Structural basis for UTP specificity of RNA editing TUTases from Trypanosoma brucei

Identification and Expression Analysis of the Nucleotidyl Transferase Protein (NTP) Family in Soybean (Glycine max) under Various Abiotic Stresses

Nucleotidyl transferases (NTPs) are common transferases in eukaryotes and play a crucial role in nucleotide modifications at the 3' end of RNA. In plants, NTPs can regulate RNA stability by influencing 3' end modifications, which in turn affect plant growth, development, stress responses, and disease resistance. Although the functions of NTP family members have been extensively studied in Arabidopsis, rice, and maize, there is limited knowledge about NTP genes in soybeans. In this study, we identified 16 members of the NTP family in soybeans, including two subfamilies (G1 and G2) with distinct secondary structures, conserved motifs, and domain distributions at the protein level. Evolutionary analysis of genes in the NTP family across multiple species and gene collinearity analysis revealed a relatively conserved evolutionary pattern. Analysis of the tertiary structure of the proteins showed that NTPs have three conserved aspartic acids that bind together to form a possible active site. Tissue-specific expression analysis indicated that some NTP genes exhibit tissue-specific expression, likely due to their specific functions. Stress expression analysis showed significant differences in the expression levels of NTP genes under high salt, drought, and cold stress. Additionally, RNA-seq analysis of soybean plants subjected to salt and drought stress further confirmed the association of soybean NTP genes with abiotic stress responses. Subcellular localization experiments revealed that GmNTP2 and GmNTP14, which likely have similar functions to HESO1 and URT1, are located in the nucleus. These research findings provide a foundation for further investigations into the functions of NTP family genes in soybeans.

Multiple domains of the integral KREPA3 protein are critical for the structure and precise functions of RNA editing catalytic complexes in Trypanosoma brucei

The gRNA directed U-insertion and deletion editing of mitochondrial mRNAs that is essential in different life-cycle stages for the protozoan parasite Trypanosoma brucei is performed by three similar multiprotein catalytic complexes (CCs) that contain the requisite enzymes. These CCs also contain a common set of eight proteins that have no apparent direct catalytic function, including six that have an OB-fold domain. We show here that one of these OB-fold proteins, KREPA3 (A3), has structural homology to other editing proteins, is essential for editing, and is multifunctional. We investigated A3 function by analyzing the effects of single amino acid loss of function mutations, most of which were identified by screening bloodstream form (BF) parasites for loss of growth following random mutagenesis. Mutations in the zinc fingers (ZFs), an intrinsically disordered region (IDR), and several within or near the carboxy-terminal OB-fold domain variably impacted CC structural integrity and editing. Some mutations resulted in almost complete loss of CCs and its proteins and editing, whereas others retained CCs but had aberrant editing. All but a mutation which is near the OB-fold affected growth and editing in BF but not procyclic form (PF) parasites. These data indicate that multiple positions within A3 have essential functions that contribute to the structural integrity of CCs, the precision of editing and the developmental differences in editing between BF and PF stages.

Structural basis of gRNA stabilization and mRNA recognition in trypanosomal RNA editing

In Trypanosoma brucei, the editosome, composed of RNA-editing substrate-binding complex (RESC) and RNA-editing catalytic complex (RECC), orchestrates guide RNA (gRNA)-programmed editing to recode cryptic mitochondrial transcripts into messenger RNAs (mRNAs). The mechanism of information transfer from gRNA to mRNA is unclear owing to a lack of high-resolution structures for these complexes. With cryo-electron microscopy and functional studies, we have captured gRNA-stabilizing RESC-A and gRNA-mRNA-binding RESC-B and RESC-C particles. RESC-A sequesters gRNA termini, thus promoting hairpin formation and blocking mRNA access. The conversion of RESC-A into RESC-B or -C unfolds gRNA and allows mRNA selection. The ensuing gRNA-mRNA duplex protrudes from RESC-B, likely exposing editing sites to RECC-catalyzed cleavage, uridine insertion or deletion, and ligation. Our work reveals a remodeling event facilitating gRNA-mRNA hybridization and assembly of a macromolecular substrate for the editosome's catalytic modality.

Lexis and Grammar of Mitochondrial RNA Processing in Trypanosomes

Trypanosoma brucei spp. cause African human and animal trypanosomiasis, a burden on health and economy in Africa. These hemoflagellates are distinguished by a kinetoplast nucleoid containing mitochondrial DNAs of two kinds: maxicircles encoding ribosomal RNAs (rRNAs) and proteins and minicircles bearing guide RNAs (gRNAs) for mRNA editing. All RNAs are produced by a phage-type RNA polymerase as 3' extended precursors, which undergo exonucleolytic trimming. Most pre-mRNAs proceed through 3' adenylation, uridine insertion/deletion editing, and 3' A/U-tailing. The rRNAs and gRNAs are 3' uridylated. Historically, RNA editing has attracted major research effort, and recently essential pre- and postediting processing events have been discovered. Here, we classify the key players that transform primary transcripts into mature molecules and regulate their function and turnover.

Structural insights into a unique preference for 3' terminal guanine of mirtron in Drosophila TUTase tailor

Terminal uridylyl transferase (TUTase) is one type of enzyme that modifies RNA molecules by facilitating the post-transcriptional addition of uridyl ribonucleotides to their 3' ends. Recent researches have reported that Drosophila TUTase, Tailor, exhibits an intrinsic preference for RNA substrates ending in 3'G, distinguishing it from any other known TUTases. Through this unique feature, Tailor plays a crucial role as the repressor in the biogenesis pathway of splicing-derived mirtron pre-miRNAs. Here we describe crystal structures of core catalytic domain of Tailor and its complexes with RNA stretches 5'-AGU-3' and 5'-AGUU-3'. We demonstrate that R327 and N347 are two key residues contributing cooperatively to Tailor's preference for 3'G, and R327 may play an extra role in facilitating the extension of polyuridylation chain. We also demonstrate that conformational stability of the exit of RNA-binding groove also contributes significantly to Tailor's activity. Overall, our work reveals useful insights to explain why Drosophila Tailor can preferentially select RNA substrates ending in 3'G and provides important values for further understanding the biological significances of biogenesis pathway of mirtron in flies.

Structural basis for acceptor RNA substrate selectivity of the 3' terminal uridylyl transferase Tailor

Non-templated 3′-uridylation of RNAs has emerged as an important mechanism for regulating the processing, stability and biological function of eukaryotic transcripts. In Drosophila, oligouridine tailing by the terminal uridylyl transferase (TUTase) Tailor of numerous RNAs induces their degradation by the exonuclease Dis3L2, which serves functional roles in RNA surveillance and mirtron RNA biogenesis. Tailor preferentially uridylates RNAs terminating in guanosine or uridine nucleotides but the structural basis underpinning its RNA substrate selectivity is unknown. Here, we report crystal structures of Tailor bound to a donor substrate analog or mono- and oligouridylated RNA products. These structures reveal specific amino acid residues involved in donor and acceptor substrate recognition, and complementary biochemical assays confirm the critical role of an active site arginine in conferring selectivity toward 3′-guanosine terminated RNAs. Notably, conservation of these active site features suggests that other eukaryotic TUTases, including mammalian TUT4 and TUT7, might exhibit similar, hitherto unknown, substrate selectivity. Together, these studies provide critical insights into the specificity of 3′-uridylation in eukaryotic post-transcriptional gene regulation.

