Electron microscopy of functional ribosome complexes

Cryoelectron microscopy has made a number of significant contributions to our understanding of the translation process. The method of single-particle reconstruction is particularly well suited for the study of the dynamics of ribosome-ligand interactions. This review follows the events of the functional cycle and discusses the findings in the context provided by the recently published x-ray structures.     

Structural changes in the ribosome during the elongation cycle

Ample data on structural changes that arise in the ribosome during translation have been accumulated. The most interesting information on such changes has been obtained by cryoelectron microscopy of ribosome complexes with various ligands and by rRNA site-directed mutagenesis combined with a structural analysis of the ribosome by a chemical modification technique (chemical probing). The review considers the best-known structural changes that arise in the translating ribosome upon its interactions with tRNA and the elongation factors. The changes are discussed in the context of interactions between the functional centers of the ribosome. A universal system of rRNA helices and proteins is described in detail. The system integrates the functional centers of the ribosome and allows transduction of allosteric conformational signals. Biochemical data are considered in terms of the structures and interactions of ribosomal elements, and a hypothesis is advanced that the position of the GTPase-associated center in the ribosome regulates the binding of the elongation factors.     

The structural changes in the ribosome during the elongation cycle

The plenty of data about structural changes in the ribosome during its functioning has been accumulated. The most interesting information on such changes was obtained by cryo-EM of various ribosomal complexes with the ligands and by combination of rRNA site-directed mutagenesis with the analysis of structural changes in ribosome by chemical modification technique (chemical probing). The most studied structural transformations of the ribosome interacting with tRNAs and elongation factors are considered in this review. The structural rearrangements are discussed in the context of interactions between the functional centers of the ribosome. We also describe the system of tertiary contacts between the rRNA helices and proteins which forms the universal structure in the ribosome. We pay attention that by means of such system the allosteric conformational signal can be transmitted between the functional centers. Besides the discussion of different biochemical data in the scope of structural data we also consider the hypothesis that the position of GTPase associated center (GAC) in the ribosome regulates the binding of elongation factors.     

Effect of buffer conditions on the position of tRNA on the 70 S ribosome as visualized by cryoelectron microscopy

The effect of buffer conditions on the binding position of tRNA on the Escherichia coli 70 S ribosome have been studied by means of three-dimensional (3D) cryoelectron microscopy. Either deacylated tRNAfMet or fMet-tRNAfMet were bound to the 70 S ribosomes, which were programmed with a 46-nucleotide mRNA having AUG codon in the middle, under two different buffer conditions (conventional buffer: containing Tris and higher Mg2+ concentration [10-15 mM]; and polyamine buffer: containing Hepes, lower Mg2+ concentration [6 mM], and polyamines). Difference maps, obtained by subtracting 3D maps of naked control ribosome in the corresponding buffer from the 3D maps of tRNA.ribosome complexes, reveal the distinct locations of tRNA on the ribosome. The position of deacylated tRNAfMet depends on the buffer condition used, whereas that of fMet-tRNAfMet remains the same in both buffer conditions. The acylated tRNA binds in the classical P site, whereas deacylated tRNA binds mostly in an intermediate P/E position under the conventional buffer condition and mostly in the position corresponding to the classical P site, i. e. in the P/P state, under the polyamine buffer conditions.     

Visualization of tRNA movements on the Escherichia coli 70S ribosome during the elongation cycle

Three-dimensional cryomaps have been reconstructed for tRNA-ribosome complexes in pre- and posttranslocational states at 17-A resolution. The positions of tRNAs in the A and P sites in the pretranslocational complexes and in the P and E sites in the posttranslocational complexes have been determined. Of these, the P-site tRNA position is the same as seen earlier in the initiation-like fMet-tRNA(f)(Met)-ribosome complex, where it was visualized with high accuracy. Now, the positions of the A- and E-site tRNAs are determined with similar accuracy. The positions of the CCA end of the tRNAs at the A site are different before and after peptide bond formation. The relative positions of anticodons of P- and E-site tRNAs in the posttranslocational state are such that a codon-anticodon interaction at the E site appears feasible.     

