPases (WACA), is unique to Bacteria and Archaea. ParA, a WACA family protein, is involved in DNA partitioning and is more widespread. A centromere-like sequence parS, in the DNA is bound by ParB, an adaptor protein with CTPase activity to form the segregation complex. The ParA ATPase, interacts with the segregation complex and partitions the DNA into the daughter cells. Furthermore, the Walker A motif-containing ParA superfamily of proteins is associated with a diverse set of functions ranging from DNA segregation to cell division, cell polarity, chemotaxis cluster assembly, cellulose biosynthesis and carboxysome maintenance. Unifying principles underlying the varied range of cellular roles in which the ParA superfamily of proteins function are outlined. Here, we provide an overview of the recent findings on the structure and function of the ParB adaptor protein and review the current models and mechanisms by which the ParA family of proteins function in the partitioning of the replicated DNA into the newly born daughter cells.

How Do MinC-D Copolymers Act on Z-Ring Localization Regulation? A New Model of Bacillus subtilis Min System

Division site selection in rod-shaped bacteria is strictly regulated spatially by the Min system. Although many sophisticated studies, including in vitro recombination, have tried to explain these regulations, the precise mechanisms are still unclear. A previous model suggested that the concentration gradient of MinC, an FtsZ inhibitor, regulates the position of the Z-ring in the cell. In Escherichia coli, the oscillation of MinCDE proteins leads to a gradient of Min proteins with the average concentration being lowest in the middle and highest near the poles. In contrast to the Min system of E. coli, the Min system of Bacillus subtilis lacks MinE and exhibits a stable concentration distribution, which is regulated by the binding of DivIVA to the negative curvature membrane. The Min proteins first accumulate at the poles of the cell and relocalize near the division site when the membrane invagination begins. It is inconsistent with the previous model of high concentrations of MinC inhibiting Z-ring formation. Our preliminary data here using electron microscopy and light scattering technology reported that B. subtilis MinC (BsMinC) and MinD (BsMinD) also assembled into large straight copolymers in the presence of ATP, similar to the Min proteins of E. coli. Their assembly is fast and dominated by MinD concentration. When BsMinD is 5 μM, a clear light scattering signal can be observed even at 0.3 μM BsMinC. Here, we propose a new model based on the MinC-D copolymers. In our hypothesis, it is not the concentration gradient of MinC, but the MinC-D copolymer assembled in the region of high concentration MinD that plays a key role in the regulation of Z-ring positioning. In B. subtilis, the regions with high MinD concentration are initially at both ends of the cell and then appear at midcell when cell division began. MinC-D copolymer will polymerize and form a complex with MinJ and DivIVA. These complexes capture FtsZ protofilaments to prevent their diffusion away from the midcell and narrow the Z-ring in the middle of the cell.

The endophilin curvature-sensitive motif requires electrostatic guidance to recycle synaptic vesicles in vivo

Curvature-sensing mechanisms assist proteins in executing particular actions on various membrane organelles. Here, we investigate the functional specificity of curvature-sensing amphipathic motifs in Caenorhabditis elegans through the study of endophilin, an endocytic protein for synaptic vesicle recycling. We generate chimeric endophilin proteins by replacing the endophilin amphipathic motif H0 with other curvature-sensing amphipathic motifs. We find that the role of amphipathic motifs cannot simply be extrapolated from the identity of their parental proteins. For example, the amphipathic motif of the nuclear pore complex protein NUP133 functionally replaces the synaptic role of endophilin H0. Interestingly, non-functional endophilin chimeras have similar defects-producing fewer synaptic vesicles but more endosomes-and this indicates that the curvature-sensing motifs in these chimeras have a common deficiency for reforming synaptic vesicles. Finally, we convert non-functional endophilin chimeras into functional proteins by changing the cationic property of amphipathic motifs, successfully reprogramming the functional specificity of curvature-sensing motifs in vivo.

