Kti12, a PSTK-like tRNA dependent ATPase essential for tRNA modification by Elongator

Cryo-EM structures of the human Elongator complex at work

tRNA modifications affect ribosomal elongation speed and co-translational folding dynamics. The Elongator complex is responsible for introducing 5-carboxymethyl at wobble uridine bases (cm5U34) in eukaryotic tRNAs. However, the structure and function of human Elongator remain poorly understood. In this study, we present a series of cryo-EM structures of human ELP123 in complex with tRNA and cofactors at four different stages of the reaction. The structures at resolutions of up to 2.9 Å together with complementary functional analyses reveal the molecular mechanism of the modification reaction. Our results show that tRNA binding exposes a universally conserved uridine at position 33 (U33), which triggers acetyl-CoA hydrolysis. We identify a series of conserved residues that are crucial for the radical-based acetylation of U34 and profile the molecular effects of patient-derived mutations. Together, we provide the high-resolution view of human Elongator and reveal its detailed mechanism of action.

Evolutionary Conservation in Protein-Protein Interactions and Structures of the Elongator Sub-Complex ELP456 from Arabidopsis and Yeast

The Elongator complex plays a pivotal role in the wobble uridine modification of the tRNA anticodon. Comprising two sets of six distinct subunits, namely, Elongator proteins (ELP1-ELP6) and associated proteins, the holo-Elongator complex demonstrates remarkable functional and structural conservation across eukaryotes. However, the precise details of the evolutionary conservation of the holo-Elongator complex and its individual sub-complexes (i.e., ELP123; ELP456) in plants remain limited. In this study, we conducted an in vivo analysis of protein-protein interactions among Arabidopsis ELP4, ELP5, and ELP6 proteins. Additionally, we predicted their structural configurations and performed a comparative analysis with the structure of the yeast Elp456 sub-complex. Protein-protein interaction analysis revealed that AtELP4 interacts with AtELP6 but not directly with AtELP5. Furthermore, we found that the Arabidopsis Elongator-associated protein, Deformed Roots and Leaves 1 (DRL1), did not directly bind to AtELP proteins. The structural comparison of the ELP456 sub-complex between Arabidopsis and yeast demonstrated high similarity, encompassing the RecA-ATPase fold and the positions of hydrogen bonds, despite their relatively low sequence homology. Our findings suggest that Arabidopsis ELP4, ELP5, and ELP6 proteins form a heterotrimer, with ELP6 serving as a bridge, indicating high structural conservation between the ELP456 sub-complexes from Arabidopsis and yeast.

Experience with German Research Consortia in the Field of Chemical Biology of Native Nucleic Acid Modifications

The chemical biology of native nucleic acid modifications has seen an intense upswing, first concerning DNA modifications in the field of epigenetics and then concerning RNA modifications in a field that was correspondingly rebaptized epitranscriptomics by analogy. The German Research Foundation (DFG) has funded several consortia with a scientific focus in these fields, strengthening the traditionally well-developed nucleic acid chemistry community and inciting it to team up with colleagues from the life sciences and data science to tackle interdisciplinary challenges. This Perspective focuses on the genesis, scientific outcome, and downstream impact of the DFG priority program SPP1784 and offers insight into how it fecundated further consortia in the field. Pertinent research was funded from mid-2015 to 2022, including an extension related to the coronavirus pandemic. Despite being a detriment to research activity in general, the pandemic has resulted in tremendously boosted interest in the field of RNA and RNA modifications as a consequence of their widespread and successful use in vaccination campaigns against SARS-CoV-2. Funded principal investigators published over 250 pertinent papers with a very substantial impact on the field. The program also helped to redirect numerous laboratories toward this dynamic field. Finally, SPP1784 spawned initiatives for several funded consortia that continue to drive the fields of nucleic acid modification.

