RNase W, a conserved ribonuclease family with a novel active site.

Ribosome biogenesis is a complex process requiring multiple precursor ribosomal RNA (rRNA) cleavage steps. In archaea, the full set of ribonucleases (RNases) involved in rRNA processing remains to be discovered. A previous study suggested that FAU-1, a conserved protein containing an RNase G/E-like protein domain fused to a domain of unknown function (DUF402), acts as an RNase in archaea. However, the molecular basis of this activity remained so far elusive. Here, we report two X-ray crystallographic structures of RNase G/E-like-DUF402 hybrid proteins from Pyrococcus furiosus and Sulfolobus acidocaldarius, at 2.1 and 2.0 Å, respectively. The structures highlight a structural homology with the 5' RNA recognition domain of Escherichia coli RNase E but no homology with other known catalytic nuclease domains. Surprisingly, we demonstrate that the C-terminal domain of this hybrid protein, annotated as a putative diphosphatase domain, harbors the RNase activity. Our functional analysis also supports a model by which the RNase G/E-like domain acts as a regulatory subunit of the RNase activity. Finally, in vivo experiments in Haloferax volcanii suggest that this RNase participates in the maturation of pre-16S rRNA. Together, our study defines a new RNase family, which we termed the RNase W family, as the first archaea-specific contributor to archaeal ribosome biogenesis.

Hepatitis C virus core protein uses triacylglycerols to fold onto the endoplasmic reticulum membrane.

Lipid droplets (LDs) are involved in viral infections, but exactly how remains unclear. Here, we study the hepatitis C virus (HCV) whose core capsid protein binds to LDs but is also involved in the assembly of virions at the endoplasmic reticulum (ER) bilayer. We found that the amphipathic helix-containing domain of core, D2, senses triglycerides (TGs) rather than LDs per se. In the absence of LDs, D2 can bind to the ER membrane but only if TG molecules are present in the bilayer. Accordingly, the pharmacological inhibition of the diacylglycerol O-acyltransferase enzymes, mediating TG synthesis in the ER, inhibits D2 association with the bilayer. We found that TG molecules enable D2 to fold into alpha helices. Sequence analysis reveals that D2 resembles the apoE lipid-binding region. Our data support that TG in LDs promotes the folding of core, which subsequently relocalizes to contiguous ER regions. During this motion, core may carry TG molecules to these regions where HCV lipoviroparticles likely assemble. Consistent with this model, the inhibition of Arf1/COPI, which decreases LD surface accessibility to proteins and ER-LD material exchange, severely impedes the assembly of virions. Altogether, our data uncover a critical function of TG in the folding of core and HCV replication and reveals, more broadly, how TG accumulation in the ER may provoke the binding of soluble amphipathic helix-containing proteins to the ER bilayer.

Chaperoning 5S RNA assembly.

In eukaryotes, three of the four ribosomal RNAs (rRNAs)­the 5.8S, 18S, and 25S/28S rRNAs­are processed from a single pre-rRNA transcript and assembled into ribosomes. The fourth rRNA, the 5S rRNA, is transcribed by RNA polymerase III and is assembled into the 5S ribonucleoprotein particle (RNP), containing ribosomal proteins Rpl5/uL18 and Rpl11/uL5, prior to its incorporation into preribosomes. In mammals, the 5S RNP is also a central regulator of the homeostasis of the tumor suppressor p53. The nucleolar localization of the 5S RNP and its assembly into preribosomes are performed by a specialized complex composed of Rpf2 and Rrs1 in yeast or Bxdc1 and hRrs1 in humans. Here we report the structural and functional characterization of the Rpf2-Rrs1 complex alone, in complex with the 5S RNA, and within pre-60S ribosomes. We show that the Rpf2-Rrs1 complex contains a specialized 5S RNA E-loop-binding module, contacts the Rpl5 protein, and also contacts the ribosome assembly factor Rsa4 and the 25S RNA. We propose that the Rpf2-Rrs1 complex establishes a network of interactions that guide the incorporation of the 5S RNP in preribosomes in the initial conformation prior to its rotation to form the central protuberance found in the mature large ribosomal subunit.

A Photoalkylative Fluorogenic Probe of Guttiferone A for Live Cell Imaging and Proteome Labeling in Plasmodium falciparum.

