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This review highlights the emerging biochemistry and biology of the metazoan tRNA-ligase complex (tRNA-LC). We begin with an overview of the cleavage-ligation pathways dependent on the tRNA-LC, epitomised by the essential process of pre-tRNA splicing, but also critical for the unfolded protein response and the struggle against transposons. The catalytic core of the tRNA-LC is the non-conventional RNA ligase RTCB, found in all domains of life. We start from the universal principles of its catalytic cycle, entailing GTP-dependent ligation of RNA molecules with 2’,3’-cyclic phosphate and 5’-hydroxyl ends. We then focus on the new findings that govern trafficking, protection, regulation and degradation of the tRNA-LC. These new modalities arise from an expanded set of subunits that to a large extent specifically associate with RTCB in Eukarya . We present how co-purification and co-evolutionary analyses converged to guide sequential discoveries of these proteins and illuminated their biochemical roles. We detail how the choice of paralogue of the auxiliary subunit FAM98 and the recruitment of Ashwin determine RTCB localisation, discriminating between its cytoplasmic and nuclear roles. We then pay particular attention to discoveries emerging from the latest structural works. These include the Archease-mediated mechanism of metazoan RTCB guanylylation, and PYROXD1-mediated protection of the tRNA-LC in the “resting state” from oxidative inactivation. We illustrate how metal ions play critical roles in both of these processes, alongside their direct roles in catalysing RNA ligation and, potentially, in the degradation of the tRNA-LC through a novel and still mysterious mechanism. We place particular emphasis on the subunits with contested functions, and on the interplay between their role in the tRNA-LC and their other cellular tasks. Along the way, we highlight the links between the subunits of the tRNA-LC, in particular their pathogenic variants or their mis-localisation, and human diseases including neurodegeneration and myopathies. The increasing understanding of the tRNA-LC, including its expanded set of subunits, illustrates how branching out from a single enzyme uncovered new biology ranging from the previously unknown congenital myopathies to novel proteasome targeting pathways.

Background/Aims: Cholangiocarcinoma (CCA) is a primary malignant neoplasm with an extremely poor prognosis. While combined chemoradiotherapy has been demonstrated to delay CCA progression to a certain extent, the absence of specific molecular biomarkers or targets significantly hinders the diagnosis and treatment of CCA. Methods: Through cross-analysis of proteomics and ADMA modificationomics, we identified DDX1 overexpressed in CCA with elevated R602-ADMA modifications. HPLC-MS/MS identified PRMT1 as the methyltransferase and USP10 as the deubiquitinating enzyme for DDX1. Immunofluorescence and nuclear-cytoplasmic partitioning experiments confirmed DDX1’s nuclear localization. GO and KEGG analyses clarify the biological functions of DDX1 in response to hypoxia. RNA-seq transcriptomics analyzed key pathways influenced by DDX1. A hydrodynamic in situ CCA mouse model was established to validate the chemopreventive effects of the PRMT1-specific inhibitor GSK715 on CCA development. Results: DDX1 promotes CCA progression both in vivo and in vitro and can be inhibited by GSK715. Mechanistically, PRMT1 mediates ADMA modification at position R602 of DDX1. This modification promotes DDX1 nuclear localization by recruiting USP10 to deubiquitinate DDX1, while simultaneously inhibiting PRMT1 degradation. DDX1 promotes the transcription of PRMT1 and USP10 by binding to the mRNA 3’UTR region, establishing a positive feedback regulatory pathway. This mechanism promotes the occurrence and development of CCA and can serve as a target for the inhibitor GSK715 to suppress CCA progression. Conclusions: Our study identified DDX1-R602-ADMA modification as a novel ADMA modification in CCA. It further confirmed its pivotal role in CCA progression. Targeting the USP10-PRMT1-DDX1 axis may represent a significant therapeutic approach for CCA. (Clin Mol Hepatol 2026;32:843-865)

The nucleocapsid protein Np of SARS-CoV-2 is involved in the replication, transcription, and packaging of the viral genome, but it also plays a role in the modulation of the host cell innate immunity and inflammation response. Ectopic expression of Np alone was able to induce significant changes in the proteome of human cells. The cellular RNA helicase DDX1 was among the proteins whose levels were increased by Np expression. DDX1 and its related helicase DDX3X were found to physically interact with Np and to increase 2- to 4-fold its affinity for double-stranded RNA in a helicase-independent manner. Conversely, Np inhibited the RNA helicase activity of both proteins. These functional interactions among Np and DDX1 and DDX3X highlight novel possible roles played by these host RNA helicases in the viral life cycle.