Localization of functional centers on the prokaryotic ribosome: immuno-electron microscopy approach

Immuno-electron microscopy studies on the morphology of the Escherichia coli ribosome and the topography of its functional centers are summarized. A three-dimensional model of the ribosome with indications of the mRNA-binding site, the peptidyl-transferase center, the EF-Tu and EF-G-binding sites and the tRNA-binding sites in given. Thus, the mutual arrangement of the centers corresponding to the main functional activities of the ribosome during the process of translation is presented.     

Messenger RNA orientation on the ribosome. Placement by electron microscopy of antibody-complementary oligodeoxynucleotide complexes

Messenger RNA orients on the small ribosomal subunit by base pairing with a complementary sequence in ribosomal RNA. We have positioned this ribosomal RNA segment and thus oriented the mRNA using a new technique--localization of an antibody-recognizable modified complementary oligodeoxynucleotide by electron microscopy. A synthetic oligodeoxynucleotide complementary to the message-positioning ribosomal RNA sequence was modified at either or both ends with different antigenic markers. Electron microscopy of subunit-oligodeoxynucleotide-antibody complexes allowed separate placement of each terminal marker of the oligodeoxynucleotide probe. The 5'-end of the complementary sequence contacts the subunit at the platform tip (rRNA nucleotide 1542). The message then extends along the interior side of the platform to the level of the fork of the cleft separating the platform from the subunit body, and displaced slightly to the convex side of the platform (rRNA nucleotide 1531). Based on our results and data from other laboratories, we propose a model for the positioning of messenger RNA on the 30 S subunit.     

Generation of ribosome nascent chain complexes for structural and functional studies

Biochemical and structural studies of co-translational folding, targeting and translocation depend on an efficient methodology to prepare ribosome nascent chain complexes (RNCs). Here we present our approach for the generation of homogenous and stable RNCs involving in vitro translation and affinity purification. Fusing the SecM arrest sequence, which tightly interacts with the ribosomal tunnel, to the nascent polypeptide chain significantly enhanced the stability of the RNCs. We have been able to increase the yield of the affinity purification step by engineering a tag with higher affinity. The RNCs generated with this approach have been successfully used to obtain 3D cryo-electron microscopic reconstructions of complexes with the signal recognition particle and the translocon. The established procedure is highly efficient and if scaled up could yield milligram amounts of RNCs sufficient for crystallization experiments.     

Three-dimensional structures of the flagellar dynein-microtubule complex by cryoelectron microscopy

The outer dynein arms (ODAs) of the flagellar axoneme generate forces needed for flagellar beating. Elucidation of the mechanisms underlying the chemomechanical energy conversion by the dynein arms and their orchestrated movement in cilia/flagella is of great importance, but the nucleotide-dependent three-dimensional (3D) movement of dynein has not yet been observed. In this study, we establish a new method for reconstructing the 3D structure of the in vitro reconstituted ODA-microtubule complex and visualize nucleotide-dependent conformational changes using cryoelectron microscopy and image analysis. As the complex went from the rigor state to the relaxed state, the head domain of the beta heavy chain shifted by 3.7 nm toward the B tubule and inclined 44 degrees inwards. These observations suggest that there is a mechanism that converts head movement into the axonemal sliding motion.     

Identification of the beta1-integrin binding site on alpha-actinin by cryoelectron microscopy

Cell-matrix adhesions in migrating cells are usually mediated by integrins, alpha-beta heterodimeric transmembrane proteins that link extracellular matrix molecules such as fibronectin to the cytoskeleton. We have synthesized the cytoplasmic domain of the beta1-integrin (residues H738-K778) with a histidine tag at its N-terminus. The binding of this peptide to a lipid monolayer containing a chelated-nickel group (dimyristoylphosphatidyl choline-suberimide-nitriloacetic acid:nickel salt) mimics the native environment at the cytoplasmic leaflet of the plasma membrane. A Nanogold particle was covalently linked to cysteines introduced at the C-terminus and after residue T757 on the integrin peptide, and co-crystallized with chicken smooth muscle alpha-actinin. The 2-D arrays of the beta1-integrin-alpha-actinin complex were examined by cryoelectron microscopy, with and without the gold label. Averaged projections were calculated for each specimen along with a difference map to determine the relative position of the gold-labeled beta1-integrin peptide. The difference maps indicate that the beta1-integrin cytoplasmic domain binds alpha-actinin between the first and second, 3-helix motifs in the central rod domain.     

