Localization of FtsL to the Escherichia coli septal ring

YgbQ, a cell division protein in Escherichia coli and Vibrio cholerae, localizes in codependent fashion with FtsL to the division site

YgbQ is a cell division protein in Escherichia coli and Vibrio cholerae. In E. coli the ygbQ gene was discovered as a result of a computer search of the E. coli genome designed to find potential interacting partners for cell division protein FtsL. In V. cholerae, ygbQ was identified as an essential gene by using a transposon that fuses genes to an arabinose promoter. The role of YgbQ in cell division is supported by the following. Cells depleted of YgbQ in both organisms form long filaments, but DNA segregation is not affected. YgbQ localizes to the constriction site in wild-type E. coli cells. Localization of E. coli YgbQ to the constriction site depends on cell division proteins FtsQ and FtsL but not FtsW and FtsI, placing YgbQ in the sequential dependency order of proteins localizing to the division site. Localization of green fluorescent protein-FtsL also depends on YgbQ, indicating that FtsL and YgbQ colocalize to the division site in E. coli. Our results show colocalization of proteins to the bacterial midcell in E. coli and raise the possibility that these proteins interact in a coiled-coil structure.

Partitioning of chromosomal DNA during establishment of cellular asymmetry in Bacillus subtilis

The switch from symmetric to asymmetric cell division is a key feature of development in many organisms, including Bacillus subtilis sporulation. Here we demonstrate that, prior to the onset of asymmetric cell division, the B. subtilis chromosome is partitioned into two unequally sized domains, with the origin-proximal one-third of the future forespore chromosome condensed near one pole of the cell. Asymmetric chromosome partitioning is independent of polar division, as it occurs in cells depleted of FtsZ but depends on two transcription factors that govern the initiation of sporulation, sigma(H) and Spo0A-P. It is also independent of chromosome partitioning proteins Spo0J and Soj, suggesting the existence of a novel mechanism controlling chromosome structure. Thus, our results demonstrate that, during sporulation, two separable events prepare B. subtilis for asymmetric cell division: the relocation of cell division sites to the cell poles and the asymmetric partitioning of the future forespore chromosome.

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.

Analysis of ftsQ mutant alleles in Escherichia coli: complementation, septal localization, and recruitment of downstream cell division proteins

FtsQ, a 276-amino-acid, bitopic membrane protein, is one of the nine proteins known to be essential for cell division in gram-negative bacterium Escherichia coli. To define residues in FtsQ critical for function, we performed random mutagenesis on the ftsQ gene and identified four alleles (ftsQ2, ftsQ6, ftsQ15, and ftsQ65) that fail to complement the ftsQ1(Ts) mutation at the restrictive temperature. Two of the mutant proteins, FtsQ6 and FtsQ15, are functional at lower temperatures but are unable to localize to the division site unless wild-type FtsQ is depleted, suggesting that they compete poorly with the wild-type protein for septal targeting. The other two mutants, FtsQ2 and FtsQ65, are nonfunctional at all temperatures tested and have dominant-negative effects when expressed in an ftsQ1(Ts) strain at the permissive temperature. FtsQ2 and FtsQ65 localize to the division site in the presence or absence of endogenous FtsQ, but they cannot recruit downstream cell division proteins, such as FtsL, to the septum. These results suggest that FtsQ2 and FtsQ65 compete efficiently for septal targeting but fail to promote the further assembly of the cell division machinery. Thus, we have separated the localization ability of FtsQ from its other functions, including recruitment of downstream cell division proteins, and are beginning to define regions of the protein responsible for these distinct capabilities.

The Escherichia coli cell division protein FtsW is required to recruit its cognate transpeptidase, FtsI (PBP3), to the division site

The bacterial cell division protein FtsW has been suggested to perform two functions: stabilize the FtsZ cytokinetic ring, and facilitate septal peptidoglycan synthesis by the transpeptidase FtsI (penicillin-binding protein 3). We show here that depleting Escherichia coli cells of FtsW had little effect on the abundance of FtsZ rings but abrogated recruitment of FtsI to potential division sites. Analysis of FtsW localization confirmed and extended these results; septal localization of FtsW required FtsZ, FtsA, FtsQ, and FtsL but not FtsI. Thus, FtsW is a late recruit to the division site and is essential for subsequent recruitment of its cognate transpeptidase FtsI but not for stabilization of FtsZ rings. We suggest that a primary function of FtsW homologues--which are found in almost all bacteria and appear to work in conjunction with dedicated transpeptidases involved in division, elongation, or sporulation--is to recruit their cognate transpeptidases to the correct subcellular location.