Guttiferone A (GA) 1, a polycyclic polyprenylated acylphloroglucinol (PPAP) isolated from the plant Symphonia globulifera (Clusiaceae), constitutes a novel hit in antimalarial drug discovery. PPAPs do not possess identified biochemical targets in malarial parasites up to now. Towards this aim, we designed and evaluated a natural product-derived photoactivatable probe AZC-GA 5, embedding a photoalkylative fluorogenic motif of the 7-azidocoumarin (AZC) type, devoted to studying the affinity proteins interacting with GA in Plasmodium falciparum. Probe 5 manifested a number of positive functional and biological features, such as (i) inhibitory activity in vitro against P. falciparum blood-stages that was superimposable to that of GA 1, dose-response photoalkylative fluorogenic properties (ii) in model conditions using bovine serum albumin (BSA) as an affinity protein surrogate, (iii) in live P. falciparum-infected erythrocytes, and (iv) in fresh P. falciparum cell lysate. Fluorogenic signals by photoactivated AZC-GA 5 in biological settings were markedly abolished in the presence of excess GA 1 as a competitor, indicating significant pharmacological specificity of the designed molecular probe relative to the native PPAP. These results open the way to identify the detected plasmodial proteins as putative drug targets for the natural product 1 by means of proteomic analysis.

Functional link between DEAH/RHA helicase Prp43 activation and ATP base binding.

The DEAH box helicase Prp43 is a bifunctional enzyme from the DEAH/RHA helicase family required both for the maturation of ribosomes and for lariat intron release during splicing. It interacts with G-patch domain containing proteins which activate the enzymatic activity of Prp43 in vitro by an unknown mechanism. In this work, we show that the activation by G-patch domains is linked to the unique nucleotide binding mode of this helicase family. The base of the ATP molecule is stacked between two residues, R159 of the RecA1 domain (R-motif) and F357 of the RecA2 domain (F-motif). Using Prp43 F357A mutants or pyrimidine nucleotides, we show that the lack of stacking of the nucleotide base to the F-motif decouples the NTPase and helicase activities of Prp43. In contrast the R159A mutant (R-motif) showed reduced ATPase and helicase activities. We show that the Prp43 R-motif mutant induces the same phenotype as the absence of the G-patch protein Gno1, strongly suggesting that the processing defects observed in the absence of Gno1 result from a failure to activate the Prp43 helicase. Overall we propose that the stacking between the R- and F-motifs and the nucleotide base is important for the activity and regulation of this helicase family.

The H/ACA RNP assembly factor SHQ1 functions as an RNA mimic.

SHQ1 is an essential assembly factor for H/ACA ribonucleoproteins (RNPs) required for ribosome biogenesis, pre-mRNA splicing, and telomere maintenance. SHQ1 binds dyskerin/NAP57, the catalytic subunit of human H/ACA RNPs, and this interaction is modulated by mutations causing X-linked dyskeratosis congenita. We report the crystal structure of the C-terminal domain of yeast SHQ1, Shq1p, and its complex with yeast dyskerin/NAP57, Cbf5p, lacking its catalytic domain. The C-terminal domain of Shq1p interacts with the RNA-binding domain of Cbf5p and, through structural mimicry, uses the RNA-protein-binding sites to achieve a specific protein-protein interface. We propose that Shq1p operates as a Cbf5p chaperone during RNP assembly by acting as an RNA placeholder, thereby preventing Cbf5p from nonspecific RNA binding before association with an H/ACA RNA and the other core RNP proteins.

RNA mimicry by the fap7 adenylate kinase in ribosome biogenesis.

During biogenesis of the 40S and 60S ribosomal subunits, the pre-40S particles are exported to the cytoplasm prior to final cleavage of the 20S pre-rRNA to mature 18S rRNA. Amongst the factors involved in this maturation step, Fap7 is unusual, as it both interacts with ribosomal protein Rps14 and harbors adenylate kinase activity, a function not usually associated with ribonucleoprotein assembly. Human hFap7 also regulates Cajal body assembly and cell cycle progression via the p53-MDM2 pathway. This work presents the functional and structural characterization of the Fap7-Rps14 complex. We report that Fap7 association blocks the RNA binding surface of Rps14 and, conversely, Rps14 binding inhibits adenylate kinase activity of Fap7. In addition, the affinity of Fap7 for Rps14 is higher with bound ADP, whereas ATP hydrolysis dissociates the complex. These results suggest that Fap7 chaperones Rps14 assembly into pre-40S particles via RNA mimicry in an ATP-dependent manner. Incorporation of Rps14 by Fap7 leads to a structural rearrangement of the platform domain necessary for the pre-rRNA to acquire a cleavage competent conformation.

