Use of chemical modification and mass spectrometry to identify substrate-contacting sites in proteinaceous RNase P, a tRNA processing enzyme

The protein-only RNase Ps, endonucleases that cleave pre-tRNA: Biological relevance, molecular architectures, substrate recognition and specificity, and protein interactomes

Protein-only RNase P (PRORP) is an essential enzyme responsible for the 5' maturation of precursor tRNAs (pre-tRNAs). PRORPs are classified into three categories with unique molecular architectures, although all three classes of PRORPs share a mechanism and have similar active sites. Single subunit PRORPs, like those found in plants, have multiple isoforms with different localizations, substrate specificities, and temperature sensitivities. Most recently, Arabidopsis thaliana PRORP2 was shown to interact with TRM1A and B, highlighting a new potential role between these enzymes. Work with At PRORPs led to the development of a ribonuclease that is being used to protect against plant viruses. The mitochondrial RNase P complex, found in metazoans, consists of PRORP, TRMT10C, and SDR5C1, and has also been shown to have substrate specificity, although the cause is unknown. Mutations in mitochondrial tRNA and mitochondrial RNase P have been linked to human disease, highlighting the need to continue understanding this complex. The last class of PRORPs, homologs of Aquifex RNase P (HARPs), is found in thermophilic archaea and bacteria. This most recently discovered type of PRORP forms a large homo-oligomer complex. Although numerous structures of HARPs have been published, it is still unclear how HARPs bind pre-tRNAs and in what ratio. There is also little investigation into the substrate specificity and ideal conditions for HARPs. Moving forward, further work is required to fully characterize each of the three classes of PRORP, the pre-tRNA binding recognition mechanism, the rules of substrate specificity, and how these three distinct classes of PRORP evolved. This article is categorized under: RNA Structure and Dynamics > RNA Structure, Dynamics and Chemistry RNA Structure and Dynamics > Influence of RNA Structure in Biological Systems.

How RNases Shape Mitochondrial Transcriptomes

Mitochondria are the power houses of eukaryote cells. These endosymbiotic organelles of prokaryote origin are considered as semi-autonomous since they have retained a genome and fully functional gene expression mechanisms. These pathways are particularly interesting because they combine features inherited from the bacterial ancestor of mitochondria with characteristics that appeared during eukaryote evolution. RNA biology is thus particularly diverse in mitochondria. It involves an unexpectedly vast array of factors, some of which being universal to all mitochondria and others being specific from specific eukaryote clades. Among them, ribonucleases are particularly prominent. They play pivotal functions such as the maturation of transcript ends, RNA degradation and surveillance functions that are required to attain the pool of mature RNAs required to synthesize essential mitochondrial proteins such as respiratory chain proteins. Beyond these functions, mitochondrial ribonucleases are also involved in the maintenance and replication of mitochondrial DNA, and even possibly in the biogenesis of mitochondrial ribosomes. The diversity of mitochondrial RNases is reviewed here, showing for instance how in some cases a bacterial-type enzyme was kept in some eukaryotes, while in other clades, eukaryote specific enzymes were recruited for the same function.

Mass Spectrometry of Nucleic Acid Noncovalent Complexes

Nucleic acids have been among the first targets for antitumor drugs and antibiotics. With the unveiling of new biological roles in regulation of gene expression, specific DNA and RNA structures have become very attractive targets, especially when the corresponding proteins are undruggable. Biophysical assays to assess target structure as well as ligand binding stoichiometry, affinity, specificity, and binding modes are part of the drug development process. Mass spectrometry offers unique advantages as a biophysical method owing to its ability to distinguish each stoichiometry present in a mixture. In addition, advanced mass spectrometry approaches (reactive probing, fragmentation techniques, ion mobility spectrometry, ion spectroscopy) provide more detailed information on the complexes. Here, we review the fundamentals of mass spectrometry and all its particularities when studying noncovalent nucleic acid structures, and then review what has been learned thanks to mass spectrometry on nucleic acid structures, self-assemblies (e.g., duplexes or G-quadruplexes), and their complexes with ligands.

The many faces of RNA-based RNase P, an RNA-world relic

RNase P is an essential enzyme that catalyzes removal of the 5' leader from precursor transfer RNAs. The ribonucleoprotein (RNP) form of RNase P is present in all domains of life and comprises a single catalytic RNA (ribozyme) and a variable number of protein cofactors. Recent cryo-electron microscopy structures of representative archaeal and eukaryotic (nuclear) RNase P holoenzymes bound to tRNA substrate/product provide high-resolution detail on subunit organization, topology, and substrate recognition in these large, multisubunit catalytic RNPs. These structures point to the challenges in understanding how proteins modulate the RNA functional repertoire and how the structure of an ancient RNA-based catalyst was reshaped during evolution by new macromolecular associations that were likely necessitated by functional/regulatory coupling.