Green fluorescent protein--a bright idea for the study of bacterial protein localization

Use of the green fluorescent protein (GFP) of Aequorea victoria as a reporter for protein and DNA localization has provided sensitive, new approaches for studying the organization of the bacterial cell, leading to new insights into diverse cellular processes. GFP has many characteristics that make it useful for localization studies in bacteria, primarily its ability to fluoresce when fused to target polypeptides without the addition of exogenously added substrates. As an alternative to immunofluorescence microscopy, the expression of gfp gene fusions has been used to probe the function of cellular components fundamental for DNA replication, translation, protein export, and signal transduction, that heretofore have been difficult to study in living cells. Moreover, protein and DNA localization can now be monitored in real time, revealing that several proteins important for cell division, development and sporulation are dynamically localized throughout the cell cycle. The use of additional GFP variants that permit the labeling of multiple components within the same cell, and the use of GFP for genetic screens, should continue to make this a valuable tool for addressing complex questions about the bacterial cell.

FtsQ, FtsL and FtsI require FtsK, but not FtsN, for co-localization with FtsZ during Escherichia coli cell division

During cell division in Gram-negative bacteria, the cell envelope invaginates and constricts at the septum, eventually severing the cell into two compartments, and separating the replicated genetic materials. In Escherichia coli, at least nine essential gene products participate directly in septum formation: FtsA, FtsI, FtsL, FtsK, FtsN, FtsQ, FtsW, FtsZ and ZipA. All nine proteins have been localized to the septal ring, an equatorial ring structure at the division site. We used translational fusions to green fluorescent protein (GFP) to demonstrate that FtsQ, FtsL and FtsI localize to potential division sites in filamentous cells depleted of FtsN, but not in those depleted of FtsK. We also constructed translational fusions of FtsZ, FtsA, FtsQ, FtsL and FtsI to enhanced cyan or yellow fluorescent protein (ECFP or EYFP respectively), GFP variants with different fluorescence spectra. Examination of cells expressing different combinations of the fusions indicated that FtsA, FtsQ, FtsL and FtsI co-localize with FtsZ in filaments depleted of FtsN. These localization results support the model that E. coli cell division proteins assemble sequentially as a multimeric complex at the division site: first FtsZ, then FtsA and ZipA independently of each other, followed successively by FtsK, FtsQ, FtsL, FtsW, FtsI and FtsN.

A set of ftsZ mutants blocked at different stages of cell division in Caulobacter

FtsZ is required throughout the cell division process in eubacteria and in archaea. We report the isolation of novel mutants of the FtsZ gene in Caulobacter crescentus. Clusters of charged amino acids were changed to alanine to minimize mutations that affect protein folding. Molecular modelling indicated that all the clustered-charged-to-alanine mutations had altered amino acids at the surface of the protein. Of 13 such mutants, four were recessive-lethal, three were dominant-lethal, and six had no discernible phenotype. An FtsZ depletion strain of Caulobacter was constructed to analyse the phenotype of the recessive-lethal mutations and used to show that they blocked cell division at distinct stages. One mutation blocked the initiation of cell division, two mutations blocked cell division randomly, and one mutation blocked both early and late stages of cell division. The effect of the recessive mutations on the subcellular localization of FtsZ was determined. Models to explain the various mutant phenotypes are discussed. This is the first set of recessive alleles of ftsZ blocked at different stages of cell division.

