In-Gel Digestion for Mass Spectrometric Characterization of RNA from Fluorescently Stained Polyacrylamide Gels

Cryo-EM structure and rRNA modification sites of a plant ribosome

Protein synthesis in crop plants contributes to the balance of food and fuel on our planet, which influences human metabolic activity and lifespan. Protein synthesis can be regulated with respect to changing environmental cues via the deposition of chemical modifications into rRNA. Here, we present the structure of a plant ribosome from tomato and a quantitative mass spectrometry analysis of its rRNAs. The study reveals fine features of the ribosomal proteins and 71 plant-specific rRNA modifications, and it re-annotates 30 rRNA residues in the available sequence. At the protein level, isoAsp is found in position 137 of uS11, and a zinc finger previously believed to be universal is missing from eL34, suggesting a lower effect of zinc deficiency on protein synthesis in plants. At the rRNA level, the plant ribosome differs markedly from its human counterpart with respect to the spatial distribution of modifications. Thus, it represents an additional layer of gene expression regulation, highlighting the molecular signature of a plant ribosome. The results provide a reference model of a plant ribosome for structural studies and an accurate marker for molecular ecology.

Instrumental analysis of RNA modifications

Organisms from all domains of life invest a substantial amount of energy for the introduction of RNA modifications into nearly all transcripts studied to date. Instrumental analysis of RNA can focus on the modified residues and reveal the function of these epitranscriptomic marks. Here, we will review recent advances and breakthroughs achieved by NMR spectroscopy, sequencing, and mass spectrometry of the epitranscriptome.

Direct Determination of Pseudouridine in RNA by Mass Spectrometry Coupled with Stable Isotope Labeling

Pseudouridine (Ψ) is the only "mass-silent" nucleoside produced by post-transcriptional RNA modification. We developed a mass spectrometry (MS)-based technique coupled with in vivo deuterium (D) labeling of uridines for direct determination of Ψs in cellular RNA and applied it to the comprehensive analysis of post-transcriptional modifications in human ribosomal RNAs. The method utilizes human TK6/mouse FM3A cells deficient in uridine monophosphate synthase using a CRISPR-Cas9 technique to turn off de novo uridine synthesis and fully labels uridines with D at uracil positions 5 and 6 by cultivating the cells in a medium containing uridine-5,6-D₂. The pseudouridylation reaction in those cells results in the exchange of the D at the C5 of uracil with hydrogen from solvent, which produces a -1 Da mass shift, thus allowing MS-based determination of RNA Ψs. We present here the experimental details of this method and show that it allows the identification of all Ψs in human major nuclear and nucleolar RNAs, including several previously unknown Ψs. Because the method allows direct determination of Ψs at the femtomole level of RNA, it will serve as a useful tool for structure/function studies of a wide variety of noncoding RNAs.

Mapping of Posttranscriptional tRNA Modifications by Two-Dimensional Gel Electrophoresis Mass Spectrometry

RNA modification mapping by mass spectrometry (MS) is based on the use of specific ribonucleases (RNases) that generate short oligonucleotide digestion products which are further separated by nano-liquid chromatography and analyzed by MS and MS/MS. Recent developments in MS instrumentation allow the possibility to deeply explore posttranscriptional modifications. Notably, development of nano-liquid chromatography and nano-electrospray drastically increases the detection sensitivity and allows the identification and sequencing of RNA digested fragments separated and extracted from two-dimensional polyacrylamide gels, as long as the mapping and characterization of ribonucleotide modifications.

TDP-43 regulates site-specific 2'-O-methylation of U1 and U2 snRNAs via controlling the Cajal body localization of a subset of C/D scaRNAs

TDP-43 regulates cellular levels of Cajal bodies (CBs) that provide platforms for the assembly and RNA modifications of small nuclear ribonucleoproteins (snRNPs) involved in pre-mRNA splicing. Alterations in these snRNPs may be linked to pathogenesis of amyotrophic lateral sclerosis. However, specific roles for TDP-43 in CBs remain unknown. Here, we demonstrate that TDP-43 regulates the CB localization of four UG-rich motif-bearing C/D-box-containing small Cajal body-specific RNAs (C/D scaRNAs; i.e. scaRNA2, 7, 9 and 28) through the direct binding to these scaRNAs. TDP-43 enhances binding of a CB-localizing protein, WD40-repeat protein 79 (WDR79), to a subpopulation of scaRNA2 and scaRNA28; the remaining population of the four C/D scaRNAs was localized to CB-like structures even with WDR79 depletion. Depletion of TDP-43, in contrast, shifted the localization of these C/D scaRNAs, mainly into the nucleolus, as well as destabilizing scaRNA2, and reduced the site-specific 2'-O-methylation of U1 and U2 snRNAs, including at 70A in U1 snRNA and, 19G, 25G, 47U and 61C in U2 snRNA. Collectively, we suggest that TDP-43 and WDR79 have separate roles in determining CB localization of subsets of C/D and H/ACA scaRNAs.

