Crystallographic studies of elongation factor G

Structural and Functional Analysis of BipA, a Regulator of Virulence in Enteropathogenic Escherichia coli

The translational GTPase BipA regulates the expression of virulence and pathogenicity factors in several eubacteria. BipA-dependent expression of virulence factors occurs under starvation conditions, such as encountered during infection of a host. Under these conditions, BipA associates with the small ribosomal subunit. BipA also has a second function to promote the efficiency of late steps in biogenesis of large ribosomal subunits at low temperatures, presumably while bound to the ribosome. During starvation, the cellular concentration of stress alarmone guanosine-3′, 5′-bis pyrophosphate (ppGpp) is increased. This increase allows ppGpp to bind to BipA and switch its binding specificity from ribosomes to small ribosomal subunits. A conformational change of BipA upon ppGpp binding could explain the ppGpp regulation of the binding specificity of BipA. Here, we present the structures of the full-length BipA from Escherichia coli in apo, GDP-, and ppGpp-bound forms. The crystal structure and small-angle x-ray scattering data of the protein with bound nucleotides, together with a thermodynamic analysis of the binding of GDP and of ppGpp to BipA, indicate that the ppGpp-bound form of BipA adopts the structure of the GDP form. This suggests furthermore, that the switch in binding preference only occurs when both ppGpp and the small ribosomal subunit are present. This molecular mechanism would allow BipA to interact with both the ribosome and the small ribosomal subunit during stress response.

Modeling the mechanisms of biological GTP hydrolysis

Enzymes that hydrolyze GTP are currently in the spotlight, due to their molecular switch mechanism that controls many cellular processes. One of the best-known classes of these enzymes are small GTPases such as members of the Ras superfamily, which catalyze the hydrolysis of the γ-phosphate bond in GTP. In addition, the availability of an increasing number of crystal structures of translational GTPases such as EF-Tu and EF-G have made it possible to probe the molecular details of GTP hydrolysis on the ribosome. However, despite a wealth of biochemical, structural and computational data, the way in which GTP hydrolysis is activated and regulated is still a controversial topic and well-designed simulations can play an important role in resolving and rationalizing the experimental data. In this review, we discuss the contributions of computational biology to our understanding of GTP hydrolysis on the ribosome and in small GTPases.

Conformational transitions in the γ subunit of the archaeal translation initiation factor 2

In eukaryotes and archaea, the heterotrimeric translation initiation factor 2 (e/aIF2) is pivotal for the delivery of methionylated initiator tRNA (Met-tRNA(i)) to the ribosome. It acts as a molecular switch that cycles between inactive (GDP-bound) and active (GTP-bound) states. Recent studies show that eIF2 can also exist in a long-lived eIF2γ-GDP-P(i) (inorganic phosphate) active state. Here, four high-resolution crystal structures of aIF2γ from Sulfolobus solfataricus are reported: aIF2γ-GDPCP (a nonhydrolyzable GTP analogue), aIF2γ-GDP-formate (in which a formate ion possibly mimics P(i)), aIF2γ-GDP and nucleotide-free aIF2γ. The structures describe the different states of aIF2γ and demonstrate the conformational transitions that take place in the aIF2γ `life cycle'.

Visualization of the eEF2-80S ribosome transition-state complex by cryo-electron microscopy

In an attempt to understand ribosome-induced GTP hydrolysis on eEF2, we determined a 12.6-A cryo-electron microscopy reconstruction of the eEF2-bound 80S ribosome in the presence of aluminum tetrafluoride and GDP, with aluminum tetrafluoride mimicking the gamma-phosphate during hydrolysis. This is the first visualization of a structure representing a transition-state complex on the ribosome. Tight interactions are observed between the factor's G domain and the large ribosomal subunit, as well as between domain IV and an intersubunit bridge. In contrast, some of the domains of eEF2 implicated in small subunit binding display a large degree of flexibility. Furthermore, we find support for a transition-state model conformation of the switch I region in this complex where the reoriented switch I region interacts with a conserved rRNA region of the 40S subunit formed by loops of the 18S RNA helices 8 and 14. This complex is structurally distinct from the eEF2-bound 80S ribosome complexes previously reported, and analysis of this map sheds light on the GTPase-coupled translocation mechanism.