GTPase activation of elongation factors Tu and G on the ribosome

The GTPase activity of elongation factors Tu and G is stimulated by the ribosome. The factor binding site is located on the 50S ribosomal subunit and comprises proteins L7/12, L10, L11, the L11-binding region of 23S rRNA, and the sarcin-ricin loop of 23S rRNA. The role of these ribosomal elements in factor binding, GTPase activation, or functions in tRNA binding and translocation, and their relative contributions, is not known. By comparing ribosomes depleted of L7/12 and reconstituted ribosomes, we show that, for both factors, interactions with L7/12 and with other ribosomal residues contribute about equally and additively to GTPase activation, resulting in an overall 10(7)-fold stimulation. Removal of L7/12 has little effect on factor binding to the ribosome. Effects on other factor-dependent functions, i.e., A-site binding of aminoacyl-tRNA and translocation, are fully explained by the inhibition of GTP hydrolysis. Based on these results, we propose that L7/12 stimulates the GTPase activity of both factors by inducing the catalytically active conformation of the G domain. This effect appears to be augmented by interactions of other structural elements of the large ribosomal subunit with the switch regions of the factors.     

Cryoelectron microscopy and cryoelectron tomography of the nuclear pre-mRNA processing machine

Large nuclear ribonucleoprotein particles, which can be viewed as the naturally assembled precursor messenger RNA (pre-mRNA) processing machine, were analyzed in frozen-hydrated preparations by cryoelectron microscopy. A general and reproducible strategy for preparing ice-embedded large nuclear ribonucleoprotein (lnRNP) particles at sufficiently high concentration was developed. Taking advantage of their negatively charged components, the lnRNP particles are adsorbed and thus concentrated on a positively charged lipid monolayer while preserving their native structure. Using this approach we carried out cryoelectron tomography and three-dimensional image reconstruction of individual lnRNP particles. The study revealed a structure similar to that of negatively stained particles studied previously, yet with additional features. The small additional domain visualized in negative stain appeared to be larger in the ice preparations. In addition, using image restoration from focus series of ice-embedded lnRNP particles, new features such as holes within the subunits were visualized in two dimensions, and it was shown that the subunits are interconnected via a fiber, very likely formed by the pre-mRNA. This finding supports the model that each subunit represents a spliceosome that splices out the intron wound around it.     

Locating the thapsigargin-binding site on Ca(2+)-ATPase by cryoelectron microscopy

Thapsigargin (TG) is a potent inhibitor of Ca(2+)-ATPase from sarcoplasmic and endoplasmic reticula. Previous enzymatic studies have concluded that Ca(2+)-ATPase is locked in a dead-end complex upon binding TG with an affinity of <1 nM and that this complex closely resembles the E(2) enzymatic state. We have studied the structural effects of TG binding by cryoelectron microscopy of tubular crystals, which have previously been shown to comprise Ca(2+)-ATPase molecules in the E(2) conformation. In particular, we have compared 3D reconstructions of Ca(2+)-ATPase in the absence and presence of either TG or its dansylated derivative. The overall molecular shape of Ca(2+)-ATPase in the reconstructions is very similar, demonstrating that the TG/Ca(2+)-ATPase complex does indeed physically resemble the E(2) conformation, in contrast to massive domain movements that appear to be induced by Ca(2+) binding. Difference maps reveal a consistent difference on the lumenal side of the membrane, which we conclude corresponds to the thapsigargin-binding site. Modeling the atomic structure for Ca(2+)-ATPase into our density maps reveals that this binding site is composed of the loops between transmembrane segments M3/M4 and M7/M8. Indirect effects are proposed to explain the effects of the S3 stalk segment on thapsigargin affinity as well as thapsigargin-induced changes in ATP affinity. Indeed, a second difference density was observed at the decavanadate-binding site within the three cytoplasmic domains, which we believe reflects an altered affinity as a result of the long-range conformational coupling that drives the reaction cycle of this family of ATP-dependent ion pumps.     