Yeast gene KTI13 (alias DPH8) operates in the initiation step of diphthamide synthesis on elongation factor 2

In yeast, Elongator-dependent tRNA modifications are regulated by the Kti11•Kti13 dimer and hijacked for cell killing by zymocin, a tRNase ribotoxin. Kti11 (alias Dph3) also controls modification of elongation factor 2 (EF2) with diphthamide, the target for lethal ADP-ribosylation by diphtheria toxin (DT). Diphthamide formation on EF2 involves four biosynthetic steps encoded by the DPH1-DPH7 network and an ill-defined KTI13 function. On further examining the latter gene in yeast, we found that kti13Δ null-mutants maintain unmodified EF2 able to escape ADP-ribosylation by DT and to survive EF2 inhibition by sordarin, a diphthamide-dependent antifungal. Consistently, mass spectrometry shows kti13Δ cells are blocked in proper formation of amino-carboxyl-propyl-EF2, the first diphthamide pathway intermediate. Thus, apart from their common function in tRNA modification, both Kti11/Dph3 and Kti13 share roles in the initiation step of EF2 modification. We suggest an alias KTI13/DPH8 nomenclature indicating dual-functionality analogous to KTI11/DPH3.

The life and times of a tRNA

The study of eukaryotic tRNA processing has given rise to an explosion of new information and insights in the last several years. We now have unprecedented knowledge of each step in the tRNA processing pathway, revealing unexpected twists in biochemical pathways, multiple new connections with regulatory pathways, and numerous biological effects of defects in processing steps that have profound consequences throughout eukaryotes, leading to growth phenotypes in the yeast Saccharomyces cerevisiae and to neurological and other disorders in humans. This review highlights seminal new results within the pathways that comprise the life of a tRNA, from its birth after transcription until its death by decay. We focus on new findings and revelations in each step of the pathway including the end-processing and splicing steps, many of the numerous modifications throughout the main body and anticodon loop of tRNA that are so crucial for tRNA function, the intricate tRNA trafficking pathways, and the quality control decay pathways, as well as the biogenesis and biology of tRNA-derived fragments. We also describe the many interactions of these pathways with signaling and other pathways in the cell.

Cryo-EM structure of the fully assembled Elongator complex

Transfer RNA (tRNA) molecules are essential to decode messenger RNA codons during protein synthesis. All known tRNAs are heavily modified at multiple positions through post-transcriptional addition of chemical groups. Modifications in the tRNA anticodons are directly influencing ribosome decoding and dynamics during translation elongation and are crucial for maintaining proteome integrity. In eukaryotes, wobble uridines are modified by Elongator, a large and highly conserved macromolecular complex. Elongator consists of two subcomplexes, namely Elp123 containing the enzymatically active Elp3 subunit and the associated Elp456 hetero-hexamer. The structure of the fully assembled complex and the function of the Elp456 subcomplex have remained elusive. Here, we show the cryo-electron microscopy structure of yeast Elongator at an overall resolution of 4.3 Å. We validate the obtained structure by complementary mutational analyses in vitro and in vivo. In addition, we determined various structures of the murine Elongator complex, including the fully assembled mouse Elongator complex at 5.9 Å resolution. Our results confirm the structural conservation of Elongator and its intermediates among eukaryotes. Furthermore, we complement our analyses with the biochemical characterization of the assembled human Elongator. Our results provide the molecular basis for the assembly of Elongator and its tRNA modification activity in eukaryotes.

Transcriptomic response of bioengineered human cartilage to parabolic flight microgravity is sex-dependent

Spaceflight and simulated spaceflight microgravity induced osteoarthritic-like alterations at the transcriptomic and proteomic levels in the articular and meniscal cartilages of rodents. But little is known about the effect of spaceflight or simulated spaceflight microgravity on the transcriptome of tissue-engineered cartilage developed from human cells. In this study, we investigate the effect of simulated spaceflight microgravity facilitated by parabolic flights on tissue-engineered cartilage developed from in vitro chondrogenesis of human bone marrow mesenchymal stem cells obtained from age-matched female and male donors. The successful induction of cartilage-like tissue was confirmed by the expression of well-demonstrated chondrogenic markers. Our bulk transcriptome data via RNA sequencing demonstrated that parabolic flight altered mostly fundamental biological processes, and the modulation of the transcriptome profile showed sex-dependent differences. The secretome profile analysis revealed that two genes (WNT7B and WNT9A) from the Wnt-signaling pathway, which is implicated in osteoarthritis development, were only up-regulated for female donors. The results of this study showed that the engineered cartilage tissues responded to microgravity in a sex-dependent manner, and the reported data offers a strong foundation to further explore the underlying mechanisms.