How Streptomyces thrive: Advancing our understanding of classical development and uncovering new behaviors

Streptomyces are soil- and marine-dwelling microbes that need to survive dramatic fluctuations in nutrient levels and environmental conditions. Here, we explore the advances made in understanding how Streptomyces bacteria can thrive in their natural environments. We examine their classical developmental cycle, and the intricate regulatory cascades that govern it. We discuss alternative growth strategies and behaviors, like the rapid expansion and colonization properties associated with exploratory growth, the release of membrane vesicles and S-cells from hyphal tips, and the acquisition of exogenous DNA along the lateral walls. We further investigate Streptomyces interactions with other organisms through the release of volatile compounds that impact nutrient levels, microbial growth, and insect behavior. Finally, we explore the increasingly diverse strategies employed by Streptomyces species in escaping and thwarting phage infections.

Spontaneous local membrane curvature induced by transmembrane proteins

The (local) curvature of cellular membranes acts as a driving force for the targeting of membrane-associated proteins to specific membrane domains, as well as a sorting mechanism for transmembrane proteins, e.g., by accumulation in regions of matching spontaneous curvature. The latter measure was previously experimentally employed to study the curvature induced by the potassium channel KvAP and by aquaporin AQP0. However, the direction of the reported spontaneous curvature levels as well as the molecular driving forces governing the membrane curvature induced by these integral transmembrane proteins could not be addressed experimentally. Here, using both coarse-grained and atomistic molecular dynamics (MD) simulations, we report induced spontaneous curvature values for the homologous potassium channel Kv 1.2/2.1 Chimera (KvChim) and AQP0 embedded in unrestrained lipid bicelles that are in very good agreement with experiment. Importantly, the direction of curvature could be directly assessed from our simulations: KvChim induces a strong positive membrane curvature (≈0.036 nm^-1) whereas AQP0 causes a comparably small negative curvature (≈-0.019 nm^-1). Analyses of protein-lipid interactions within the bicelle revealed that the potassium channel shapes the surrounding membrane via structural determinants. Differences in shape of the protein-lipid interface of the voltage-gating domains between the extracellular and cytosolic membrane leaflets induce membrane stress and thereby promote a protein-proximal membrane curvature. In contrast, the water pore AQP0 displayed a high structural stability and an only faint effect on the surrounding membrane environment that is connected to its wedge-like shape.

Subcellular Dynamics of a Conserved Bacterial Polar Scaffold Protein

In order to survive, bacterial cells rely on precise spatiotemporal organization and coordination of essential processes such as cell growth, chromosome segregation, and cell division. Given the general lack of organelles, most bacteria are forced to depend on alternative localization mechanisms, such as, for example, geometrical cues. DivIVA proteins are widely distributed in mainly Gram-positive bacteria and were shown to bind the membrane, typically in regions of strong negative curvature, such as the cell poles and division septa. Here, they have been shown to be involved in a multitude of processes: from apical cell growth and chromosome segregation in actinobacteria to sporulation and inhibition of division re-initiation in firmicutes. Structural analyses revealed that DivIVA proteins can form oligomeric assemblies that constitute a scaffold for recruitment of other proteins. However, it remained unclear whether interaction with partner proteins influences DivIVA dynamics. Using structured illumination microscopy (SIM), single-particle tracking (SPT) microscopy, and fluorescent recovery after photobleaching (FRAP) experiments, we show that DivIVA from Corynebacterium glutamicum is mobilized by its binding partner ParB. In contrast, we show that the interaction between Bacillus subtilis DivIVA and its partner protein MinJ reduces DivIVA mobility. Furthermore, we show that the loss of the rod-shape leads to an increase in DivIVA dynamics in both organisms. Taken together, our study reveals the modulation of the polar scaffold protein by protein interactors and cell morphology. We reason that this leads to a very simple, yet robust way for actinobacteria to maintain polar growth and their rod-shape. In B. subtilis, however, the DivIVA protein is tailored towards a more dynamic function that allows quick relocalization from poles to septa upon division.

