Akiko Noma

Biosynthesis of wybutosine, a hyper‐modified nucleoside in eukaryotic phenylalanine tRNA

Wybutosine (yW) is a tricyclic nucleoside with a large side chain found at the 3′‐position adjacent to the anticodon of eukaryotic phenylalanine tRNA. yW supports codon recognition by stabilizing codon–anticodon interactions during decoding on the ribosome. To identify genes responsible for yW synthesis from uncharacterized genes of Saccharomyces cerevisiae, we employed a systematic reverse genetic approach combined with mass spectrometry (‘ribonucleome analysis’). Four genes YPL207w, YML005w, YGL050w and YOL141w (named TYW1, TYW2, TYW3 and TYW4, respectively) were essential for yW synthesis. Mass spectrometric analysis of each modification intermediate of yW revealed its sequential biosynthetic pathway. TYW1 is an iron–sulfur (Fe–S) cluster protein responsible for the tricyclic formation. Multistep enzymatic formation of yW from yW‐187 could be reconstituted in vitro using recombinant TYW2, TYW3 and TYW4 with S‐adenosylmethionine, suggesting that yW synthesis might proceed through sequential reactions in a complex formed by multiple components assembled with the precursor tRNA. This hypothesis is also supported by the fact that plant ortholog is a large fusion protein consisting of TYW2 and TYW3 with the C‐terminal domain of TYW4.

Mechanistic characterization of the sulfur-relay system for eukaryotic 2-thiouridine biogenesis at tRNA wobble positions

The wobble modification in tRNAs, 5-methoxycarbonylmethyl-2-thiouridine (mcm5s2U), is required for the proper decoding of NNR codons in eukaryotes. The 2-thio group confers conformational rigidity of mcm5s2U by largely fixing the C3′-endo ribose puckering, ensuring stable and accurate codon–anticodon pairing. We have identified five genes in Saccharomyces cerevisiae, YIL008w (URM1), YHR111w (UBA4), YOR251c (TUM1), YNL119w (NCS2) and YGL211w (NCS6), that are required for 2-thiolation of mcm5s2U. An in vitro sulfur transfer experiment revealed that Tum1p stimulated the cysteine desulfurase of Nfs1p, and accepted persulfide sulfurs from Nfs1p. URM1 is a ubiquitin-related modifier, and UBA4 is an E1-like enzyme involved in protein urmylation. The carboxy-terminus of Urm1p was activated as an acyl-adenylate (-COAMP), then thiocarboxylated (-COSH) by Uba4p. The activated thiocarboxylate can be utilized in the subsequent reactions for 2-thiouridine formation, mediated by Ncs2p/Ncs6p. We could successfully reconstitute the 2-thiouridine formation in vitro using recombinant proteins. This study revealed that 2-thiouridine formation shares a pathway and chemical reactions with protein urmylation. The sulfur-flow of eukaryotic 2-thiouridine formation is distinct mechanism from the bacterial sulfur-relay system which is based on the persulfide chemistry.

Expanding Role of the Jumonji C Domain as an RNA Hydroxylase

JmjC (Jumonji C) domain-containing proteins are known to be an extensive family of Fe(II)/2-oxoglutarate-dependent oxygenases involved in epigenetic regulation of gene expression by catalyzing oxidative demethylation of methylated histones. We report here that a human JmjC protein named Tyw5p (TYW5) unexpectedly acts in the biosynthesis of a hypermodified nucleoside, hydroxywybutosine, in tRNAPhe by catalyzing hydroxylation. The finding provides an insight into the expanding role of JmjC protein as an RNA hydroxylase.

Actin-binding protein ABP140 is a methyltransferase for 3-methylcytidine at position 32 of tRNAs in Saccharomyces cerevisiae

Transfer RNAs contain various modified nucleotides that are introduced enzymatically at the post-transcriptional level. In Saccharomyces cerevisiae, 3-methylcytidine (m³C) is found at position 32 of the tRNAs for Thr and Ser. We used a systematic reverse genetic approach combined with mass spectrometry (ribonucleome analysis), and identified the actin-binding protein ABP140 as the protein responsible for m³C formation in both tRNA(Thr1) and tRNA(Ser1). ABP140 consists of an N-terminal actin-binding sequence and a C-terminal S-adenosylmethionine (Ado-Met) binding motif. Deletion of the actin-binding sequence in ABP140 did not affect m³C formation, indicating that subcellular localization of ABP140 to actin filaments is not involved in tRNA modification. m³C formation in tRNA(Thr1) could be reconstituted using recombinant Abp140p in the presence of Ado-Met, whereas m³C did not form in tRNA(Ser1) in vitro, indicating the absence of a factor(s) required for tRNA(Ser1) m³C formation. Thus, ABP140 has been designated TRM140 according to the preferred nomenclature. In addition, we observed a specific reduction of m³C formation in HeLa cells by siRNA-mediated knock down of the human ortholog of TRM140.

