Structural insights into SRP RNA: an induced fit mechanism for SRP assembly

Translational and rotational diffusion of short ribonucleic acids

Inevitable precondition for ribonucleic acids to regulate gene expression and to perform gene editing is diffusion. Free three-dimensional translational diffusion velocity of RNA of up to 200 nucleotides could be predicted with high accuracy by the empirical model D = 4.58 10^-10 N^-0.39 m²s^-1. Furthermore, the biological function of ribonucleic acids is determined by rotational diffusion. In the presented work, an empirical model is derived applying atom-level shell-modeling of electron density maps, D_r = 1.62 10⁹ N^-1.20 s^-1, to predict the rotational diffusion coefficient of short ribonucleic acids based on the polymer size.

Structure of a bacterial OapB protein with its OLE RNA target gives insights into the architecture of the OLE ribonucleoprotein complex

Bacterial noncoding RNAs (ncRNAs) play key roles in many biological processes including gene regulation, RNA processing and modification, and protein synthesis and translocation. OLE RNAs, found in many Gram-positive species, are one of the largest highly structured ncRNA classes whose biochemical functions remain unknown. In Bacillus halodurans, OLE RNAs interact with at least two proteins, OapA and OapB, which are required to assemble a functional OLE ribonucleoprotein (RNP) complex contributing to cellular responses to certain environmental stresses. We established X-ray structural models that reveal the sequence elements and tertiary structural features of OLE RNA that are critical for its specific recognition by OapB, which will aid future exploration of the biological and biochemical functions of the unusual OLE RNP complex.

The OLE (ornate, large, and extremophilic) RNA class is one of the most complex and well-conserved bacterial noncoding RNAs known to exist. This RNA is known to be important for bacterial responses to stress caused by short-chain alcohols, cold, and elevated Mg²⁺ concentrations. These biological functions have been shown to require the formation of a ribonucleoprotein (RNP) complex including at least two protein partners: OLE-associated protein A (OapA) and OLE-associated protein B (OapB). OapB directly binds OLE RNA with high-affinity and specificity and is believed to assist in assembling the functional OLE RNP complex. To provide the atomic details of OapB–OLE RNA interaction and to potentially reveal previously uncharacterized protein–RNA interfaces, we determined the structure of OapB from Bacillus halodurans alone and in complex with an OLE RNA fragment at resolutions of 1.0 Å and 2.0 Å, respectively. The structure of OapB exhibits a K-shaped overall architecture wherein its conserved KOW motif and additional unique structural elements of OapB form a bipartite RNA-binding surface that docks to the P13 hairpin and P12.2 helix of OLE RNA. These high-resolution structures elucidate the molecular contacts used by OapB to form a stable RNP complex and explain the high conservation of sequences and structural features at the OapB–OLE RNA-binding interface. These findings provide insight into the role of OapB in the assembly and biological function of OLE RNP complex and can guide the exploration of additional possible OLE RNA-binding interactions present in OapB.

Footprinting SHAPE-eCLIP Reveals Transcriptome-wide Hydrogen Bonds at RNA-Protein Interfaces

Discovering the interaction mechanism and location of RNA-binding proteins (RBPs) on RNA is critical for understanding gene expression regulation. Here, we apply selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) on in vivo transcripts compared to protein-absent transcripts in four human cell lines to identify transcriptome-wide footprints (fSHAPE) on RNA. Structural analyses indicate that fSHAPE precisely detects nucleobases that hydrogen bond with protein. We demonstrate that fSHAPE patterns predict binding sites of known RBPs, such as iron response elements in both known loci and previously unknown loci in CDC34, SLC2A4RG, COASY, and H19. Furthermore, by integrating SHAPE and fSHAPE with crosslinking and immunoprecipitation (eCLIP) of desired RBPs, we interrogate specific RNA-protein complexes, such as histone stem-loop elements and their nucleotides that hydrogen bond with stem-loop-binding proteins. Together, these technologies greatly expand our ability to study and understand specific cellular RNA interactions in RNA-protein complexes.

