Localization of the membrane binding sites of Era in Escherichia coli

Era, a GTPase-like protein of the Ras family, does not control ribosome assembly in Mycobacterium tuberculosis

Era GTPase is universally present in microbes including Mycobacterium tuberculosis (Mtb) complex bacteria. While Era is known to regulate ribosomal assembly in Escherichia coli and predicted to be essential for in vitro growth, its function in mycobacteria remains obscured. Herein, we show that Era ortholog in the attenuated Mtb H37Ra strain, MRA_2388 (annotated as Era^MT) is a cell envelope localized protein harbouring critical GTP-binding domains, which interacts with several envelope proteins of Mtb. The purified Era from M. smegmatis (annotated as Era^MS) exhibiting ~90 % sequence similarity with Era^MT, exists in monomeric conformation. While it is co-purified with RNA upon overexpression in E. coli, the presence of RNA does not modulate the GTPase activity of the Era^MS as against its counterpart from other organisms. CRISPRi silencing of eraMT does not show any substantial effect on the in vitro growth of Mtb H37Ra, which suggests a redundant function of Era in mycobacteria. Notably, no effect on ribosome assembly, protein synthesis or bacterial susceptibility to protein synthesis inhibitors was observed upon depletion of Era^MT in Mtb H37Ra, further indicating a divergent role of Era GTPase in mycobacteria.

Protein Assistants of Small Ribosomal Subunit Biogenesis in Bacteria

Ribosome biogenesis is a fundamental and multistage process. The basic steps of ribosome assembly are the transcription, processing, folding, and modification of rRNA; the translation, folding, and modification of r-proteins; and consecutive binding of ribosomal proteins to rRNAs. Ribosome maturation is facilitated by biogenesis factors that include a broad spectrum of proteins: GTPases, RNA helicases, endonucleases, modification enzymes, molecular chaperones, etc. The ribosome assembly factors assist proper rRNA folding and protein–RNA interactions and may sense the checkpoints during the assembly to ensure correct order of this process. Inactivation of these factors is accompanied by severe growth phenotypes and accumulation of immature ribosomal subunits containing unprocessed rRNA, which reduces overall translation efficiency and causes translational errors. In this review, we focus on the structural and biochemical analysis of the 30S ribosomal subunit assembly factors RbfA, YjeQ (RsgA), Era, KsgA (RsmA), RimJ, RimM, RimP, and Hfq, which take part in the decoding-center folding.

Overproduction of a Dominant Mutant of the Conserved Era GTPase Inhibits Cell Division in Escherichia coli

Cell growth and division are coordinated, ensuring homeostasis under any given growth condition, with division occurring as cell mass doubles. The signals and controlling circuit(s) between growth and division are not well understood; however, it is known in Escherichia coli that the essential GTPase Era, which is growth rate regulated, coordinates the two functions and may be a checkpoint regulator of both. We have isolated a mutant of Era that separates its effect on growth and division. When overproduced, the mutant protein Era647 is dominant to wild-type Era and blocks division, causing cells to filament. Multicopy suppressors that prevent the filamentation phenotype of Era647 either increase the expression of FtsZ or decrease the expression of the Era647 protein. Excess Era647 induces complete delocalization of Z rings, providing an explanation for why Era647 induces filamentation, but this effect is probably not due to direct interaction between Era647 and FtsZ. The hypermorphic ftsZ* allele at the native locus can suppress the effects of Era647 overproduction, indicating that extra FtsZ is not required for the suppression, but another hypermorphic allele that accelerates cell division through periplasmic signaling, ftsL*, cannot. Together, these results suggest that Era647 blocks cell division by destabilizing the Z ring.IMPORTANCE All cells need to coordinate their growth and division, and small GTPases that are conserved throughout life play a key role in this regulation. One of these, Era, provides an essential function in the assembly of the 30S ribosomal subunit in Escherichia coli, but its role in regulating E. coli cell division is much less well understood. Here, we characterize a novel dominant negative mutant of Era (Era647) that uncouples these two activities when overproduced; it inhibits cell division by disrupting assembly of the Z ring, without significantly affecting ribosome production. The unique properties of this mutant should help to elucidate how Era regulates cell division and coordinates this process with ribosome biogenesis.