Cytoskeletal proteins: Lessons learned from bacteria

Cytoskeletal proteins are classified as a group that is defined functionally, whose members are capable of polymerizing into higher order structures, either dynamically or statically, to perform structural roles during a variety of cellular processes. In eukaryotes, the most well-studied cytoskeletal proteins are actin, tubulin, and intermediate filaments, and are essential for cell shape and movement, chromosome segregation, and intracellular cargo transport. Prokaryotes often harbor homologs of these proteins, but in bacterial cells, these homologs are usually not employed in roles that can be strictly defined as "cytoskeletal". However, several bacteria encode other proteins capable of polymerizing which, although they do not appear to have a eukaryotic counterpart, nonetheless appear to perform a more traditional "cytoskeletal" function. In this review, we discuss recent reports that cover the structure and functions of prokaryotic proteins that are broadly termed as cytoskeletal, either by sequence homology or by function, to highlight how the enzymatic properties of traditionally studied cytoskeletal proteins may be used for other types of cellular functions; and to demonstrate how truly "cytoskeletal" functions may be performed by uniquely bacterial proteins that do not display homology to eukaryotic proteins.

Single-cell growth inference of Corynebacterium glutamicum reveals asymptotically linear growth

Regulation of growth and cell size is crucial for the optimization of bacterial cellular function. So far, single bacterial cells have been found to grow predominantly exponentially, which implies the need for tight regulation to maintain cell size homeostasis. Here, we characterize the growth behavior of the apically growing bacterium Corynebacterium glutamicum using a novel broadly applicable inference method for single-cell growth dynamics. Using this approach, we find that C. glutamicum exhibits asymptotically linear single-cell growth. To explain this growth mode, we model elongation as being rate-limited by the apical growth mechanism. Our model accurately reproduces the inferred cell growth dynamics and is validated with elongation measurements on a transglycosylase deficient ΔrodA mutant. Finally, with simulations we show that the distribution of cell lengths is narrower for linear than exponential growth, suggesting that this asymptotically linear growth mode can act as a substitute for tight division length and division symmetry regulation.

Localized Proteolysis for the Construction of Intracellular Asymmetry in Escherichia coli

Protein-level regulations have gained importance in building synthetic circuits, as they offer a potential advantage in the speed of operation compared to gene regulation circuits. In nature, localized protein degradation is prevalent in polarizing cellular signaling. We, therefore, set out to systematically investigate whether localized proteolysis can be employed to construct intracellular asymmetry in Escherichia coli. We demonstrate that, by inserting a cognate cleavage site between the reporter and C-terminal degron, the unstable reporter can be stabilized in the presence of the tobacco etch virus protease. Furthermore, the split protease can be functionally reconstituted by the PopZ-based polarity system to exert localized proteolysis. Selective stabilization of the unstable reporter at the PopZ pole can lead to intracellular asymmetry in E. coli. Our study provides complementary evidence to support that localized proteolysis may be a strategy for polarization in developmental cell biology. Circuits designed in this study may also help to expand the synthetic biology repository for the engineering of synthetic morphogenesis, particularly for processes that require rapid control of local protein abundance.

Cardiolipin-Containing Lipid Membranes Attract the Bacterial Cell Division Protein DivIVA

DivIVA is a protein initially identified as a spatial regulator of cell division in the model organism Bacillus subtilis, but its homologues are present in many other Gram-positive bacteria, including Clostridia species. Besides its role as topological regulator of the Min system during bacterial cell division, DivIVA is involved in chromosome segregation during sporulation, genetic competence, and cell wall synthesis. DivIVA localizes to regions of high membrane curvature, such as the cell poles and cell division site, where it recruits distinct binding partners. Previously, it was suggested that negative curvature sensing is the main mechanism by which DivIVA binds to these specific regions. Here, we show that Clostridioides difficile DivIVA binds preferably to membranes containing negatively charged phospholipids, especially cardiolipin. Strikingly, we observed that upon binding, DivIVA modifies the lipid distribution and induces changes to lipid bilayers containing cardiolipin. Our observations indicate that DivIVA might play a more complex and so far unknown active role during the formation of the cell division septal membrane.

Dynamics of the Bacillus subtilis Min System

The molecular mechanisms that help to place the division septum in bacteria is of fundamental importance to ensure cell proliferation and maintenance of cell shape and size. The Min protein system, found in many rod-shaped bacteria, is thought to play a major role in division site selection.