How RNA-Binding Proteins Interact with RNA: Molecules and Mechanisms

RNA-binding proteins (RBPs) comprise a large class of over 2,000 proteins that interact with transcripts in all manner of RNA-driven processes. The structures and mechanisms that RBPs use to bind and regulate RNA are incredibly diverse. In this review, we take a look at the components of protein-RNA interaction, from the molecular level to multi-component interaction. We first summarize what is known about protein-RNA molecular interactions based on analyses of solved structures. We additionally describe software currently available for predicting protein-RNA interaction and other resources useful for the study of RBPs. We then review the structure and function of seventeen known RNA-binding domains and analyze the hydrogen bonds adopted by protein-RNA structures on a domain-by-domain basis. We conclude with a summary of the higher-level mechanisms that regulate protein-RNA interactions.

Sequential activation of human signal recognition particle by the ribosome and signal sequence drives efficient protein targeting

Signal recognition particle (SRP) is a universally conserved targeting machine that mediates the targeted delivery of ∼30% of the proteome. The molecular mechanism by which eukaryotic SRP achieves efficient and selective protein targeting remains elusive. Here, we describe quantitative analyses of completely reconstituted human SRP (hSRP) and SRP receptor (SR). Enzymatic and fluorescence analyses showed that the ribosome, together with a functional signal sequence on the nascent polypeptide, are required to activate SRP for rapid recruitment of the SR, thereby delivering translating ribosomes to the endoplasmic reticulum. Single-molecule fluorescence spectroscopy combined with cross-complementation analyses reveal a sequential mechanism of activation whereby the ribosome unlocks the hSRP from an autoinhibited state and primes SRP to sample a variety of conformations. The signal sequence further preorganizes the mammalian SRP into the optimal conformation for efficient recruitment of the SR. Finally, the use of a signal sequence to activate SRP for receptor recruitment is a universally conserved feature to enable efficient and selective protein targeting, and the eukaryote-specific components confer upon the mammalian SRP the ability to sense and respond to ribosomes.

Structural Changes of RNA in Complex with Proteins in the SRP

The structural flexibility of RNA allows it to exist in several shapes and sizes. Thus, RNA is functionally diverse and is known to be involved in processes such as catalysis, ligand binding, and most importantly, protein recognition. RNA can adopt different structures, which can often dictate its functionality. When RNA binds onto protein to form a ribonucleoprotein complex (RNP), multiple interactions and conformational changes occur with the RNA and protein. However, there is the question of whether there is a specific pattern for these changes to occur upon recognition. In particular when RNP complexity increases with the addition of multiple proteins/RNA, it becomes difficult to structurally characterize the overall changes using the current structural determination techniques. Hence, there is a need to use a combination of biochemical, structural and computational modeling to achieve a better understanding of the processes that RNPs are involved. Nevertheless, there are well-characterized systems that are evolutionarily conserved [such as the signal recognition particle (SRP)] that give us important information on the structural changes of RNA and protein upon complex formation.

ProbeAlign: incorporating high-throughput sequencing-based structure probing information into ncRNA homology search

Background

Recent advances in RNA structure probing technologies, including the ones based on high-throughput sequencing, have improved the accuracy of thermodynamic folding with quantitative nucleotide-resolution structural information.

Results

In this paper, we present a novel approach, ProbeAlign, to incorporate the reactivities from high-throughput RNA structure probing into ncRNA homology search for functional annotation. To reduce the overhead of structure alignment on large-scale data, the specific pairing patterns in the query sequences are ignored. On the other hand, the partial structural information of the target sequences embedded in probing data is retrieved to guide the alignment. Thus the structure alignment problem is transformed into a sequence alignment problem with additional reactivity information. The benchmark results show that the prediction accuracy of ProbeAlign outperforms filter-based CMsearch with high computational efficiency. The application of ProbeAlign to the FragSeq data, which is based on genome-wide structure probing, has demonstrated its capability to search ncRNAs in a large-scale dataset from high-throughput sequencing.

Conclusions

By incorporating high-throughput sequencing-based structure probing information, ProbeAlign can improve the accuracy and efficiency of ncRNA homology search. It is a promising tool for ncRNA functional annotation on genome-wide datasets.

Availability

The source code of ProbeAlign is available at http://genome.ucf.edu/ProbeAlign.

