Wolfgang Piendl

Structure of the L1 protuberance in the ribosome.

The L1 protuberance of the 50S ribosomal subunit is implicated in the release/disposal of deacylated tRNA from the E site. The apparent mobility of this ribosomal region has thus far prevented an accurate determination of its three-dimensional structure within either the 50S subunit or the 70S ribosome. Here we report the crystal structure at 2.65 Å resolution of ribosomal protein L1 from Sulfolobus acidocaldarius in complex with a specific 55-nucleotide fragment of 23S rRNA from Thermus thermophilus. This structure fills a major gap in current models of the 50S ribosomal subunit. The conformations of L1 and of the rRNA fragment differ dramatically from those within the crystallographic model of the T. thermophilus 70S ribosome. Incorporation of the L1–rRNA complex into the structural models of the T. thermophilus 70S ribosome and the Deinococcus radiodurans 50S subunit gives a reliable representation of most of the L1 protuberance within the ribosome.

Translation termination factor aRF1 from the archaeon Methanococcus jannaschii is active with eukaryotic ribosomes.

Class‐1 translation termination factors (release factors (RFs)) from Eukarya (eRF1) and Archaea (aRF1) exhibit a high degree of amino acid sequence homology and share many common motifs. In contrast to eRF1, function(s) of aRF1 have not yet been studied in vitro. Here, we describe for the first time the cloning and expression in Escherichia coli of the gene encoding the peptide chain RF from the hyperthermophilic archaeon Methanococcus jannaschii (MjaRF1). In an in vitro assay with mammalian ribosomes, MjaRF1, which was overproduced in E. coli, was active as a RF with all three termination codon‐containing tetraplets, demonstrating the functional resemblance of aRF1 and eRF1. This observation confirms the earlier prediction that eRF1 and aRF1 form a common structural–functional eRF1/aRF1 protein family, originating from a common ancient ancestor.

Archaeal ribosomal protein L1: the structure provides new insights into RNA binding of the L1 protein family

BACKGROUND L1 is an important primary rRNA-binding protein, as well as a translational repressor that binds mRNA. It was shown that L1 proteins from some bacteria and archaea are functionally interchangeable within the ribosome and in the repression of translation. The crystal structure of bacterial L1 from Thermus thermophilus (TthL1) has previously been determined. RESULTS We report here the first structure of a ribosomal protein from archaea, L1 from Methanococcus jannaschii (MjaL1). The overall shape of the two-domain molecule differs dramatically from that of its bacterial counterpart (TthL1) because of the different relative orientations of the domains. Two strictly conserved regions of the amino acid sequence, each belonging to one of the domains and positioned close to each other in the interdomain cavity of TthL1, are separated by about 25 A in MjaL1 owing to a significant opening of the structure. These regions are structurally highly conserved and are proposed to be the specific RNA-binding sites. CONCLUSIONS The unusually high RNA-binding affinity of MjaL1 might be explained by the exposure of its highly conserved regions. The open conformation of MjaL1 is strongly stabilized by nonconserved interdomain interactions and suggests that the closed conformations of L1 (as in TthL1) open upon RNA binding. Comparison of the two L1 protein structures reveals a high conformational variability of this ribosomal protein. Determination of the MjaL1 structure offers an additional variant for fitting the L1 protein into electron-density maps of the 50S ribosomal subunit.

MvaL1 autoregulates the synthesis of the three ribosomal proteins encoded on the MvaL1 operon of the archaeon Methanococcus vannielii by inhibiting its own translation before or at the formation of the first peptide bond

The control of ribosomal protein synthesis has been investigated extensively in Eukarya and Bacteria. In Archaea, only the regulation of the MvaL1 operon (encoding ribosomal proteins MvaL1, MvaL10 and MvaL12) of Methanococcus vannielii has been studied in some detail. As in Escherichia coli, regulation takes place at the level of translation. MvaL1, the homologue of the regulatory protein L1 encoded by the L11 operon of E. coli, was shown to be an autoregulator of the MvaL1 operon. The regulatory MvaL1 binding site on the mRNA is located about 30 nucleotides downstream of the ATG start codon, a sequence that is not in direct contact with the initiating ribosome. Here, we demonstrate that autoregulation of MvaL1 occurs at or before the formation of the first peptide bond of MvaL1. Specific interaction of purified MvaL1 with both 23S RNA and its own mRNA is confirmed by filter binding studies. In vivo expression experiments reveal that translation of the distal MvaL10 and MvaL12 cistrons is coupled to that of the MvaL1 cistron. A mRNA secondary structure resembling a canonical L10 binding site and preliminary in vitro regulation experiments had suggested a co‐regulatory function of MvaL10, the homologue of the regulatory protein L10 of the β‐operon of E. coli. However, we show that MvaL10 does not have a regulatory function.

Affinity of ribosomal protein S8 from mesophilic and (hyper)thermophilic archaea and bacteria for 16S rRNA correlates with the growth temperatures of the organisms

The ribosomal protein S8 plays a pivotal role in the assembly of the 30S ribosomal subunit. Using filter binding assays, S8 proteins from mesophilic, and (hyper)thermophilic species of the archaeal genus Methanococcus and from the bacteria Escherichia coli and Thermus thermophilus were tested for their affinity to their specific 16S rRNA target site. S8 proteins from hyperthermophiles exhibit a 100‐fold and S8 from thermophiles exhibit a 10‐fold higher affinity than their mesophilic counterparts. Thus, there is a striking correlation of affinity of S8 proteins for their specific RNA binding site and the optimal growth temperatures of the respective organisms. The stability of individual rRNA‐protein complexes might modulate the stability of the ribosome, providing a maximum of thermostability and flexibility at the growth temperature of the organism.