Multi-domain utilization by TUT4 and TUT7 in control of let-7 biogenesis

The uridyl transferases TUT4 and TUT7 switch between two modes of activity that either promotes let-7 expression (monoU) or marks it for degradation (oligoU). Lin28 modulates the switch via recruitment of TUT4(7) to pre-let-7 in stem cells and human cancers. We found TUT4(7) utilize two multi-domain functional modules during the switch from mono- to oligoU. The catalytic module (CM) is essential for both activities, while the Lin28-interacting module (LIM) is indispensible for oligoU. The TUT7 CM structure trapped in the monoU state, revealed a duplex RNA binding pocket that orients group II pre-let-7 hairpins to position the 1-nt overhang favor monoU addition. Conversely, the switch to oligoU requires the ZK domain of Lin28 to drive the formation of a stable ternary complex between pre-let-7 and the inactive LIM. Finally, ZK2 of TUT4(7) aids oligoU addition by engaging the growing oligoU tail through uracil-specific interactions.

A Comparative Study of the Structural Dynamics of Four Terminal Uridylyl Transferases

African trypanosomiasis occurs in 36 countries in sub-Saharan Africa with 10,000 reported cases annually. No definitive remedy is currently available and if left untreated, the disease becomes fatal. Structural and biochemical studies of trypanosomal terminal uridylyl transferases (TUTases) demonstrated their functional role in extensive uridylate insertion/deletion of RNA. Trypanosoma brucei RNA Editing TUTase 1 (TbRET1) is involved in guide RNA 3’ end uridylation and maturation, while TbRET2 is responsible for U-insertion at RNA editing sites. Two additional TUTases called TbMEAT1 and TbTUT4 have also been reported to share similar function. TbRET1 and TbRET2 are essential enzymes for the parasite viability making them potential drug targets. For this study, we clustered molecular dynamics (MD) trajectories of four TUTases based on active site shape measured by Pocket Volume Measurer (POVME) program. Among the four TUTases, TbRET1 exhibited the largest average pocket volume, while TbMEAT1’s and TbTUT4’s active sites displayed the most flexibility. A side pocket was also identified within the active site in all TUTases with TbRET1 having the most pronounced. Our results indicate that TbRET1’s larger side pocket can be exploited to achieve selective inhibitor design as FTMap identifies it as a druggable pocket.

Tipping the balance of RNA stability by 3' editing of the transcriptome

BACKGROUND

The regulation of active microRNAs (miRNAs) and maturation of messenger RNAs (mRNAs) that are competent for translation is a crucial point in the control of all cellular processes, with established roles in development and differentiation. Terminal nucleotidyltransferases (TNTases) are potent regulators of RNA metabolism. TNTases promote the addition of single or multiple nucleotides to an RNA transcript that can rapidly alter transcript stability. The well-known polyadenylation promotes transcript stability while the newly discovered but ubiquitious 3'-end polyuridylation marks RNA for degradation. Monoadenylation and uridylation are essential control mechanisms balancing mRNA and miRNA homeostasis.

SCOPE OF REVIEW

This review discusses the multiple functions of non-canonical TNTases, focusing on their substrate range, biological functions, and evolution. TNTases directly control mRNA and miRNA levels, with diverse roles in transcriptome stabilization, maturation, silencing, or degradation. We will summarize the current state of knowledge on non-canonical nucleotidyltransferases and their function in regulating miRNA and mRNA metabolism. We will review the discovery of uridylation as an RNA degradation pathway and discuss the evolution of nucleotidyltransferases along with their use in RNA labeling and future applications as therapeutic targets.

MAJOR CONCLUSIONS

The biochemically and evolutionarily highly related adenylyl- and uridylyltransferases play antagonizing roles in the cell. In general, RNA adenylation promotes stability, while uridylation marks RNA for degradation. Uridylyltransferases evolved from adenylyltransferases in multiple independent evolutionary events by the insertion of a histidine residue into the active site, altering nucleotide, but not RNA specificity.

GENERAL SIGNIFICANCE

Understanding the mechanisms regulating RNA stability in the cell and controlling the transcriptome is essential for efforts aiming to influence cellular fate. Selectively enhancing or reducing RNA stability allows for alterations in the transcriptome, proteome, and downstream cellular processes. Genetic, biochemical, and clinical data suggest TNTases are potent targets for chemotherapeutics and have been exploited for RNA labeling applications. This article is part of a Special Issue entitled "Biochemistry of Synthetic Biology - Recent Developments" Guest Editor: Dr. Ilka Heinemann and Dr. Patrick O'Donoghue.

Constructive edge of uridylation-induced RNA degradation

RNA uridylation is a significant transcriptome-shaping factor in protists, fungi, metazoans, and plants. The 3' U-additions are catalyzed by terminal uridyltransferases (TUTases), a diverse group of enzymes that along with non-canonical poly(A) polymerases form a distinct group in the superfamily of DNA polymerase β-like nucleotidyl transferases. Within and across studied organisms and subcellular compartments, TUTases differ in nucleotide triphosphate selectivity, interacting partners, and RNA targets. A general premise linking RNA uridylation to 3'-5' degradation received support from several studies of small RNAs and mRNA turnover. However, recent work on kinetoplastid protists typified by Trypanosoma brucei provides evidence that RNA uridylation may play a more nuanced role in generating functional small RNAs. In this pathogen's mitochondrion, most mRNAs are internally edited by U-insertions and deletions, and subjected to 3' adenylation/uridylation; guide RNAs (gRNAs) required for editing are U-tailed. The prominent role of uridylation in mitochondrial RNA metabolism stimulated identification of the first TUTase, RNA editing TUTase 1 (RET1). Here we discuss functional studies of mitochondrial uridylation in trypanosomes that have revealed an unorthodox pathway of small RNA biogenesis. The current model accentuates physical coupling of RET1 and 3'-5' RNase II/RNB-type exonuclease DSS1 within a stable complex termed the mitochondrial 3' processome (MPsome). In the confines of this complex, RET1 initially uridylates a long precursor to activate its 3'-5' degradation by DSS1, and then uridylates trimmed guide RNA to disengage the processing complex from the mature molecule. We also discuss a potential role of antisense transcription in the MPsome pausing at a fixed distance from gRNA's 5' end. This step likely defines the mature 3' end by enabling kinetic competition between TUTase and exonuclease activities.

The Potential of Secondary Metabolites from Plants as Drugs or Leads against Protozoan Neglected Diseases-Part III: In-Silico Molecular Docking Investigations

Malaria, leishmaniasis, Chagas disease, and human African trypanosomiasis continue to cause considerable suffering and death in developing countries. Current treatment options for these parasitic protozoal diseases generally have severe side effects, may be ineffective or unavailable, and resistance is emerging. There is a constant need to discover new chemotherapeutic agents for these parasitic infections, and natural products continue to serve as a potential source. This review presents molecular docking studies of potential phytochemicals that target key protein targets in Leishmania spp., Trypanosoma spp., and Plasmodium spp.

RNA Editing TUTase 1: structural foundation of substrate recognition, complex interactions and drug targeting

Terminal uridyltransferases (TUTases) execute 3′ RNA uridylation across protists, fungi, metazoan and plant species. Uridylation plays a particularly prominent role in RNA processing pathways of kinetoplastid protists typified by the causative agent of African sleeping sickness, Trypanosoma brucei. In mitochondria of this pathogen, most mRNAs are internally modified by U-insertion/deletion editing while guide RNAs and rRNAs are U-tailed. The founding member of TUTase family, RNA editing TUTase 1 (RET1), functions as a subunit of the 3′ processome in uridylation of gRNA precursors and mature guide RNAs. Along with KPAP1 poly(A) polymerase, RET1 also participates in mRNA translational activation. RET1 is divergent from human TUTases and is essential for parasite viability in the mammalian host and the insect vector. Given its robust in vitro activity, RET1 represents an attractive target for trypanocide development. Here, we report high-resolution crystal structures of the RET1 catalytic core alone and in complex with UTP analogs. These structures reveal a tight docking of the conserved nucleotidyl transferase bi-domain module with a RET1-specific C2H2 zinc finger and RNA recognition (RRM) domains. Furthermore, we define RET1 region required for incorporation into the 3′ processome, determinants for RNA binding, subunit oligomerization and processive UTP incorporation, and predict druggable pockets.