The (p)ppGpp-binding GTPase Era promotes rRNA processing and cold adaptation in Staphylococcus aureus

Ribosome assembly cofactors are widely conserved across all domains of life. One such group, the ribosome-associated GTPases (RA-GTPase), act as molecular switches to coordinate ribosome assembly. We previously identified the Staphylococcus aureus RA-GTPase Era as a target for the stringent response alarmone (p)ppGpp, with binding leading to inhibition of GTPase activity. Era is highly conserved throughout the bacterial kingdom and is essential in many species, although the function of Era in ribosome assembly is unclear. Here we show that Era is not essential in S. aureus but is important for 30S ribosomal subunit assembly. Protein interaction studies reveal that Era interacts with the 16S rRNA endonuclease YbeY and the DEAD-box RNA helicase CshA. We determine that both Era and CshA are required for growth at suboptimal temperatures and rRNA processing. Era and CshA also form direct interactions with the (p)ppGpp synthetase Rel_Sau, with Rel_Sau positively impacting the GTPase activity of Era but negatively affecting the helicase activity of CshA. We propose that in its GTP-bound form, Era acts as a hub protein on the ribosome to direct enzymes involved in rRNA processing/degradation and ribosome subunit assembly to their site of action. This activity is impeded by multiple components of the stringent response, contributing to the slowed growth phenotype synonymous with this stress response pathway.

The bacterial ribosome is an essential cellular component and as such is the target for a number of currently used antimicrobials. Correct assembly of this complex macromolecule requires a number of accessory enzymes, the functions of which are poorly characterised. Here we examine the function of Era, a GTPase enzyme involved in 30S ribosomal subunit biogenesis in the important human pathogen S. aureus. We uncover that Era is not an essential enzyme in S. aureus, as it is in many other species, but is important for correct ribosome assembly. In a bid to determine a function for this enzyme in ribosomal assembly, we identify a number of protein interaction partners with roles in ribosomal RNA maturation or degradation, supporting the idea that Era acts as a hub protein facilitating ribosomal biogenesis. We also uncover a link between Era and the (p)ppGpp synthetase Rel_Sau, revealing an additional level of control of rRNA processing by the stringent response. With this study we elaborate on the functions of GTPases in ribosomal assembly, processes that are controlled at multiple points by the stringent response.

The universally conserved prokaryotic GTPases

Members of the large superclass of P-loop GTPases share a core domain with a conserved three-dimensional structure. In eukaryotes, these proteins are implicated in various crucial cellular processes, including translation, membrane trafficking, cell cycle progression, and membrane signaling. As targets of mutation and toxins, GTPases are involved in the pathogenesis of cancer and infectious diseases. In prokaryotes also, it is hard to overestimate the importance of GTPases in cell physiology. Numerous papers have shed new light on the role of bacterial GTPases in cell cycle regulation, ribosome assembly, the stress response, and other cellular processes. Moreover, bacterial GTPases have been identified as high-potential drug targets. A key paper published over 2 decades ago stated that, "It may never again be possible to capture [GTPases] in a family portrait" (H. R. Bourne, D. A. Sanders, and F. McCormick, Nature 348:125-132, 1990) and indeed, the last 20 years have seen a tremendous increase in publications on the subject. Sequence analysis identified 13 bacterial GTPases that are conserved in at least 75% of all bacterial species. We here provide an overview of these 13 protein subfamilies, covering their cellular functions as well as cellular localization and expression levels, three-dimensional structures, biochemical properties, and gene organization. Conserved roles in eukaryotic homologs will be discussed as well. A comprehensive overview summarizing current knowledge on prokaryotic GTPases will aid in further elucidating the function of these important proteins.