Truncated forms of U2 snRNA (U2-tfs) are shunted toward a novel uridylylation pathway that differs from the degradation pathway for U1-tfs

During the biogenesis of U1 small nuclear ribonucleoprotein, a small population of U1 snRNA molecules acquires an extra methylation at the first transcribed nucleotide and a nucleolytic cleavage to remove the 3' structured region including the Sm protein-binding site and stem-loop 4. These modifications occur before hypermethylation of the monomethylated 5' cap, whereby producing truncated forms of U1 snRNA (U1-tfs) that are diverted from the normal pathway to a processing body-associated degradation pathway. Here, we demonstrate that a small population of U2 snRNA molecules receives post-transcriptional modifications similar to those of U1 to yield U2-tfs. Like U1-tfs, U2-tfs molecules were produced from transcripts of the U2 snRNA gene having all cis-elements or lacking the 3' box. Unlike U1-tfs, however, a portion of U2-tfs received additional uridylylation of up to 5 nucleotides in length at position 87 (designated as U2-tfs-polyU) and formed an Sm protein-binding site-like structure that was stabilized by the small nuclear ribonucleoprotein SmB/B' probably as a part of heptameric Sm core complex that associates to the RNA. Both U2-tfs and U2-tfs-polyU were degraded by a nuclease distinct from the canonical Dis3L2 by a process catalyzed by terminal uridylyltransferase 7. Collectively, our data suggest that U2 snRNA biogenesis is regulated, at least in part, by a novel degradation pathway to ensure that defective U2 molecules are not incorporated into the spliceosome.

Structural insights into Gemin5-guided selection of pre-snRNAs for snRNP assembly

Xu et al. show that the WD40 domain of Gemin5 is both necessary and sufficient for binding the Sm site of pre-snRNAs. They also determined the crystal structures of the WD40 domain of Gemin5 in complex with the Sm site or m7G cap of pre-snRNA.

In cytoplasm, the survival of motor neuron (SMN) complex delivers pre-small nuclear RNAs (pre-snRNAs) to the heptameric Sm ring for the assembly of the ring complex on pre-snRNAs at the conserved Sm site [A(U)_4–6G]. Gemin5, a WD40 protein component of the SMN complex, is responsible for recognizing pre-snRNAs. In addition, Gemin5 has been reported to specifically bind to the m⁷G cap. In this study, we show that the WD40 domain of Gemin5 is both necessary and sufficient for binding the Sm site of pre-snRNAs by isothermal titration calorimetry (ITC) and mutagenesis assays. We further determined the crystal structures of the WD40 domain of Gemin5 in complex with the Sm site or m⁷G cap of pre-snRNA, which reveal that the WD40 domain of Gemin5 recognizes the Sm site and m⁷G cap of pre-snRNAs via two distinct binding sites by respective base-specific interactions. In addition, we also uncovered a novel role of Gemin5 in escorting the truncated forms of U1 pre-snRNAs for proper disposal. Overall, the elucidated Gemin5 structures will contribute to a better understanding of Gemin5 in small nuclear ribonucleic protein (snRNP) biogenesis as well as, potentially, other cellular activities.

A mass spectrometry-based method for comprehensive quantitative determination of post-transcriptional RNA modifications: the complete chemical structure of Schizosaccharomyces pombe ribosomal RNAs

We present a liquid chromatography–mass spectrometry (LC-MS)-based method for comprehensive quantitative identification of post-transcriptional modifications (PTMs) of RNA. We incorporated an in vitro-transcribed, heavy isotope-labeled reference RNA into a sample RNA solution, digested the mixture with a number of RNases and detected the post-transcriptionally modified oligonucleotides quantitatively based on shifts in retention time and the MS signal in subsequent LC-MS. This allowed the determination and quantitation of all PTMs in Schizosaccharomyces pombe ribosomal (r)RNAs and generated the first complete PTM maps of eukaryotic rRNAs at single-nucleotide resolution. There were 122 modified sites, most of which appear to locate at the interface of ribosomal subunits where translation takes place. We also identified PTMs at specific locations in rRNAs that were altered in response to growth conditions of yeast cells, suggesting that the cells coordinately regulate the modification levels of RNA.

