Domain arrangement of Der, a switch protein containing two GTPase domains

Conserved P-loop GTPases of unknown function in bacteria: an emerging and vital ensemble in bacterial physiology

Establishing the roles of conserved gene products in bacteria is of fundamental importance to our understanding of the core protein complement necessary to sustain cellular life. P-loop GTPases and related ATPases represent an abundant and remarkable group of proteins in bacteria that, in many cases, have evaded characterization. Here, efforts aimed at understanding the cellular function of a group of 8 conserved, poorly characterized genes encoding P-loop GTPases, era, obg, trmE, yjeQ, engA, yihA, hflX, ychF, and a related ATPase, yjeE, are reviewed in considerable detail. While concrete cellular roles remain elusive for all of these genes and considerable pleiotropy has plagued their study, experiments to date have frequently implicated the ribosome. In the case of era, obg, yjeQ, and engA, the evidence is most consistent with roles in ribosome biogenesis, though the prediction is necessarily putative. While the protein encoded in trmE clearly has a catalytic function in tRNA modification, the participation of its GTPase domain remains obscure, as do the functions of the remaining proteins. A full understanding of the cellular functions of all of these important proteins remains the goal of ongoing studies of cellular phenotype and protein biochemistry.

A common dimerization interface in bacterial response regulators KdpE and TorR

Bacterial response regulators are key regulatory proteins that function as the final elements of so-called two-component signaling systems. The activities of response regulators in vivo are modulated by phosphorylation that results from interactions between the response regulator and its cognate histidine protein kinase. The level of response regulator phosphorylation, which is regulated by intra-or extracellular signals sensed by the histidine protein kinase, ultimately determines the output response that is initiated or carried out by the response regulator. We have recently hypothesized that in the OmpR/PhoB subfamily of response regulator transcription factors, this activation involves a common mechanism of dimerization using a set of highly conserved residues in the alpha4-beta5-alpha5 face. Here we report the X-ray crystal structures of the regulatory domains of response regulators TorR (1.8 A), Ca(2+)-bound KdpE (2.0 A), and Mg(2+)/BeF(3)(-)-bound KdpE (2.2 A), both members of the OmpR/ PhoB subfamily from Escherichia coli. Both regulatory domains form symmetric dimers in the asymmetric unit that involve the alpha4-beta5-alpha5 face. As observed previously in other OmpR/PhoB response regulators, the dimer interfaces are mediated by highly conserved residues within this subfamily. These results provide further evidence that most all response regulators of the OmpR/ PhoB subfamily share a common mechanism of activation by dimerization.

Malaria-suppressible expression of the anti-apoptotic triple GTPase mGIMAP8

The IMAP/IAN family of AIG1-like GTPases is conserved among vertebrates and angiosperm plants and has been postulated to regulate apoptosis, particularly in context with diseases such as cancer, diabetes, and infections. The human genes were recently renamed as gimap for GTPase of the immunity associated protein (GIMAP) family. Here we extend this new nomenclature to the murine gimap gene family. All gimap genes of the mouse are clustered on chromosome 6B with eight functional members and one pseudogene. The mGIMAP proteins contain one GTP-binding site and display molecular masses between 33 and 38 kDa except for the very unusual 77 kDa mGIMAP8 protein, which is the first characterized protein containing three GTP-binding domains. Northern blot analysis revealed expression of mgimap8 predominantly in the thymus. The low expression level observed in the spleen was further suppressed by Plasmodium chabaudi malaria. Confocal laser scanning microscopy demonstrated localization of mGIMAP8 at ER, Golgi, and mitochondria. Overexpression of mGIMAP8 could significantly impair anisomycin-induced activation of caspase 3. Our data support the view that mGIMAP8 exerts an anti-apoptotic effect in the immune system and is involved in responses to infections.

Signalling the genome: the Ras-like small GTPase family of trypanosomatids

The genomes of the three principle experimental-model species of Kinetoplastida -Trypanosoma brucei brucei, Trypanosoma cruzi and Leishmania major - are now complete, providing both a milestone for trypanosome biology and an opportunity to consider a multitude of questions at the genome level. Of the >40 members of the Ras-like GTPase family in T. brucei, at least 30 are involved in intracellular transport, whereas fewer than eight are likely to have a classical role in signal transduction. There are no true members of the Ras or Rho subfamilies but divergent Ras- or Rho-like GTPases are present, suggesting that signalling mechanisms in trypanosomatids are highly unusual. Comparisons of T. brucei with T. cruzi and L. major indicate a high degree of conservation among the species. These analyses provide a framework for the functional investigation of small-GTPase-mediated signalling processes in trypanosomes.