The Architecture of Trypanosoma brucei editosomes

Uridine insertion and deletion RNA editing generates functional mitochondrial mRNAs in Trypanosoma brucei Editing is catalyzed by three distinct ∼20S editosomes that have a common set of 12 proteins, but are typified by mutually exclusive RNase III endonucleases with distinct cleavage specificities and unique partner proteins. Previous studies identified a network of protein-protein interactions among a subset of common editosome proteins, but interactions among the endonucleases and their partner proteins, and their interactions with common subunits were not identified. Here, chemical cross-linking and mass spectrometry, comparative structural modeling, and genetic and biochemical analyses were used to define the molecular architecture and subunit organization of purified editosomes. We identified intra- and interprotein cross-links for all editosome subunits that are fully consistent with editosome protein structures and previously identified interactions, which we validated by genetic and biochemical studies. The results were used to create a highly detailed map of editosome protein domain proximities, leading to identification of molecular interactions between subunits, insights into the functions of noncatalytic editosome proteins, and a global understanding of editosome architecture.

Selective inhibitors of trypanosomal uridylyl transferase RET1 establish druggability of RNA post-transcriptional modifications

Non-coding RNAs are crucial regulators for a vast array of cellular processes and have been implicated in human disease. These biological processes represent a hitherto untapped resource in our fight against disease. In this work we identify small molecule inhibitors of a non-coding RNA uridylylation pathway. The TUTase family of enzymes is important for modulating non-coding RNA pathways in both human cancer and pathogen systems. We demonstrate that this new class of drug target can be accessed with traditional drug discovery techniques. Using the Trypanosoma brucei TUTase, RET1, we identify TUTase inhibitors and lay the groundwork for the use of this new target class as a therapeutic opportunity for the under-served disease area of African Trypanosomiasis. In a broader sense this work demonstrates the therapeutic potential for targeting RNA post-transcriptional modifications with small molecules in human disease.

U-Insertion/Deletion mRNA-Editing Holoenzyme: Definition in Sight

RNA editing is a process that alters DNA-encoded sequences and is distinct from splicing, 5' capping, and 3' additions. In 30 years since editing was discovered in mitochondria of trypanosomes, several functionally and evolutionarily unrelated mechanisms have been described in eukaryotes, archaea, and viruses. Editing events are predominantly post-transcriptional and include nucleoside insertions and deletions, and base substitutions and modifications. Here, we review the mechanism of uridine insertion/deletion mRNA editing in kinetoplastid protists typified by Trypanosoma brucei. This type of editing corrects frameshifts, introduces translation punctuation signals, and often adds hundreds of uridines to create protein-coding sequences. We focus on protein complexes responsible for editing reactions and their interactions with other elements of the mitochondrial gene expression pathway.

Structure of mitochondrial poly(A) RNA polymerase reveals the structural basis for dimerization, ATP selectivity and the SPAX4 disease phenotype

Polyadenylation, performed by poly(A) polymerases (PAPs), is a ubiquitous post-transcriptional modification that plays key roles in multiple aspects of RNA metabolism. Although cytoplasmic and nuclear PAPs have been studied extensively, the mechanism by which mitochondrial PAP (mtPAP) selects adenosine triphosphate over other nucleotides is unknown. Furthermore, mtPAP is unique because it acts as a dimer. However, mtPAP's dimerization requirement remains enigmatic. Here, we show the structural basis for mtPAP's nucleotide selectivity, dimerization and catalysis. Our structures reveal an intricate dimerization interface that features an RNA-recognition module formed through strand complementation. Further, we propose the structural basis for the N478D mutation that drastically reduces the length of poly(A) tails on mitochondrial mRNAs in patients with spastic ataxia 4 (SPAX4), a severe and progressive neurodegenerative disease.

Structure and intrinsic disorder of the proteins of the Trypanosoma brucei editosome

Mitochondrial pre-mRNAs in trypanosomatids undergo RNA editing to be converted into translatable mRNAs. The reaction is characterized by the insertion and deletion of uridine residues and is catalyzed by a macromolecular protein complex called the editosome. Despite intensive research, structural information for the majority of editosome proteins is still missing and no high resolution structure for the editosome exists. Here we present a comprehensive structural bioinformatics analysis of all proteins of the Trypanosoma brucei editosome. We specifically focus on the interplay between intrinsic order and disorder. According to computational predictions, editosome proteins involved in the basal reaction steps of the processing cycle are mostly ordered. By contrast, thirty percent of the amino acid content of the editosome is intrinsically disordered, which includes most prominently proteins with OB-fold domains. Based on the data we suggest a functional model, in which the structurally disordered domains of the complex are correlated with the RNA binding and RNA unfolding activity of the T. brucei editosome.

Comparison of the Mitochondrial Genomes and Steady State Transcriptomes of Two Strains of the Trypanosomatid Parasite, Leishmania tarentolae

U-insertion/deletion RNA editing is a post-transcriptional mitochondrial RNA modification phenomenon required for viability of trypanosomatid parasites. Small guide RNAs encoded mainly by the thousands of catenated minicircles contain the information for this editing. We analyzed by NGS technology the mitochondrial genomes and transcriptomes of two strains, the old lab UC strain and the recently isolated LEM125 strain. PacBio sequencing provided complete minicircle sequences which avoided the assembly problem of short reads caused by the conserved regions. Minicircles were identified by a characteristic size, the presence of three short conserved sequences, a region of inherently bent DNA and the presence of single gRNA genes at a fairly defined location. The LEM125 strain contained over 114 minicircles encoding different gRNAs and the UC strain only ~24 minicircles. Some LEM125 minicircles contained no identifiable gRNAs. Approximate copy numbers of the different minicircle classes in the network were determined by the number of PacBio CCS reads that assembled to each class. Mitochondrial RNA libraries from both strains were mapped against the minicircle and maxicircle sequences. Small RNA reads mapped to the putative gRNA genes but also to multiple regions outside the genes on both strands and large RNA reads mapped in many cases over almost the entire minicircle on both strands. These data suggest that minicircle transcription is complete and bidirectional, with 3’ processing yielding the mature gRNAs. Steady state RNAs in varying abundances are derived from all maxicircle genes, including portions of the repetitive divergent region. The relative extents of editing in both strains correlated with the presence of a cascade of cognate gRNAs. These data should provide the foundation for a deeper understanding of this dynamic genetic system as well as the evolutionary variation of editing in different strains.

U-insertion/deletion RNA editing is a unique post-transcriptional mRNA modification process that occurs in trypanosomatid parasites and is required for viability. The participation of guide RNAs which are transcribed from the thousands of catenated minicircles in determining the precise sites and number of U’s inserted and deleted to create translatable mRNAs is novel and significant in terms of the recently realized importance of small RNAs in biology. This study contributes the necessary bioinformatics foundation for a deeper understanding of this important genetic system in molecular detail using a model trypanosomatid, Leishmania tarentolae. We used Next Generation Sequencing methods to determine the complete maxicircle and minicircle genomes and to map maxicircle pre-edited and edited transcripts and minicircle transcripts. The transcription of minicircle-encoded guide RNAs was confirmed and novel information about minicircle gene expression was obtained. The biological context involved a comparison of two strains of the parasites, one recently isolated and having an intact mitochondrial genetic system and the other an old lab strain that has developed a partially defective mitochondrial genome. The data are important for an understanding of the mitochondrial genomic complexity and expression of this dynamic genetic system.