AtELP4 a subunit of the Elongator complex in Arabidopsis, mediates cell proliferation and dorsoventral polarity during leaf morphogenesis

The Elongator complex in eukaryotes has conserved tRNA modification functions and contributes to various physiological processes such as transcriptional control, DNA replication and repair, and chromatin accessibility. ARABIDOPSIS ELONGATOR PROTEIN 4 (AtELP4) is one of the six subunits (AtELP1-AtELP6) in Arabidopsis Elongator. In addition, there is an Elongator-associated protein, DEFORMED ROOTS AND LEAVES 1 (DRL1), whose homolog in yeast (Kti12) binds tRNAs. In this study, we explored the functions of AtELP4 in plant-specific aspects such as leaf morphogenesis and evolutionarily conserved ones between yeast and Arabidopsis. ELP4 comparison between yeast and Arabidopsis revealed that plant ELP4 possesses not only a highly conserved P-loop ATPase domain but also unknown plant-specific motifs. ELP4 function is partially conserved between Arabidopsis and yeast in the growth sensitivity toward caffeine and elevated cultivation temperature. Either single Atelp4 or drl1-102 mutants and double Atelp4 drl1-102 mutants exhibited a reduction in cell proliferation and changed the adaxial-abaxial polarity of leaves. In addition, the single Atelp4 and double Atelp4 drl1-102 mutants showed remarkable downward curling at the whole part of leaf blades in contrast to wild-type leaf blades. Furthermore, our genetic study revealed that AtELP4 might epistatically act on DRL1 in the regulation of cell proliferation and dorsoventral polarity in leaves. Taken together, we suggest that AtELP4 as part of the plant Elongator complex may act upstream of a regulatory pathway for adaxial-abaxial polarity and cell proliferation during leaf development.

E2/E3-independent ubiquitin-like protein conjugation by Urm1 is directly coupled to cysteine persulfidation

Post-translational modifications by ubiquitin-like proteins (UBLs) are essential for nearly all cellular processes. Ubiquitin-related modifier 1 (Urm1) is a unique UBL, which plays a key role in tRNA anticodon thiolation as a sulfur carrier protein (SCP) and is linked to the noncanonical E1 enzyme Uba4 (ubiquitin-like protein activator 4). While Urm1 has also been observed to conjugate to target proteins like other UBLs, the molecular mechanism of its attachment remains unknown. Here, we reconstitute the covalent attachment of thiocarboxylated Urm1 to various cellular target proteins in vitro, revealing that, unlike other known UBLs, this process is E2/E3-independent and requires oxidative stress. Furthermore, we present the crystal structures of the peroxiredoxin Ahp1 before and after the covalent attachment of Urm1. Surprisingly, we show that urmylation is accompanied by the transfer of sulfur to cysteine residues in the target proteins, also known as cysteine persulfidation. Our results illustrate the role of the Uba4-Urm1 system as a key evolutionary link between prokaryotic SCPs and the UBL modifications observed in modern eukaryotes.

Gamma-irradiation fluctuates the mRNA N6-methyladenosine (m6A) spectrum of bone marrow in hematopoietic injury

Humans benefit from nuclear technologies but consequently experience nuclear disasters or side effects of iatrogenic radiation. Hematopoietic system injury first arises upon radiation exposure. As an intricate new layer of genetic control, the posttranscriptional m⁶A modification of RNA has recently come under investigation and has been demonstrated to play pivotal roles in multiple physiological and pathological processes. However, how the m⁶A methylome functions in the hematopoietic system after irradiation remains ambiguous. Here, we uncovered the time-varying epitranscriptome-wide m⁶A methylome and transcriptome alterations in γ-ray-exposed mouse bone marrow. 4 Gy γ-irradiation rapidly (5 min and 2 h) and severely impaired the mouse hematopoietic system, including spleen and thymus weight, blood components, tissue inflammation and malondialdehyde (MDA) levels. The m⁶A content and expression of m⁶A related enzymes were altered. Gamma-irradiation triggered dynamic and reversible m⁶A modification profiles and altered mRNA expression, where both m⁶A fold-enrichment and mRNA expression most followed the (5 min_up/2 h_down) pattern. The CDS enrichment region preferentially upregulated m⁶A peaks at 5 min. Moreover, the main GO and KEGG pathways were closely related to metabolism and the classical radiation response. Finally, m⁶A modifications correlated with transcriptional regulation of genes in multiple aspects. Blocking the expression of m⁶A demethylases FTO and ALKBH5 mitigated radiation hematopoietic toxicity. Together, our findings present the comprehensive landscape of mRNA m⁶A methylation in the mouse hematopoietic system in response to γ-irradiation, shedding light on the significance of m⁶A modifications in mammalian radiobiology. Regulation of the epitranscriptome may be exploited as a strategy against radiation damage.