Crystal structure of a novel JmjC-domain-containing protein, TYW5, involved in tRNA modification

Wybutosine (yW) is a hypermodified nucleoside found in position 37 of tRNAPhe, and is essential for correct phenylalanine codon translation. yW derivatives widely exist in eukaryotes and archaea, and their chemical structures have many species-specific variations. Among them, its hydroxylated derivative, hydroxywybutosine (OHyW), is found in eukaryotes including human, but the modification mechanism remains unknown. Recently, we identified a novel Jumonji C (JmjC)-domain-containing protein, TYW5 (tRNA yW-synthesizing enzyme 5), which forms the OHyW nucleoside by carbon hydroxylation, using Fe(II) ion and 2-oxoglutarate (2-OG) as cofactors. In this work, we present the crystal structures of human TYW5 (hTYW5) in the free and complex forms with 2-OG and Ni(II) ion at 2.5 and 2.8 Å resolutions, respectively. The structure revealed that the catalytic domain consists of a β-jellyroll fold, a hallmark of the JmjC domains and other Fe(II)/2-OG oxygenases. hTYW5 forms a homodimer through C-terminal helix bundle formation, thereby presenting a large, positively charged patch involved in tRNA binding. A comparison with the structures of other JmjC-domain-containing proteins suggested a mechanism for substrate nucleotide recognition. Functional analyses of structure-based mutants revealed the essential Arg residues participating in tRNA recognition by TYW5. These findings extend the repertoire of the tRNA modification enzyme into the Fe(II)/2-OG oxygenase superfamily.

Structural basis of AdoMet-dependent aminocarboxypropyl transfer reaction catalyzed by tRNA-wybutosine synthesizing enzyme, TYW2

S-adenosylmethionine (AdoMet) is a methyl donor used by a wide variety of methyltransferases, and it is also used as the source of an α-amino-α-carboxypropyl (“acp”) group by several enzymes. tRNA-yW synthesizing enzyme-2 (TYW2) is involved in the biogenesis of a hypermodified nucleotide, wybutosine (yW), and it catalyzes the transfer of the “acp” group from AdoMet to the C7 position of the imG-14 base, a yW precursor. This modified nucleoside yW is exclusively located at position 37 of eukaryotic tRNAPhe, and it ensures the anticodon-codon pairing on the ribosomal decoding site. Although this “acp” group has a significant role in preventing decoding frame shifts, the mechanism of the “acp” group transfer by TYW2 remains unresolved. Here we report the crystal structures and functional analyses of two archaeal homologs of TYW2 from Pyrococcus horikoshii and Methanococcus jannaschii. The in vitro mass spectrometric and radioisotope-labeling analyses confirmed that these archaeal TYW2 homologues have the same activity as yeast TYW2. The crystal structures verified that the archaeal TYW2 contains a canonical class-I methyltransferase (MTase) fold. However, their AdoMet-bound structures revealed distinctive AdoMet-binding modes, in which the “acp” group, instead of the methyl group, of AdoMet is directed to the substrate binding pocket. Our findings, which were confirmed by extensive mutagenesis studies, explain why TYW2 transfers the “acp” group, and not the methyl group, from AdoMet to the nucleobase.

Structural basis of tRNA modification with CO2 fixation and methylation by wybutosine synthesizing enzyme TYW4

Wybutosine (yW), one of the most complicated modified nucleosides, is found in the anticodon loop of eukaryotic phenylalanine tRNA. This hypermodified nucleoside ensures correct codon recognition by stabilizing codon-anticodon pairings during the decoding process in the ribosome. TYW4 is an S-adenosylmethionine (SAM)-dependent enzyme that catalyzes the final step of yW biosynthesis, methylation and methoxycarbonylation. However, the structural basis for the catalytic mechanism by TYW4, and especially that for the methoxycarbonylation, have remained elusive. Here we report the apo and cofactor-bound crystal structures of yeast TYW4. The structures revealed that the C-terminal domain folds into a β-propeller structure, forming part of the binding pocket for the target nucleoside. A comparison of the apo, SAM-bound, and S-adenosylhomocysteine-bound structures of TYW4 revealed a drastic structural change upon cofactor binding, which may sequester solvent from the catalytic site during the reaction and facilitate product release after the reaction. In conjunction with the functional analysis, our results suggest that TYW4 catalyzes both methylation and methoxycarbonylation at a single catalytic site, and in the latter reaction, the methoxycarbonyl group is formed through the fixation of carbon dioxide.