Pentatricopeptide repeats of protein-only RNase P use a distinct mode to recognize conserved bases and structural elements of pre-tRNA

Pentatricopeptide repeat (PPR) motifs are α-helical structures known for their modular recognition of single-stranded RNA sequences with each motif in a tandem array binding to a single nucleotide. Protein-only RNase P 1 (PRORP1) in Arabidopsis thaliana is an endoribonuclease that uses its PPR domain to recognize precursor tRNAs (pre-tRNAs) as it catalyzes removal of the 5'-leader sequence from pre-tRNAs with its NYN metallonuclease domain. To gain insight into the mechanism by which PRORP1 recognizes tRNA, we determined a crystal structure of the PPR domain in complex with yeast tRNAPhe at 2.85 Å resolution. The PPR domain of PRORP1 bound to the structurally conserved elbow of tRNA and recognized conserved structural features of tRNAs using mechanisms that are different from the established single-stranded RNA recognition mode of PPR motifs. The PRORP1 PPR domain-tRNAPhe structure revealed a conformational change of the PPR domain upon tRNA binding and moreover demonstrated the need for pronounced overall flexibility in the PRORP1 enzyme conformation for substrate recognition and catalysis. The PRORP1 PPR motifs have evolved strategies for protein-tRNA interaction analogous to tRNA recognition by the RNA component of ribonucleoprotein RNase P and other catalytic RNAs, indicating convergence on a common solution for tRNA substrate recognition.

Determination of protein-only RNase P interactome in Arabidopsis mitochondria and chloroplasts identifies a complex between PRORP1 and another NYN domain nuclease

The essential type of endonuclease that removes 5' leader sequences from transfer RNA precursors is called RNase P. While ribonucleoprotein RNase P enzymes containing a ribozyme are found in all domains of life, another type of RNase P called 'PRORP', for 'PROtein-only RNase P', is composed of protein that occurs only in a wide variety of eukaryotes, in organelles and in the nucleus. Here, to find how PRORP functions integrate with other cell processes, we explored the protein interaction network of PRORP1 in Arabidopsis mitochondria and chloroplasts. Although PRORP proteins function as single subunit enzymes in vitro, we found that PRORP1 occurs in protein complexes and is present in high-molecular-weight fractions that contain mitochondrial ribosomes. The analysis of immunoprecipitated protein complexes identified proteins involved in organellar gene expression processes. In particular, direct interaction was established between PRORP1 and MNU2 a mitochondrial nuclease. A specific domain of MNU2 and a conserved signature of PRORP1 were found to be directly accountable for this protein interaction. Altogether, results revealed the existence of an RNA maturation complex in Arabidopsis mitochondria and suggested that PRORP proteins cooperated with other gene expression factors for RNA maturation in vivo.

Involvement of PIN-like domain nucleases in tRNA processing and translation regulation

Transfer RNAs require essential maturation steps to become functional. Among them, RNase P removes 5' leader sequences of pre-tRNAs. Although RNase P was long thought to occur universally as ribonucleoproteins, different types of protein-only RNase P enzymes were discovered in both eukaryotes and prokaryotes. Interestingly, all these enzymes belong to the super-group of PilT N-terminal-like nucleases (PIN)-like ribonucleases. This wide family of enzymes can be subdivided into major subgroups. Here, we review recent studies at both functional and mechanistic levels on three PIN-like ribonucleases groups containing enzymes connected to tRNA maturation and/or translation regulation. The evolutive distribution of these proteins containing PIN-like domains as well as their organization and fusion with various functional domains is discussed and put in perspective with the diversity of functions they acquired during evolution, for the maturation and homeostasis of tRNA and a wider array of RNA substrates. © 2019 IUBMB Life, 2019 © 2019 IUBMB Life, 71(8):1117-1125, 2019.

Both kinds of RNase P in all domains of life: surprises galore

RNase P, an essential housekeeping endonuclease needed for 5'-processing of tRNAs, exists in two distinct forms: one with an RNA- and the other with a protein-based active site. The notion that the protein form of RNase P exists only in eukaryotes has been upended by the recent discovery of a protein-only variant in Bacteria and Archaea. The use of these two divergent scaffolds, shaped by convergent evolution, in all three domains of life inspires questions relating to the ancestral form of RNase P, as well as their origins and function(s) in vivo. Results from our analysis of publicly available bacterial and archaeal genomes suggest that the widespread RNA-based ribonucleoprotein variant is likely the ancient form. We also discuss the possible genetic origins and function of RNase P, including how the simultaneous presence of its variants may contribute to the fitness of their host organisms.