Characterization of the 16S rRNA- and membrane-binding domains of Streptococcus pneumoniae Era GTPase: structural and functional implications

Era is a highly conserved GTPase essential for bacterial growth. The N-terminal part of Era contains a conserved GTPase domain, whereas the C-terminal part of the protein contains an RNA- and membrane-binding domain, the KH domain. To investigate whether the binding of Era to 16S rRNA and membrane requires its GTPase activity and whether the GTPase domain is essential for these activities, the N- and C-terminal parts of the Streptococcus pneumoniae Era - Era-N (amino acids 1-185) and Era-C (amino acids 141-299), respectively - were expressed and purified. Era-C, which had completely lost GTPase activity, bound to the cytoplasmic membrane and 16S rRNA. In contrast, Era-N, which retained GTPase activity, failed to bind to RNA or membrane. These results therefore indicate that the binding of Era to RNA and membrane does not require the GTPase activity of the protein and that the RNA-binding domain is an independent, functional domain. The physiological effects of the overexpression of Era-C were assessed. The Escherichia coli cells overexpressing Era and Era-N exhibited the same growth rate as wild-type E. coli cells. In contrast, the E. coli cells overexpressing Era-C exhibited a reduced growth rate, indicating that the overexpression of Era-C inhibits cell growth. Furthermore, overexpression of era-N and era-C resulted in morphological changes. Finally, purified Era and Era-C were able to bind to poly(U) RNA, and the binding of Era to poly(U) RNA was significantly inhibited by liposome, as the amount of Era bound to the RNA decreased proportionally with the increase of liposome in the assay. Therefore, this study provides the first biochemical evidence that both binding sites are overlapping. Together, these results indicate that the RNA- and membrane-binding domain of Era is a separate, functional entity and does not require the GTPase activity or the GTPase domain of the protein for activity.

Analysis of the interaction of 16S rRNA and cytoplasmic membrane with the C-terminal part of the Streptococcus pneumoniae Era GTPase

Era, an essential GTPase, plays a regulatory role in several cellular processes. The Era protein of Streptococcus pneumoniae has recently been shown to bind to 16S rRNA and the cytoplasmic membrane. However, exact locations of Era responsible for RNA- and membrane-binding were unknown. To identify the regions in Era that interact with the RNA and membrane, the C-terminal part of S. pneumoniae Era was systematically deleted while the N-terminal part, responsible for the GTPase activity of the protein, was kept intact. The resulting truncated Era proteins were purified and characterized. The C-terminal deletion of 9 or 19 amino-acid residues did not affect 16S rRNA-binding activity while further deletions of the C-terminus (29-114 amino-acid residues) abolished the activity. These results indicate that the integrity of the putative KH domain of Era, spanning the amino-acid residues between approximately 22-83 from the C-terminus, is required for 16S rRNA-binding. Furthermore, the Era proteins with a deletion up to 45 residues from the C-terminus retained membrane-binding activity, but longer deletions significantly reduced the activity. These results indicate that part of the putative KH domain is also required for membrane-binding. Thus, these results indicate for the first time that the regions critical for the membrane- and 16S rRNA-binding activities of Era overlap. The era gene with a deletion of 9 or 19 codons from its 3' terminus complemented an Escherichia coli mutant strain deficient in Era production whereas the genes with longer deletions failed to do so, thereby indicating that the KH domain is essential for Era function. Taken together, the results of this study indicate that the putative KH domain is required for 16S rRNA-binding activity and that part of the KH domain is also required for membrane-binding activity. The results also suggest that the interaction between Era and 16S rRNA is essential for bacterial growth.

Analysis of guanine nucleotide binding and exchange kinetics of the Escherichia coli GTPase Era

Era is an essential Escherichia coli guanine nucleotide binding protein that appears to play a number of cellular roles. Although the kinetics of Era guanine nucleotide binding and hydrolysis have been described, guanine nucleotide exchange rates have never been reported. Here we describe a kinetic analysis of guanine nucleotide binding, exchange, and hydrolysis by Era using the fluorescent mant (N-methyl-3'-O-anthraniloyl) guanine nucleotide analogs. The equilibrium binding constants (K(D)) for mGDP and mGTP (0.61 +/- 0. 12 microgM and 3.6 +/- 0.80 microM, respectively) are similar to those of the unmodified nucleotides. The single turnover rates for mGTP hydrolysis by Era were 3.1 +/- 0.2 mmol of mGTP hydrolyzed/min/mol in the presence of 5 mM MgCl(2) and 5.6 +/- 0.3 mmol of mGTP hydrolyzed/min/mol in the presence of 0.2 mM MgCl(2). Moreover, Era associates with and exchanges guanine nucleotide rapidly (on the order of seconds) in both the presence and absence of Mg(2+). We suggest that models of Era function should reflect the rapid exchange of nucleotides in addition to the GTPase activity inherent to Era.