Signal recognition particle: an essential protein-targeting machine

The signal recognition particle (SRP) and its receptor compose a universally conserved and essential cellular machinery that couples the synthesis of nascent proteins to their proper membrane localization. The past decade has witnessed an explosion in in-depth mechanistic investigations of this targeting machine at increasingly higher resolutions. In this review, we summarize recent work that elucidates how the SRP and SRP receptor interact with the cargo protein and the target membrane, respectively, and how these interactions are coupled to a novel GTPase cycle in the SRP·SRP receptor complex to provide the driving force and enhance the fidelity of this fundamental cellular pathway. We also discuss emerging frontiers in which important questions remain to be addressed.

Archaea signal recognition particle shows the way

Archaea SRP is composed of an SRP RNA molecule and two bound proteins named SRP19 and SRP54. Regulated by the binding and hydrolysis of guanosine triphosphates, the RNA-bound SRP54 protein transiently associates not only with the hydrophobic signal sequence as it emerges from the ribosomal exit tunnel, but also interacts with the membrane-associated SRP receptor (FtsY). Comparative analyses of the archaea genomes and their SRP component sequences, combined with structural and biochemical data, support a prominent role of the SRP RNA in the assembly and function of the archaea SRP. The 5e motif, which in eukaryotes binds a 72 kilodalton protein, is preserved in most archaea SRP RNAs despite the lack of an archaea SRP72 homolog. The primary function of the 5e region may be to serve as a hinge, strategically positioned between the small and large SRP domain, allowing the elongated SRP to bind simultaneously to distant ribosomal sites. SRP19, required in eukaryotes for initiating SRP assembly, appears to play a subordinate role in the archaea SRP or may be defunct. The N-terminal A region and a novel C-terminal R region of the archaea SRP receptor (FtsY) are strikingly diverse or absent even among the members of a taxonomic subgroup.

Crossing the membrane in Archaea, the third domain of life

Many of the recent advancements in the field of protein translocation, particularly from the structural perspective, have relied on Archaea. For instance, the solved structures of the translocon from the methanoarchaeon Methanocaldococcus jannaschii of the ribosomal large subunit from the haloarchaeon Haloarcula marismortui and of components of the SRP pathway from several archaeal species have provided novel insight into various aspects of the translocation event. Given the major contribution that Archaea have made to our understanding of how proteins enter and traverse membranes, it is surprising that relatively little is known of protein translocation in Archaea in comparison to the well-defined translocation pathways of Eukarya and Bacteria. What is known, however, points to archaeal translocation as comprising a mosaic of eukaryal and bacterial traits together with aspects of the process seemingly unique to this, the third domain of life. Here, current understanding of archaeal protein translocation is considered. This article is part of a Special Issue entitled Protein translocation across or insertion into membranes.

Structural insights into the assembly of the human and archaeal signal recognition particles

The signal recognition particle (SRP) is a conserved ribonucleoprotein (RNP) complex that co-translationally targets membrane and secretory proteins to membranes. The assembly of the particle depends on the proper folding of the SRP RNA, which in mammalia and archaea involves an induced-fit mechanism within helices 6 and 8 in the S domain of SRP. The two helices are juxtaposed and clamped together upon binding of the SRP19 protein to their apices. In the current assembly paradigm, archaeal SRP19 causes the asymmetric loop of helix 8 to bulge out and expose the binding platform for the key player SRP54. Based on a heterologous archaeal SRP19–human SRP RNA structure, mammalian SRP19 was thought not to be able to induce this change, thus explaining the different requirements of SRP19 for SRP54 recruitment. In contrast, the crystal structures of a crenarchaeal and the all-human SRP19–SRP RNA binary complexes presented here show that the asymmetric loop is bulged out in both binary complexes. Differences in SRP assembly between mammalia and archaea are therefore independent of SRP19 and are based on differences in SRP RNA itself. A new SRP-assembly scheme is presented.