Division site selection is a vital process to ensure generation of viable offspring. In many rod-shaped bacteria, a dynamic protein system, termed the Min system, acts as a central regulator of division site placement. The Min system is best studied in Escherichia coli, where it shows a remarkable oscillation from pole to pole with a time-averaged density minimum at midcell. Several components of the Min system are conserved in the Gram-positive model organism Bacillus subtilis. However, in B. subtilis, it is commonly believed that the system forms a stationary bipolar gradient from the cell poles to midcell. Here, we show that the Min system of B. subtilis localizes dynamically to active sites of division, often organized in clusters. We provide physical modeling using measured diffusion constants that describe the observed enrichment of the Min system at the septum. Mathematical modeling suggests that the observed localization pattern of Min proteins corresponds to a dynamic equilibrium state. Our data provide evidence for the importance of ongoing septation for the Min dynamics, consistent with a major role of the Min system in controlling active division sites but not cell pole areas.

Construction of intracellular asymmetry and asymmetric division in Escherichia coli

The design principle of establishing an intracellular protein gradient for asymmetric cell division is a long-standing fundamental question. While the major molecular players and their interactions have been elucidated via genetic approaches, the diversity and redundancy of natural systems complicate the extraction of critical underlying features. Here, we take a synthetic cell biology approach to construct intracellular asymmetry and asymmetric division in Escherichia coli, in which division is normally symmetric. We demonstrate that the oligomeric PopZ from Caulobacter crescentus can serve as a robust polarized scaffold to functionalize RNA polymerase. Furthermore, by using another oligomeric pole-targeting DivIVA from Bacillus subtilis, the newly synthesized protein can be constrained to further establish intracellular asymmetry, leading to asymmetric division and differentiation. Our findings suggest that the coupled oligomerization and restriction in diffusion may be a strategy for generating a spatial gradient for asymmetric cell division.

Identification of a Potential Membrane-Targeting Sequence in the C-Terminus of the F Plasmid Segregation Protein SopA

Stable maintenance and partitioning of the 'Fertility' plasmid or the F plasmid in its host Escherichia coli require the function of a ParA superfamily of proteins known as SopA. The mechanism by which SopA mediates plasmid segregation is well studied. SopA is a nucleoid-binding protein and binds DNA in an ATP-dependent but sequence non-specific manner. ATP hydrolysis stimulated by the binding of the SopBC complex mediates the release of SopA from the nucleoid. Cycles of ATP-binding and hydrolysis generate an ATPase gradient that moves the plasmid through a chemophoresis force. Nucleoid binding of SopA thus assumes a central role in its plasmid-partitioning function. However, earlier work also suggests that the F plasmid can be partitioned into anucleate cells, thus implicating nucleoid independent partitioning. Interestingly, SopA is also reported to be associated with the inner membrane of the bacteria. Here, we report the identification of a possible membrane-targeting sequence, a predicted amphipathic helix, at the C-terminus of SopA. Molecular dynamics simulations indicate that the predicted amphipathic helical motif of SopA has weak affinity for membranes. Moreover, we experimentally show that SopA can associate with bacterial membranes, is detectable in the membrane fractions of bacterial lysates, and is sensitive to the membrane potential. Further, unlike the wild-type SopA, a deletion of the C-terminal 29 amino acids results in the loss of F plasmids from bacterial cells.

Molecular interactions and their predictive roles in cell pole determination in bacteria

Bacterial cell cycle is divided into well-coordinated phases; chromosome duplication and segregation, cell elongation, septum formation, and cytokinesis. The temporal separation of these phases depends upon the growth rates and doubling time in different bacteria. The entire process of cell division starts with the assembly of divisome complex at mid-cell position followed by constriction of the cell wall and septum formation. In the mapping of mid-cell position for septum formation, the gradient of oscillating Min proteins across the poles plays a pivotal role in several bacteria genus. The cues in the cell that defines the poles and plane of cell division are not fully characterized in cocci. Recent studies have shed some lights on molecular interactions at the poles and the underlying mechanisms involved in pole determination in non-cocci. In this review, we have brought forth recent findings on these aspects together, which would suggest a model to explain the mechanisms of pole determination in rod shaped bacteria and could be extrapolated as a working model in cocci.