Cytological and biochemical characterization of the FtsA cell division protein of Bacillus subtilis

The actin-like protein FtsA is present in many eubacteria, and genetic experiments have shown that it plays an important, sometimes essential, role in cell division. Here, we show that Bacillus subtilis FtsA is targeted to division sites in both vegetative and sporulating cells. As in other organisms FtsA is probably recruited immediately after FtsZ. In sporulating cells of B. subtilis FtsZ is recruited to potential division sites at both poles of the cell, but asymmetric division occurs at only one pole. We have now found that FtsA is recruited to only one cell pole, suggesting that it may play an important role in the generation of asymmetry in this system. FtsA is present in much higher quantities in B. subtilis than in Escherichia coli, with approximately one molecule of FtsA for five of FtsZ. This means that there is sufficient FtsA to form a complete circumferential ring at the division site. Therefore, FtsA may have a direct structural role in cell division. We have purified FtsA and shown that it behaves as a dimer and that it has both ATP-binding and ATP-hydrolysis activities. This suggests that ATP hydrolysis by FtsA is required, together with GTP hydrolysis by FtsZ, for cell division in B. subtilis (and possibly in most eubacteria).

Crystal structure of the cell division protein FtsA from Thermotoga maritima

Bacterial cell division requires formation of a septal ring. A key step in septum formation is polymerization of FtsZ. FtsA directly interacts with FtsZ and probably targets other proteins to the septum. We have solved the crystal structure of FtsA from Thermotoga maritima in the apo and ATP-bound form. FtsA consists of two domains with the nucleotide-binding site in the interdomain cleft. Both domains have a common core that is also found in the actin family of proteins. Structurally, FtsA is most homologous to actin and heat-shock cognate protein (Hsc70). An important difference between FtsA and the actin family of proteins is the insertion of a subdomain in FtsA. Movement of this subdomain partially encloses a groove, which could bind the C-terminus of FtsZ. FtsZ is the bacterial homologue of tubulin, and the FtsZ ring is functionally similar to the contractile ring in dividing eukaryotic cells. Elucidation of the crystal structure of FtsA shows that another bacterial protein involved in cytokinesis is structurally related to a eukaryotic cytoskeletal protein involved in cytokinesis.

Themes and variations in prokaryotic cell division

Perhaps the biggest single task facing a bacterial cell is to divide into daughter cells that contain the normal complement of chromosomes. Recent technical and conceptual breakthroughs in bacterial cell biology, combined with the flood of genome sequence information and the excellent genetic tools in several model systems, have shed new light on the mechanism of prokaryotic cell division. There is good evidence that in most species, a molecular machine, organized by the tubulin-like FtsZ protein, assembles at the site of division and orchestrates the splitting of the cell. The determinants that target the machine to the right place at the right time are beginning to be understood in the model systems, but it is still a mystery how the machine actually generates the constrictive force necessary for cytokinesis. Moreover, although some cell division determinants such as FtsZ are present in a broad spectrum of prokaryotic species, the lack of FtsZ in some species and different profiles of cell division proteins in different families suggests that there are diverse mechanisms for regulating cell division.

Green fluorescent protein functions as a reporter for protein localization in Escherichia coli

The use of green fluorescent protein (GFP) as a reporter for protein localization in Escherichia coli was explored by creating gene fusions between malE (encoding maltose-binding protein [MBP]) and a variant of gfp optimized for fluorescence in bacteria (GFPuv). These constructs encode hybrid proteins composed of GFP fused to the carboxy-terminal end of MBP. Fluorescence was not detected when the hybrid protein was synthesized with the MBP signal sequence. In contrast, when the MBP signal sequence was deleted, fluorescence was observed. Cell fractionation studies showed that the fluorescent MBP-GFP hybrid protein was localized in the cytoplasm, whereas the nonfluorescent version was localized to the periplasmic space. Smaller MBP-GFP hybrid proteins, however, exhibited abnormal fractionation. Expression of the gene fusions in different sec mutants, as well as signal sequence processing assays, confirmed that the periplasmically localized hybrid proteins were exported by the sec-dependent pathway. The distinction between fluorescent and nonfluorescent colonies was exploited as a scorable phenotype to isolate malE signal sequence mutations. While expression of hybrid proteins comprised of full-length MBP did not result in overproduction lethality characteristic of some exported beta-galactosidase hybrid proteins, synthesis of shorter, exported hybrid proteins was toxic to the cells. Purification of MBP-GFP hybrid protein from the different cellular compartments indicated that GFP is improperly folded when localized outside of the cytoplasm. These results suggest that GFP could serve as a useful reporter for genetic analysis of bacterial protein export and of protein folding.