Polyuridylation in Eukaryotes: A 3'-End Modification Regulating RNA Life

In eukaryotes, mRNA polyadenylation is a well-known modification that is essential for many aspects of the protein-coding RNAs life cycle. However, modification of the 3′ terminal nucleotide within various RNA molecules is a general and conserved process that broadly modulates RNA function in all kingdoms of life. Numerous types of modifications have been characterized, which are generally specific for a given type of RNA such as the CCA addition found in tRNAs. In recent years, the addition of nontemplated uridine nucleotides or uridylation has been shown to occur in various types of RNA molecules and in various cellular compartments with significantly different outcomes. Indeed, uridylation is able to alter RNA half-life both in positive and in negative ways, highlighting the importance of the enzymes in charge of performing this modification. The present review aims at summarizing the current knowledge on the various processes leading to RNA 3′-end uridylation and on their potential impacts in various diseases.

Improved crystallization and diffraction of caffeine-induced death suppressor protein 1 (Cid1)

The use of truncation and RNA-binding mutations of caffeine induced death suppressor protein 1 (Cid1) as a means to enhance crystallogenesis leading to an improvement of X-ray diffraction resolution by 1.5 Å is reported.

The post-transcriptional addition of uridines to the 3′-end of RNAs is an important regulatory process that is critical for coding and noncoding RNA stability. In fission yeast and metazoans this untemplated 3′-uridylylation is catalysed by a single family of terminal uridylyltransferases (TUTs) whose members are adapted to specific RNA targets. In Schizosaccharomyces pombe the TUT Cid1 is responsible for the uridylylation of polyadenylated mRNAs, targeting them for destruction. In metazoans, the Cid1 orthologues ZCCHC6 and ZCCHC11 uridylate histone mRNAs, targeting them for degradation, but also uridylate microRNAs, altering their maturation. Cid1 has been studied as a model TUT that has provided insights into the larger and more complex metazoan enzyme system. In this paper, two strategies are described that led to improvements both in the crystallogenesis of Cid1 and in the resolution of diffraction by ∼1.5 Å. These advances have allowed high-resolution crystallo­graphic studies of this TUT system to be initiated.

Structural plasticity of Cid1 provides a basis for its distributive RNA terminal uridylyl transferase activity

Terminal uridylyl transferases (TUTs) are responsible for the post-transcriptional addition of uridyl residues to RNA 3′ ends, leading in some cases to altered stability. The Schizosaccharomyces pombe TUT Cid1 is a model enzyme that has been characterized structurally at moderate resolution and provides insights into the larger and more complex mammalian TUTs, ZCCHC6 and ZCCHC11. Here, we report a higher resolution (1.74 Å) crystal structure of Cid1 that provides detailed evidence for uracil selection via the dynamic flipping of a single histidine residue. We also describe a novel closed conformation of the enzyme that may represent an intermediate stage in a proposed product ejection mechanism. The structural insights gained, combined with normal mode analysis and biochemical studies, demonstrate that the plasticity of Cid1, particularly about a hinge region (N164–N165), is essential for catalytic activity, and provide an explanation for its distributive uridylyl transferase activity. We propose a model clarifying observed differences between the in vitro apparently processive activity and in vivo distributive monouridylylation activity of Cid1. We suggest that modulating the flexibility of such enzymes—for example by the binding of protein co-factors—may allow them alternatively to add single or multiple uridyl residues to the 3′ termini of RNA molecules.

Computer-aided discovery of Trypanosoma brucei RNA-editing terminal uridylyl transferase 2 inhibitors

Human African trypanosomiasis (HAT) is a major health problem in sub-Saharan Africa caused by Trypanosoma brucei infection. Current HAT drugs are difficult to administer and not effective against all parasite species at different stages of the disease which indicates an unmet pharmaceutical need. TbRET2 is an indispensable enzyme for the parasite and is targeted here using a computational approach that combines molecular dynamics simulations and virtual screening. The compounds prioritized are then tested in T. brucei via Alamar blue cell viability assays. This work identified 20 drug-like compounds which are candidates for further testing in the drug discovery process.

Mitochondrial RNA editing in trypanosomes: small RNAs in control

Mitochondrial mRNA editing in trypanosomes is a posttranscriptional processing pathway thereby uridine residues (Us) are inserted into, or deleted from, messenger RNA precursors. By correcting frameshifts, introducing start and stop codons, and often adding most of the coding sequence, editing restores open reading frames for mitochondrially-encoded mRNAs. There can be hundreds of editing events in a single pre-mRNA, typically spaced by few nucleotides, with U-insertions outnumbering U-deletions by approximately 10-fold. The mitochondrial genome is composed of ∼50 maxicircles and thousands of minicircles. Catenated maxi- and minicircles are packed into a dense structure called the kinetoplast; maxicircles yield rRNA and mRNA precursors while guide RNAs (gRNAs) are produced predominantly from minicircles, although varying numbers of maxicircle-encoded gRNAs have been identified in kinetoplastids species. Guide RNAs specify positions and the numbers of inserted or deleted Us by hybridizing to pre-mRNA and forming series of mismatches. These 50-60 nucleotide (nt) molecules are 3' uridylated by RET1 TUTase and stabilized via association with the gRNA binding complex (GRBC). Editing reactions of mRNA cleavage, U-insertion or deletion, and ligation are catalyzed by the RNA editing core complex (RECC). To function in mitochondrial translation, pre-mRNAs must further undergo post-editing 3' modification by polyadenylation/uridylation. Recent studies revealed a highly compound nature of mRNA editing and polyadenylation complexes and their interactions with the translational machinery. Here we focus on mechanisms of RNA editing and its functional coupling with pre- and post-editing 3' mRNA modification and gRNA maturation pathways.

A critical switch in the enzymatic properties of the Cid1 protein deciphered from its product-bound crystal structure

The addition of uridine nucleotide by the poly(U) polymerase (PUP) enzymes has a demonstrated impact on various classes of RNAs such as microRNAs (miRNAs), histone-encoding RNAs and messenger RNAs. Cid1 protein is a member of the PUP family. We solved the crystal structure of Cid1 in complex with non-hydrolyzable UMPNPP and a short dinucleotide compound ApU. These structures revealed new residues involved in substrate/product stabilization. In particular, one of the three catalytic aspartate residues explains the RNA dependence of its PUP activity. Moreover, other residues such as residue N165 or the β-trapdoor are shown to be critical for Cid1 activity. We finally suggest that the length and sequence of Cid1 substrate RNA influence the balance between Cid1's processive and distributive activities. We propose that particular processes regulated by PUPs require the enzymes to switch between the two types of activity as shown for the miRNA biogenesis where PUPs can either promote DICER cleavage via short U-tail or trigger miRNA degradation by adding longer poly(U) tail. The enzymatic properties of these enzymes may be critical for determining their particular function in vivo.

Explorations of linked editosome domains leading to the discovery of motifs defining conserved pockets in editosome OB-folds

Trypanosomatids form a group of protozoa which contain parasites of human, animals and plants. Several of these species cause major human diseases, including Trypanosoma brucei which is the causative agent of human African trypanosomiasis, also called sleeping sickness. These organisms have many highly unusual features including a unique U-insertion/deletion RNA editing process in the single mitochondrion. A key multi-protein complex, called the ∼20S editosome, or editosome, carries out a cascade of essential RNA-modifying reactions and contains a core of 12 different proteins of which six are the interaction proteins A1 to A6. Each of these interaction proteins comprises a C-terminal OB-fold and the smallest interaction protein A6 has been shown to interact with four other editosome OB-folds. Here we report the results of a "linked OB-fold" approach to obtain a view of how multiple OB-folds might interact in the core of the editosome. Constructs with variants of linked domains in 25 expression and co-expression experiments resulted in 13 soluble multi-OB-fold complexes. In several instances, these complexes were more homogeneous in size than those obtained from corresponding unlinked OB-folds. The crystal structure of A3(OB) linked to A6 could be elucidated and confirmed the tight interaction between these two OB domains as seen also in our recent complex of A3(OB) and A6 with nanobodies. In the current crystal structure of A3(OB) linked to A6, hydrophobic side chains reside in well-defined pockets of neighboring OB-fold domains. When analyzing the available crystal structures of editosome OB-folds, it appears that in five instances "Pocket 1" of A1(OB), A3(OB) and A6 is occupied by a hydrophobic side chain from a neighboring protein. In these three different OB-folds, Pocket 1 is formed by two conserved sequence motifs and an invariant arginine. These pockets might play a key role in the assembly or mechanism of the editosome by interacting with hydrophobic side chains from other proteins.