Characterization of an A-kinase anchoring protein-like suggests an alternative way of PKA anchoring in Plasmodium falciparum.

BACKGROUND: The asexual intra-erythrocytic multiplication of the malaria parasite Plasmodium falciparum is regulated by various molecular mechanisms. In eukaryotic cells, protein kinases are known to play key roles in cell cycle regulation and signaling pathways. The activity of cAMP-dependent protein kinase (PKA) depends on A-kinase anchoring proteins (AKAPs) through protein interactions. While several components of the cAMP dependent pathway-including the PKA catalytic and regulatory subunits-have been characterized in P. falciparum, whether AKAPs are involved in this pathway remains unclear. Here, PfAKAL, an open reading frame of a potential AKAP-like protein in the P. falciparum genome was identified, and its protein partners and putative cellular functions characterized. METHODS: The expression of PfAKAL throughout the erythrocytic cycle of the 3D7 strain was assessed by RT-qPCR and the presence of the corresponding protein by immunofluorescence assays. In order to study physical interactions between PfAKAL and other proteins, pull down experiments were performed using a recombinant PfAKAL protein and parasite protein extracts, or with recombinant proteins. These interactions were also tested by combining biochemical and proteomic approaches. As phosphorylation could be involved in the regulation of protein complexes, both PfAKAL and Pf14-3-3I phosphorylation was studied using a radiolabel kinase activity assay. Finally, to identify a potential function of the protein, PfAKAL sequence was aligned and structurally modeled, revealing a conserved nucleotide-binding pocket; confirmed by qualitative nucleotide binding experiments. RESULTS: PfAKAL is the first AKAP-like protein in P. falciparum to be identified, and shares 23 % sequence identity with the central domain of human AKAP18δ. PfAKAL is expressed in mature asexual stages, merozoites and gametocytes. In spite of homology to AKAP18, biochemical and immunochemical analyses demonstrated that PfAKAL does not interact directly with the P. falciparum PKA regulatory subunit (PfPKA-R), but instead binds and colocalizes with Pf14-3-3I, which in turn interacts with PfPKA-R. In vivo, these different interactions could be regulated by phosphorylation, as PfPKA-R and Pf14-3-3I, but not PfAKAL, are phosphorylated in vitro by PKA. Interestingly, PfAKAL binds nucleotides such as AMP and cAMP, suggesting that this protein may be involved in the AMP-activated protein kinase (AMPK) pathway, or associated with phosphodiesterase activities. CONCLUSION: PfAKAL is an atypical AKAP that shares common features with human AKAP18, such as nucleotides binding. The interaction of PfAKAL with PfPKA-R could be indirectly mediated through a join interaction with Pf14-3-3I. Therefore, PfPKA localization could not depend on PfAKAL, but rather involves other partners.

Characterization of the interaction between protein Snu13p/15.5K and the Rsa1p/NUFIP factor and demonstration of its functional importance for snoRNP assembly.

The yeast Snu13p protein and its 15.5K human homolog both bind U4 snRNA and box C/D snoRNAs. They also bind the Rsa1p/NUFIP assembly factor, proposed to scaffold immature snoRNPs and to recruit the Hsp90-R2TP chaperone complex. However, the nature of the Snu13p/15.5K-Rsa1p/NUFIP interaction and its exact role in snoRNP assembly remained to be elucidated. By using biophysical, molecular and imaging approaches, here, we identify residues needed for Snu13p/15.5K-Rsa1p/NUFIP interaction. By NMR structure determination and docking approaches, we built a 3D model of the Snup13p-Rsa1p interface, suggesting that residues R249, R246 and K250 in Rsa1p and E72 and D73 in Snu13p form a network of electrostatic interactions shielded from the solvent by hydrophobic residues from both proteins and that residue W253 of Rsa1p is inserted in a hydrophobic cavity of Snu13p. Individual mutations of residues in yeast demonstrate the functional importance of the predicted interactions for both cell growth and snoRNP formation. Using archaeal box C/D sRNP 3D structures as templates, the association of Snu13p with Rsa1p is predicted to be exclusive of interactions in active snoRNPs. Rsa1p and NUFIP may thus prevent premature activity of pre-snoRNPs, and their removal may be a key step for active snoRNP production.

Activated B-Cells enhance epitope spreading to support successful cancer immunotherapy.