Compositional properties and thermal adaptation of SRP-RNA in bacteria and archaea

Previous studies have reported a positive correlation between the GC content of the double-stranded regions of structural RNAs and the optimal growth temperature (OGT) in prokaryotes. These observations led to the hypothesis that natural selection favors an increase in GC content to ensure the correct folding and the structural stability of the molecule at high temperature. To date these studies have focused mainly on ribosomal and transfer RNAs. Therefore, we addressed the question of the relationship between GC content and OGT in a different and universally conserved structural RNA, the RNA component of the signal recognition particle (SRP). To this end we generated the secondary structures of SRP-RNAs for mesophilic, thermophilic, and hyperthermophilic bacterial and archaeal species. The analysis of the GC content in the stems and loops of the SRP-RNA of these organisms failed to detect a relationship between the GC contents in the stems of this structural RNA and the growth temperature of bacteria. By contrast, we found that in archaea the GC content in the stem regions of SRP-RNA is highest in hyperthermophiles, intermediate in thermophiles, and lower in mesophiles. In these organisms, we demonstrated a clear positive correlation between the GC content of the stem regions of their SRP-RNAs and their OGT. This correlation was confirmed by a phylogenetic nonindependence analysis. Thus we conclude that in archaea the increase in GC content in the stem regions of SRP-RNA is an adaptation response to environmental temperature.

Three-way RNA junctions with remote tertiary contacts: a recurrent and highly versatile fold

Three-way junction RNAs adopt a recurrent Y shape when two of the helices form a coaxial stack and the third helix establishes one or more tertiary contacts several base pairs away from the junction. In this review, the structure, distribution, and functional relevance of these motifs are examined. Structurally, the folds exhibit conserved junction topologies, and the distal tertiary interactions play a crucial role in determining the final shape of the structures. The junctions and remote tertiary contacts behave as flexible hinge motifs that respond to changes in the other region, providing these folds with switching mechanisms that have been shown to be functionally useful in a variety of contexts. In addition, the juxtaposition of RNA domains at the junction and at the distal tertiary complexes enables the RNA helices to adopt unusual conformations that are frequently used by proteins, RNA molecules, and antibiotics as platforms for specific binding. As a consequence of these properties, Y-shaped junctions are widely distributed in all kingdoms of life, having been observed in small naked RNAs such as riboswitches and ribozymes or embedded in complex ribonucleoprotein systems like ribosomal RNAs, RNase P, or the signal recognition particle. In all cases, the folds were found to play an essential role for the functioning or assembly of the RNA or ribonucleoprotein systems that contain them.

Transposable elements in disease-associated cryptic exons

Transposable elements (TEs) make up a half of the human genome, but the extent of their contribution to cryptic exon activation that results in genetic disease is unknown. Here, a comprehensive survey of 78 mutation-induced cryptic exons previously identified in 51 disease genes revealed the presence of TEs in 40 cases (51%). Most TE-containing exons were derived from short interspersed nuclear elements (SINEs), with Alus and mammalian interspersed repeats (MIRs) covering >18 and >16% of the exonized sequences, respectively. The majority of SINE-derived cryptic exons had splice sites at the same positions of the Alu/MIR consensus as existing SINE exons and their inclusion in the mRNA was facilitated by phylogenetically conserved changes that improved both traditional and auxiliary splicing signals, thus marking intronic TEs amenable for pathogenic exonization. The overrepresentation of MIRs among TE exons is likely to result from their high average exon inclusion levels, which reflect their strong splice sites, a lack of splicing silencers and a high density of enhancers, particularly (G)AA(G) motifs. These elements were markedly depleted in antisense Alu exons, had the most prominent position on the exon-intron gradient scale and are proposed to promote exon definition through enhanced tertiary RNA interactions involving unpaired (di)adenosines. The identification of common mechanisms by which the most dynamic parts of the genome contribute both to new exon creation and genetic disease will facilitate detection of intronic mutations and the development of computational tools that predict TE hot-spots of cryptic exon activation.

Protein transport across and into cell membranes in bacteria and archaea

In the three domains of life, the Sec, YidC/Oxa1, and Tat translocases play important roles in protein translocation across membranes and membrane protein insertion. While extensive studies have been performed on the endoplasmic reticular and Escherichia coli systems, far fewer studies have been done on archaea, other Gram-negative bacteria, and Gram-positive bacteria. Interestingly, work carried out to date has shown that there are differences in the protein transport systems in terms of the number of translocase components and, in some cases, the translocation mechanisms and energy sources that drive translocation. In this review, we will describe the different systems employed to translocate and insert proteins across or into the cytoplasmic membrane of archaea and bacteria.