An Oscillating MinD Protein Determines the Cellular Positioning of the Motility Machinery in Archaea

MinD proteins are well studied in rod-shaped bacteria such as E. coli, where they display self-organized pole-to-pole oscillations that are important for correct positioning of the Z-ring at mid-cell for cell division. Archaea also encode proteins belonging to the MinD family, but their functions are unknown. MinD homologous proteins were found to be widespread in Euryarchaeota and form a sister group to the bacterial MinD family, distinct from the ParA and other related ATPase families. We aimed to identify the function of four archaeal MinD proteins in the model archaeon Haloferax volcanii. Deletion of the minD genes did not cause cell division or size defects, and the Z-ring was still correctly positioned. Instead, one of the deletions (ΔminD4) reduced swimming motility and hampered the correct formation of motility machinery at the cell poles. In ΔminD4 cells, there is reduced formation of the motility structure and chemosensory arrays, which are essential for signal transduction. In bacteria, several members of the ParA family can position the motility structure and chemosensory arrays via binding to a landmark protein, and consequently these proteins do not oscillate along the cell axis. However, GFP-MinD4 displayed pole-to-pole oscillation and formed polar patches or foci in H. volcanii. The MinD4 membrane-targeting sequence (MTS), homologous to the bacterial MinD MTS, was essential for the oscillation. Surprisingly, mutant MinD4 proteins failed to form polar patches. Thus, MinD4 from H. volcanii combines traits of different bacterial ParA/MinD proteins.

Cell-Cycle-Associated Expression Patterns Predict Gene Function in Mycobacteria

Although the major events in prokaryotic cell cycle progression are likely to be coordinated with transcriptional and metabolic changes, these processes remain poorly characterized. Unlike many rapidly growing bacteria, DNA replication and cell division are temporally resolved in mycobacteria, making these slow-growing organisms a potentially useful system to investigate the prokaryotic cell cycle. To determine whether cell-cycle-dependent gene regulation occurs in mycobacteria, we characterized the temporal changes in the transcriptome of synchronously replicating populations of Mycobacterium tuberculosis (Mtb). By enriching for genes that display a sinusoidal expression pattern, we discover 485 genes that oscillate with a period consistent with the cell cycle. During cytokinesis, the timing of gene induction could be used to predict the timing of gene function, as mRNA abundance was found to correlate with the order in which proteins were recruited to the developing septum. Similarly, the expression pattern of primary metabolic genes could be used to predict the relative importance of these pathways for different cell cycle processes. Pyrimidine synthetic genes peaked during DNA replication, and their depletion caused a filamentation phenotype that phenocopied defects in this process. In contrast, the inosine monophasphate dehydrogenase dedicated to guanosine synthesis, GuaB2, displayed the opposite expression pattern and its depletion perturbed septation. Together, these data imply obligate coordination between primary metabolism and cell division and identify periodically regulated genes that can be related to specific cell biological functions.

Ultrastructure of Exospore Formation in Streptomyces Revealed by Cryo-Electron Tomography

Many bacteria form spores in response to adverse environmental conditions. Several sporulation pathways have evolved independently and occur through distinctive mechanisms. Here, using cryo-electron tomography (cryo-ET), we examine all stages of growth and exospore formation in the model organism Streptomyces albus. Our data reveal the native ultrastructure of vegetative hyphae, including the likely structures of the polarisome and cytoskeletal filaments. In addition, we observed septal junctions in vegetative septa, predicted to be involved in protein and DNA translocation between neighboring cells. During sporulation, the cell envelope undergoes dramatic remodeling, including the formation of a spore wall and two protective proteinaceous layers. Mature spores reveal the presence of a continuous spore coat and an irregular rodlet sheet. Together, these results provide an unprecedented examination of the ultrastructure in Streptomyces and further our understanding of the structural complexity of exospore formation.