Transmissible gastroenteritis virus (TGEV), an enteropathogenic coronavirus (CoV) of porcine, causes lethal watery diarrhea and severe dehydration in piglets and leads to severe economic losses in the swine industry. Unlike most CoVs that antagonize type I interferon (IFN) production, previous studies showed that TGEV infection induces IFN-I production both and . However, the underlying mechanism(s) remain largely unknown. In this study, we found that TGEV infection significantly facilitated IFN-β production as well as activation of the transcription factors IFN regulatory factor 3 (IRF3) and nuclear factor-kappaB (NF-κB) in porcine kidney (PK-15) cells. Screening of TGEV-encoded proteins demonstrated that non-structural protein 14 (nsp14) was the most potent IFN-β inducer and induced IFN-β production mainly by activating NF-κB but not IRF3. Further analysis showed that nsp14 interacted with DDX1, a member of the DExD/H helicase family. Knockdown of DDX1 by specific small interfering RNA (siRNA) significantly decreased nsp14-induced IFN-β production and NF-κB activation. Furthermore, TGEV-induced IFN-β production and IFN-stimulated gene (ISG) expression were decreased in cells transfected with DDX1-specific siRNA, indicating the vital role of DDX1 to TGEV-induced IFN-β responses. In summary, our data revealed a potential coactivator role of host RNA helicase DDX1 to the induction of IFN-β response initiated by TGEV and demonstrated that nsp14 is an important IFN inducer among the TGEV-encoded proteins.

Chickens are the natural host of Newcastle disease virus (NDV) and avian influenza virus (AIV). The discovery that the RIG-I gene, the primary RNA virus pattern recognition receptor (PRR) in mammals, is naturally absent in chickens has directed attention to studies of chicken RNA PRRs and their functions in antiviral immune responses. Here, we identified Asp-Glu-Ala-Asp (DEAD)-box helicase 1 (DDX1) as an essential RNA virus PRR in chickens and investigated its functions in anti-RNA viral infections. The chDDX1 gene was cloned, and cross-species sequence alignment and phylogenetic tree analyses revealed high conservation of DDX1 among vertebrates. A quantitative RT-PCR showed that chDDX1 mRNA are widely expressed in different tissues in healthy chickens. In addition, chDDX1 was significantly upregulated after infection with AIV, NDV, or GFP-expressing vesicular stomatitis virus (VSV-GFP). Overexpression of chDDX1 in DF-1 cells induced the expression of IFN-β, IFN-stimulated genes (ISGs), and proinflammatory cytokines; it also inhibited NDV and VSV replications. The knockdown of chDDX1 increased the viral yield of NDV and VSV and decreased the production of IFN-β, which was induced by RNA analog polyinosinic-polycytidylic acid (poly[I:C]), by AIV, and by NDV. We used a chicken IRF7 (chIRF7) knockout DF-1 cell line in a series of experiments to demonstrate that chDDX1 activates IFN signaling via the chIRF7 pathway. Finally, an in-vitro pulldown assay showed a strong and direct interaction between poly(I:C) and the chDDX1 protein, indicating that chDDX1 may act as an RNA PRR during IFN activation. In brief, our results suggest that chDDX1 is an important mediator of IFN-β and is involved in RNA- and RNA virus-mediated chDDX1-IRF7-IFN-β signaling pathways.

The discovery of discontiguous tRNA genes triggered studies dissecting the process of tRNA splicing. As a result, we have gained detailed mechanistic knowledge on enzymatic removal of tRNA introns catalyzed by endonuclease and ligase proteins. In addition to the elucidation of tRNA processing, these studies facilitated the discovery of additional functions of RNA ligases such as RNA repair and non-conventional mRNA splicing events. Recently, the identification of a new type of RNA ligases in bacteria, archaea, and humans closed a long-standing gap in the field of tRNA processing. This review summarizes past and recent findings in the field of tRNA splicing with a focus on RNA ligation as it preferentially occurs in archaea and humans. In addition to providing an integrated view of the types and phyletic distribution of RNA ligase proteins known to date, this survey also aims at highlighting known and potential accessory biological functions of RNA ligases.

Hepatocellular carcinoma (HCC) is one of the leading lethal malignant tumors worldwide. DEAD-box (DDX) family helicases are implicated in numerous human cancers. However, the role of DDX1 in HCC has not yet been fully elucidated. We downloaded gene expression data and clinical information data of HCC from The Cancer Genome Atlas and International Cancer Genome Consortium (ICGC) database and conducted subsequent analyses using the R package and online portal. The results revealed that HCC tissues had higher DDX1 expression compared with either paired or unpaired normal tissues. The increased DDX1 expression was closely related to the advanced pathological grade and histologic grade of HCC. Further analysis suggested that patients with high DDX1 expression contributed to poor prognosis The Cox regression analysis revealed that the expression level of DDX1 was an independent prognostic factor for HCC. In addition, an ICGC cohort was used for external validation. The cBio-Portal, MethSurv, and UALCAN database were used for evaluating the genomic mechanism. Moreover, the Tumor Immune Estimation Resource dataset and QUANTISEQ algorithm revealed that DDX1 expression positively correlates with immune infiltrating cells. We also identified the DDX1-related differentially expressed genes (DEGs) and explored their biological functions by GO, KEGG, and GSEA analyses, which indicated that DDX1 may regulate the progression of HCC. In general, increased DDX1 expression predicts a poor prognosis and drives the progression of HCC.