Era GTPase of Escherichia coli: binding to 16S rRNA and modulation of GTPase activity by RNA and carbohydrates

Era, an essential GTPase, appears to play an important role in the regulation of the cell cycle and protein synthesis of bacteria and mycoplasmas. In this study, native Era, His-tagged Era (His-Era) and glutathione S-transferase (GST)-fusion Era (GST-Era) proteins from Escherichia coli were expressed and purified. It was shown that the GST-Era and His-Era proteins purified by 1-step affinity column chromatographic methods were associated with RNA and exhibited a higher GTPase activity. However, the native Era protein purified by a 3-step column chromatographic method had a much lower GTPase activity and was not associated with RNA which had been removed during purification. Purified GST-Era protein was shown to be present as a high- and a low-molecular-mass forms. The high-molecular-mass form of GST-Era was associated with RNA and exhibited a much higher GTPase activity. Removal of the RNA associated with GST-Era resulted in a significant reduction in the GTPase activity. The RNA associated with GST-Era was shown to be primarily 16S rRNA. A purified native Era protein preparation, when mixed with total cellular RNA, was found to bind to some of the RNA. The native Era protein isolated directly from the cells of a wild-type E. coli strain was also present as a high-molecular-mass form complexed with RNA and RNase treatment converted the high-molecular-mass form into a 32 kDa low-molecular-mass form, a monomer of Era. Furthermore, a C-terminally truncated Era protein, when expressed in E. coli, did not bind RNA. Finally, the GTPase activity of the Era protein free of RNA, but not the Era protein associated with the RNA, was stimulated by acetate and 3-phosphoglycerate. These carbohydrates, however, failed to activate the GTPase activity of the C-terminally truncated Era protein. Thus, the results of this study establish that the C-terminus of Era is essential for the RNA-binding activity and that the RNA and carbohydrates modulate the GTPase activity of Era possibly through a similar mechanism.

16S rRNA is bound to era of Streptococcus pneumoniae

Era is an essential membrane-associated GTPase that is present in bacteria and mycoplasmas. Era appears to play an important role in the regulation of the bacterial cell cycle. In this study, we expressed the native and glutathione S-transferase (GST) fusion forms of Streptococcus pneumoniae Era in Escherichia coli and purified both proteins to homogeneity. We showed that RNA was copurified with the GST-Era protein of S. pneumoniae during affinity purification and remained associated with the protein after removal of the GST tag by thrombin cleavage. The thrombin-treated and untreated GST-Era proteins could bind and hydrolyze GTP and exhibited similar kinetic properties (dissociation constant [kD], Km, and Vmax). However, the native Era protein purified by using different chromatographic columns had a much lower GTPase activity than did GST-Era, although it had a similar k(D). In addition, RNA was not associated with the protein. Purified GST-Era protein was shown to be present as high (600-kDa)- and low (120-kDa)-molecular-mass forms. The high-molecular-mass form of GST-Era was associated with RNA and exhibited a very high GTPase activity. Approximately 40% of purified GST-Era protein was associated with RNA, and removal of the RNA resulted in a significant reduction in GTPase activity. The RNA associated with GST-Era was shown to be predominantly 16S rRNA. The native Era protein isolated directly from S. pneumoniae was also present as a high-molecular-mass species (600 kDa) complexed with RNA. Together, our results suggest that 16S rRNA is associated with Era and might stimulate its GTPase activity.