Structures of SRP54 and SRP19, the two proteins that organize the ribonucleic core of the signal recognition particle from Pyrococcus furiosus

In all organisms the Signal Recognition Particle (SRP), binds to signal sequences of proteins destined for secretion or membrane insertion as they emerge from translating ribosomes. In Archaea and Eucarya, the conserved ribonucleoproteic core is composed of two proteins, the accessory protein SRP19, the essential GTPase SRP54, and an evolutionarily conserved and essential SRP RNA. Through the GTP-dependent interaction between the SRP and its cognate receptor SR, ribosomes harboring nascent polypeptidic chains destined for secretion are dynamically transferred to the protein translocation apparatus at the membrane. We present here high-resolution X-ray structures of SRP54 and SRP19, the two RNA binding components forming the core of the signal recognition particle from the hyper-thermophilic archaeon Pyrococcus furiosus (Pfu). The 2.5 Å resolution structure of free Pfu-SRP54 is the first showing the complete domain organization of a GDP bound full-length SRP54 subunit. In its ras-like GTPase domain, GDP is found tightly associated with the protein. The flexible linker that separates the GTPase core from the hydrophobic signal sequence binding M domain, adopts a purely α-helical structure and acts as an articulated arm allowing the M domain to explore multiple regions as it scans for signal peptides as they emerge from the ribosomal tunnel. This linker is structurally coupled to the GTPase catalytic site and likely to propagate conformational changes occurring in the M domain through the SRP RNA upon signal sequence binding. Two different 1.8 Å resolution crystal structures of free Pfu-SRP19 reveal a compact, rigid and well-folded protein even in absence of its obligate SRP RNA partner. Comparison with other SRP19•SRP RNA structures suggests the rearrangement of a disordered loop upon binding with the RNA through a reciprocal induced-fit mechanism and supports the idea that SRP19 acts as a molecular scaffold and a chaperone, assisting the SRP RNA in adopting the conformation required for its optimal interaction with the essential subunit SRP54, and proper assembly of a functional SRP.

Structural principles from large RNAs

Since the year 2000 a number of large RNA three-dimensional structures have been determined by X-ray crystallography. Structures composed of more than 100 nucleotide residues include the signal recognition particle RNA, group I intron, the GlmS ribozyme, RNAseP RNA, and ribosomal RNAs from Haloarcula morismortui, Escherichia coli, Thermus thermophilus, and Deinococcus radiodurans. These large RNAs are constructed from the same secondary and tertiary structural motifs identified in smaller RNAs but appear to have a larger organizational architecture. They are dominated by long continuous interhelical base stacking, tend to segregate into domains, and are planar in overall shape as opposed to their globular protein counterparts. These findings have consequences in RNA folding, intermolecular interaction, and packing, in addition to studies of design and engineering and structure prediction.

Interaction of signal-recognition particle 54 GTPase domain and signal-recognition particle RNA in the free signal-recognition particle

The signal-recognition particle (SRP) is a ubiquitous protein-RNA complex that targets proteins to cellular membranes for insertion or secretion. A key player in SRP-mediated protein targeting is the evolutionarily conserved core consisting of the SRP RNA and the multidomain protein SRP54. Communication between the SRP54 domains is critical for SRP function, where signal sequence binding at the M domain directs receptor binding at the GTPase domain (NG domain). These SRP activities are linked to domain rearrangements, for which the role of SRP RNA is not clear. In free SRP, a direct interaction of the GTPase domain with SRP RNA has been proposed but has never been structurally verified. In this study, we present the crystal structure at 2.5-A resolution of the SRP54-SRP19-SRP RNA complex of Methanococcus jannaschii SRP. The structure reveals an RNA-bound conformation of the SRP54 GTPase domain, in which the domain is spatially well separated from the signal peptide binding site. The association of both the N and G domains with SRP RNA in free SRP provides further structural evidence for the pivotal role of SRP RNA in the regulation of the SRP54 activity.

