Cross-linking FtsZ polymers into coherent Z rings

A key event in bacterial cytokinesis is the formation of the Z ring, which serves as a mechanical scaffold that recruits other cytokinetic proteins to establish functional divisomes. This scaffolding function of Z rings is essential throughout cytokinesis, but the underlying molecular interactions are poorly understood. Here we report that a widely conserved FtsZ binding protein, ZapA, has cytological, biochemical and biophysical properties that argue for the importance of cross-linking interactions between FtsZ polymers in the coherence of Z rings. Escherichia coli zapA null mutant cells have Z rings that are structurally looser and many helical precursors of Z rings fail to coalesce into coherent rings. Biophysical behaviour of FtsZ in the presence of ZapA reveals that ZapA not only bundles, but also cross-links FtsZ polymers, which makes it the first cross-linking protein of the bacterial cytoskeleton. Cross-linking in vitro occurs at the stoichiometry of FtsZ-ZapA interaction at the Z rings in vivo, where nearly all intracellular ZapA is dynamically associated. ZapA also stabilizes longitudinal bonds between FtsZ monomers since it promotes the polymerization of FtsZ mutants with lesions at the polymerization interface and since it reverses the inhibitory effects of SulA, a known antagonist of FtsZ longitudinal interactions.

Examination of the interaction between FtsZ and MinCN in E. coli suggests how MinC disrupts Z rings

In Escherichia coli the Min system prevents Z ring assembly at cell poles by topologically regulating the division inhibitor MinC. The MinC protein has two domains of equal size and both domains can target FtsZ and block cell division in the proper context. Recently, we have shown that, along with MinD, the C-terminal domain of MinC (MinC(C)) competes with FtsA, and to a lesser extent with ZipA, for interaction with the C-terminal tail of FtsZ to block division. Here we explored the interaction between the N-terminal domain of MinC (MinC(N)) and FtsZ. A search for mutations in ftsZ that confer resistance to MinC(N) identified an alpha-helix at the interface of FtsZ subunits as being critical for the activity of MinC(N). Focusing on one such mutant FtsZ-N280D, we showed that it greatly reduced the FtsZ-MinC interaction and was resistant to MinC(N) both in vivo and in vitro. With these results, an updated model for the action of MinC on FtsZ is proposed: MinC interacts with FtsZ to disrupt two interactions, FtsZ-FtsA/ZipA and FtsZ-FtsZ, both of which are essential for Z ring formation.

The conserved C-terminal tail of FtsZ is required for the septal localization and division inhibitory activity of MinC(C)/MinD

The Escherichia coli Min system contributes to spatial regulation of cytokinesis by preventing assembly of the Z ring away from midcell. MinC is a cell division inhibitor whose activity is spatially regulated by MinD and MinE. MinC has two functional domains of similar size, both of which have division inhibitory activity in the proper context. However, the molecular mechanism of the inhibitory action of either domain is not very clear. Here, we report that the septal localization and division inhibitory activity of MinC(C)/MinD requires the conserved C-terminal tail of FtsZ. This tail also mediates interaction with two essential division proteins, ZipA and FtsA, to link FtsZ polymers to the membrane. Overproduction of MinC(C)/MinD displaces FtsA from the Z ring and eventually disrupts the Z ring, probably because it also displaces ZipA. These results support a model for the division inhibitory action of MinC/MinD. MinC/MinD binds to ZipA and FtsA decorated FtsZ polymers located at the membrane through the MinC(C)/MinD-FtsZ interaction. This binding displaces FtsA and/or ZipA, and more importantly, positions MinC(N) near the FtsZ polymers making it a more effective inhibitor.