The widely conserved Era G-protein contains an RNA-binding domain required for Era function in vivo

Era is a small G-protein widely conserved in eubacteria and eukaryotes. Although essential for bacterial growth and implicated in diverse cellular processes, its actual function remains unclear. Several lines of evidence suggest that Era may be involved in some aspect of RNA biology. The GTPase domain contains features in common with all G-proteins and is required for Era function in vivo. The C-terminal domain (EraCTD) bears scant similarity to proteins outside the Era subfamily. On the basis of sequence comparisons, we argue that the EraCTD is similar to, but distinct from, the KH RNA-binding domain. Although both contain the consensus VIGxxGxxI RNA-binding motif, the protein folds are probably different. We show that bacterial Era binds RNA in vitro and can form higher-order RNA-protein complexes. Mutations in the VIGxxGxxI motif and other conserved residues of the Escherichia coli EraCTD decrease RNA binding in vitro and have corresponding effects on Era function in vivo, including previously described effects on cell division and chromosome partitioning. Importantly, mutations in L-66, located in the predicted switch II region of the E. coli Era GTPase domain, also perturb binding, leading us to propose that the GTPase domain regulates RNA binding in response to unknown cellular cues. The possible biological significance of Era RNA binding is discussed.

Characterization of mutations affecting the Escherichia coli essential GTPase era that suppress two temperature-sensitive dnaG alleles

Two suppressor mutations of the temperature-sensitive DNA primase mutant dnaG2903 have been characterized. The gene responsible for suppression, era, encodes an essential GTPase of Escherichia coli. One mutation, rnc-15, is an insertion of an IS1 element within the leader region of the rnc operon and causes a polar defect on the downstream genes of the operon. A previously described polar mutation, rnc-40, was also able to suppress dnaG2903. The other mutation, era-1, causes a single amino acid substitution (P17R) in the G1 region of the GTP-binding domain of Era. Analysis of the GTPase activity of the Era-1 mutant protein showed a four- to five-fold decrease in the ability to convert GTP to GDP. Thus, lowered expression of wild-type Era caused by the polar mutations and reduced GTPase activity caused by the era-1 mutation suppresses dnaG2903 as well as a second dnaG allele, parB. Phenotypic analysis of the era-1 mutant at 25 degrees C showed that 10% of the cells contain four segregated nucleoids, indicative of a delay in cell division. Possible mechanisms of suppression of dnaG and roles for Era are discussed.

Mutational analysis of Era, an essential GTP-binding protein of Escherichia coli

Era is an essential GTP-binding protein of an unknown function in Escherichia coli. On the basis of its sequence similarities to other GTP-binding proteins such as E. coli EF-Tu, EF-G, IF2 and eukaryotic Ras proteins, it has been suggested that the Era function is activated by GTP binding, and that subsequent conversion of bound GTP to GDP by the intrinsic GTPase activity modulates its function. Two Era mutants, one dominant negative mutant (dE), which has a deletion mutation from Ala40 to Gly49, and the other non-functional mutant (T42A/T43A), which has two substitution mutations, Thr42 to Ala and Thr43 to Ala, were analyzed for their abilities of GTP-binding and GTPase activity. It was found that the dE mutant lost the GTP-binding ability, while it still retained the GTPase activity. On the other hand, the T42A/T43A mutant retained both the GTP-crosslinking and GTPase activities. However, the Km values for GTPase activity increased 5- and 12-fold for dE and T42A/T43A mutants, respectively. These results indicate that both the GTP-binding and GTPase activities are important for the Era function.

Structure and regulation of the Salmonella typhimurium rnc-era-recO operon

The Escherichia coli rnc-era-recO operon encodes ribonuclease III (RNase III; a dsRNA endonuclease involved in rRNA and mRNA processing and decay), Era (an essential G-protein of unknown functions and RecO (involved in the RecF homologous recombination pathway). Expression of the rnc and era genes is negatively autoregulated: RNase III cleaves the rncO 'operator' in the untranslated leader, destabilizing the operon mRNA. As part of a larger effort to understand RNase III and Era structure and function, we characterized rnc operon structure, function and regulation in the closely related bacterium Salmonella typhimurium. Construction of a S typhimurium strain conditionally defective for RNase III and Era expression showed that Era is essential for cell growth. This mutant strain also enabled selection of recombinant clones containing the intact S typhimurium rnc-era-recO operon, whose nucleotide sequence, predicted protein sequence, and predicted rncO RNA secondary structure were all highly conserved with those of E coli. Furthermore, genetic and biochemical analysis revealed that S typhimurium rnc gene expression is negatively autoregulated by a mechanism very similar or identical to that in E coli, and that the cleavage specificities of RNase IIIs.t. and RNase IIIE.c. are indistinguishable with regard to rncO cleavage and S typhimurium 23S rRNA fragmentation in vivo.