Advances on systems metabolic engineering of Bacillus subtilis as a chassis cell

The Gram-positive model bacterium Bacillus subtilis, has been broadly applied in various fields because of its low pathogenicity and strong protein secretion ability, as well as its well-developed fermentation technology. B. subtilis is considered as an attractive host in the field of metabolic engineering, in particular for protein expression and secretion, so it has been well studied and applied in genetic engineering. In this review, we discussed why B. subtilis is a good chassis cell for metabolic engineering. We also summarized the latest research progress in systematic biology, synthetic biology and evolution-based engineering of B. subtilis, and showed systemic metabolic engineering expedite the harnessing B. subtilis for bioproduction.

A mycobacterial DivIVA domain-containing protein involved in cell length and septation

Mycobacterial cells elongate via polar deposition of cell wall material, similar to the filamentous Streptomyces species, which contain a tip-organizing centre. Coiled-coiled proteins such as DivIVA play an important role in this process. The genome of Mycobacterium tuberculosis, the causative agent of tuberculosis, encodes many coiled-coil proteins that are homologous to DivIVA with a potential role in mycobacterial cell elongation. Here we describe studies on Mycobacterium smegmatis MSMEG_2416, a homologue of M. tuberculosis Rv2927c. Two previous independent studies showed that MSMEG_2416 was involved in septation (subsequently referred to as sepIVA). Contrary to these previous reports, we found sepIVA to be dispensable for growth in laboratory media by generating a viable null mutant. The mutant strain did, however, show a number of differences, including a change in colony morphology and biofilm formation that could be reversed on complementation with sepIVA as well as Rv2927c, the sepIVA homologue from M. tuberculosis. However, analysis of cell wall lipids did not reveal any alterations in lipid profiles of the mutant strain. Microscopic examination of the mutant revealed longer cells with more septa, which occurred at irregular intervals, often generating mini-compartments, a profile similar to that observed in the previous studies following conditional depletion, highlighting a role for sepIVA in mycobacterial growth.

Structural basis of self-assembly in the lipid-binding domain of mycobacterial polar growth factor Wag31

Wag31, or DivIVA, is an essential protein and a drug target in the human pathogen Mycobacterium tuberculosis that self-assembles at the negatively curved membrane surface to form a higher-order structural scaffold, maintains rod-shaped cellular morphology and localizes key cell-wall synthesizing enzymes at the pole for exclusive polar growth. The crystal structure of the N-terminal lipid-binding domain of mycobacterial Wag31 was determined at 2.3 Å resolution. The structure revealed a highly polar surface lined with several conserved charged residues that suggest probable sites for interactions with membrane lipids. Crystal-packing analysis revealed a previously unseen 'dimer-of-dimers' assembly state of N-terminal Wag31, which is formed by antiparallel stacking of two coiled-coil dimers. Size-exclusion column-chromatography-coupled small-angle solution X-ray scattering data revealed a tetrameric form as a major assembly state of N-terminal Wag31 in solution, further supporting the crystal structure. The results suggest that, in addition to lipid binding, the N-terminal Wag31 can participate in self-assembly to form filamentous structures. Plausible models of linear self-assembly and branching of Wag31 filaments consistent with available data are suggested.

Cell division protein FtsZ: from structure and mechanism to antibiotic target

Antimicrobial resistance to virtually all clinically applied antibiotic classes severely limits the available options to treat bacterial infections. Hence, there is an urgent need to develop and evaluate new antibiotics and targets with resistance-breaking properties. Bacterial cell division has emerged as a new antibiotic target pathway to counteract multidrug-resistant pathogens. New approaches in antibiotic discovery and bacterial cell biology helped to identify compounds that either directly interact with the major cell division protein FtsZ, thereby perturbing the function and dynamics of the cell division machinery, or affect the structural integrity of FtsZ by inducing its degradation. The impressive antimicrobial activities and resistance-breaking properties of certain compounds validate the inhibition of bacterial cell division as a promising strategy for antibiotic intervention.