Overview of cell shape: cytoskeletons shape bacterial cells

An evolving hypothesis is that bacterial cell shape is determined by cytoskeletal elements that localize peptidoglycan synthetic machineries. In most bacteria FtsZ assembles into the Z ring which recruits the machinery necessary for cytokinesis. Most rod shaped cells require MreB which assembles into cables that run between the poles of the cell and distribute various components of peptidoglycan metabolism along the cell length. Cells with other shapes have additional cytoskeletal elements that either localize synthetic machineries or possibly influence their activity.

Assembly dynamics of the bacterial MinCDE system and spatial regulation of the Z ring

The positioning of a cytoskeletal element that dictates the division plane is a fundamental problem in biology. The assembly and positioning of this cytoskeletal element has to be coordinated with DNA segregation and cell growth to ensure that equal-sized progeny cells are produced, each with a copy of the chromosome. In most prokaryotes, cytokinesis involves positioning a Z ring assembled from FtsZ, the ancestral homologue of tubulin. The position of the Z ring is determined by a gradient of negative regulators of Z-ring assembly. In Escherichia coli, the Min system consists of three proteins that cooperate to position the Z ring through a fascinating oscillation, which inhibits the formation of the Z ring away from midcell. Additional gradients of negative regulators of FtsZ assembly are used by E. coli and other bacteria to achieve spatial control of Z-ring assembly.

Identification of a region of FtsA required for interaction with FtsZ

The assembly of the Z ring is the earliest step in bacterial cell division. In Escherichia coli this assembly requires either FtsA or ZipA which bind to a conserved, C-terminal 17 amino acid motif in FtsZ and to the membrane. The FtsZ-ZipA interaction is well characterized; however, nothing is known about the region of FtsA involved in the interaction with FtsZ even though the FtsA-FtsZ interaction is nearly ubiquitous in Eubacteria. FtsA is proposed to bind to the membrane through its conserved C-terminal amphiphatic helix before efficiently interacting with FtsZ. Based upon this model we designed a genetic screen to identify mutants specifically impaired for the FtsA-FtsZ interaction. The mutants obtained retain the ability to be targeted to the membrane but fail to be recruited to the Z ring or interact with FtsZ in the yeast two-hybrid system. These mutants do not complement an ftsA-depletion strain. Through this approach we have identified a region of FtsA containing some invariant residues which is required for binding to FtsZ. The results support our model that FtsA is targeted to the membrane before it interacts with FtsZ and demonstrates that this interaction plays an essential role in E. coli cell division.

Z ring as executor of bacterial cell division

It has become apparent that bacteria possess ancestors of the major eukaryotic cytoskeletal proteins. FtsZ, the ancestral homologue of tubulin, assembles into a cytoskeletal structure associated with cell division, designated the Z ring. Formation of the Z ring represents a major point of both spatial and temporal regulation of cell division. Here we discuss findings concerning the structure and the formation of the ring as well as its spatial and temporal regulation.

Tethering the Z ring to the membrane through a conserved membrane targeting sequence in FtsA

The cytokinetic Z ring is required for bacterial cell division. It consists of polymers of FtsZ, the bacterial ancestor of eukaryotic tubulin, linked to the cytoplasmic membrane. Formation of a Z ring in Escherichia coli occurs as long as one of two proteins, ZipA or FtsA, is present. Both of these proteins bind FtsZ suggesting that they might function to tether FtsZ filaments to the membrane. Although ZipA has a transmembrane domain and therefore can function as a membrane anchor, interaction of FtsA with the membrane has not been explored. In this study we demonstrate that FtsA, which is structurally related to eukaryotic actin, has a conserved C-terminal amphipathic helix that is essential for FtsA function. It is required to target FtsA to the membrane and subsequently to the Z ring. As FtsA is much more widely conserved in bacteria than ZipA, it is likely that FtsA serves as the principal membrane anchor for the Z ring.