Mitochondrial poly(A) polymerase and polyadenylation

Polyadenylation of mitochondrial RNAs in higher eukaryotic organisms have diverse effects on their function and metabolism. Polyadenylation completes the UAA stop codon of a majority of mitochondrial mRNAs in mammals, regulates the translation of the mRNAs, and has diverse effects on their stability. In contrast, polyadenylation of most mitochondrial mRNAs in plants leads to their degradation, consistent with the bacterial origin of this organelle. PAPD1 (mtPAP, TUTase1), a noncanonical poly(A) polymerase (ncPAP), is responsible for producing the poly(A) tails in mammalian mitochondria. The crystal structure of human PAPD1 was reported recently, offering molecular insights into its catalysis. This article is part of a Special Issue entitled: Mitochondrial Gene Expression.

Crystal structures of the Cid1 poly (U) polymerase reveal the mechanism for UTP selectivity

Polyuridylation is emerging as a ubiquitous post-translational modification with important roles in multiple aspects of RNA metabolism. These poly (U) tails are added by poly (U) polymerases with homology to poly (A) polymerases; nevertheless, the selection for UTP over ATP remains enigmatic. We report the structures of poly (U) polymerase Cid1 from Schizoscaccharomyces pombe alone and in complex with UTP, CTP, GTP and 3′-dATP. These structures reveal that each of the 4 nt can be accommodated at the active site; however, differences exist that suggest how the polymerase selects UTP over the other nucleotides. Furthermore, we find that Cid1 shares a number of common UTP recognition features with the kinetoplastid terminal uridyltransferases. Kinetic analysis of Cid1’s activity for its preferred substrates, UTP and ATP, reveal a clear preference for UTP over ATP. Ultimately, we show that a single histidine in the active site plays a pivotal role for poly (U) activity. Notably, this residue is typically replaced by an asparagine residue in Cid1-family poly (A) polymerases. By mutating this histidine to an asparagine residue in Cid1, we diminished Cid1’s activity for UTP addition and improved ATP incorporation, supporting that this residue is important for UTP selectivity.

NF45 dimerizes with NF90, Zfr and SPNR via a conserved domain that has a nucleotidyltransferase fold

Nuclear factors NF90 and NF45 form a complex involved in a variety of cellular processes and are thought to affect gene expression both at the transcriptional and translational level. In addition, this complex affects the replication of several viruses through direct interactions with viral RNA. NF90 and NF45 dimerize through their common 'DZF' domain (domain associated with zinc fingers). NF90 has additional double-stranded RNA-binding domains that likely mediate its association with target RNAs. We present the crystal structure of the NF90/NF45 dimerization complex at 1.9-Å resolution. The DZF domain shows structural similarity to the template-free nucleotidyltransferase family of RNA modifying enzymes. However, both NF90 and NF45 have lost critical catalytic residues during evolution and are therefore not functional enzymes. Residues on NF90 that make up its interface with NF45 are conserved in two related proteins, spermatid perinuclear RNA-binding protein (SPNR) and zinc-finger RNA-binding protein (Zfr). Using a co-immunoprecipitation assay and site-specific mutants, we demonstrate that NF45 is also able to recognize SPNR and Zfr through the same binding interface, revealing that NF45 is able to form a variety of cellular complexes with other DZF-domain proteins.

Structural basis for the activity of a cytoplasmic RNA terminal uridylyl transferase

Cytoplasmic terminal uridylyl transferases comprise a conserved family of enzymes that negatively regulate the stability or biological activity of a variety of eukaryotic RNAs, including mRNAs and tumor-suppressor let-7 microRNAs. Here we describe crystal structures of the Schizosaccharomyces pombe cytoplasmic terminal uridylyl transferase Cid1 in two apo conformers and bound to UTP. We demonstrate that a single histidine residue, conserved in mammalian Cid1 orthologs, is responsible for discrimination between UTP and ATP. We also describe a new high-affinity RNA substrate-binding mechanism of Cid1, which is essential for enzymatic activity and is mediated by three basic patches across the surface of the enzyme. Overall, our structures provide a basis for understanding the activity of Cid1 and a mechanism of UTP selectivity conserved in its human orthologs, suggesting potential implications for anticancer drug design.

Functional implications from the Cid1 poly(U) polymerase crystal structure

In eukaryotes, mRNA degradation begins with poly(A) tail removal, followed by decapping, and the mRNA body is degraded by exonucleases. In recent years, the major influence of 3'-end uridylation as a regulatory step within several RNA degradation pathways has generated significant attention toward the responsible enzymes, which are called poly(U) polymerases (PUPs). We determined the atomic structure of the Cid1 protein, the founding member of the PUP family, in its UTP-bound form, allowing unambiguous positioning of the UTP molecule. Our data also suggest that the RNA substrate accommodation and product translocation by the Cid1 protein rely on local and global movements of the enzyme. Supplemented by point mutations, the atomic model is used to propose a catalytic cycle. Our study underlines the Cid1 RNA binding properties, a feature with critical implications for miRNAs, histone mRNAs, and, more generally, cellular RNA degradation.

Elements of nucleotide specificity in the Trypanosoma brucei mitochondrial RNA editing enzyme RET2

The causative agent of African sleeping sickness, Trypanosoma brucei , undergoes an unusual mitochondrial RNA editing process that is essential for its survival. RNA editing terminal uridylyl transferase 2 of T. brucei (TbRET2) is an indispensable component of the editosome machinery that performs this editing. TbRET2 is required to maintain the vitality of both the insect and bloodstream forms of the parasite, and with its high-resolution crystal structure, it poses as a promising pharmaceutical target. Neither the exclusive requirement of uridine 5'-triphosphate (UTP) for catalysis, nor the RNA primer preference of TbRET2 is well-understood. Using all-atom explicitly solvated molecular dynamics (MD) simulations, we investigated the effect of UTP binding on TbRET2 structure and dynamics, as well as the determinants governing TbRET2's exclusive UTP preference. Through our investigations of various nucleoside triphosphate substrates (NTPs), we show that UTP preorganizes the binding site through an extensive water-mediated H-bonding network, bringing Glu424 and Arg144 side chains to an optimum position for RNA primer binding. In contrast, cytosine 5'-triphosphate (CTP) and adenosine 5'-triphosphate (ATP) cannot achieve this preorganization and thus preclude productive RNA primer binding. Additionally, we have located ligand-binding "hot spots" of TbRET2 based on the MD conformational ensembles and computational fragment mapping. TbRET2 reveals different binding pockets in the apo and UTP-bound MD simulations, which could be targeted for inhibitor design.