An RNase P-Based Assay for Accurate Determination of the 5'-Deoxy-5'-azidoguanosine-Modified Fraction of in Vitro-Transcribed RNAs

Chemoenzymatic approaches are important for generating site-specific, chemically modified RNAs, a cornerstone for RNA structure-function correlation studies. T7 RNA polymerase (T7RNAP)-mediated in vitro transcription (IVT) of a DNA template containing the G-initiating class III Φ6.5 promoter is typically used to generate 5'-chemically modified RNAs by including a guanosine analogue (G analogue) initiator in the IVT. However, the yield of 5'-G analogue-initiated RNA is often poor and variable due to the high ratios of G analogue:GTP used in IVT. We recently reported that a T7RNAP P266L mutant afforded an approximately three-fold increase in fluorescent 5'-thienoguanosine-initiated pre-tRNA^Cys compared to the wild type T7RNAP. We have further explored the use of T7RNAP P266L to generate 5'-deoxy-5'-azidoguanosine (^az G)-initiated RNA and found that the mutant yielded approximately four times more ^az G-initiated pre-tRNA^Cys than the wild type in an IVT containing a 10:1 ratio of ^az G:GTP. For accurate quantitation of the 5'-^az G-initiated RNA fraction, we employed RNase P, an endonuclease that catalyzes the removal of the 5'-leader in pre-tRNAs. Importantly, we show herein how RNase P can be leveraged for assessing 5'-G analogue incorporation in any RNA by rendering the target RNA, upon its binding to a customized external guide sequence RNA, into an unnatural substrate of RNase P. Such an approach in conjunction with T7RNAP P266L-based IVT should aid chemoenzymatic methods that are designed to generate 5'-chemically modified RNAs.

Biochemical Studies Provide Insights into the Necessity for Multiple Arabidopsis thaliana Protein-Only RNase P Isoenzymes

RNase P catalyzes removal of the 5' leader from precursor tRNAs (pre-tRNAs) in all three domains of life. Some eukaryotic cells contain multiple forms of the protein-only RNase P (PRORP) variant, prompting efforts to unravel this seeming redundancy. Previous studies concluded that there were only modest differences in the processing of typical pre-tRNAs by the three isoforms in Arabidopsis thaliana [AtPRORP1 (organellar), AtPRORP2 and AtPRORP3 (nuclear)]. Here, we investigated if different physical attributes of the three isoforms might engender payoffs under specific conditions. Our temperature-activity profiling studies revealed that AtPRORPs display substrate-identity dependent behavior at elevated temperatures (37-45 °C), with the organellar variant outperforming the nuclear counterparts. Echoing these findings, molecular dynamics simulations revealed that AtPRORP2 relative to AtPRORP1 samples a wider conformational ensemble that deviates from the crystal structure. Results from our biochemical studies and molecular dynamics simulations support the idea that AtPRORPs have overlapping but not necessarily redundant attributes and inspire new perspectives on the suitability of each variant to perform its function(s) in a specific cellular locale.

A T7 RNA Polymerase Mutant Enhances the Yield of 5'-Thienoguanosine-Initiated RNAs

Spectroscopic methods, which are used to establish RNA structure-function relationships, require strategies for post-synthetic, site-specific incorporation of chemical probes into target RNAs. For RNAs larger than 50 nt, the enzymatic incorporation of a nucleoside or nucleotide monophosphate guanosine analogue (G analogue) at their 5'-end is routinely achieved by T7 RNA polymerase (T7RNAP)-mediated in vitro transcription (IVT) of the appropriate DNA template containing a GTP-initiating class III Φ6.5 promoter. However, when high G analogue:GTP ratios are used to bias G analogue incorporation at the 5'-end, RNA yield is compromised. Here, we show that the use of a T7RNAP P266L mutant in IVT with 10:1 thienoguanosine (^th G):GTP increased the percent incorporation and yield of 5'-^th G-initiated precursor tRNA for a net ≈threefold gain compared to IVT with wild-type T7RNAP. We also demonstrated that a one-pot multienzyme approach, consisting of transcription by T7RNAP P266L and post-transcriptional cleanup by polyphosphatase and an exonuclease, led to essentially near-homogeneous 5'-^th G-modified transcripts. This approach should be of broad utility in preparing 5'-modified RNAs.