Background/Aims: Cholangiocarcinoma (CCA) is a primary malignant neoplasm with an extremely poor prognosis. While combined chemoradiotherapy has been demonstrated to delay CCA progression to a certain extent, the absence of specific molecular biomarkers or targets significantly hinders the diagnosis and treatment of CCA.Methods: Through cross-analysis of proteomics and ADMA modificationomics, we identified DDX1 overexpressed in CCA with elevated R602-ADMA modifications. HPLC-MS/MS identified PRMT1 as the methyltransferase and USP10 as the deubiquitinating enzyme for DDX1. Immunofluorescence and nuclear-cytoplasmic partitioning experiments confirmed DDX1’s nuclear localization. GO and KEGG analyses clarify the biological functions of DDX1 in response to hypoxia. RNA-seq transcriptomics analyzed key pathways influenced by DDX1. A hydrodynamic in situ CCA mouse model was established to validate the chemopreventive effects of the PRMT1-specific inhibitor GSK715 on CCA development.Results: DDX1 promotes CCA progression both in vivo and in vitro and can be inhibited by GSK715. Mechanistically, PRMT1 mediates ADMA modification at position R602 of DDX1. This modification promotes DDX1 nuclear localization by recruiting USP10 to deubiquitinate DDX1, while simultaneously inhibiting PRMT1 degradation. DDX1 promotes the transcription of PRMT1 and USP10 by binding to the mRNA 3’UTR region, establishing a positive feedback regulatory pathway. This mechanism promotes the occurrence and development of CCA and can serve as a target for the inhibitor GSK715 to suppress CCA progression.Conclusions: Our study identified DDX1-R602-ADMA modification as a novel ADMA modification in CCA. It further confirmed its pivotal role in CCA progression. Targeting the USP10-PRMT1-DDX1 axis may represent a significant therapeutic approach for CCA.

Renal interstitial fibrosis (RIF) is a classic pathophysiological process of chronic kidney disease (CKD). However, the mechanisms underlying RIF remain unclear. The present study found that a novel circular RNA, cirInpp5b, might be involved in RIF by high-throughput sequencing. Subsequent experiments revealed that circInpp5b was reduced in UUO mouse kidney tissues and TGF-β1-treated proximal tubular cells. The overexpression of circInpp5b inhibited RIF in UUO mice and prevented extracellular matrix (ECM) deposition in TGF-β1-treated proximal tubular cells. Furthermore, overexpression of circInpp5b down-regulated the protein level of DDX1. Mechanistically, circInpp5b bound to the DDX1 protein and promoted its lysosomal degradation. Collectively, the findings of our study demonstrate that circInpp5b ameliorates RIF by binding to the DDX1 protein and promoting its lysosomal degradation.

Background Previously we described a transdominant negative mutant of the HIV-1 Tat protein, termed Nullbasic, that downregulated the steady state levels of unspliced and singly spliced viral mRNA, an activity caused by inhibition of HIV-1 Rev activity. Nullbasic also altered the subcellular localizations of Rev and other cellular proteins, including CRM1, B23 and C23 in a Rev-dependent manner, suggesting that Nullbasic may disrupt Rev function and trafficking by intervening with an unidentified component of the Rev nucleocytoplasmic transport complex. Results To seek a possible mechanism that could explain how Nullbasic inhibits Rev activity, we used a proteomics approach to identify host cellular proteins that interact with Nullbasic. Forty-six Nullbasic-binding proteins were identified by mass spectrometry including the DEAD-box RNA helicase, DDX1. To determine the effect of DDX1 on Nullbasic-mediated Rev activity, we performed cell-based immunoprecipitation assays, Rev reporter assays and bio-layer interferometry (BLI) assays. Interaction between DDX1 and Nullbasic was observed by co-immunoprecipitation of Nullbasic with endogenous DDX1 from cell lysates. BLI assays showed a direct interaction between Nullbasic and DDX1. Nullbasic affected DDX1 subcellular distribution in a Rev-independent manner. Interestingly overexpression of DDX1 in cells not only restored Rev-dependent mRNA export and gene expression in a Rev reporter assay but also partly reversed Nullbasic-induced Rev subcellular mislocalization. Moreover, HIV-1 wild type Tat co-immunoprecipitated with DDX1 and overexpression of Tat could rescue the unspliced viral mRNA levels inhibited by Nullbasic in HIV-1 expressing cells. Conclusions Nullbasic was used to further define the complex mechanisms involved in the Rev-dependent nuclear export of the 9 kb and 4 kb viral RNAs. All together, these data indicate that DDX1 can be sequestered by Nullbasic leading to destabilization of the Rev nucleocytoplasmic transport complex and decreased levels of Rev-dependent viral transcripts. The outcomes support a role for DDX1 in maintenance of a Rev nuclear complex that transports viral RRE-containing mRNA to the cytoplasm. To our knowledge Nullbasic is the first anti-HIV protein that specifically targets the cellular protein DDX1 to block Rev’s activity. Furthermore, our research raises the possibility that wild type Tat may play a previously unrecognized but very important role in Rev function.