Cold-sensitive conditional mutations in Era, an essential Escherichia coli GTPase, isolated by localized random polymerase chain reaction mutagenesis

Conditional cold-sensitive mutations in Era, an essential Escherichia coli GTPase, were isolated. Localized random polymerase chain reaction (PCR) mutagenesis employing Taq and T7 DNA polymerases under error prone amplification conditions was exploited to generate mutations in the era gene. A plasmid exchange technique was used to identify conditional cold-sensitive mutations in Era that give rise to defective cell growth below 30 degrees C. Three recessive missense mutations in Era, N26S, A156D, and E200K, were isolated. All three mutations are located at residues conserved in Era homologues from Streptococcus mutans and Coxiella burnetti.

Clustering of fibronectin adhesins toward Treponema denticola tips upon contact with immobilized fibronectin

Treponema denticola has been shown to bind to immobilized fibronectin (Fn) by its tips. Yet labeling of cells in suspension with an Fn-gold conjugate to localize the Fn adhesins shows that they are distributed in patches along the entire cell length. Subsequent experiments have shown that the number and proportion of tip-oriented cells increase with time, suggesting that Fn contact stimulates T. denticola to rearrange adhesins toward its tips. To test this hypothesis, T. denticola cells were allowed to migrate in a 2% methylcellulose column toward nitrocellulose filters coated with Fn, laminin, bovine serum albumin, or phosphate-buffered saline. Cells close to and distant from the filters were collected, labeled with Fn-gold probes, and examined by transmission electron microscopy. The number of gold particles on each of 20 cells was counted, dividing each cell into thirds along its length: the end third with the most label (end 1), the middle third, and the end with the least label (end 2). The mean number (+/- standard deviation) of gold probes per third was calculated. Fn-gold probes clustered toward one end of T. denticola cells when in contact with Fn-coated nitrocellulose, with > 55% of probes in end 1. In contrast, no clustering toward T. denticola ends occurred with laminin-, bovine serum albumin, or phosphate-buffered saline-coated filters or in the absence of a filter. Blocking access of the T. denticola cells to the Fn-coated nitrocellulose filter by placing an uncoated filter between them prevented clustering of Fn-gold. Removal of T. denticola cells from direct contact with the Fn-coated filter did not promote redistribution of clustered probes. These data suggest that T. denticola is stimulated to cluster Fn adhesins irreversibly toward its tips when it migrates into contact with immobilized Fn. This might be significant for establishing multiple adhesive interactions with host cells and ligands.

In vivo selection of conditional-lethal mutations in the gene encoding elongation factor G of Escherichia coli

The ribosome translocation step that occurs during protein synthesis is a highly conserved, essential activity of all cells. The precise movement of one codon that occurs following peptide bond formation is regulated by elongation factor G (EF-G) in eubacteria or elongation factor 2 (EF-2) in eukaryotes. To begin to understand molecular interactions that regulate this process, a genetic selection was developed with the aim of obtaining conditional-lethal alleles of the gene (fusA) that encodes EF-G in Escherichia coli. The genetic selection depends on the observation that resistant strains arose spontaneously in the presence of sublethal concentrations of the antibiotic kanamycin. Replica plating was performed to obtain mutant isolates from this collection that were restrictive for growth at 42 degrees C. Two tightly temperature-sensitive strains were characterized in detail and shown to harbor single-site missense mutations within fusA. The fusA100 mutant encoded a glycine-to-aspartic acid change at codon 502. The fusA101 allele encoded a glutamine-to-proline alteration at position 495. Induction kinetics of beta-galactosidase activity suggested that both mutations resulted in slower elongation rates in vivo. These missense mutations were very near a small group of conserved amino acid residues (positions 483 to 493) that occur in EF-G and EF-2 but not EF-Tu. It is concluded that these sequences encode a specific domain that is essential for efficient translocase function.