Inhibitors of RNA editing as potential chemotherapeutics against trypanosomatid pathogens

The related trypanosomatid pathogens, Trypanosoma brucei spp., Trypanosoma cruzi and Leishmania spp. cause devastating diseases in humans and animals and continue to pose a major challenge in drug development. Mitochondrial RNA editing, catalyzed by multi-protein complexes known as editosomes, has provided an opportunity for development of efficient and specific chemotherapeutic targets against trypanosomatid pathogens. This review will discuss both methods for discovery of RNA editing inhibitors, as well as inhibitors against the T. brucei editosome that were recently discovered through creative virtual and high throughput screening methods. In addition, the use of these inhibitors as agents that can block or perturb one or more steps of the RNA editing process will be discussed. These inhibitors can potentially be used to study the dynamic processing and assembly of the editosome proteins. A thorough understanding of the mechanisms and specificities of these new inhibitors is needed in order to contribute to both the functional studies of an essential gene expression mechanism and to the possibility of future drug development against the trypanosomatid pathogens.

Crystal structure of a heterodimer of editosome interaction proteins in complex with two copies of a cross-reacting nanobody

The parasite Trypanosoma brucei, the causative agent of sleeping sickness across sub-Saharan Africa, depends on a remarkable U-insertion/deletion RNA editing process in its mitochondrion. A approximately 20 S multi-protein complex, called the editosome, is an essential machinery for editing pre-mRNA molecules encoding the majority of mitochondrial proteins. Editosomes contain a common core of twelve proteins where six OB-fold interaction proteins, called A1-A6, play a crucial role. Here, we report the structure of two single-strand nucleic acid-binding OB-folds from interaction proteins A3 and A6 that surprisingly, form a heterodimer. Crystal growth required the assistance of an anti-A3 nanobody as a crystallization chaperone. Unexpectedly, this anti-A3 nanobody binds to both A3(OB) and A6, despite only ~40% amino acid sequence identity between the OB-folds of A3 and A6. The A3(OB)-A6 heterodimer buries 35% more surface area than the A6 homodimer. This is attributed mainly to the presence of a conserved Pro-rich loop in A3(OB). The implications of the A3(OB)-A6 heterodimer, and of a dimer of heterodimers observed in the crystals, for the architecture of the editosome are profound, resulting in a proposal of a 'five OB-fold center' in the core of the editosome.

Uridine insertion/deletion editing in trypanosomes: a playground for RNA-guided information transfer

RNA editing is a collective term referring to enzymatic processes that change RNA sequence apart from splicing, 5' capping or 3' extension. In this article, we focus on uridine insertion/deletion mRNA editing found exclusively in mitochondria of kinetoplastid protists. This type of editing corrects frameshifts, introduces start and stops codons, and often adds much of the coding sequence to create an open reading frame. The mitochondrial genome of trypanosomatids, the most extensively studied clade within the order Kinetoplastida, is composed of ∼50 maxicircles with limited coding capacity and thousands of minicircles. To produce functional mRNAs, a multitude of nuclear-encoded factors mediate interactions of maxicircle-encoded pre-mRNAs with a vast repertoire of minicircle-encoded guide RNAs. Editing reactions of mRNA cleavage, U-insertions or U-deletions, and ligation are catalyzed by the RNA editing core complex (RECC, the 20S editosome) while each step of this enzymatic cascade is directed by guide RNAs. These 50-60 nucleotide (nt) molecules are 3' uridylated by RET1 TUTase and stabilized via association with the gRNA binding complex (GRBC). Remarkably, the information transfer between maxicircle and minicircle transcriptomes does not rely on template-dependent polymerization of nucleic acids. Instead, intrinsic substrate specificities of key enzymes are largely responsible for the fidelity of editing. Conversely, the efficiency of editing is enhanced by assembling enzymes and RNA binding proteins into stable multiprotein complexes. WIREs RNA 2011 2 669-685 DOI: 10.1002/wrna.82 For further resources related to this article, please visit the WIREs website.

Structures of a key interaction protein from the Trypanosoma brucei editosome in complex with single domain antibodies

Several major global diseases are caused by single-cell parasites called trypanosomatids. These organisms exhibit many unusual features including a unique and essential U-insertion/deletion RNA editing process in their single mitochondrion. Many key RNA editing steps occur in ∼20S editosomes, which have a core of 12 proteins. Among these, the "interaction protein" KREPA6 performs a central role in maintaining the integrity of the editosome core and also binds to ssRNA. The use of llama single domain antibodies (VHH domains) accelerated crystal growth of KREPA6 from Trypanosoma brucei dramatically. All three structures obtained are heterotetramers with a KREPA6 dimer in the center, and one VHH domain bound to each KREPA6 subunit. Two of the resultant heterotetramers use complementarity determining region 2 (CDR2) and framework residues to form a parallel pair of beta strands with KREPA6 - a mode of interaction not seen before in VHH domain-protein antigen complexes. The third type of VHH domain binds in a totally different manner to KREPA6. Intriguingly, while KREPA6 forms tetramers in solution adding either one of the three VHH domains results in the formation of a heterotetramer in solution, in perfect agreement with the crystal structures. Biochemical solution studies indicate that the C-terminal tail of KREPA6 is involved in the dimerization of KREPA6 dimers to form tetramers. The implications of these crystallographic and solution studies for possible modes of interaction of KREPA6 with its many binding partners in the editosome are discussed.

Structural basis for dimerization and activity of human PAPD1, a noncanonical poly(A) polymerase

Poly(A) polymerases (PAPs) are found in most living organisms and have important roles in RNA function and metabolism. Here, we report the crystal structure of human PAPD1, a noncanonical PAP that can polyadenylate RNAs in the mitochondria (also known as mtPAP) and oligouridylate histone mRNAs (TUTase1). The overall structure of the palm and fingers domains is similar to that in the canonical PAPs. The active site is located at the interface between the two domains, with a large pocket that can accommodate the substrates. The structure reveals the presence of a previously unrecognized domain in the N-terminal region of PAPD1, with a backbone fold that is similar to that of RNP-type RNA binding domains. This domain (named the RL domain), together with a β-arm insertion in the palm domain, contributes to dimerization of PAPD1. Surprisingly, our mutagenesis and biochemical studies show that dimerization is required for the catalytic activity of PAPD1.

iCODA: RNAi-based inducible knock-in system in Trypanosoma brucei

In vivo mutational analysis is often required to characterize enzymes that function as subunits of the U-insertion/deletion RNA editing core complex (RECC) in mitochondria of Trypanosoma brucei. The mutations may skew phenotypic manifestation of a dominant negative overexpression if complex association is disrupted. Conditional knockouts and knock-ins of essential mitochondrial genes are time consuming and restricted to the bloodstream form parasites, thus limiting biochemical analysis. We have combined CODA (computationally optimized DNA assembly) technology with RNA interference to develop an iCODA inducible knock-in system for expeditious phenotype assessment and affinity purification of the RECC bearing a mutant subunit. For functional knock-in, the gene region targeted by RNAi is replaced with a synthetic sequence bearing at least one silent mutation per 12 contiguous base pairs. Upon co-expression of the double-stranded RNA targeting the endogenous transcript and modified mRNA in a stable cell line, the endogenous mRNA is destroyed and the cell survives on the RNAi-resistant transcript encoding the same polypeptide. In this chapter, we describe the generation of procyclic (insect) transgenic cell lines, RNAi rescue, complex purification, and validation methods for RNA editing TUTase 2 (RET2). These methods should be readily applicable for any gene in T. brucei.

Fragment-based discovery of novel thymidylate synthase leads by NMR screening and group epitope mapping

Solution-state nuclear magnetic resonance (NMR) is a versatile tool for the study of binding interactions between small molecules and macromolecular targets. We applied ligand-based NMR techniques to the study of human thymidylate synthase (hTS) using known nanomolar inhibitors and a library of small molecule fragments. Screening by NMR led to the rapid identification of ligand pairs that bind in proximal sites within the cofactor-binding pocket of hTS. Screening hits were used as search criteria within commercially available sources, and a subset of catalog analogs were tested for potency by in vitro assay and binding affinity by quantitative saturation transfer difference (STD)-NMR titration. Two compounds identified by this approach possess low micromolar affinity and potency, as well as excellent binding efficiency against hTS. Relative binding orientations for both leads were modeled using AutoDock, and the most likely bound conformations were validated using experimentally derived STD-NMR binding epitope data. These ligands represent novel starting points for fragment-based drug design of non-canonical TS inhibitors, and their binding epitopes highlight important and previously unexploited interactions with conserved residues in the cofactor-binding site.