Septal localization of the membrane-bound division proteins of Bacillus subtilis DivIB and DivIC is codependent only at high temperatures and requires FtsZ

Using immunofluorescence microscopy, we have examined the dependency of localization among three Bacillus subtilis division proteins, FtsZ, DivIB, and DivIC, to the division site. DivIC is required for DivIB localization. However, DivIC localization is dependent on DivIB only at high growth temperatures, at which DivIB is essential for division. FtsZ localization is required for septal recruitment of DivIB and DivIC, but FtsZ can be recruited independently of DivIB. These localization studies suggest a more specific role for DivIB in division, involving interaction with DivIC.

The Bacillus subtilis cell division protein FtsL localizes to sites of septation and interacts with DivIC

FtsL is a small bitopic membrane protein required for vegetative cell division and sporulation in Bacillus subtilis. We investigated its localization by fluorescence microscopy using a green fluorescent protein (GFP) fusion. GFP-FtsL was localized at mid-cell in vegetative cells and at the asymmetric septum in sporulating cells. We also show that FtsL forms a ring-like structure at the division site and that it remains localized at mid-cell during the whole septation process. By yeast two-hybrid analysis and non-denaturing polyacrylamide gel electrophoresis (PAGE) with purified proteins, FtsL was found to interact with another membrane-bound division protein, the FtsL-like DivIC protein.

Intrinsic instability of the essential cell division protein FtsL of Bacillus subtilis and a role for DivIB protein in FtsL turnover

Cell division in most eubacteria is driven by an assembly of about eight conserved division proteins. These proteins form a ring structure that constricts in parallel with the formation of the division septum. Here, we show that one of the division proteins, FtsL, is highly unstable. We also show that the protein is targeted to the ring structure and that targeting occurs in concert with the recruitment of several other membrane-associated division proteins. FtsL stability is further reduced in the absence of DivIB protein (probably homologous to E. coli FtsQ) at high temperature, suggesting that DivIB is involved in the control of FtsL turnover. The reduced stability of FtsL may explain the temperature dependence of divIB mutants, because their phenotype can be suppressed by overexpression of FtsL. The results provide new insights into the roles of the FtsL and DivIB proteins in bacterial cell division.

Bacterial cell division

Formation of the bacterial division septum is catalyzed by a number of essential proteins that assemble into a ring structure at the future division site. Assembly of proteins into the cytokinetic ring appears to occur in a hierarchial order that is initiated by the FtsZ protein, a structural and functional analog of eukaryotic tubulins. Placement of the division site at its correct location in Escherichia coli requires a division inhibitor (MinC), that is responsible for preventing septation at unwanted sites near the cell poles, and a topological specificity protein (MinE), that forms a ring at midcell and protects the midcell site from the division inhibitor. However, the mechanism responsible for identifying the position of the midcell site or the polar sites used for spore septum formation is still unclear. Regulation of the division process and its coordination with other cell cycle events, such as chromosome replication, are poorly understood. However, a protein has been identified in Caulobacter (CtrA) that regulates both the initiation of chromosome regulation and the transcription of ftsZ, and that may play an important role in the coordination process.

Non-hydrolysable GTP-gamma-S stabilizes the FtsZ polymer in a GDP-bound state

FtsZ, a tubulin homologue, forms a cytokinetic ring at the site of cell division in prokaryotes. The ring is thought to consist of polymers that assemble in a strictly GTP-dependent way. GTP, but not guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S), has been shown to induce polymerization of FtsZ, whereas in vitro Ca2+ is known to inhibit the GTP hydrolysis activity of FtsZ. We have studied FtsZ dynamics at limiting GTP concentrations in the presence of 10 mM Ca2+. GTP and its non-hydrolysable analogue GTP-gamma-S bind FtsZ with similar affinity, whereas the non-hydrolysable analogue guanylyl-imidodiphosphate (GMP-PNP) is a poor substrate. Preformed FtsZ polymers can be stabilized by GTP-gamma-S and are destabilized by GDP. As more than 95% of the nucleotide associated with the FtsZ polymer is in the GDP form, it is concluded that GTP hydrolysis by itself does not trigger FtsZ polymer disassembly. Strikingly, GTP-gamma-S exchanges only a small portion of the FtsZ polymer-bound GDP. These data suggest that FtsZ polymers are stabilized by a small fraction of GTP-containing FtsZ subunits. These subunits may be located either throughout the polymer or at the polymer ends, forming a GTP cap similar to tubulin.

