The hypermorph FtsA* protein has an in vivo role in relieving the Escherichia coli proto-ring block caused by excess ZapC

Cell Cycle-Dependent Recruitment of FtsN to the Divisome in Escherichia coli

Cell division in Escherichia coli starts with the formation of an FtsZ protofilament network at midcell, the Z ring. However, only after a considerable lag period does the cell start to form a midcell constriction. The onset of constriction depends upon the arrival of so-called late divisome proteins, among which, FtsN is the last essential one. The timing and dependency of FtsN arrival to the divisome, along with genetic evidence, suggests it triggers cell division. In this study, we used high-throughput fluorescence microscopy to determine the arrival of FtsN and the early divisome protein ZapA to midcell at a single-cell level during the cell cycle. Our data show while the recruitment of ZapA/FtsZ is gradual in the cell cycle, recruitment of FtsN is rapid and begins at about the onset of constriction. At this time, the fraction of ZapA/FtsZ in the Z ring approaches its peak value. We also find a second increase in FtsN recruitment to the divisome, which begins once the amount of ZapA/FtsZ at midcell starts decreasing. Increasing hypermorphic FtsA* (FtsA R286W), but not FtsA, accelerates FtsN recruitment but not constriction. This finding is consistent with FtsA* recruiting FtsN with some other divisome component being rate-limiting for constriction under these conditions. Finally, our data support the recently proposed idea that ZapA/FtsZ and FtsN are part of physically separate complexes in midcell throughout the whole septation process. IMPORTANCE Cell division in most bacteria starts with the formation of an FtsZ protofilament network at midcell, the Z ring. However, cells only start to constrict after a considerable lag. A factor thought to trigger the onset of constriction in Escherichia coli is FtsN, which is the last essential protein to be recruited to the Z ring. Using a high-throughput quantitative fluorescence microscopy, we determine the cell cycle-dependent recruitment of FtsN to the Z ring. Our data show rapid accumulation of FtsN to the Z ring about a quarter of the cell cycle after the formation of the Z ring. This initial wave is followed by another increase in FtsN recruitment once the FtsZ protofilament network starts to disassemble. The presence of FtsA* accelerates FtsN recruitment to the Z ring but does not lead to earlier constrictions. Our data furthermore suggest FtsZ and FtsN are part of physically separate complexes throughout the division process.

Localization, Assembly, and Activation of the Escherichia coli Cell Division Machinery

Decades of research, much of it in Escherichia coli, have yielded a wealth of insight into bacterial cell division. Here, we provide an overview of the E. coli division machinery with an emphasis on recent findings. We begin with a short historical perspective into the discovery of FtsZ, the tubulin homolog that is essential for division in bacteria and archaea. We then discuss assembly of the divisome, an FtsZ-dependent multiprotein platform, at the midcell septal site. Not simply a scaffold, the dynamic properties of polymeric FtsZ ensure the efficient and uniform synthesis of septal peptidoglycan. Next, we describe the remodeling of the cell wall, invagination of the cell envelope, and disassembly of the division apparatus culminating in scission of the mother cell into two daughter cells. We conclude this review by highlighting some of the open questions in the cell division field, emphasizing that much remains to be discovered, even in an organism as extensively studied as E. coli.

FtsZ dynamics in bacterial division: What, how, and why?

Bacterial cell division is orchestrated by the divisome, a protein complex centered on the tubulin homolog FtsZ. FtsZ polymerizes into a dynamic ring that defines the division site, recruits downstream proteins, and directs peptidoglycan synthesis to drive constriction. Recent studies have documented treadmilling of FtsZ polymer clusters both in cells and in vitro. Emerging evidence suggests that FtsZ dynamics are regulated largely by intrinsic properties of FtsZ itself and by the membrane anchoring protein FtsA. Although FtsZ dynamics are broadly required for Z-ring assembly, their role(s) during constriction may vary among bacterial species. These recent advances set the stage for future studies to investigate how FtsZ dynamics are physically and/or functionally coupled to peptidoglycan metabolic enzymes to direct efficient division.

Direct Interaction between the Two Z Ring Membrane Anchors FtsA and ZipA

The initiation of Escherichia coli cell division requires three proteins, FtsZ, FtsA, and ZipA, which assemble in a dynamic ring-like structure at midcell. Along with the transmembrane protein ZipA, the actin-like FtsA helps to tether treadmilling polymers of tubulin-like FtsZ to the membrane. In addition to forming homo-oligomers, FtsA and ZipA interact directly with the C-terminal conserved domain of FtsZ. Gain-of-function mutants of FtsA are deficient in forming oligomers and can bypass the need for ZipA, suggesting that ZipA may normally function to disrupt FtsA oligomers, although no direct interaction between FtsA and ZipA has been reported. Here, we use in vivo cross-linking to show that FtsA and ZipA indeed interact directly. We identify the exposed surface of FtsA helix 7, which also participates in binding to ATP through its internal surface, as a key interface needed for the interaction with ZipA. This interaction suggests that FtsZ's membrane tethers may regulate each other's activities.IMPORTANCE To divide, most bacteria first construct a protein machine at the plane of division and then recruit the machinery that will synthesize the division septum. In Escherichia coli, this first stage involves the assembly of FtsZ polymers at midcell, which directly bind to membrane-associated proteins FtsA and ZipA to form a discontinuous ring structure. Although FtsZ directly binds both FtsA and ZipA, it is unclear why FtsZ requires two separate membrane tethers. Here, we uncover a new direct interaction between the tethers, which involves a helix within FtsA that is adjacent to its ATP binding pocket. Our findings imply that in addition to their known roles as FtsZ membrane anchors, FtsA and ZipA may regulate each other's structure and function.