Same but different - Molecular comparison of human KTI12 and PSTK

Kti12 and PSTK are closely related and highly similar proteins implicated in different aspects of tRNA metabolism. Kti12 has been identified as an essential regulatory factor of the Elongator complex, involved in the modification of uridine bases in eukaryotic tRNAs. PSTK phosphorylates the tRNA^Sec-bound amino acid serine, which is required to synthesize selenocysteine. Kti12 and PSTK have previously been studied independently in various organisms, but only appear simultaneously in some animalia, including humans. As Kti12- and PSTK-related pathways are clinically relevant, it is of prime importance to understand their biological functions and mutual relationship in humans. Here, we use different tRNA substrates to directly compare the enzymatic activities of purified human KTI12 and human PSTK proteins. Our complementary Co-IP and BioID2 approaches in human cells confirm that Elongator is the main interaction partner of KTI12 but additionally indicate potential links to proteins involved in vesicular transport, RNA metabolism and deubiquitination. Moreover, we identify and validate a yet uncharacterized interaction between PSTK and γ-taxilin. Foremost, we demonstrate that human KTI12 and PSTK do not share interactors or influence their respective biological functions. Our data provide a comprehensive analysis of the regulatory networks controlling the activity of the human Elongator complex and selenocysteine biosynthesis.

Effects of Peptide Thanatin on the Growth and Transcriptome of Penicillium digitatum

Penicillium digitatum is the most damaging pathogen provoking green mold in citrus fruit during storage, and there is an urgent need for novel antifungal agents with high efficiency. The aim of this study was to investigate the antifungal effects of peptide thanatin against P. digitatum and the molecular mechanisms. Results showed that peptide thanatin had a prominent inhibitory effect on P. digitatum by in vitro and in vivo test. A total of 938 genes, including 556 downregulated and 382 upregulated genes, were differentially expressed, as revealed by RNA-seq of whole P. digitatum genomes analysis with or without thanatin treatment. The downregulated genes mainly encoded RNA polymerase, ribosome biogenesis, amino acid metabolism, and major facilitator superfamily. The genes associated with heat shock proteins and antioxidative systems were widely expressed in thanatin-treated group. DNA, RNA, and the protein content of P. digitatum were significantly decreased after thanatin treatment. In conclusion, thanatin could inhibit the growth of P. digitatum, and the underlying mechanism might be the genetic information processing and stress response were affected. The research will provide more precise and directional clues to explore the inhibitory mechanism of thanatin on growth of P. digitatum.

PtKTI12 genes influence wobble uridine modifications and drought stress tolerance in hybrid poplar

The multisubunit Elongator complex plays key roles in transcription by interacting with RNA polymerase II and chromatin modeling. Kti proteins have been identified as the auxiliary protein for the Elongator complex. However, our knowledge of Kti proteins in woody plants remains limited. In this study, in total 16 KTI gene homologs were identified in Populus trichocarpa. Among them, the two KTI12 candidates were named PtKTI12A and PtKTI12B. Although PtKTI12A and PtKTI12B were largely different in gene expression level and tissue specificity, both genes were induced by heat and drought stresses. PtKTI12A and PtKTI12B RNAi transgenic poplar plants showed reduced levels of modified nucleosides, in particular 5-carbamoylmethyluridine and 5-methoxycarbonylmethyl-2-thiouridine. Meanwhile, their tolerance to drought was improved when subjected to withdrawal of watering. Also, the protein products of PtKTI12A and PtKTI12B had similar subcellular localization and predicted tertiary structure. The results suggest that Kti12 proteins are involved in tRNA wobble uridine modification, stress response and drought stress tolerance in hybrid poplar.