Cell division in Escherichia coli: role of FtsL domains in septal localization, function, and oligomerization

In Escherichia coli, nine essential cell division proteins are known to localize to the division septum. FtsL is a 13-kDa bitopic membrane protein with a short cytoplasmic N-terminal domain, a membrane-spanning segment, and a periplasmic domain that has a repeated heptad motif characteristic of leucine zippers. Here, we identify the requirements for FtsL septal localization and function. We used green fluorescent protein fusions to FtsL proteins where domains of FtsL had been exchanged with analogous domains from either its Haemophilus influenzae homologue or the unrelated MalF protein to show that both the membrane-spanning segment and the periplasmic domain of FtsL are required for localization to the division site. Mutagenesis of the periplasmic heptad repeat motif severely impaired both localization and function as well as the ability of FtsL to drive the formation of sodium dodecyl sulfate-resistant multimers in vitro. These results are consistent with the predicted propensity of the FtsL periplasmic domain to adopt a coiled-coiled structure. This coiled-coil motif is conserved in all gram-negative and gram-positive FtsL homologues identified so far. Our data suggest that most of the FtsL molecule is a helical coiled coil involved in FtsL multimerization.

Role of penicillin-binding protein PBP 2B in assembly and functioning of the division machinery of Bacillus subtilis

We have characterized the role of the penicillin-binding protein PBP 2B in cell division of Bacillus subtilis. We have shown that depletion of the protein results in an arrest in division, but that this arrest is slow, probably because the protein is relatively stable. PBP 2B-depleted filaments contained, at about their mid-points, structures resembling partially formed septa, into which most, if not all, of the division proteins had assembled. Although clearly deficient in wall material, membrane invagination seemed to continue, indicating that membrane and wall ingrowth can be uncoupled. At other potential division sites along the filaments, no visible ingrowths were observed, although FtsZ rings assembled at regular intervals. Thus, PBP 2B is apparently required for both the initiation of division and continued septal ingrowth. Immunofluorescence microscopy showed that the protein is recruited to the division site. The pattern of localization suggested that this recruitment occurs continually during septal ingrowth. During sporulation, PBP 2B was present transiently in the asymmetrical septum of sporulating cells, and its availability may play a role in the regulation of sporulation septation.

Timing of FtsZ assembly in Escherichia coli

The timing of the appearance of the FtsZ ring at the future site of division in Escherichia coli was determined by in situ immunofluorescence microscopy for two strains grown under steady-state conditions. The strains, B/rA and K-12 MC4100, differ largely in the duration of the D period, the time between termination of DNA replication and cell division. In both strains and under various growth conditions, the assembly of the FtsZ ring was initiated approximately simultaneously with the start of the D period. This is well before nucleoid separation or initiation of constriction as determined by fluorescence and phase-contrast microscopy. The durations of the Z-ring period, the D period, and the period with a visible constriction seem to be correlated under all investigated growth conditions in these strains. These results suggest that (near) termination of DNA replication could provide a signal that initiates the process of cell division.

Localization of FtsL to the Escherichia coli septal ring

In Escherichia coli, nine gene products are known to be essential for assembly of the division septum. One of these, FtsL, is a bitopic membrane protein whose precise function is not understood. Here we use fluorescence microscopy to study the subcellular localization of FtsL, both in a wild-type strain and in a merodiploid strain that expresses a GFP-FtsL fusion protein. We show that FtsL localizes to the cell septum where it forms a ring analogous to the cytoplasmic FtsZ ring. FtsL localization is dependent upon the function of FtsZ, FtsA and FtsQ, but not FtsI. In a reverse approach, we use fusions of green fluorescent protein (GFP) to FtsZ, FtsA and ZipA to show that these proteins localize to the division site in an FtsL-independent fashion. We propose that FtsL is a relatively late recruit to the ring structure that mediates septation.