Expression of the Ian family of putative GTPases during T cell development and description of an Ian with three sets of GTP/GDP-binding motifs

Reports suggest that two members of the novel immune-associated nucleotide (Ian) GTPase family, Ian1 and Ian5, play roles in T cell development. We performed real-time PCR analysis of the expression of Ian genes of the rat during T cell maturation, in macrophages and in cell lines. We found that all of the genes were expressed at relatively low levels at the early double-negative thymocyte stage but were expressed more strongly at later cell stages. Our study also revealed the fact that the previously reported Ian9, Ian10 and Ian11 genes are, instead, parts of a single gene for which we retain the name Ian9, potentially encoding a GTPase with a highly unusual triplicated structure. Antisera were developed against both Ian1 and Ian9. We established that Ian9 is produced as an approximately 75-kDa protein in both T cells and thymocytes. We observed that levels of both Ian1 and Ian9 proteins are profoundly reduced in T cells from lymphopenic rats as compared with wild-type rats. It was demonstrated that thymocytes and B cells from lymphopenic rats (Ian5 null) did not show enhanced sensitivity to gamma-irradiation-induced apoptosis.

Structural analysis and solution studies of the activated regulatory domain of the response regulator ArcA: a symmetric dimer mediated by the alpha4-beta5-alpha5 face

Escherichia coli react to changes from aerobic to anaerobic conditions of growth using the ArcA-ArcB two-component signal transduction system. This system, in conjunction with other proteins, regulates the respiratory metabolic pathways in the organism. ArcA is a member of the OmpR/PhoB subfamily of response regulator transcription factors that are known to regulate transcription by binding in tandem to target DNA direct repeats. It is still unclear in this subfamily how activation by phosphorylation of the regulatory domain of response regulators stimulates DNA binding by the effector domain and how dimerization and domain orientation, as well as intra- and intermolecular interactions, affect this process. In order to address these questions we have solved the crystal structures of the regulatory domain of ArcA in the presence and absence of the phosphoryl analog, BeF3-. In the crystal structures, the regulatory domain of ArcA forms a symmetric dimer mediated by the alpha4-beta5-alpha5 face of the protein and involving a number of residues that are highly conserved in the OmpR/PhoB subfamily. It is hypothesized that members of this subfamily use a common mechanism of regulation by dimerization. Additional biophysical studies were employed to probe the oligomerization state of ArcA, as well as its individual domains, in solution. The solution studies show the propensity of the individual domains to associate into oligomers larger than the dimer observed for the intact protein, and suggest that the C-terminal DNA-binding domain also plays a role in oligomerization.

Comparative analysis of the human gimap gene cluster encoding a novel GTPase family

There is a growing family of novel GTPases conserved among higher plants and vertebrates, abbreviated as AIG1, IAP, IMAP, and IAN, respectively. Here, we comparatively analyze the human gene family encoding GTPases of the immunity-associated protein family recently re-termed GIMAP. Chromosome 7q36.1 contains, within 300 kb, a gimap gene cluster with seven functional genes and one pseudogene (hgimap3). The six genes hgimap1, hgimap2, hgimap4, hgimap5, hgimap6, and hgimap7 encode 33-46 kDa proteins with one GTP-binding domain, whereas hgimap8 encodes a very unusual 75-kDa protein with three GTP-binding domains. All hgimap genes except hgimap2 have orthologs in the mouse. Major expression sites of hgimap mRNAs are the spleen and lymph nodes, but also other organs such as muscle, heart, placenta, and digestive tract display detectable hgimap mRNA levels. The proteins hGIMAP4 and hGIMAP7 can be localized at ER and Golgi apparatus, but not in mitochondria, lysosomes and nuclei. All hgimap genes were expressed at very low levels-if at all-in diverse cancer cell lines. Our data support the view that the GIMAP proteins are involved in the control of cell survival not only in cells of the immune system as commonly anticipated.

The crystal structure of YloQ, a circularly permuted GTPase essential for Bacillus subtilis viability

yloQ is one of 11 essential genes in Bacillus subtilis with unknown roles in the physiology of the cell. It encodes a polypeptide of 298 residues with motifs characteristic of GTPases. As a contribution to elucidating its indispensable cellular function, we have solved the crystal structure of YloQ to 1.6 A spacing, revealing a three-domain organisation. At the heart of the molecule is the putative GTPase domain, which exhibits a classical alpha/beta nucleotide-binding fold with a topology very similar to that of Ras and Era. However, as anticipated from the order in which the conserved G protein motifs appear in the sequence, the GTPase domain fold in YloQ is circularly permuted with respect to the classical GTPases. The nucleotide-binding pocket in YloQ is unoccupied, and analysis of the phosphate-binding (P) loop indicates that conformational changes in this region would be needed to accommodate GTP. The GTPase domain is flanked at its N terminus by a beta-barrel domain with an oligonucleotide/oligosaccharide-binding (OB) fold, and at its C terminus by an alpha-helical domain containing a coordinated zinc ion. This combination of protein modules is unique to YloQ and its orthologues. Sequence comparisons reveal a clustering of conserved basic and aromatic residues on one face of the OB domain, perhaps pointing to a role for YloQ in nucleic acid binding. The zinc ion in the alpha-helical domain is coordinated by three cysteine residues and a histidine residue in a novel ligand organisation. The juxtaposition of the switch I and switch II regions of the G domain and the OB and zinc-binding domains suggests that chemical events at the GTPase active site may be transduced into relative movements of these domains. The pattern of conserved residues and electrostatic surface potential calculations suggest that the OB and/or Zn-binding domains participate in nucleic acid binding consistent with a possible role for YloQ at some stage during mRNA translation.