The structure of LepA, the ribosomal back translocase

LepA is a highly conserved elongation factor that promotes the back translocation of tRNAs on the ribosome during the elongation cycle. We have determined the crystal structure of LepA from Escherichia coli at 2.8-A resolution. The high degree of sequence identity between LepA and EF-G is reflected in the structural similarity between the individual homologous domains of LepA and EF-G. However, the orientation of domains III and V in LepA differs from their orientations in EF-G. LepA also contains a C-terminal domain (CTD) not found in EF-G that has a previously unobserved protein fold. The high structural similarity between LepA and EF-G enabled us to derive a homology model for LepA bound to the ribosome using a 7.3-A cryo-EM structure of a complex between EF-G and the 70S ribosome. In this model, the very electrostatically positive CTD of LepA is placed in the direct vicinity of the A site of the large ribosomal subunit, suggesting a possible interaction between the CTD and the back translocated tRNA or 23S rRNA.

The location and the significance of a cross-link between the sarcin/ricin domain of ribosomal RNA and the elongation factor-G

During translocation peptidyl-tRNA moves from the A-site to the P-site and mRNA is displaced by three nucleotides in the 3' direction. This reaction is catalyzed by elongation factor-G (EF-G) and is associated with ribosome-dependent hydrolysis of GTP. The molecular basis of translocation is the most important unsolved problem with respect to ribosome function. A critical question, one that might provide a clue to the mechanism of translocation, is the precise identity of the contacts between EF-G and ribosome components. To make the identification, a covalent bond was formed, by ultraviolet irradiation, between EF-G and a sarcin/ricin domain (SRD) oligoribonucleotide containing 5-iodouridine. The cross-link was established, by mass spectroscopy and by Edman degradation, to be between a tryptophan at position 127 in the G domain in EF-G and either one of two 5-iodouridine nucleotides in the sequence UAG2655U in the SRD. G2655 is a critical identity element for the recognition of the factor's ribosomal binding site. The site of the cross-link provides the first direct evidence that the SRD is in close proximity to the EF-G catalytic center. The proximity suggests that the SRD RNA has a role in the activation of GTP hydrolysis that leads to a transition in the conformation of the factor and to its release from the ribosome.

Sordarin inhibits fungal protein synthesis by blocking translocation differently to fusidic acid

Sordarin derivatives are selective inhibitors of fungal protein synthesis, which specifically impair elongation factor 2 (EF-2) function. We have studied the effect of sordarin on the ribosome-dependent GTPase activity of EF-2 from Candida albicans in the absence of any other component of the translation system. The effect of sordarin turned out to be dependent both on the ratio of ribosomes to EF-2 and on the nature of the ribosomes. When the amount of EF-2 exceeded that of ribosomes sordarin inhibited the GTPase activity following an inverted bell-shaped dose-response curve, whereas when EF-2 and ribosomes were in equimolar concentrations sordarin yielded a typical sigmoidal dose-dependent inhibition. However, when ricin-treated ribosomes were used, sordarin stimulated the hydrolysis of GTP. These results were compared with those obtained with fusidic acid, showing that both drugs act in a different manner. All these data are consistent with sordarin blocking the elongation cycle at the initial steps of translocation, prior to GTP hydrolysis. In agreement with this conclusion, sordarin prevented the formation of peptidyl-[(3)H]puromycin on polysomes from Candida albicans.

Domain IV of elongation factor G from Thermus thermophilus is strictly required for translocation

Two truncated variants of elongation factor G from Thermus thermophilus with deletion of its domain IV have been constructed and the mutated genes were expressed in Escherichia coli. The truncated factors were produced in a soluble form and retained a high thermostability. It was demonstrated that mutated factors possessed (1) a reduced affinity to the ribosomes with an uncleavable GTP analog and (2) a specific ribosome-dependent GTPase activity. At the same time, in contrast to the wild-type elongation factor G, they were incapable to promote translocation. The conclusions are drawn that (1) domain IV is not involved in the GTPase activity of elongation factor G, (2) it contributes to the binding of elongation factor G with the ribosome and (3) is strictly required for translocation. These results suggest that domain IV might be directly involved in translocation and GTPase activity of the factor is not directly coupled with translocation.