DivIVA Controls Progeny Morphology and Diverse ParA Proteins Regulate Cell Division or Gliding Motility in Bdellovibrio bacteriovorus

The predatory bacterium B. bacteriovorus grows and divides inside the periplasm of Gram-negative bacteria, forming a structure known as a bdelloplast. Cell division of predators inside the dead prey cell is not by binary fission but instead by synchronous division of a single elongated filamentous cell into odd or even numbers of progeny cells. Bdellovibrio replication and cell division processes are dependent on the finite level of nutrients available from inside the prey bacterium. The filamentous growth and division process of the predator maximizes the number of progeny produced by the finite nutrients in a way that binary fission could not. To learn more about such an unusual growth profile, we studied the role of DivIVA in the growing Bdellovibrio cell. This protein is well known for its link to polar cell growth and spore formation in Gram-positive bacteria, but little is known about its function in a predatory growth context. We show that DivIVA is expressed in the growing B. bacteriovorus cell and controls cell morphology during filamentous cell division, but not the number of progeny produced. Bacterial Two Hybrid (BTH) analysis shows DivIVA may interact with proteins that respond to metabolic indicators of amino-acid biosynthesis or changes in redox state. Such changes may be relevant signals to the predator, indicating the consumption of prey nutrients within the sealed bdelloplast environment. ParA, a chromosome segregation protein, also contributes to bacterial septation in many species. The B. bacteriovorus genome contains three ParA homologs; we identify a canonical ParAB pair required for predatory cell division and show a BTH interaction between a gene product encoded from the same operon as DivIVA with the canonical ParA. The remaining ParA proteins are both expressed in Bdellovibrio but are not required for predator cell division. Instead, one of these ParA proteins coordinates gliding motility, changing the frequency at which the cells reverse direction. Our work will prime further studies into how one bacterium can co-ordinate its cell division with the destruction of another bacterium that it dwells within.

Chromosome organization by a conserved condensin-ParB system in the actinobacterium Corynebacterium glutamicum

Higher-order chromosome folding and segregation are tightly regulated in all domains of life. In bacteria, details on nucleoid organization regulatory mechanisms and function remain poorly characterized, especially in non-model species. Here, we investigate the role of DNA-partitioning protein ParB and SMC condensin complexes in the actinobacterium Corynebacterium glutamicum. Chromosome conformation capture reveals SMC-mediated long-range interactions around ten centromere-like parS sites clustered at the replication origin (oriC). At least one oriC-proximal parS site is necessary for reliable chromosome segregation. We use chromatin immunoprecipitation and photoactivated single-molecule localization microscopy to show the formation of distinct, parS-dependent ParB-nucleoprotein subclusters. We further show that SMC/ScpAB complexes, loaded via ParB at parS sites, mediate chromosomal inter-arm contacts (as previously shown in Bacillus subtilis). However, the MukBEF-like SMC complex MksBEFG does not contribute to chromosomal DNA-folding; instead, this complex is involved in plasmid maintenance and interacts with the polar oriC-tethering factor DivIVA. Our results complement current models of ParB-SMC/ScpAB crosstalk and show that some condensin complexes evolved functions that are apparently uncoupled from chromosome folding.

The regulation of higher-order chromosome folding and segregation in bacteria is poorly understood. Here, Böhm et al. provide insights into the roles of DNA partitioning protein ParB and SMC condensin complexes in Corynebacterium glutamicum.

Global phenotypic profiling identifies a conserved actinobacterial cofactor for a bifunctional PBP-type cell wall synthase

Members of the Corynebacterineae suborder of Actinobacteria have a unique cell surface architecture and, unlike most well-studied bacteria, grow by tip-extension. To investigate the distinct morphogenic mechanisms shared by these organisms, we performed a genome-wide phenotypic profiling analysis using Corynebacterium glutamicum as a model. A high-density transposon mutagenized library was challenged with a panel of antibiotics and other stresses. The fitness of mutants in each gene under each condition was then assessed by transposon-sequencing. Clustering of the resulting phenotypic fingerprints revealed a role for several genes of previously unknown function in surface biogenesis. Further analysis identified CofA (Cgp_0016) as an interaction partner of the peptidoglycan synthase PBP1a that promotes its stable accumulation at sites of polar growth. The related Mycobacterium tuberculosis proteins were also found to interact, highlighting the utility of our dataset for uncovering conserved principles of morphogenesis for this clinically relevant bacterial suborder.