Structure of the ribosomal protein L1-mRNA complex at 2.1 A resolution: common features of crystal packing of L1-RNA complexes.

The crystal structure of a hybrid complex between the bacterial ribosomal protein L1 from Thermus thermophilus and a Methanococcus vannielii mRNA fragment containing an L1-binding site was determined at 2.1 A resolution. It was found that all polar atoms involved in conserved protein-RNA hydrogen bonds have high values of density in the electron-density map and that their hydrogen-bonding capacity is fully realised through interactions with protein atoms, water molecules and K(+) ions. Intermolecular contacts were thoroughly analyzed in the present crystals and in crystals of previously determined L1-RNA complexes. It was shown that extension of the RNA helices providing canonical helix stacking between open-open or open-closed ends of RNA fragments is a common feature of these and all known crystals of complexes between ribosomal proteins and RNAs. In addition, the overwhelming majority of complexes between ribosomal proteins and RNA molecules display crystal contacts formed by the central parts of the RNA fragments. These contacts are often very extensive and strong and it is proposed that they are formed in the saturated solution prior to crystal formation.

Stability of the ‘L12 stalk’ in ribosomes from mesophilic and (hyper)thermophilic Archaea and Bacteria

The ribosomal stalk complex, consisting of one molecule of L10 and four or six molecules of L12, is attached to 23S rRNA via protein L10. This complex forms the so-called ‘L12 stalk’ on the 50S ribosomal subunit. Ribosomal protein L11 binds to the same region of 23S rRNA and is located at the base of the ‘L12 stalk’. The ‘L12 stalk’ plays a key role in the interaction of the ribosome with translation factors. In this study stalk complexes from mesophilic and (hyper)thermophilic species of the archaeal genus Methanococcus and from the Archaeon Sulfolobus solfataricus, as well as from the Bacteria Escherichia coli, Geobacillus stearothermophilus and Thermus thermophilus, were overproduced in E.coli and purified under non-denaturing conditions. Using filter-binding assays the affinities of the archaeal and bacterial complexes to their specific 23S rRNA target site were analyzed at different pH, ionic strength and temperature. Affinities of both archaeal and bacterial complexes for 23S rRNA vary by more than two orders of magnitude, correlating very well with the growth temperatures of the organisms. A cooperative effect of binding to 23S rRNA of protein L11 and the L10/L124 complex from mesophilic and thermophilic Archaea was shown to be temperature-dependent.

Structure of ribosomal protein L1 from Methanococcus thermolithotrophicus. Functionally important structural invariants on the L1 surface

The crystal structure of ribosomal protein L1 from the archaeon Methanococcus thermolithotrophicus has been determined at 2.7 A resolution. The crystals belong to space group P2(1)2(1)2(1), with unit-cell parameters a = 67.0, b = 70.1, c = 106.3 A and two molecules per asymmetric unit. The structure was solved by the molecular-replacement method with AMoRe and refined with CNS to an R value of 18.9% and an R(free) of 25.4% in the resolution range 30-2.7 A. Comparison of this structure with those obtained previously for two L1 proteins from other sources (the bacterium Thermus thermophilus and the archaeon M. jannaschii) as well as detailed analysis of intermolecular contacts in the corresponding L1 crystals reveal structural invariants on the molecular surface which are probably important for binding the 23S ribosomal RNA and protein function within the ribosome.

Domain II of Thermus thermophilus Ribosomal Protein L1 Hinders Recognition of Its mRNA

The two-domain ribosomal protein L1 has a dual function as a primary rRNA-binding ribosomal protein and as a translational repressor that binds its own mRNA. Here, we report the crystal structure of a complex between the isolated domain I of L1 from the bacterium Thermus thermophilus and a specific mRNA fragment from Methanoccocus vannielii. In parallel, we report kinetic characteristics measured for complexes formed by intact TthL1 and its domain I with the specific mRNA fragment. Although, there is a close similarity between the RNA-protein contact regions in both complexes, the association rate constant is higher in the case of the complex formed by the isolated domain I. This finding demonstrates that domain II hinders mRNA recognition by the intact TthL1.

Disruption of shape complementarity in the ribosomal protein L1–RNA contact region does not hinder specific recognition of the RNA target site

The formation of a specific and stable complex between two (macro)molecules implies complementary contact surface regions. We used ribosomal protein L1, which specifically binds a target site on 23S rRNA, to study the influence of surface modifications on the protein−RNA affinity. The threonine residue in the universally conserved triad Thr−Met−Gly significant for RNA recognition and binding was substituted by phenylalanine, valine and alanine, respectively. The crystal structure of the mutant Thr217Val of the isolated domain I of L1 from Thermus thermophilus (TthL1) was determined. This structure and that of two other mutants, which had been determined earlier, were analysed and compared with the structure of the wild type L1 proteins. The influence of structural changes in the mutant L1 proteins on their affinity for the specific 23S rRNA fragment was tested by kinetic experiments using surface plasmon resonance (SPR) biosensor analysis. Association rate constants undergo minor changes, whereas dissociation rate constants displayed significantly higher values in comparison with that for the wild type protein. The analysed L1 mutants recognize the specific RNA target site, but the mutant L1−23S rRNA complexes are less stable compared to the wild type complexes. Copyright