How Elongator Acetylates tRNA Bases

Elp3, the catalytic subunit of the eukaryotic Elongator complex, is a lysine acetyltransferase that acetylates the C5 position of wobble-base uridines (U34) in transfer RNAs (tRNAs). This Elongator-dependent RNA acetylation of anticodon bases affects the ribosomal translation elongation rates and directly links acetyl-CoA metabolism to both protein synthesis rates and the proteome integrity. Of note, several human diseases, including various cancers and neurodegenerative disorders, correlate with the dysregulation of Elongator's tRNA modification activity. In this review, we focus on recent findings regarding the structure of Elp3 and the role of acetyl-CoA during its unique modification reaction.

Plant Elongator-Protein Complex of Diverse Activities Regulates Growth, Development, and Immune Responses

Contrary to the conserved Elongator composition in yeast, animals, and plants, molecular functions and catalytic activities of the complex remain controversial. Elongator was identified as a component of elongating RNA polymerase II holoenzyme in yeast, animals, and plants. Furthermore, it was suggested that Elonagtor facilitates elongation of transcription via histone acetyl transferase activity. Accordingly, phenotypes of Arabidopsis elo mutants, which show development, growth, or immune response defects, correlate with transcriptional downregulation and the decreased histone acetylation in the coding regions of crucial genes. Plant Elongator was also implicated in other processes: transcription and processing of miRNA, regulation of DNA replication by histone acetylation, and acetylation of alpha-tubulin. Moreover, tRNA modification, discovered first in yeast and confirmed in plants, was claimed as the main activity of Elongator, leading to specificity in translation that might also result indirectly in a deficiency in transcription. Heterologous overexpression of individual Arabidopsis Elongator subunits and their respective phenotypes suggest that single Elongator subunits might also have another function next to being a part of the complex. In this review, we shall present the experimental evidence of all molecular mechanisms and catalytic activities performed by Elongator in nucleus and cytoplasm of plant cells, which might explain how Elongator regulates growth, development, and immune responses.

Misactivation of multiple starvation responses in yeast by loss of tRNA modifications

Previously, combined loss of different anticodon loop modifications was shown to impair the function of distinct tRNAs in Saccharomyces cerevisiae. Surprisingly, each scenario resulted in shared cellular phenotypes, the basis of which is unclear. Since loss of tRNA modification may evoke transcriptional responses, we characterized global transcription patterns of modification mutants with defects in either tRNAGlnUUG or tRNALysUUU function. We observe that the mutants share inappropriate induction of multiple starvation responses in exponential growth phase, including derepression of glucose and nitrogen catabolite-repressed genes. In addition, autophagy is prematurely and inadequately activated in the mutants. We further demonstrate that improper induction of individual starvation genes as well as the propensity of the tRNA modification mutants to form protein aggregates are diminished upon overexpression of tRNAGlnUUG or tRNALysUUU, the tRNA species that lack the modifications of interest. Hence, our data suggest that global alterations in mRNA translation and proteostasis account for the transcriptional stress signatures that are commonly triggered by loss of anticodon modifications in different tRNAs.

Fungal Kti12 proteins display unusual linker regions and unique ATPase p-loops

Kti12 (Kluyveromyces lactis toxin insensitive 12) is an evolutionary highly conserved ATPase, crucial for the tRNA-modification activity of the eukaryotic Elongator complex. The protein consists of an N-terminal ATPase and a C-terminal tRNA-binding domain, which are connected by a flexible linker. The precise role of the linker region and its involvement in the communication between the two domains and their activities remain elusive. Here, we analyzed all available Kti12 protein sequences and report the discovery of a subset of Kti12 proteins with abnormally long linker regions. These Kti12 proteins are characterized by a co-occurring lysine to leucine substitution in their Walker A motif, previously thought to be invariable. We show that the K14L substitution lowers the affinity to ATP, but does not affect the catalytic activity of Kti12 at high ATP concentrations. We compare the activity of mutated variants of Kti12 in vitro with complementation assays in vivo in yeast. Ultimately, we compared Kti12 to other known p-loop ATPase family members known to carry a similar deviant Walker A motif. Our data establish Kti12 of Eurotiomycetes as an example of eukaryotic ATPase harboring a significantly deviating but still functional Walker A motif.