Escherichia coli ZipA Organizes FtsZ Polymers into Dynamic Ring-Like Protofilament Structures

ZipA is an essential cell division protein in Escherichia coli. Together with FtsA, ZipA tethers dynamic polymers of FtsZ to the cytoplasmic membrane, and these polymers are required to guide synthesis of the cell division septum. This dynamic behavior of FtsZ has been reconstituted on planar lipid surfaces in vitro, visible as GTP-dependent chiral vortices several hundred nanometers in diameter, when anchored by FtsA or when fused to an artificial membrane binding domain. However, these dynamics largely vanish when ZipA is used to tether FtsZ polymers to lipids at high surface densities. This, along with some in vitro studies in solution, has led to the prevailing notion that ZipA reduces FtsZ dynamics by enhancing bundling of FtsZ filaments. Here, we show that this is not the case. When lower, more physiological levels of the soluble, cytoplasmic domain of ZipA (sZipA) were attached to lipids, FtsZ assembled into highly dynamic vortices similar to those assembled with FtsA or other membrane anchors. Notably, at either high or low surface densities, ZipA did not stimulate lateral interactions between FtsZ protofilaments. We also used E. coli mutants that are either deficient or proficient in FtsZ bundling to provide evidence that ZipA does not directly promote bundling of FtsZ filaments in vivo. Together, our results suggest that ZipA does not dampen FtsZ dynamics as previously thought, and instead may act as a passive membrane attachment for FtsZ filaments as they treadmill.

Bacterial cells use a membrane-attached ring of proteins to mark and guide formation of a division septum at midcell that forms a wall separating the two daughter cells and allows cells to divide. The key protein in this ring is FtsZ, a homolog of tubulin that forms dynamic polymers. Here, we use electron microscopy and confocal fluorescence imaging to show that one of the proteins required to attach FtsZ polymers to the membrane during E. coli cell division, ZipA, can promote dynamic swirls of FtsZ on a lipid surface in vitro. Importantly, these swirls are observed only when ZipA is present at low, physiologically relevant surface densities. Although ZipA has been thought to enhance bundling of FtsZ polymers, we find little evidence for bundling in vitro. In addition, we present several lines of in vivo evidence indicating that ZipA does not act to directly bundle FtsZ polymers.

Delineating FtsQ-mediated regulation of cell division in Mycobacterium tuberculosis

Identifying and characterizing the individual contributors to bacterial cellular elongation and division will improve our understanding of their impact on cell growth and division. Here, we delineated the role of ftsQ, a terminal gene of the highly conserved division cell wall (dcw) operon, in growth, survival, and cell length maintenance in the human pathogen Mycobacterium tuberculosis (Mtb). We found that FtsQ overexpression significantly increases the cell length and number of multiseptate cells. FtsQ depletion in Mtb resulted in cells that were shorter than WT cells during the initial growth stages (4 days after FtsQ depletion) but were longer than WT cells at later stages (10 days after FtsQ depletion) and compromised the survival in vitro and in differentiated THP1 macrophages. Overexpression of N- and C-terminal FtsQ regions altered the cell length, and the C-terminal domain alone complemented the FtsQ depletion phenotype. MS analyses suggested robust FtsQ phosphorylation on Thr-24, and although phosphoablative and -mimetic mutants rescued the FtsQ depletion-associated cell viability defects, they failed to complement the cell length defects. MS and coimmunoprecipitation experiments identified 63 FtsQ-interacting partners, and we show that the interaction of FtsQ with the recently identified cell division protein SepIVA is independent of FtsQ phosphorylation and suggests a role of FtsQ in modulating cell division. FtsQ exhibited predominantly septal localization in both the presence and absence of SepIVA. Our results suggest a role for FtsQ in modulating the length, division, and survival of Mtb cells both in vitro and in the host.

Unite to divide: Oligomerization of tubulin and actin homologs regulates initiation of bacterial cell division

To generate two cells from one, bacteria such as Escherichia coli use a complex of membrane-embedded proteins called the divisome that synthesize the division septum. The initial stage of cytokinesis requires a tubulin homolog, FtsZ, which forms polymers that treadmill around the cell circumference. The attachment of these polymers to the cytoplasmic membrane requires an actin homolog, FtsA, which also forms dynamic polymers that directly bind to FtsZ. Recent evidence indicates that FtsA and FtsZ regulate each other’s oligomeric state in E. coli to control the progression of cytokinesis, including the recruitment of septum synthesis proteins. In this review, we focus on recent advances in our understanding of protein-protein association between FtsZ and FtsA in the initial stages of divisome function, mainly in the well-characterized E. coli system.