RNA cytidine acetyltransferase of small-subunit ribosomal RNA: identification of acetylation sites and the responsible acetyltransferase in fission yeast, Schizosaccharomyces pombe

The eukaryotic small-subunit (SSU) ribosomal RNA (rRNA) has two evolutionarily conserved acetylcytidines. However, the acetylation sites and the acetyltransferase responsible for the acetylation have not been identified. We performed a comprehensive MS-based analysis covering the entire sequence of the fission yeast, Schizosaccharomyces pombe, SSU rRNA and identified two acetylcytidines at positions 1297 and 1815 in the 3′ half of the rRNA. To identify the enzyme responsible for the cytidine acetylation, we searched for an S. pombe gene homologous to TmcA, a bacterial tRNA N-acetyltransferase, and found one potential candidate, Nat10. A temperature-sensitive strain of Nat10 with a mutation in the Walker A type ATP-binding motif abolished the cytidine acetylation in SSU rRNA, and the wild-type Nat10 supplemented to this strain recovered the acetylation, providing evidence that Nat10 is necessary for acetylation of SSU rRNA. The Nat10 mutant strain showed a slow-growth phenotype and was defective in forming the SSU rRNA from the precursor RNA, suggesting that cytidine acetylation is necessary for ribosome assembly.

RNase MRP cleaves pre-tRNASer-Met in the tRNA maturation pathway

Ribonuclease mitochondrial RNA processing (RNase MRP) is a multifunctional ribonucleoprotein (RNP) complex that is involved in the maturation of various types of RNA including ribosomal RNA. RNase MRP consists of a potential catalytic RNA and several protein components, all of which are required for cell viability. We show here that the temperature-sensitive mutant of rmp1, the gene for a unique protein component of RNase MRP, accumulates the dimeric tRNA precursor, pre-tRNA^Ser-Met. To examine whether RNase MRP mediates tRNA maturation, we purified the RNase MRP holoenzyme from the fission yeast Schizosaccharomyces pombe and found that the enzyme directly and selectively cleaves pre-tRNA^Ser-Met, suggesting that RNase MRP participates in the maturation of specific tRNA in vivo. In addition, mass spectrometry–based ribonucleoproteomic analysis demonstrated that this RNase MRP consists of one RNA molecule and 11 protein components, including a previously unknown component Rpl701. Notably, limited nucleolysis of RNase MRP generated an active catalytic core consisting of partial mrp1 RNA fragments, which constitute “Domain 1” in the secondary structure of RNase MRP, and 8 proteins. Thus, the present study provides new insight into the structure and function of RNase MRP.

Identification of truncated forms of U1 snRNA reveals a novel RNA degradation pathway during snRNP biogenesis

The U1 small nuclear ribonucleoprotein (snRNP) plays pivotal roles in pre-mRNA splicing and in regulating mRNA length and isoform expression; however, the mechanism of U1 snRNA quality control remains undetermined. Here, we describe a novel surveillance pathway for U1 snRNP biogenesis. Mass spectrometry-based RNA analysis showed that a small population of SMN complexes contains truncated forms of U1 snRNA (U1-tfs) lacking the Sm-binding site and stem loop 4 but containing a 7-monomethylguanosine 5′ cap and a methylated first adenosine base. U1-tfs form a unique SMN complex, are shunted to processing bodies and have a turnover rate faster than that of mature U1 snRNA. U1-tfs are formed partly from the transcripts of U1 genes and partly from those lacking the 3′ box elements or having defective SL4 coding regions. We propose that U1 snRNP biogenesis is under strict quality control: U1 transcripts are surveyed at the 3′-terminal region and U1-tfs are diverted from the normal U1 snRNP biogenesis pathway.

Denaturing reversed phase liquid chromatographic separation of non-coding ribonucleic acids on macro-porous polystyrene-divinylbenzene resins

The ability of denaturing ion-paired reversed phase LC to separate RNA was assessed using macro-porous polystyrene-divinylbenzene resins as the stationary phase. Using the three stationary phases with different pore size and a mobile phase containing phosphate, we separated RNAs of 20-8000 nucleotides with extremely high sensitivity, e.g., 50pg for an RNA 20 nucleotides in length, S/N=5. The method was used to separate non-coding RNAs obtained from biological sources and is suited for use with direct MS-based chemical characterization.