Rearrangement of competing U2 RNA helices within the spliceosome promotes multiple steps in splicing

Nuclear pre-messenger RNA (pre-mRNA) splicing requires multiple spliceosomal small nuclear RNA (snRNA) and pre-mRNA rearrangements. Here we reveal a new snRNA conformational switch in which successive roles for two competing U2 helices, stem IIa and stem IIc, promote distinct splicing steps. When stem IIa is stabilized by loss of stem IIc, rapid ATP-independent and Cus2p-insensitive prespliceosome formation occurs. In contrast, hyperstabilized stem IIc improves the first splicing step on aberrant branchpoint pre-mRNAs and rescues temperature-sensitive U6-U57C, a U6 mutation that also suppresses first-step splicing defects of branchpoint mutations. A second, later role for stem IIa is revealed by its suppression of a cold-sensitive allele of the second-step splicing factor PRP16. Our data expose a spliceosomal progression cycle of U2 stem IIa formation, disruption by stem IIc, and then reformation of stem IIa before the second catalytic step. We propose that the competing stem IIa and stem IIc helices are key spliceosomal RNA elements that optimize juxtaposition of the proper reactive sites during splicing.

A threefold RNA-protein interface in the signal recognition particle gates native complex assembly

Intermediate states play well-established roles in the folding and misfolding reactions of individual RNA and protein molecules. In contrast, the roles of transient structural intermediates in multi-component ribonucleoprotein (RNP) assembly processes and their potential for misassembly are largely unexplored. The SRP19 protein is unstructured but forms a compact core domain and two extended RNA-binding loops upon binding the signal recognition particle (SRP) RNA. The SRP54 protein subsequently binds to the fully assembled SRP19-RNA complex to form an intimate threefold interface with both SRP19 and the RNA and without significantly altering the structure of SRP19. We show, however, that the presence of SRP54 during SRP19-RNA assembly dramatically alters the folding energy landscape to create a non-native folding pathway that leads to an aberrant SRP19-RNA conformation. The misassembled complex arises from the surprising ability of SRP54 to bind rapidly to an SRP19-RNA assembly intermediate and to interfere with subsequent folding of one of the RNA binding loops at the three-way protein-RNA interface. An incorrect temporal order of assembly thus readily yields a non-native three-component ribonucleoprotein particle. We propose there may exist a general requirement to regulate the order of interaction in multi-component RNP assembly reactions by spatial or temporal compartmentalization of individual constituents in the cell.

An allosteric-feedback mechanism for protein-assisted group I intron splicing

The I-AniI maturase facilitates self-splicing of a mitochondrial group I intron in Aspergillus nidulans. Binding occurs in at least two steps: first, a specific but labile encounter complex rapidly forms and then this intermediate is slowly resolved into a native, catalytically active RNA/protein complex. Here we probe the structure of the RNA throughout the assembly pathway. Although inherently unstable, the intron core, when bound by I-AniI, undergoes rapid folding to a near-native state in the encounter complex. The next transition includes the slow destabilization and docking into the core of the peripheral stacked helix that contains the 5' splice site. Mutational analyses confirm that both transitions are important for native complex formation. We propose that protein-driven destabilization and docking of the peripheral stacked helix lead to subtle changes in the I-AniI binding site that facilitate native complex formation. These results support an allosteric-feedback mechanism of RNA-protein recognition in which proteins engaged in an intermediate complex can influence RNA structure far from their binding sites. The linkage of these changes to stable binding ensures that the protein and RNA do not get sequestered in nonfunctional complexes.

Solution structure of a GAAG tetraloop in helix 6 of SRP RNA from Pyrococcus furiosus

The NMR structure of a 12-mer RNA derived from the helix 6 of SRP RNA from Pyrococcus furiosus, whose loop-closing base pair is U.G, was determined, and the structural and thermodynamic properties of the RNA were compared with those of a mutant RNA with the C:G closing base pair. Although the structures of the two RNAs are similar to each other and adopt the GNRR motif the conformational stabilities are significantly different to each other It was suggested that weaker stacking interaction of the GAAG loop with the U:G closing base pair in 12-mer RNA causes the lower conformational stability.