MinC mutants deficient in MinD- and DicB-mediated cell division inhibition due to loss of interaction with MinD, DicB, or a septal component

The min locus encodes a negative regulatory system that limits formation of the cytokinetic Z ring to midcell by preventing its formation near the poles. Of the three Min proteins, MinC is the inhibitor and prevents Z-ring formation by interacting directly with FtsZ. MinD activates MinC by recruiting it to the membrane and conferring a higher affinity on the MinCD complex for a septal component. MinE regulates the cellular location of MinCD by inducing MinD, and thereby MinC, to oscillate between the poles of the cell, resulting in a time-averaged concentration of MinCD on the membrane that is lowest at midcell. MinC can also be activated by the prophage-encoded protein DicB, which targets MinC to the septum without recruiting it first to the membrane. Previous studies have shown that the C-terminal domain of MinC is responsible for the interaction with MinD, DicB, and the septal component. In the present study, we isolated mutations in the C-terminal domain of MinC that affected its interaction with MinD, DicB, and the septal component. Among the mutations isolated, R133A and S134A are specifically deficient in the interaction with MinD, E156A is primarily affected in the interaction with DicB, and R172A is primarily deficient in the interaction with the septum. These mutations differentiate the interactions of MinC with its partners and further support the model of MinCD- and MinC-DicB-mediated cell division inhibition.

Analysis of MinD mutations reveals residues required for MinE stimulation of the MinD ATPase and residues required for MinC interaction

The MinD ATPase is critical to the oscillation of the Min proteins, which limits formation of the Z ring to midcell. In the presence of ATP, MinD binds to the membrane and recruits MinC, forming a complex that can destabilize the cytokinetic Z ring. MinE, which is also recruited to the membrane by MinD, displaces MinC and stimulates the MinD ATPase, resulting in the oscillation of the Min proteins. In this study we have investigated the role of lysine 11, present in the deviant Walker A motif of MinD, and the three residues in helix 7 (E146, S148, and D152) that interact electrostatically with lysine 11. Lysine 11 is required for interaction of MinD with the membrane, MinC, MinE, and itself. In contrast, the three residues in helix 7 that interact with lysine 11 are not required for binding to the membrane or activation of MinC. They are also not required for MinE binding; however, they are required for MinE to stimulate the MinD ATPase. Interestingly, the D152A mutant self-interacts, binds to the membrane, and recruits MinC and MinE in the presence of ADP as well as ATP. This mutant provides evidence that dimerization of MinD is sufficient for MinD to bind the membrane and recruit its partners.

The switch I and II regions of MinD are required for binding and activating MinC

MinD and MinC cooperate to form an efficient inhibitor of Z-ring formation that is spatially regulated by MinE. MinD activates MinC by recruiting it to the membrane and targeting it to a septal component. To better understand this activation, we have isolated loss-of-function mutations in minD and carried out site-directed mutagenesis. Many of these mutations block MinC-MinD interaction; however, they also prevent MinD self-interaction and membrane binding, suggesting that they affect nucleotide interaction or protein folding. Two mutations in the switch I region (MinD box) and one mutation in the switch II region had little affect on most MinD functions, such as MinD self-interaction, membrane binding, and MinE stimulation; however, they did eliminate MinD-MinC interaction. Two additional mutations in the switch II region did not affect MinC binding. Further study revealed that one of these allowed the MinCD complex to target to the septum but was still deficient in blocking division. These results indicate that the switch I and II regions of MinD are required for interaction with MinC but not MinE and that the switch II region has a role in activating MinC.

Another cytoskeleton in the closet

Many eukaryotic cells contain up to three families of cytoskeletal proteins that are responsible for the spatial organization of the cell. Although the prokaryotic origins of the actin and tubulin families have now been established, the origin of the third was unknown. In this issue of Cell, provide evidence that the third family, comprising the intermediate filaments, also has origins in bacteria and is responsible for producing curved cells.