Biophysical analysis of Arabidopsis protein-only RNase P alone and in complex with tRNA provides a refined model of tRNA binding

RNase P is a universal enzyme that removes 5' leader sequences from tRNA precursors. The enzyme is therefore essential for maturation of functional tRNAs and mRNA translation. RNase P represents a unique example of an enzyme that can occur either as ribonucleoprotein or as protein alone. The latter form of the enzyme, called protein-only RNase P (PRORP), is widespread in eukaryotes in which it can provide organellar or nuclear RNase P activities. Here, we have focused on Arabidopsis nuclear PRORP2 and its interaction with tRNA substrates. Affinity measurements helped assess the respective importance of individual pentatricopeptide repeat motifs in PRORP2 for RNA binding. We characterized the PRORP2 structure by X-ray crystallography and by small-angle X-ray scattering in solution as well as that of its complex with a tRNA precursor by small-angle X-ray scattering. Of note, our study reports the first structural data of a PRORP-tRNA complex. Combined with complementary biochemical and biophysical analyses, our structural data suggest that PRORP2 undergoes conformational changes to accommodate its substrate. In particular, the catalytic domain and the RNA-binding domain can move around a central hinge. Altogether, this work provides a refined model of the PRORP-tRNA complex that illustrates how protein-only RNase P enzymes specifically bind tRNA and highlights the contribution of protein dynamics to achieve this specific interaction.

Cleavage of Model Substrates by Arabidopsis thaliana PRORP1 Reveals New Insights into Its Substrate Requirements

Two broad classes of RNase P trim the 5' leader of precursor tRNAs (pre-tRNAs): ribonucleoprotein (RNP)- and proteinaceous (PRORP)-variants. These two RNase P types, which use different scaffolds for catalysis, reflect independent evolutionary paths. While the catalytic RNA-based RNP form is present in all three domains of life, the PRORP family is restricted to eukaryotes. To obtain insights on substrate recognition by PRORPs, we examined the 5' processing ability of recombinant Arabidopsis thaliana PRORP1 (AtPRORP1) using a panel of pre-tRNASer variants and model hairpin-loop derivatives (pATSer type) that consist of the acceptor-T-stem stack and the T-/D-loop. Our data indicate the importance of the identity of N-1 (the residue immediately 5' to the cleavage site) and the N-1:N+73 base pair for cleavage rate and site selection of pre-tRNASer and pATSer. The nucleobase preferences that we observed mirror the frequency of occurrence in the complete suite of organellar pre-tRNAs in eight algae/plants that we analyzed. The importance of the T-/D-loop in pre-tRNASer for tight binding to AtPRORP1 is indicated by the 200-fold weaker binding of pATSer compared to pre-tRNASer, while the essentiality of the T-loop for cleavage is reflected by the near-complete loss of activity when a GAAA-tetraloop replaced the T-loop in pATSer. Substituting the 2'-OH at N-1 with 2'-H also resulted in no detectable cleavage, hinting at the possible role of this 2'-OH in coordinating Mg2+ ions critical for catalysis. Collectively, our results indicate similarities but also key differences in substrate recognition by the bacterial RNase P RNP and AtPRORP1: while both forms exploit the acceptor-T-stem stack and the elbow region in the pre-tRNA, the RNP form appears to require more recognition determinants for cleavage-site selection.

Mechanistic and Structural Studies of Protein-Only RNase P Compared to Ribonucleoproteins Reveal the Two Faces of the Same Enzymatic Activity

RNase P, the essential activity that performs the 5′ maturation of tRNA precursors, can be achieved either by ribonucleoproteins containing a ribozyme present in the three domains of life or by protein-only enzymes called protein-only RNase P (PRORP) that occur in eukaryote nuclei and organelles. A fast growing list of studies has investigated three-dimensional structures and mode of action of PRORP proteins. Results suggest that similar to ribozymes, PRORP proteins have two main domains. A clear functional analogy can be drawn between the specificity domain of the RNase P ribozyme and PRORP pentatricopeptide repeat domain, and between the ribozyme catalytic domain and PRORP N4BP1, YacP-like Nuclease domain. Moreover, both types of enzymes appear to dock with the acceptor arm of tRNA precursors and make specific contacts with the corner of pre-tRNAs. While some clear differences can still be delineated between PRORP and ribonucleoprotein (RNP) RNase P, the two types of enzymes seem to use, fundamentally, the same catalytic mechanism involving two metal ions. The occurrence of PRORP and RNP RNase P represents a remarkable example of convergent evolution. It might be the unique witness of an ongoing replacement of catalytic RNAs by proteins for enzymatic activities.