The A26G replacement in the consensus sequence A-X-X-X-X-G-K-[T,S] of the guanine nucleotide binding site activates the intrinsic GTPase of the elongation factor 2 from the archaeon Sulfolobus solfataricus

A recombinant form of the elongation factor 2 from the archaeon Sulfolobus solfataricus (SsEF-2), carrying the A26G substitution, has been produced and characterized. The amino acid replacement converted the guanine nucleotide binding consensus sequences A-X-X-X-X-G-K-[T,S] of the elongation factors EF-G or EF-2 into the corresponding G-X-X-X-X-G-K-[T,S] motif which is present in all the other GTP-binding proteins. The rate of poly(U)-directed poly(Phe) synthesis and the ribosome-dependent GTPase activity of A26GSsEF-2 were decreased compared to SsEF-2, thus indicating that the A26G replacement partially affected the function of SsEF-2 during translocation. In contrast, the A26G substitution enhanced the catalytic efficiency of the intrinsic SsEF-2 GTPase triggered by ethylene glycol [Raimo, G., Masullo, M., Scarano, G., & Bocchini, V. (1997) Biochimie 78, 832-837]. Surprisingly, A26GSsEF-2 was able to hydrolyse GTP even in the absence of ethylene glycol; furthermore, the alcohol increased the affinity for GTP without modifying the catalytic constant of A26GSsEF-2 GTPase. Compared to SsEF-2, the affinity of A26GSsEF-2 for [3H]GDP was significantly reduced. These findings suggest that A26 is a regulator of the biochemical functions of SsEF-2. The involvement of this alanine residue in the guanine nucleotide-binding pocket of EF-2 or EF-G is discussed.

Dynamics of translation on the ribosome: molecular mechanics of translocation

The translocation step of protein elongation entails a large-scale rearrangement of the tRNA-mRNA-ribosome complex. Recent years have seen major advances in unraveling the mechanism of the process on the molecular level. A number of intermediate states have been defined and, in part, characterized structurally. The article reviews the recent evidence that suggests a dynamic role of the ribosome and its ligands during translocation. The focus is on dynamic aspects of tRNA movement and on the role of elongation factor G and GTP hydrolysis in translocation catalysis. The significance of structural changes of the ribosome induced by elongation factor G as well the role of ribosomal RNA are addressed. A functional model of elongation factor G as a motor protein driven by GTP hydrolysis is discussed.

EF-G-catalyzed translocation of anticodon stem-loop analogs of transfer RNA in the ribosome

Translocation, catalyzed by elongation factor EF-G, is the precise movement of the tRNA-mRNA complex within the ribosome following peptide bond formation. Here we examine the structural requirement for A- and P-site tRNAs in EF-G-catalyzed translocation by substituting anticodon stem-loop (ASL) analogs for the respective tRNAs. Translocation of mRNA and tRNA was monitored independently; mRNA movement was assayed by toeprinting, while tRNA and ASL movement was monitored by hydroxyl radical probing by Fe(II) tethered to the ASLs and by chemical footprinting. Translocation depends on occupancy of both A and P sites by tRNA bound in a mRNA-dependent fashion. The requirement for an A-site tRNA can be satisfied by a 15 nucleotide ASL analog comprising only a 4 base pair (bp) stem and a 7 nucleotide anticodon loop. Translocation of the ASL is both EF-G- and GTP-dependent, and is inhibited by the translocational inhibitor thiostrepton. These findings show that the D, T and acceptor stem regions of A-site tRNA are not essential for EF-G-dependent translocation. In contrast, no translocation occurs if the P-site tRNA is substituted with an ASL, indicating that other elements of P-site tRNA structure are required for translocation. We also tested the effect of increasing the A-site ASL stem length from 4 to 33 bp on translocation from A to P site. Translocation efficiency decreases as the ASL stem extends beyond 22 bp, corresponding approximately to the maximum dimension of tRNA along the anticodon-D arm axis. This result suggests that a structural feature of the ribosome between the A and P sites, interferes with movement of tRNA analogs that exceed the normal dimensions of the coaxial tRNA anticodon-D arm.