Structure and function of the polymerase core of TRAMP, a RNA surveillance complex

The Trf4p/Air2p/Mtr4p polyadenylation (TRAMP) complex recognizes aberrant RNAs in Saccharomyces cerevisiae and targets them for degradation. A TRAMP subcomplex consisting of a noncanonical poly(A) RNA polymerase in the Pol ss superfamily of nucleotidyl transferases, Trf4p, and a zinc knuckle protein, Air2p, mediates initial substrate recognition. Trf4p and related eukaryotic poly(A) and poly(U) polymerases differ from other characterized enzymes in the Pol ss superfamily both in sequence and in the lack of recognizable nucleic acid binding motifs. Here we report, at 2.7-A resolution, the structure of Trf4p in complex with a fragment of Air2p comprising two zinc knuckle motifs. Trf4p consists of a catalytic and central domain similar in fold to those of other noncanonical Pol beta RNA polymerases, and the two zinc knuckle motifs of Air2p interact with the Trf4p central domain. The interaction surface on Trf4p is highly conserved across eukaryotes, providing evidence that the Trf4p/Air2p complex is conserved in higher eukaryotes as well as in yeast and that the TRAMP complex may also function in RNA surveillance in higher eukaryotes. We show that Air2p, and in particular sequences encompassing a zinc knuckle motif near its N terminus, modulate Trf4p activity, and we present data supporting a role for this zinc knuckle in RNA binding. Finally, we show that the RNA 3' end plays a role in substrate recognition.

Mechanism of U-insertion RNA editing in trypanosome mitochondria: characterization of RET2 functional domains by mutational analysis

3'-Terminal uridylyl transferases (TUTases) selectively bind uridine 5'-triphosphate (UTP) and catalyze the addition of uridine 5'-monophosphate to the 3'-hydroxyl of RNA substrates in a template-independent manner. RNA editing TUTase 1 and RNA editing TUTase 2 (RET2) play central roles in uridine insertion/deletion RNA editing, which is an essential part of mitochondrial RNA processing in trypanosomes. Although the conserved N-terminal (catalytic) domain and C-terminal (nucleotide base recognition) domain are readily distinguished in all known TUTases, nucleotide specificity, RNA substrate preference, processivity, quaternary structures, and auxiliary domains vary significantly among enzymes of divergent biological functions. RET2 acts as a subunit of the RNA editing core complex to carry out guide-RNA-dependent U-insertion into mitochondrial mRNA. By correlating mutational effects on RET2 activity as recombinant protein and as RNA editing core complex subunit with RNAi-based knock-in phenotypes, we have assessed the UTP and RNA binding sites in RET2. Here we demonstrate functional conservation of key UTP-binding and metal-ion-coordinating residues and identify amino acids involved in RNA substrate recognition. Invariant arginine residues 144 and 435 positioned in the vicinity of the UTP binding site are critical for RET2 activity on single-stranded and double-stranded RNAs, as well as function in vivo. Recognition of a double-stranded RNA, which resembles a guide RNA/mRNA duplex, is further facilitated by multipoint contacts across the RET2-specific middle domain.

Mechanism of U insertion RNA editing in trypanosome mitochondria: the bimodal TUTase activity of the core complex

Expression of the trypanosomal mitochondrial genome requires the insertion and deletion of uridylyl residues at specific sites in pre-mRNAs. RET2 terminal uridylyl transferase is an integral component of the RNA editing core complex (RECC) and is responsible for the guide-RNA-dependent U insertion reaction. By analyzing RNA-interference-based knock-in Trypanosoma brucei cell lines, purified editing complex, and individual protein, we have investigated RET2's association with the RECC. In addition, the U insertion activity exhibited by RET2 as an RECC subunit was compared with characteristics of the monomeric protein. We show that interaction of RET2 with RECC is accomplished via a protein-protein contact between its middle domain and a structural subunit, MP81. The recombinant RET2 catalyzes a faithful editing on gapped (precleaved) double-stranded RNA substrates, and this reaction requires an internal monophosphate group at the 5' end of the mRNA 3' cleavage fragment. However, RET2 processivity is limited to insertion of three Us. Incorporation into the RECC voids the internal phosphate requirement and allows filling of longer gaps similar to those observed in vivo. Remarkably, monomeric and RECC-embedded enzymes display a similar bimodal activity: the distributive insertion of a single uracil is followed by a processive extension limited by the number of guiding nucleotides. Based on the RNA substrate specificity of RET2 and the purine-rich nature of U insertion sites, we propose that the distributive +1 insertion creates a substrate for the processive gap-filling reaction. Upon base-pairing of the +1 extended 5' cleavage fragment with a guiding nucleotide, this substrate is recognized by RET2 in a different mode compared to the product of the initial nucleolytic cleavage. Therefore, RET2 distinguishes base pairs in gapped RNA substrates which may constitute an additional checkpoint contributing to overall fidelity of the editing process.

Structure of the mitochondrial editosome-like complex associated TUTase 1 reveals divergent mechanisms of UTP selection and domain organization

RNA uridylylation reactions catalyzed by terminal uridylyl transferases (TUTases) play critical roles in the formation of the mitochondrial transcriptome in trypanosomes. Two mitochondrial RNA editing TUTases have been described: RNA editing TUTase 1 catalyzes guide RNA, ribosomal RNA, and mRNA 3'-uridylylation, and RNA editing TUTase 2 acts as a subunit of the RNA editing core complex (also referred to as the 20S editosome) to perform guided U-insertion mRNA editing. Although RNA editing TUTase 1 and RNA editing TUTase 2 carry out distinct functions and possess dissimilar enzymatic properties, their catalytic N-terminal domain and base recognition C-terminal domain display a high degree of similarity, while their middle domains are less conserved. MEAT1 (mitochondrial editosome-like complex associated TUTase 1), which interacts with an editosome-like assembly and is exclusively U-specific, nonetheless shows limited similarity with editing TUTases and lacks the middle domain. The crystal structures of apo MEAT1 and UTP-bound MEAT1 refined to 1.56 A and 1.95 A, respectively, reveal an unusual mechanism of UTP selection and domain organization previously unseen in TUTases. In addition to established invariant UTP-binding determinants, we have identified and verified critical contributions of MEAT1-specific residues using mutagenesis. Furthermore, MEAT1 possesses a novel bridging domain, which extends from the C-terminal domain and makes hydrophobic contacts with the N-terminal domain, thereby creating a cavity adjacent to the UTP-binding site. Unlike the minimal TUT4 TUTase, MEAT1 shows no appreciable conformational change upon UTP binding and apparently does not require RNA substrate to select a cognate nucleoside triphosphate. Because MEAT1 is essential for the viability of the bloodstream and insect forms of Trypanosoma brucei, the unique organization of its active site renders this protein an attractive target for trypanocide development.

Polyadenylation and beyond: emerging roles for noncanonical poly(A) polymerases

The addition of nontemplated nucleotides, particularly adenylyl and uridylyl residues, to the 3' ends of RNA substrates has been the focus of much attention in recent years, and these studies have generated some intriguing surprises. In addition to the well-known canonical poly(A) polymerase (PAP) that polyadenylates mRNAs prior to export from the nucleus to the cytoplasm, a separate class of noncanonical poly(A) polymerases has emerged over the past decade. Studies on various organisms have led to the realization that these noncanonical PAPs, which are conserved from yeast to mammals, play crucial and diverse roles in the regulation of gene expression. Here we review the current knowledge of these enzymes, with an emphasis on the human proteins, and highlight recent discoveries that have implications far beyond the understanding of RNA metabolism itself.