The Bacillus subtilis dCMP deaminase ComEB acts as a dynamic polar localization factor for ComGA within the competence machinery

Bacillus subtilis can import DNA from the environment by an uptake machinery that localizes to a single cell pole. We investigated the roles of ComEB and of the ATPase ComGA during the state of competence. We show that ComEB plays an important role during competence, possibly because it is necessary for the recruitment of GomGA to the cell pole. ComEB localizes to the cell poles even upon expression during exponential phase, indicating that it can serve as polar marker. ComEB is also a deoxycytidylate monophosphate (dCMP) deaminase, for the function of which a conserved cysteine residue is important. However, cysteine-mutant ComEB is still capable of natural transformation, while a comEB deletion strain is highly impaired in competence, indicating that ComEB confers two independent functions. Single-molecule tracking (SMT) reveals that both proteins exchange at the cell poles between bound and unbound in a time scale of a few milliseconds, but turnover of ComGA increases during DNA uptake, whereas the mobility of ComEB is not affected. Our data reveal a highly dynamic role of ComGA during DNA uptake and an unusual role for ComEB as a mediator of polar localization, localizing by diffusion-capture on an extremely rapid time scale and functioning as a moonlighting enzyme.

A biphasic growth model for cell pole elongation in mycobacteria

Mycobacteria grow by inserting new cell wall material in discrete zones at the cell poles. This pattern implies that polar growth zones must be assembled de novo at each division, but the mechanisms that control the initiation of new pole growth are unknown. Here, we combine time-lapse optical and atomic force microscopy to measure single-cell pole growth in mycobacteria with nanometer-scale precision. We show that single-cell growth is biphasic due to a lag phase of variable duration before the new pole transitions from slow to fast growth. This transition and cell division are independent events. The difference between the lag and interdivision times determines the degree of single-cell growth asymmetry, which is high in fast-growing species and low in slow-growing species. We propose a biphasic growth model that is distinct from previous unipolar and bipolar models and resembles “new end take off” (NETO) dynamics of polar growth in fission yeast.

Mycobacteria grow by inserting new cell wall material at the cell poles. Here, Hannebelle et al. combine time-lapse optical and atomic force microscopy to show that single-cell growth is biphasic due to a lag phase of variable duration before the new pole transitions from slow to fast growth.

Intracellular morphogens: Specifying patterns at the subcellular scale

The notion that graded distributions of signals underlie the spatial organization of biological systems has long been a central pillar in the fields of cell and developmental biology. During morphogenesis, morphogens spread across tissues to guide development of the embryo. Similarly, a variety of dynamic gradients and pattern-forming networks have been discovered that shape subcellular organization. Here we discuss the principles of intracellular pattern formation by these intracellular morphogens and relate them to conceptually similar processes operating at the tissue scale. We will specifically review mechanisms for generating cellular asymmetry and consider how intracellular patterning networks are controlled and adapt to cellular geometry. Finally, we assess the general concept of intracellular gradients as a mechanism for positional control in light of current data, highlighting how the simple readout of fixed concentration thresholds fails to fully capture the complexity of spatial patterning processes occurring inside cells.

Effect of membrane composition on DivIVA-membrane interaction

DivIVA is a crucial membrane-binding protein that helps to localize other proteins to negatively curved membranes at cellular poles and division septa in Gram-positive bacteria. The N-terminal domain of DivIVA is responsible for membrane binding. However, to which lipids the domain binds or how it recognizes the membrane negative curvature remains elusive. Using computer simulations, we demonstrate that the N-terminal domain of Streptomyces coelicolor DivIVA adsorbs to membranes with affinity and orientation dependent on the lipid composition. The domain interacts non-specifically with lipid phosphates via its arginine-rich tip and the strongest interaction is with cardiolipin. Moreover, we observed a specific attraction between a negatively charged side patch of the domain and ethanolamine lipids, which addition caused the change of the domain orientation from perpendicular to parallel alignment to the membrane plane. Similar but less electrostatically dependent behavior was observed for the N-terminal domain of Bacillus subtilis. The domain propensity for lipids which prefer negatively curved membranes could be a mechanism for the cellular localization of DivIVA protein.