Wobble Inosine tRNA Modification Is Essential to Cell Cycle Progression in G1/S and G2/M Transitions in Fission Yeast

Inosine (I) at position 34 (wobble position) of tRNA is formed by the hydrolytic deamination of a genomically encoded adenosine (A). The enzyme catalyzing this reaction, termed tRNA A:34 deaminase, is the heterodimeric Tad2p/ADAT2·Tad3p/ADAT3 complex in eukaryotes. In budding yeast, deletion of each subunit is lethal, indicating that the wobble inosine tRNA modification is essential for viability; however, most of its physiological roles remain unknown. To identify novel cell cycle mutants in fission yeast, we isolated the tad3-1 mutant that is allelic to the tad3+ gene encoding a homolog of budding yeast Tad3p. Interestingly, the tad3-1 mutant cells principally exhibited cell cycle-specific phenotype, namely temperature-sensitive and irreversible cell cycle arrest both in G1 and G2. Further analyses revealed that in the tad3-1 mutant cells, the S257N mutation that occurred in the catalytically inactive Tad3 subunit affected its association with catalytically active Tad2 subunit, leading to an impairment in the A to I conversion at position 34 of tRNA. In tad3-1 mutant cells, the overexpression of the tad3+ gene completely suppressed the decreased tRNA inosine content. Notably, the overexpression of the tad2+ gene partially suppressed the temperature-sensitive phenotype and the decreased tRNA inosine content, indicating that the tad3-1 mutant phenotype is because of the insufficient I34 formation of tRNA. These results suggest that the wobble inosine tRNA modification is essential for cell cycle progression in the G1/S and G2/M transitions in fission yeast.

A single acetylation of 18 S rRNA is essential for biogenesis of the small ribosomal subunit in Saccharomyces cerevisiae.

Background: Post-transcriptional modifications of rRNAs play important roles in biogenesis and function of ribosome. Results: Identification of an essential RNA acetyltransferase Rra1p responsible for forming N4-acetylcytidine at position 1773 in 18 S rRNA. Conclusion: Rra1p and ac4C1773 are required for pre-18 S rRNA processing. Significance: Rra1p modulates 40 S subunit biogenesis through a single acetylation of 18 S rRNA by sensing nuclear acetyl-CoA concentration. Biogenesis of eukaryotic ribosome is a complex event involving a number of non-ribosomal factors. During assembly of the ribosome, rRNAs are post-transcriptionally modified by 2′-O-methylation, pseudouridylation, and several base-specific modifications, which are collectively involved in fine-tuning translational fidelity and/or modulating ribosome assembly. By mass-spectrometric analysis, we demonstrated that N4-acetylcytidine (ac4C) is present at position 1773 in the 18 S rRNA of Saccharomyces cerevisiae. In addition, we found an essential gene, KRE33 (human homolog, NAT10), that we renamed RRA1 (ribosomal RNA cytidine acetyltransferase 1) encoding an RNA acetyltransferase responsible for ac4C1773 formation. Using recombinant Rra1p, we could successfully reconstitute ac4C1773 in a model rRNA fragment in the presence of both acetyl-CoA and ATP as substrates. Upon depletion of Rra1p, the 23 S precursor of 18 S rRNA was accumulated significantly, which resulted in complete loss of 18 S rRNA and small ribosomal subunit (40 S), suggesting that ac4C1773 formation catalyzed by Rra1p plays a critical role in processing of the 23 S precursor to yield 18 S rRNA. When nuclear acetyl-CoA was depleted by inactivation of acetyl-CoA synthetase 2 (ACS2), we observed temporal accumulation of the 23 S precursor, indicating that Rra1p modulates biogenesis of 40 S subunit by sensing nuclear acetyl-CoA concentration.

Structure-Function Analysis of Human TYW2 Enzyme Required for the Biosynthesis of a Highly Modified Wybutosine (yW) Base in Phenylalanine-tRNA

Posttranscriptional modifications are critical for structure and function of tRNAs. Wybutosine (yW) and its derivatives are hyper-modified guanosines found at the position 37 of eukaryotic and archaeal tRNAPhe. TYW2 is an enzyme that catalyzes α-amino-α-carboxypropyl transfer activity at the third step of yW biogenesis. Using complementation of a ΔTYW2 strain, we demonstrate here that human TYW2 (hTYW2) is active in yeast and can synthesize the yW of yeast tRNAPhe. Structure-guided analysis identified several conserved residues in hTYW2 that interact with S-adenosyl-methionine (AdoMet), and mutation studies revealed that K225 and E265 are critical residues for the enzymatic activity. We previously reported that the human TYW2 is overexpressed in breast cancer. However, no difference in the tRNAPhe modification status was observed in either normal mouse tissue or a mouse tumor model that overexpresses Tyw2, indicating that hTYW2 may have a role in tumorigenesis unrelated to yW biogenesis.