Membrane binding by MinD involves insertion of hydrophobic residues within the C-terminal amphipathic helix into the bilayer

MinD binds to phospholipid vesicles in the presence of ATP and is released by MinE, which stimulates the MinD ATPase. Membrane binding requires a short conserved C-terminal region, which has the potential to form an amphipathic helix. This finding has led to a model in which the binding of ATP regulates the formation or accessibility of this helix, which then embeds in the membrane bilayer. To test this model, we replaced each of the four hydrophobic residues within this potential helix with tryptophan or a charged residue. Introduction of a negatively charged amino acid decreased membrane binding of MinD and its ability to activate MinC. In contrast, mutants with tryptophan substitutions retained the ability to bind to the membrane and activate MinC. Fluorescence emission spectroscopy analysis of the tryptophan mutants F263W, L264W, and L267W confirmed that these tryptophan residues did insert into the hydrophobic interior of the bilayer. We conclude that membrane binding by MinD involves penetration of the hydrophobic residues within the C-terminal amphipathic helix into the hydrophobic interior of the bilayer.

MinD and role of the deviant Walker A motif, dimerization and membrane binding in oscillation

The ATPase activity of MinD is required for it to oscillate between the ends of the cell and spatially regulate cell division in Escherichia coli. It is a member of a functionally diverse subgroup of ATPases which are involved in activities ranging from nitrogen fixation (NifH) to plasmid segregation (ParA). All members of the subgroup have a deviant Walker A motif which contains a conserved 'signature' lysine that characterizes this subgroup. In the NifH homodimer the signature lysines make intermonomer contact with the bound nucleotides indicating a role in ATP hydrolysis. ATP binding to NifH leads to formation of an active dimer that associates with a partner that is also a dimer. Because ATP hydrolysis is coupled to formation of the complex, the complex is only transient. In the presence of ATP MinD binds MinC and goes to the membrane, however, the ATPase is not stimulated and the complex is stable. Subsequent interaction of this complex with MinE, however, leads to ATPase stimulation and release of the Min proteins from the membrane. The sequential interaction of MinD with these two proteins, which is dictated by the membrane, is critical to the oscillatory mechanism involved in spatial regulation of division.

A conserved sequence at the C-terminus of MinD is required for binding to the membrane and targeting MinC to the septum

MinD is a key component of an oscillatory system that spatially regulates cell division in Escherichia coli. It is a peripheral membrane ATPase that recruits MinC and oscillates between the two halves of the cell in a MinE dependent manner. In vitro MinD binds to phospholipid vesicles in an ATP-dependent manner and is released through MinE-stimulated ATP hydrolysis. In this study we examined the function of the conserved C-terminus of MinD. Short truncations of three and ten amino acids dramatically decreased the ability of MinD to localize to the membrane and spatially regulate division. These truncations bound MinC but were deficient in targeting MinC to the septum. In vitro they dimerized, but were deficient in binding to phospholipid vesicles and undergoing MinE stimulation. We suggest a model in which the ATP-dependent dimerization of MinD affects the conformation of the C-terminal region, a potential amphipathic helix, triggering membrane binding.

Recruitment of MinC, an inhibitor of Z-ring formation, to the membrane in Escherichia coli: role of MinD and MinE

In Escherichia coli, the min system prevents division away from midcell through topological regulation of MinC, an inhibitor of Z-ring formation. The topological regulation involves oscillation of MinC between the poles of the cell under the direction of the MinDE oscillator. Since the mechanism of MinC involvement in the oscillation is unknown, we investigated the interaction of MinC with the other Min proteins. We observed that MinD dimerized in the presence of ATP and interacted with MinC. In the presence of a phospholipid bilayer, MinD bound to the bilayer and recruited MinC in an ATP-dependent manner. Addition of MinE to the MinCD-bilayer complex resulted in release of both MinC and MinD. The release of MinC did not require ATP hydrolysis, indicating that MinE could displace MinC from the MinD-bilayer complex. In contrast, MinC was unable to displace MinE bound to the MinD-bilayer complex. These results suggest that MinE induces a conformational change in MinD bound to the bilayer that results in the release of MinC. Also, it is argued that binding of MinD to the membrane activates MinC.