Immune checkpoint therapies (ICT) have transformed the treatment of cancer over the past decade. However, many patients do not respond or suffer relapses. Successful immunotherapy requires epitope spreading, but the slow or inefficient induction of functional antitumoral immunity delays the benefit to patients or causes resistances. Therefore, understanding the key mechanisms that support epitope spreading is essential to improve immunotherapy. In this review, we highlight the major role played by B-cells in breaking immune tolerance by epitope spreading. Activated B-cells are key Antigen-Presenting Cells (APC) that diversify the T-cell response against self-antigens, such as ribonucleoproteins, in autoimmunity but also during successful cancer immunotherapy. This has important implications for the design of future cancer vaccines.

2-D structure of the A region of Xist RNA and its implication for PRC2 association.

In placental mammals, inactivation of one of the X chromosomes in female cells ensures sex chromosome dosage compensation. The 17 kb non-coding Xist RNA is crucial to this process and accumulates on the future inactive X chromosome. The most conserved Xist RNA region, the A region, contains eight or nine repeats separated by U-rich spacers. It is implicated in the recruitment of late inactivated X genes to the silencing compartment and likely in the recruitment of complex PRC2. Little is known about the structure of the A region and more generally about Xist RNA structure. Knowledge of its structure is restricted to an NMR study of a single A repeat element. Our study is the first experimental analysis of the structure of the entire A region in solution. By the use of chemical and enzymatic probes and FRET experiments, using oligonucleotides carrying fluorescent dyes, we resolved problems linked to sequence redundancies and established a 2-D structure for the A region that contains two long stem-loop structures each including four repeats. Interactions formed between repeats and between repeats and spacers stabilize these structures. Conservation of the spacer terminal sequences allows formation of such structures in all sequenced Xist RNAs. By combination of RNP affinity chromatography, immunoprecipitation assays, mass spectrometry, and Western blot analysis, we demonstrate that the A region can associate with components of the PRC2 complex in mouse ES cell nuclear extracts. Whilst a single four-repeat motif is able to associate with components of this complex, recruitment of Suz12 is clearly more efficient when the entire A region is present. Our data with their emphasis on the importance of inter-repeat pairing change fundamentally our conception of the 2-D structure of the A region of Xist RNA and support its possible implication in recruitment of the PRC2 complex.

Detection and Labeling of Small Non-Coding RNAs by Splinted Ligation.

Discovery and characterization of microRNAs (miRNAs) and other families of small RNAs lead researchers to study their structures/functions and their expression patterns. The splinted ligation method described here is based on nucleic acid hybridization. It is optimized for the direct labeling and quantitative detection of small RNAs. A specific bridge DNA oligonucleotide is used, which is perfectly complementary to both the target small RNA and a labeled ligation nucleic acid. The target RNA is subsequently labeled by ligation, detected by analysis in denaturing conditions, and quantified by phosphorimaging. The protocol does not require any specific material, and the procedure is fast and sensitive.

A Chemically Stable Fluorescent Mimic of Dihydroartemisinin, Artemether, and Arteether with Conserved Bioactivity and Specificity Shows High Pharmacological Relevance to the Antimalarial Drugs.

Three novel tracers designed as fluorescent surrogates of artemisinin-derived antimalarial drugs (i.e., dihydroartemisinin, artemether, arteether, and artemisone) were synthesized from dihydroartemisinin. One of these tracers, corresponding to a dihydroartemisinin/artemether/arteether mimic, showed a combination of excellent physicochemical and biological properties such as hydrolytic stability, high inhibitory potency against blood-stage parasites, similar ring-stage survival assay values than the clinical antimalarials, high cytopermeability and specific labeling of live P. falciparum cells, alkylation of heme, as well as specific covalent labeling of drug-sensitive and drug-resistant P. falciparum proteomes at physiological concentrations, consistent with a multitarget action of the drugs. Our study demonstrates that probes containing the complete structural core of clinical artemisinin derivatives can be stable in biochemical and cellular settings, and recapitulate the complex mechanisms of these frontline, yet threatened, antimalarial drugs.

Ultraspecific live imaging of the dynamics of zebrafish neutrophil granules by a histopermeable fluorogenic benzochalcone probe.