Mutagenesis and functional characterization of the four domains of GlnD, a bifunctional nitrogen sensor protein

GlnD is a bifunctional uridylyltransferase/uridylyl-removing enzyme (UTase/UR) and is believed to be the primary sensor of nitrogen status in the cell by sensing the level of glutamine in enteric bacteria. It plays an important role in nitrogen assimilation and metabolism by reversibly regulating the modification of P(II) protein; P(II) in turn regulates a variety of other proteins. GlnD appears to have four distinct domains: an N-terminal nucleotidyltransferase (NT) domain; a central HD domain, named after conserved histidine and aspartate residues; and two C-terminal ACT domains, named after three of the allosterically regulated enzymes in which this domain is found. Here we report the functional analysis of these domains of GlnD from Escherichia coli and Rhodospirillum rubrum. We confirm the assignment of UTase activity to the NT domain and show that the UR activity is a property specifically of the HD domain: substitutions in this domain eliminated UR activity, and a truncated protein lacking the NT domain displayed UR activity. The deletion of C-terminal ACT domains had little effect on UR activity itself but eliminated the ability of glutamine to stimulate that activity, suggesting a role for glutamine sensing by these domains. The deletion of C-terminal ACT domains also dramatically decreased UTase activity under all conditions tested, but some of these effects are due to the competition of UTase activity with unregulated UR activity in these variants.

Comprehensive classification of nucleotidyltransferase fold proteins: identification of novel families and their representatives in human

This article presents a comprehensive review of large and highly diverse superfamily of nucleotidyltransferase fold proteins by providing a global picture about their evolutionary history, sequence-structure diversity and fulfilled functional roles. Using top-of-the-line homology detection method combined with transitive searches and fold recognition, we revised the realm of these superfamily in numerous databases of catalogued protein families and structures, and identified 10 new families of nucleotidyltransferase fold. These families include hundreds of previously uncharacterized and various poorly annotated proteins such as Fukutin/LICD, NFAT, FAM46, Mab-21 and NRAP. Some of these proteins seem to play novel important roles, not observed before for this superfamily, such as regulation of gene expression or choline incorporation into cell membrane. Importantly, within newly detected families we identified 25 novel superfamily members in human genome. Among these newly assigned members are proteins known to be involved in congenital muscular dystrophy, neurological diseases and retinal pigmentosa what sheds some new light on the molecular background of these genetic disorders. Twelve of new human nucleotidyltransferase fold proteins belong to Mab-21 family known to be involved in organogenesis and development. The determination of specific biological functions of these newly detected proteins remains a challenging task.

A protein-protein interaction map of trypanosome ~20S editosomes

Mitochondrial mRNA editing in trypanosomatid parasites involves several multiprotein assemblies, including three very similar complexes that contain the key enzymatic editing activities and sediment at ~20S on glycerol gradients. These ~20S editosomes have a common set of 12 proteins, including enzymes for uridylyl (U) removal and addition, 2 RNA ligases, 2 proteins with RNase III-like domains, and 6 proteins with predicted oligonucleotide binding (OB) folds. In addition, each of the 3 distinct ~20S editosomes contains a different RNase III-type endonuclease, 1 of 3 related proteins and, in one case, an additional exonuclease. Here we present a protein-protein interaction map that was obtained through a combination of yeast two-hybrid analysis and subcomplex reconstitution with recombinant protein. This map interlinks ten of the proteins and in several cases localizes the protein region mediating the interaction, which often includes the predicted OB-fold domain. The results indicate that the OB-fold proteins form an extensive protein-protein interaction network that connects the two trimeric subcomplexes that catalyze U removal or addition and RNA ligation. One of these proteins, KREPA6, interacts with the OB-fold zinc finger protein in each subcomplex that interconnects their two catalytic proteins. Another OB-fold protein, KREPA3, appears to link to the putative endonuclease subcomplex. These results reveal a physical organization that underlies the coordination of the various catalytic and substrate binding activities within the ~20S editosomes during the editing process.

Novel TUTase associates with an editosome-like complex in mitochondria of Trypanosoma brucei

Expression of mitochondrial genomes in Kinetoplastida protists requires massive uracil insertion/deletion mRNA editing. The cascade of editing reactions is accomplished by a multiprotein complex, the 20S editosome, and is directed by trans-acting guide RNAs. Two distinct RNA terminal uridylyl transferases (TUTases), RNA Editing TUTase 1 (RET1) and RNA Editing TUTase 2 (RET2), catalyze 3' uridylylation of guide RNAs and U-insertions into the mRNAs, respectively. RET1 is also involved in mitochondrial mRNA turnover and participates in numerous heterogeneous complexes; RET2 is an integral part of the 20S editosome, in which it forms a U-insertion subcomplex with zinc finger protein MP81 and RNA editing ligase REL2. Here we report the identification of a third mitochondrial TUTase from Trypanosoma brucei. The mitochondrial editosome-like complex associated TUTase (MEAT1) interacts with a 20S editosome-like particle, effectively substituting the U-insertion subcomplex. MEAT1 and RET2 are mutually exclusive in their respective complexes, which otherwise share several components. Similarly to RET2, MEAT1 is exclusively U-specific in vitro and is active on gapped double-stranded RNA resembling editing substrates. However, MEAT1 does not require a 5' phosphate group on the 3' mRNA cleavage fragment produced by editing endonucleases. The functional RNAi complementation experiments showed that MEAT1 is essential for viability of bloodstream and insect parasite forms. The growth inhibition phenotype in the latter can be rescued by coexpressing an RNAi-resistant gene with double-stranded RNA targeting the endogenous transcript. However, preliminary RNA analysis revealed no gross effects on RNA editing in MEAT1-depleted cells and indicated its possible role in regulating the mitochondrial RNA stability.

Identification and characterization of nuclear non-canonical poly(A) polymerases from Trypanosoma brucei

Regulation of nuclear genome expression in Trypanosoma brucei is critical for this protozoan parasite's successful transition between its vertebrate and invertebrate host environments. The canonical eukaryotic circuits such as modulation of transcription initiation, mRNA splicing and polyadenylation appear to be nearly non-existent in T. brucei suggesting that the transcriptome is primarily defined by mRNA turnover. Our previous work has highlighted sequence similarities between terminal RNA uridylyl transferases (TUTases) and non-canonical poly(A) polymerases, which are widely implicated in regulating nuclear, cytoplasmic and organellar RNA decay throughout the eukaryotic lineage. Here, we have continued characterization of TUTase-like proteins in T. brucei and identified two nuclear non-canonical poly(A) polymerases (ncPAPs). The 82kDa TbncPAP1 is essential for viability of procyclic and bloodstream forms of T. brucei. Similar to Trf4/5 proteins from budding yeast, TbncPAP1 requires protein cofactor(s) to exert poly(A) polymerase activity in vitro. The recombinant 54kDa TbncPAP2 showed a PAP activity as an individual polypeptide. Proteomic analysis of the TbncPAP1 interactions demonstrated its association with Mtr4 RNA helicase and several RNA binding proteins, including a potential ortholog of Air1p/2p proteins, which indicates the presence of a stable TRAMP-like complex in trypanosomes. Our findings suggest that TbncPAP1 may be a "quality control" nuclear PAP involved in targeting aberrant or anti-sense transcripts for degradation by the 3'-exosome. Such mechanisms are likely to play a major role in alleviating promiscuity of the transcriptional machinery.