SRP19 is a dispensable component of the signal recognition particle in Archaea

In vitro, archaeal SRP54 binds SRP RNA in the absence of SRP19, suggesting the latter to be expendable in Archaea. Accordingly, the Haloferax volcanii SRP19 gene was deleted. Although normally transcribed at a level comparable to that of the essential SRP54 gene, SRP19 deletion had no effect on cell growth, membrane protein insertion, protein secretion, or ribosome levels. The absence of SRP19 did, however, increase membrane bacterioruberin levels.

Assembly of the archaeal box C/D sRNP can occur via alternative pathways and requires temperature-facilitated sRNA remodeling

Archaeal dual-guide box C/D small nucleolar RNA-like RNAs (sRNAs) bind three core proteins in sequential order at both terminal box C/D and internal C'/D' motifs to assemble two ribonuclear protein (RNP) complexes active in guiding nucleotide methylation. Experiments have investigated the process of box C/D sRNP assembly and the resultant changes in sRNA structure or "remodeling" as a consequence of sRNP core protein binding. Hierarchical assembly of the Methanocaldococcus jannaschii sR8 box C/D sRNP is a temperature-dependent process with binding of L7 and Nop56/58 core proteins to the sRNA requiring elevated temperature to facilitate necessary RNA structural dynamics. Circular dichroism (CD) spectroscopy and RNA thermal denaturation revealed an increased order and stability of sRNA folded structure as a result of L7 binding. Subsequent binding of the Nop56/58 and fibrillarin core proteins to the L7-sRNA complex further remodeled sRNA structure. Assessment of sR8 guide region accessibility using complementary RNA oligonucleotide probes revealed significant changes in guide region structure during sRNP assembly. A second dual-guide box C/D sRNA from M. jannaschii, sR6, also exhibited RNA remodeling during temperature-dependent sRNP assembly, although core protein binding was affected by sR6's distinct folded structure. Interestingly, the sR6 sRNP followed an alternative assembly pathway, with both guide regions being continuously exposed during sRNP assembly. Further experiments using sR8 mutants possessing alternative guide regions demonstrated that sRNA folded structure induced by specific guide sequences impacted the sRNP assembly pathway. Nevertheless, assembled sRNPs were active for sRNA-guided methylation independent of the pathway followed. Thus, RNA remodeling appears to be a common and requisite feature of archaeal dual-guide box C/D sRNP assembly and formation of the mature sRNP can follow different assembly pathways in generating catalytically active complexes.

On the structural features of hairpin triloops in rRNA: from nucleotide to global conformational change upon ligand binding

RNA structure can be viewed as both a construct composed of various structural motifs and a flexible polymer that is substantially influenced by its environment. In this light, the present paper represents an attempt to reconcile the two standpoints. By using the 3D structures both of four (16S and 23S) portions of unbound 50S, H50S, and T30S ribosomal subunits and of 38 large ribonucleoligand complexes as the starting point, the behavior, which is induced by ligand binding, of 73 hairpin triloops with closing g-c and c-g base pairs was investigated using root-mean-square deviation (RMSD) approach and pseudotorsional (eta,theta) convention at the nucleotide-by-nucleotide level. Triloops were annotated in accordance with a recent proposal of geometric nomenclature. A simple measure for the determination of the strain of a triloop is introduced. It is believed that a possible classification of the interior triloops, based on the 2D eta-theta unique path, will aid to conceive their local behavior upon ligand binding. All rRNA residues in contact with ligands as well as regions of considerable conformational changes upon complex formation were identified. The analysis offers the answer to: how proximal to and how far from the actual ligand-binding sites the structural changes occur?

RNA-mediated interaction between the peptide-binding and GTPase domains of the signal recognition particle

The signal recognition particle (SRP) targets nascent proteins to cellular membranes for insertion or secretion by recognizing polypeptides containing an N-terminal signal sequence as they emerge from the ribosome. GTP-dependent binding of SRP to its receptor protein leads to controlled release of the nascent chain into a membrane-spanning translocon pore. Here we show that the association of the SRP with its receptor triggers a marked conformational change in the complex, localizing the SRP RNA and the adjacent signal peptide-binding site at the SRP-receptor heterodimer interface. The orientation of the RNA suggests how peptide binding and GTP hydrolysis can be coupled through direct structural contact during cycles of SRP-directed protein translocation.