GTPase-dependent signaling in bacteria: characterization of a membrane-binding site for era in Escherichia coli

Era is an Escherichia coli GTPase that is essential for cell viability and is peripherally associated with the cytoplasmic membrane. Both immunoelectron microscopy and subcellular-fractionation experiments have shown that Era is present in cytoplasmic as well as membrane-associated pools. These data led to speculation that the mechanism of action of Era may require cycling between membrane and cytoplasmic sites. In order to investigate this possibility, an in vitro binding assay was developed to characterize the binding of Era to membrane fractions. Competition and saturation binding experiments suggest that a site that is specific for Era and capable of binding up to 5 ng of Era per microgram of membrane protein is present in membrane preparations. The binding curve is complex, indicating that multiple equilibria describe the interaction. The binding of Era to this putative receptor is dependent on guanine nucleotides; binding cannot be measured in the absence of nucleotide, and neither ATP nor UTP can substitute. Subfractionation of cell walls showed that the guanine nucleotide-dependent binding site was present in fractions enriched in cytoplasmic membrane. These data provide evidence that Era may be involved in a GTPase-receptor-coupled membrane-signaling pathway that is essential for growth in E. coli.

Polarized distribution of Listeria monocytogenes surface protein ActA at the site of directional actin assembly

The facultative intracellular pathogen Listeria monocytogenes can infect host tissues by using directional actin assembly to propel itself from one cell into another. The movement is generated by continuous actin assembly from one end of the bacterium into a tail, which is left behind in the cytoplasm. Bacterial actin assembly requires expression of the bacterial gene actA. We have used immunocytochemistry to show that the actA gene product, ActA, is distributed asymmetrically on the bacterial surface: it is not expressed at one pole and is increasingly concentrated towards the other. This polarized distribution of ActA was linked to bacterial division: ActA protein was not, or only faintly, expressed at the pole that had been formed during the previous division. On intracellular bacteria ActA was expressed at the site of actin assembly, suggesting that ActA may be involved in actin filament nucleation off the bacterial surface. We predict that the asymmetrical distribution of this protein is required for the ability of intracellular Listeria to move in the direction of the non-ActA expressing pole.

Polar location of the chemoreceptor complex in the Escherichia coli cell

The eukaryotic cell exhibits compartmentalization of functions to various membrane-bound organelles and to specific domains within each membrane. The spatial distribution of the membrane chemoreceptors and associated cytoplasmic chemotaxis proteins in Escherichia coli were examined as a prototypic functional aggregate in bacterial cells. Bacterial chemotaxis involves a phospho-relay system brought about by ligand association with a membrane receptor, culminating in a switch in the direction of flagellar rotation. The transduction of the chemotaxis signal is initiated by a chemoreceptor-CheW-CheA ternary complex at the inner membrane. These ternary complexes aggregate predominantly at the cell poles. Polar localization of the cytoplasmic CheA and CheW proteins is dependent on membrane-bound chemoreceptor. Chemoreceptors are not confined to the cell poles in strains lacking both CheA and CheW. The chemoreceptor-CheW binary complex is polarly localized in the absence of CheA, whereas the chemoreceptor-CheA binary complex is not confined to the cell poles in strains lacking CheW. The subcellular localization of the chemotaxis proteins may reflect a general mechanism by which the bacterial cell sequesters different regions of the cell for specialized functions.

Membrane-associated GTPases in bacteria

Members of the GTPase superfamily are extremely important in regulating membrane signalling pathways in all cells. This review focuses on membrane-associated GTPases that have been described in prokaryotes. In bacteria, LepA and NodQ are very similar to protein synthesis elongation factors but apparently have membrane-related functions. The amino acid sequences of FtsY and Ffh are clearly related to eukaryotic factors involved in protein secretion. Obg and Era are not closely related to any GTPase subgroup according to amino acid sequence comparisons, but they are essential for viability. In spite of similarities to well-studied eukaryotic proteins the signalling pathways of these cellular regulators, with the exception of NodQ, have not yet been elucidated.