Detection of small interfering RNA (siRNA) by mass spectrometry procedures in doping controls

Uncovering manipulation of athletic performance via small interfering (si)RNA is an emerging field in sports drug testing. Due to the potential to principally knock down every target gene in the organism by means of the RNA interference pathway, this facet of gene doping has become a realistic scenario. In the present study, two distinct model siRNAs comprising 21 nucleotides were designed as double strands which were perfect counterparts to a sequence of the respective messenger RNA coding the muscle regulator myostatin of Rattus norvegicus. Several modified nucleotides were introduced in both the sense and the antisense strand comprising phosphothioates, 2'-O-methylation, 2'-fluoro-nucleotides, locked nucleic acids and a cholesterol tag at the 3'-end. The model siRNAs were applied to rats at 1 mg/kg (i.v.) and blood as well as urine samples were collected. After isolation of the RNA by means of a RNA purification kit, the target analytes were detected by liquid chromatography - high resolution/high accuracy mass spectrometry (LC-HRMS). Analytes were detected as modified nucleotides after alkaline hydrolysis, as intact oligonucleotide strands (top-down) and by means of denaturing SDS-PAGE analysis. The gel-separated siRNA was further subjected to in-gel hydrolysis with different RNases and subsequent identification of the fragments by untargeted LC-HRMS analysis (bottom-up, 'experimental RNomics'). Combining the results of all approaches, the identification of several 3'-truncated urinary metabolites was accomplished and target analytes were detected up to 24 h after a single administration. Simultaneously collected blood samples yielded no promising results. The methods were validated and found fit-for-purpose for doping controls.

Two steps forward--one step back: advances in affinity purification mass spectrometry of macromolecular complexes

Cellular functions are defined by the dynamic interactions of proteins within macromolecular networks. Deciphering these complex interplays is the key to getting a comprehensive picture of cellular behavior and to understanding biological systems, from a simple bacterial cell to highly regulated neuronal cells or cancerous tissue. In the last decade, affinity purification (AP) coupled to mass spectrometry has emerged as a powerful tool to comprehensively study interaction networks and their macromolecular assemblies. This review discusses recent advances in AP approaches, from cell lysis to the importance of sample preparation and the choice of AP matrix as well as the development of different epitope tags and strategies to study dynamic interactions, with an emphasis on RNA-protein interaction networks.

Informatics for mass spectrometry-based RNA analysis

Mass spectrometry (MS) allows the sensitive and direct characterization of biological macromolecules and therefore has the potential to complement the more conventional genetic and biochemical methods used for RNA characterization. Although MS has been used much less frequently for RNA research than it has been for protein research, recent technical improvements in both instrumentation and software make MS a powerful tool for RNA analysis because it can now be used to sequence, quantify, and chemically analyze RNAs. Mass spectrometry is particularly well suited for the characterization of RNAs associated with ribonucleoprotein complexes. This review focuses on the software and databases that can be used for MS-based RNA studies. Software for the processing of raw mass spectra, the identification and characterization of RNAs by mass mapping, de novo sequencing, and tandem MS-based database searching are available.

Characterization and quantification of RNA post-transcriptional modifications using stable isotope labeling of RNA in conjunction with mass spectrometry analysis

Mass spectrometry has emerged as an increasingly powerful tool for the identification and characterization of nucleic acids, in particular RNA post-transcriptional modifications. High mass accuracy instrumentation is often required to discriminate between compositional isomers of oligonucleotides. We have used stable isotope labeling ((15)N) of E. coli RNA in conjunction with mass spectrometry analysis of the combined heavy- and light-labeled RNA for the identification and quantification of oligoribonucleotides and post-transcriptional modifications. The number of nitrogen atoms in the oligoribonucleotide and fragment ions can readily be determined using this approach, enabling the discrimination between potential compositional isomers without the requirement of high mass accuracy mass spectrometers. In addition, the identification of specific fragment ions in both the unlabeled and labeled oligoribonucleotides can be used to gain further confidence in the assignment of RNA post-transcriptional modifications. Using this approach we have identified a range of post-transcriptional modifications of E. coli 16S rRNA. Furthermore, this method facilitates the rapid and accurate quantification of oligoribonucleotides, including cyclic phosphate intermediates and missed cleavages often generated from RNase digestions.