Neutrophil granules (NGs) are key components of the innate immune response and mark the development of neutrophilic granulocytes in mammals. However, there has been no specific fluorescent vital stain up to now to monitor their dynamics within a whole live organism. We rationally designed a benzochalcone fluorescent probe (HAB) featuring high tissue permeability and optimal photophysics such as elevated quantum yield, pronounced solvatochromism and target-induced fluorogenesis. Phenotypic screening identified HAB as the first cell- and organelle-specific small-molecule fluorescent tracer of NGs in live zebrafish larvae, with no labeling of other cell types or organelles. HAB staining was independent of the state of neutrophil activation, labeling NGs of both resting and phagocytically active neutrophils with equal specificity. By high-resolution live imaging, we documented the dynamics of HAB-stained NGs during phagocytosis. Upon zymosan injection, labeled NGs were rapidly recruited to the forming phagosomes. Despite being a reversible ligand, HAB could not be displaced by high concentrations of pharmacologically relevant competing chalcones, indicating that this specific labeling was the result of the HAB's precise physicochemical signature rather than a general feature of chalcones. However, one of the competitors was discovered as a promising interstitial fluorescent tracer illuminating zebrafish histology, similarly to BODIPY-ceramide. As a yellow-emitting histopermeable vital stain, HAB functionally and spectrally complements most genetically incorporated fluorescent tags commonly used in live zebrafish biology, holding promise for the study of neutrophil-dependent responses relevant to human physiopathology such as developmental defects, inflammation and infection. Furthermore, HAB intensely labeled isolated live human neutrophils at the level of granulated subcellular structures consistent with human NGs, suggesting that the labeling of NGs by HAB is not restricted to the zebrafish model but also relevant to mammalian systems.

Identification of the 'NORE' (N-Oct-3 responsive element), a novel structural motif and composite element.

N-Oct-3 is a neuronal transcription factor widely expressed in the developing mammalian central nervous system, and necessary to maintain neural cell differentiation. The key role of N-Oct-3 in the transcriptional regulation of a multiplicity of genes is primarily due to the structural plasticity of its so-called 'POU' (acronym of Pit, Oct, Unc) DNA-binding domain. We have recently reported about the unusual dual neuro-specific transcriptional regulation displayed by N-Oct-3 [Blaud,M., Vossen,C., Joseph,G., Alazard,R., Erard,M. and Nieto,L. (2004) J. Mol. Biol., 339, 1049-1058]. To elucidate the underlying molecular mechanisms, we have now made use of molecular modeling, DNA footprinting and electrophoretic mobility shift assay techniques. This combined approach has allowed us to uncover a novel mode of homodimerization adopted by the N-Oct-3 POU domain bound to the neuronal aromatic amino acids de-carboxylase and corticotropin-releasing hormone gene promoters and to demonstrate that this pattern is induced by a structural motif that we have termed 'NORE' (N-Oct-3 responsive element), comprising the 14 bp sequence element TNNRTAAATAATRN. In addition, we have been able to explain how the same structural motif can also induce the formation of a heterodimer in association with hepatocyte nuclear factor 3beta(/Forkhead box a2). Finally, we discuss the possible role of the NORE motif in relation to neuroendocrine lung tumor formation, and in particular the development of small cell lung cancer.

Sequential domain assembly of ribosomal protein S3 drives 40S subunit maturation.

Eukaryotic ribosomes assemble by association of ribosomal RNA with ribosomal proteins into nuclear precursor particles, which undergo a complex maturation pathway coordinated by non-ribosomal assembly factors. Here, we provide functional insights into how successive structural re-arrangements in ribosomal protein S3 promote maturation of the 40S ribosomal subunit. We show that S3 dimerizes and is imported into the nucleus with its N-domain in a rotated conformation and associated with the chaperone Yar1. Initial assembly of S3 with 40S precursors occurs via its C-domain, while the N-domain protrudes from the 40S surface. Yar1 is replaced by the assembly factor Ltv1, thereby fixing the S3 N-domain in the rotated orientation and preventing its 40S association. Finally, Ltv1 release, triggered by phosphorylation, and flipping of the S3 N-domain into its final position results in the stable integration of S3. Such a stepwise assembly may represent a new paradigm for the incorporation of ribosomal proteins.

Characteristic patterns of N Oct-3 binding to a set of neuronal promoters.