Dynamic proteins in bacteria

Growth of the bacterial cell involves proteins that assemble into dynamic localized structures that are required for cellular morphogenesis and division. During the past year, the continued application of fluorescence microscopy has led to the discovery of novel actin-like filaments involved in cell shape and plasmid DNA segregation, and to new insights into the regulation and dynamics of the Z-ring. Studies on the Min proteins, which rapidly oscillate between the cell poles to spatially regulate Z-ring assembly, has led to a biochemical basis for the oscillation and a suggestion that MinD assembles into dynamic filaments. These studies further demonstrate that the eukaryotic cytoskeleton had its origins in bacteria.

Dynamic assembly of MinD on phospholipid vesicles regulated by ATP and MinE

Selection of the division site in Escherichia coli is regulated by the min system and requires the rapid oscillation of MinD between the two halves of the cell under the control of MinE. In this study we have further investigated the molecular basis for this oscillation by examining the interaction of MinD with phospholipid vesicles. We found that MinD bound to phospholipid vesicles in the presence of ATP and, upon binding, assembled into a well-ordered helical array that deformed the vesicles into tubes. Stimulation of the MinD ATPase by addition of MinE led to disassembly of the tubes and the release of MinD from the vesicles. It is proposed that this MinE-regulated dynamic assembly of MinD underlies MinD oscillation.

Unexpected twist to the Z ring

Development in Bacillus subtilis involves a switch in the location of the cytokinetic Z ring from midcell to the pole. Time lapse photography of an FtsZ-GFP fusion reveals that this switch involves a spiral intermediate and allows identification of the specific sporulation functions involved.

Unique and overlapping roles for ZipA and FtsA in septal ring assembly in Escherichia coli

ZipA and FtsA are essential division proteins in Escherichia coli that are recruited to the division site by interaction with FtsZ. Utilizing a newly isolated temperature-sensitive mutation in zipA we have more fully characterized the role of ZipA. We confirmed that ZipA is not required for Z ring formation; however, we found that ZipA, like FtsA, is required for recruitment of FtsK and therefore all downstream division proteins. In the absence of FtsA or ZipA Z rings formed; however, in the absence of both, new Z rings were unable to form and preformed Z rings were destabilized. Consistent with this, we found that an FtsZ mutant unable to interact with both ZipA and FtsA was unable to assemble into Z rings. These results demonstrate that ZipA and FtsA are both required for recruitment of additional division proteins to the Z ring, but either one is capable of supporting formation and stabilization of Z rings.

Assembly of an FtsZ mutant deficient in GTPase activity has implications for FtsZ assembly and the role of the Z ring in cell division

FtsZ, the ancestral homologue of eukaryotic tubulins, assembles into the Z ring, which is required for cytokinesis in prokaryotic cells. Both FtsZ and tubulin have a GTPase activity associated with polymerization. Interestingly, the ftsZ2 mutant is viable, although the FtsZ2 mutant protein has dramatically reduced GTPase activity due to a glycine-for-aspartic acid substitution within the synergy loop. In this study, we have examined the properties of FtsZ2 and found that the reduced GTPase activity is not enhanced by DEAE-dextran-induced assembly, indicating it has a defective catalytic site. In the absence of DEAE-dextran, FtsZ2 fails to assemble unless supplemented with wild-type FtsZ. FtsZ has to be at or above the critical concentration for copolymerization to occur, indicating that FtsZ is nucleating the copolymers. The copolymers formed are relatively stable and appear to be stabilized by a GTP-cap. These results indicate that FtsZ2 cannot nucleate assembly in vitro, although it must in vivo. Furthermore, the stability of FtsZ-FtsZ2 copolymers argues that FtsZ2 polymers would be stable, suggesting that stable FtsZ polymers are able to support cell division.