A functional connection between translation elongation and protein folding at the ribosome exit tunnel in Saccharomyces cerevisiae

Proteostasis needs to be tightly controlled to meet the cellular demand for correctly de novo folded proteins and to avoid protein aggregation. While a coupling between translation rate and co-translational folding, likely involving an interplay between the ribosome and its associated chaperones, clearly appears to exist, the underlying mechanisms and the contribution of ribosomal proteins remain to be explored. The ribosomal protein uL3 contains a long internal loop whose tip region is in close proximity to the ribosomal peptidyl transferase center. Intriguingly, the rpl3[W255C] allele, in which the residue making the closest contact to this catalytic site is mutated, affects diverse aspects of ribosome biogenesis and function. Here, we have uncovered, by performing a synthetic lethal screen with this allele, an unexpected link between translation and the folding of nascent proteins by the ribosome-associated Ssb-RAC chaperone system. Our results reveal that uL3 and Ssb-RAC cooperate to prevent 80S ribosomes from piling up within the 5′ region of mRNAs early on during translation elongation. Together, our study provides compelling in vivo evidence for a functional connection between peptide bond formation at the peptidyl transferase center and chaperone-assisted de novo folding of nascent polypeptides at the solvent-side of the peptide exit tunnel.     

Determination of the alpha-actinin-binding site on actin filaments by cryoelectron microscopy and image analysis

The three-dimensional structure of actin filaments decorated with the actin-binding domain of chick smooth muscle alpha-actinin (alpha A1-2) has been determined to 21-A resolution. The shape and location of alpha A1-2 was determined by subtracting maps of F-actin from the reconstruction of decorated filaments. alpha A1-2 resembles a bell that measures approximately 38 A at its base and extends 42 A from its base to its tip. In decorated filaments, the base of alpha A1-2 is centered about the outer face of subdomain 2 of actin and contacts subdomain 1 of two neighboring monomers along the long-pitch (two-start) helical strands. Using the atomic model of F-actin (Lorenz, M., D. Popp, and K. C. Holmes. 1993. J. Mol. Biol. 234:826-836.), we have been able to test directly the likelihood that specific actin residues, which have been previously identified by others, interact with alpha A1-2. Our results indicate that residues 86-117 and 350-375 comprise distinct binding sites for alpha-actinin on adjacent actin monomers.     

Role of yeast elongation factor 3 in the elongation cycle

Investigation of the role of the polypeptide chain elongation factor 3 (EF-3) of yeast indicates that EF-3 participates in the elongation cycle by stimulating the function of EF-1 alpha in binding aminoacyl-tRNA (aa-tRNA) to the ribosome. In the yeast system, the binding of the ternary complex of EF-1 alpha.GTP.aa-tRNA to the ribosome is stoichiometric to the amount of EF-1 alpha. In the presence of EF-3, EF-1 alpha functions catalytically in the above mentioned reaction. The EF-3 effect is manifest in the presence of ATP, GTP, or ITP. A nonhydrolyzable analog of ATP does not replace ATP in this reaction, indicating a role of ATP hydrolysis in EF-3 function. The stimulatory effect of EF-3 is, in many respects, distinct from that of EF-1 beta. Factor 3 does not stimulate the formation of a binary complex between EF-1 alpha and GTP, nor does it stimulate the exchange of EF-1 alpha-bound GDP with free GTP. The formation of a ternary complex between EF-1 alpha.GTP.aa-tRNA is also not affected by EF-3. It appears that the only reaction of the elongation cycle that is stimulated by EF-3 is EF-1 alpha-dependent binding of aa-tRNA to the ribosome. Purified elongation factor 3, isolated from a temperature-sensitive mutant, failed to stimulate this reaction after exposure to a nonpermissive temperature. A heterologous combination of ribosomal subunits from yeast and wheat germ manifest the requirement for EF-3, dependent upon the source of the "40 S" ribosomal subunit. A combination of 40 S subunits from yeast and "60 S" from wheat germ showed the stimulatory effect of EF-3 in polyphenylalanine synthesis (Chakraburtty, K., and Kamath, A. (1988) Int. J. Biochem. 20, 581-590). However, we failed to demonstrate the effect of EF-3 in binding aa-tRNA to such a heterologous combination of the ribosomal subunits.