N Oct-3, a neurospecific POU protein, homodimerizes in a non-cooperative fashion on the neuronal aromatic l-amino acid decarboxylase gene promoter and generates heterodimers with HNF-3beta. Several other neuronal gene promoters, the corticotropin releasing hormone and the aldolase C gene promoters also contain overlapping binding sites for N Oct-3 and HNF-3beta. We have demonstrated that N Oct-3 presents a non-cooperative homodimerization on these two additional targets and can also give rise to heterodimers with HNF-3beta. Surprisingly, despite the high degree of conservation of the respective POU subunits, the ubiquitous POU protein Oct-1 can only form monomers even in the presence of either N Oct-3 or HNF-3beta on these DNA targets. Our data indicate that this difference is correlated with the specific ability of a portion of the N Oct-3 linker to fold as an alpha-helix, a property shared by class III POU proteins. These results suggest that this novel binding pattern permits the heterodimerization of N Oct-3 and HNF-3beta on the neuronal promoters, which could be a key issue in the development of the nervous system and possibly tumors of neural origin.

Detection and labeling of small non-coding RNAs by splinted ligation.

Discovery and characterization of microRNAs (miRNAs) and other families of small RNAs lead researchers to study their structures/functions and their expression patterns. The splinted ligation method described here is based on nucleic acid hybridization. It is optimized for the direct labeling and quantitative detection of small RNAs. A specific bridge DNA oligonucleotide is used, which is perfectly complementary to both the target small RNA and a labeled ligation nucleic acid. The target RNA is subsequently labeled by ligation, detected by analysis in denaturing conditions, and quantified by phosphorimaging. The protocol doesn't require any specific material, and the procedure is fast and sensitive.

RNA size is a critical factor for U-containing substrate selectivity and permanent pseudouridylated product release during the RNA:Ψ-synthase reaction catalyzed by box H/ACA sRNP enzyme at high temperature.

The box H/ACA small ribonucleoprotein particles (H/ACA sRNPs) are RNP enzymes that isomerize uridines (U) into pseudouridines (Ψ) in archaeal RNAs. The RNA component acts as a guide by forming base-pair interactions with the substrate RNA to specify the target nucleotide of the modification to the catalytic subunit Cbf5. Here, we have analyzed association of an H/ACA sRNP enzyme from the hyperthermophilic archaeon Pyrococcus abyssi with synthetic substrate RNAs of different length and with target nucleotide variants, and estimated their turnover at high temperature. In these conditions, we found that a short substrate, which length is restricted to the interaction with RNA guide sequence, has higher turnover rate. However, the longer substrate with additional 5' and 3' sequences non-complementary to the guide RNA is better discriminated by the U to Ψ conversion allowing the RNP enzyme to distinguish the modified product from the substrate. In addition, we identified that the conserved residue Y179 in the catalytic center of Cbf5 is crucial for substrate selectivity.

Comparative study of two box H/ACA ribonucleoprotein pseudouridine-synthases: relation between conformational dynamics of the guide RNA, enzyme assembly and activity.

Multiple RNA-guided pseudouridine synthases, H/ACA ribonucleoprotein particles (RNPs) which contain a guide RNA and four proteins, catalyze site-specific post-transcriptional isomerization of uridines into pseudouridines in substrate RNAs. In archaeal particles, the guide small RNA (sRNA) is anchored by the pseudouridine synthase aCBF5 and the ribosomal protein L7Ae. Protein aNOP10 interacts with both aCBF5 and L7Ae. The fourth protein, aGAR1, interacts with aCBF5 and enhances catalytic efficiency. Here, we compared the features of two H/ACA sRNAs, Pab21 and Pab91, from Pyrococcus abyssi. We found that aCBF5 binds much more weakly to Pab91 than to Pab21. Surprisingly, the Pab91 sRNP exhibits a higher catalytic efficiency than the Pab21 sRNP. We thus investigated the molecular basis of the differential efficiencies observed for the assembly and catalytic activity of the two enzymes. For this, we compared profiles of the extent of lead-induced cleavages in these sRNAs during a stepwise reconstitution of the sRNPs, and analyzed the impact of the absence of the aNOP10-L7Ae interaction. Such probing experiments indicated that the sRNAs undergo a series of conformational changes upon RNP assembly. These changes were also evaluated directly by circular dichroism (CD) spectroscopy, a tool highly adapted to analyzing RNA conformational dynamics. In addition, our results reveal that the conformation of helix P1 formed at the base of the H/ACA sRNAs is optimized in Pab21 for efficient aCBF5 binding and RNP assembly. Moreover, P1 swapping improved the assembly of the Pab91 sRNP. Nonetheless, efficient aCBF5 binding probably also relies on the pseudouridylation pocket which is not optimized for high activity in the case of Pab21.