Ribosomal tRNA binding sites: three-site models of translation

The first models of translation described protein synthesis in terms of two operationally defined tRNA binding sites, the P-site for the donor substrate, the peptidyl-tRNA, and the A-site for the acceptor substrates, the aminoacyl-tRNAs. The discovery and analysis of the third tRNA binding site, the E-site specific for deacylated tRNAs, resulted in the allosteric three-site model, the two major features of which are (1) the reciprocal relationship of A-site and E-site occupation, and (2) simultaneous codon-anticodon interactions of both tRNAs present at the elongating ribosome. However, structural studies do not support the three operationally defined sites in a simple fashion as three topographically fixed entities, thus leading to new concepts of tRNA binding and movement: (1) the hybrid-site model describes the tRNAs' movement through the ribosome in terms of changing binding sites on the 30S and 50S subunits in an alternating fashion. The tRNAs thereby pass through hybrid binding states. (2) The alpha-epsilon model introduces the concept of a movable tRNA-binding domain comprising two binding sites, termed alpha and epsilon. The translocation movement is seen as a result of a conformational change of the ribosome rather than as a diffusion process between fixed binding sites. The alpha-epsilon model reconciles most of the experimental data currently available.

Hydrolysis of GTP by elongation factor G drives tRNA movement on the ribosome

Elongation factor G (EF-G) is a GTPase that is involved in the translocation of bacterial ribosomes along messenger RNA during protein biosynthesis. In contrast to current models, EF-G-dependent GTP hydrolysis is shown to precede, and greatly accelerate, the rearrangement of the ribosome that leads to translocation. Domain IV of the EF-G structure is crucial for both rapid translocation and subsequent release of the factor from the ribosome. By coupling the free energy of GTP hydrolysis to translocation, EF-G serves as a motor protein to drive the directional movement of transfer and messenger RNAs on the ribosome.

Isolation, crystallization and X-ray analysis of the quaternary complex of Phe-tRNA(Phe), EF-Tu, a GTP analog and kirromycin

Kirromycin inhibits bacterial protein synthesis by acting on elongation factor Tu (EF-Tu). Complexes of the antibiotic, Phe-tRNA(Phe), the guanosine triphosphate analog GDPNP, and mesophilic (Escherichia coli), as well as thermophilic (Thermus thermophilus) EF-Tu were isolated. Crystallization was achieved at 4 degrees C, pH 6.4, using ammonium sulphate as precipitant. Crystallographic data were recorded at cryogenic temperature on crystals exposed to synchrotron radiation. Crystals of the thermophilic complex are based on a rhombohedral lattice with cell dimensions of 137.3 A, and angles of 54.0 degrees. Although related, these cell parameters are different from those found in the crystals of the recently solved structure of the ternary complex of Phe-tRNA(Phe), GDPNP, and Thermus aquaticus EF-Tu (Nissen, P., Kjeldgaard, M., Thirup, S., Polekhina, G., Reshetnikova, L., Clark, B.F. and Nyborg, J. (1995) Science 270, 1464-1472 [1]), possibly indicating some allosteric effect caused by kirromycin. Crystals of the mesophilic complex belong to the cubic space P432, with cell axis of 196.26 A. In both cases, the crystals contain one complex per asymmetric unit.

Messenger RNA translation in prokaryotes: GTPase centers associated with translational factors

During the decoding of messenger RNA, each step of the translational cycle requires the intervention of protein factors and the hydrolysis of one or more GTP molecule(s). Of the prokaryotic translational factors, IF2, EF-Tu, SELB, EF-G and RF3 are GTP-binding proteins. In this review we summarize the latest findings on the structures and the roles of these GTPases in the translational process.