Structural analysis of the domain interface in DrrB, a response regulator of the OmpR/PhoB subfamily

The N-terminal regulatory domains of bacterial response regulator proteins catalyze phosphoryl transfer and function as phosphorylation-dependent regulatory switches to control the output activities of C-terminal effector domains. Structures of numerous isolated regulatory and effector domains have been determined. However, a detailed understanding of regulatory interactions among these domains has been limited by the relative paucity of structural data for intact multidomain response regulator proteins. The first multidomain structures determined, those of transcription factor NarL and methylesterase CheB, both revealed extensive interdomain interfaces. The regulatory domains obstruct access to the functional sites of the effector domains, indicating a regulatory mechanism based on inhibition. In contrast, the recently determined structure of the OmpR/PhoB homologue DrrD revealed no significant interdomain interface, suggesting that the domains are tethered by a flexible linker and lack a fixed orientation relative to each other. To address the generality of this feature, we have determined the 1.8-A resolution crystal structure of Thermotoga maritima DrrB, providing a second structure of a multidomain response regulator of the OmpR/PhoB subfamily. The structure reveals an extensive domain interface of 751 A(2) and therefore differs greatly from that observed in DrrD. Residues that are crucial players in defining the activation state of the regulatory domain contribute to this interface, implying that conformational changes associated with phosphorylation will influence these intramolecular contacts. The DrrB and DrrD structures are suggestive of different signaling mechanisms, with intramolecular communication between N- and C-terminal domains making substantially different contributions to effector domain regulation in individual members of the OmpR/PhoB family.

Suppression of defective ribosome assembly in a rbfA deletion mutant by overexpression of Era, an essential GTPase in Escherichia coli

Era is a small GTP-binding protein and essential for cell growth in Escherichia coli. It consists of two domains: N-terminal GTP-binding and C-terminal RNA-binding KH domains. It has been shown to bind to 16S rRNAs and 30S ribosomal subunits in vitro. Here, we report that a precursor of 16S rRNA accumulates in Era-depleted cells. The accumulation of the precursors is also seen in a cold-sensitive mutant, E200K, in which the mutation site is located in the C-terminal domain. The major precursor molecule accumulated seems to be 17S rRNA, containing extra sequences at both 5' and 3' ends of 16S rRNA. Moreover, the amounts of both 30S and 50S ribosomal subunits relative to the amount of 70S monosomes increase in Era-depleted and E200K mutant cells. The C-terminal KH domain has a high structural similarity to the RbfA protein, a cold shock protein that also specifically associates with 30S ribosomal subunits. RbfA is essential for cell growth at low temperature, and a precursor of 16S rRNA accumulates in an rbfA deletion strain. The 16S rRNA precursor seems to be identical in size to that accumulated in Era mutant cells. Surprisingly, the cold-sensitive cell growth of the rbfA deletion cells was partially suppressed by overproduction of the wild-type Era. The C-terminal domain alone was not able to suppress the cold-sensitive phenotype, whereas Era-dE, which has a 10-residue deletion in a putative effector region of the N-terminal domain, functioned as a more efficient suppressor than the wild-type Era. It was found that Era-dE suppressed defective 16S rRNA maturation, resuming a normal polysome profile to reduce highly accumulated free 30S and 50S subunits in the rbfA deletion cells. These results indicate that Era is involved in 16S rRNA maturation and ribosome assembly.

Function of the universally conserved bacterial GTPases

The GTPase superfamily of cellular regulators is well represented in bacteria. A small number are universally conserved over the entire range of bacterial species. Such a pervasive taxonomic distribution suggests that these enzymes play important roles in bacterial cellular systems. Recent advances have demonstrated that bacterial GTPases are important regulators of ribosome function, and important for the distribution of DNA to daughter cells following cell division. In addition, the atomic structure of a unique GTPase, EngA, has recently been established. Unlike any other GTPase, EngA contains tandem GTP-binding domains. This structural study suggests that the GTPase cycles of the domains are regulated differentially in a manner that remains to be elucidated.