Biology subjects

Bass, B. L.

Publications and source records attributed to Bass, B. L..

8 recordsLinked to original sources

bioRxiv2026

C. elegans Dicer stacks with the RIG-I-like receptor DRH-1 to cleave dsRNA

In prior studies we showed that the nematode ancestor of Dicer's helicase domain had minimal ATP hydrolysis, translocation and dsRNA binding activity, and in extant C. elegans the RIG-I like receptor (RLR) DRH-1, and the dsRNA binding protein RDE-4, were co-opted to provide these activities. Here we report cleavage-competent cryo-EM structures of the antiviral complex (AVC; DCR-1{middle dot}DRH-1{middle dot}RDE-4), in the absence (3.2[A]) and presence (3.0[A] of ATP. A key feature of both structures is stacking of DCR-1's helicase with the helicases of two DRH-1 molecules, reminiscent of oligomerization of mammalian RLRs. dsRNA threads through all three helicases with a widened major groove at helicase-helicase interfaces and where the conserved Hel2 loop inserts into the major groove. The presence of nucleotide redistributed helicase-helicase and helicase-RNA contacts in the AVC, as for MDA5, albeit specific interactions and remodeling differed. MDA5 and RIG-I RLRs contain an unstructured linker between their CARDs and helicase domain, but the analogous linker in DRH-1 has a short-structured region positioned to interact with DCR-1. These findings provide a structural framework for understanding how DCR-1, DRH-1, and RDE-4 cooperate to cleave viral dsRNA and elucidate unique features required for antiviral defense in different animals.

immunology↗

bioRxiv2025

Reconstitution of antiviral Dicer activity in vitro reveals distinct contributions of RDE-4 dsRNA-binding motifs

In C. elegans, antiviral RNA interference (RNAi) relies on the coordinated activity of Dicer (DCR-1), the helicase DRH-1, and the double-stranded RNA (dsRNA)-binding protein, RDE-4, yet the domain-specific contributions of RDE-4 remain unclear. Here, we reconstituted the antiviral complex from independently purified DCR-1*DRH-1 and RDE-4 to define how RDE-4 stabilizes and activates the complex. Addition of recombinant RDE-4 restored ATP hydrolysis and dsRNA cleavage to levels previously observed with the pre-assembled complex, and time-course assays revealed that RDE-4 is essential for maintaining DCR-1*DRH-1 activity. Mutational analysis of RDE-4 revealed that both dsRBM2 and dsRBM3, but not dsRBM1, are required for reconstituting ATP hydrolysis and cleavage. Disruption of the KKxAK motif in dsRBM2 drastically reduced dsRNA affinity and abolished catalytic rescue despite preserving robust binding to DCR-1*DRH-1. Mass photometry and pulldown assays revealed that RDE-4 primarily forms DCR-1 containing complexes, predominantly through interaction with dsRBM3, with no evidence for stable interaction with DRH-1 alone. Functionally, RDE-4 enhanced DRH-1-driven ATP hydrolysis on both 52 and 106 base-pair dsRNAs, but cleavage efficiency showed strong length dependence, implicating dsRNA substrate length as an effector in this system. Our findings establish RDE-4 as an important stabilizer of the antiviral complex and reveal distinct roles for dsRBM2 and dsRBM3 in ATP hydrolysis and dsRNA cleavage. Furthermore, our results suggest that substrate length modulates RDE-4 function, not just alone, but within the antiviral complex. These insights refine our understanding of antiviral RNAi in C. elegans and uncover regulatory mechanisms within the antiviral complex.

biochemistry↗

bioRxiv2025

Biochemical and structural basis of Dicer helicase function unveiled by resurrecting ancient proteins

A fully functional Dicer helicase, present in the modern arthropod, uses energy generated during ATP hydrolysis to power translocation on bound dsRNA, enabling the processive dsRNA cleavage required for efficient antiviral defense. However, modern Dicer orthologs exhibit divergent helicase functions that affect their ability to contribute to antiviral defense, and moreover, mechanisms that couple ATP hydrolysis to Dicer helicase movement on dsRNA remain enigmatic. Here, we used biochemical and structural analyses of ancestrally reconstructed Dicer helicases to map evolution of dsRNA binding affinity, ATP hydrolysis and translocation. We found that loss of affinity for dsRNA occurred early in Dicer evolution, coinciding with a decline in translocation activity, despite preservation of ATP hydrolysis activity, exemplified by the ancient deuterostome Dicer. Ancestral nematode Dicer also exhibited significant decline in ATP hydrolysis and translocation, but studies of antiviral activities in the modern nematode C. elegans indicate Dicer retained a role in antiviral defense by recruiting a second helicase. Cryo-EM analyses of an ancient metazoan Dicer allowed capture of multiple helicase states revealing the mechanism that connects each step of ATP hydrolysis to unidirectional movement along dsRNA. Overall, our study rationalizes the diversity in modern Dicer helicases by connecting ancestral functions to observations in extant enzymes. Significance StatementAmong invertebrates, the contribution of Dicers helicase to recognition and elimination of viral double-stranded RNA varies from phylum to phylum. At the extreme end of the spectrum, vertebrate Dicers show no helicase activity. On the other end, an arthropod ortholog uses helicase translocation to efficiently move double-stranded RNA into Dicers cleavage site. The biochemical and structural basis of Dicers helicase function, as well as the evolutionary events that contribute to a divergence in function, have remained unknown. This study shows how ancient Dicer helicase tightly binds double-stranded RNA and couples ATP hydrolysis to movement along this substrate. In addition, the data reveal how components of this intricate system declined along different clades of animal evolution.

biochemistry↗

bioRxiv2024

Activation of PKR by a short-hairpin RNA

Recognition of viral infection often relies on the detection of double-stranded RNA (dsRNA), a process that is conserved in many different organisms. In mammals, proteins such as MDA5, RIG-I, OAS, and PKR detect viral dsRNA, but struggle to differentiate between viral and endogenous dsRNA. This study investigates an shRNA targeting DDX54s potential to activate PKR, a key player in the immune response to dsRNA. Knockdown of DDX54 by a specific shRNA induced robust PKR activation in human cells, even when DDX54 is overexpressed, suggesting an off-target mechanism. Activation of PKR by the shRNA was enhanced by knockdown of ADAR1, a dsRNA binding protein that suppresses PKR activation, indicating a dsRNA-mediated mechanism. In vitro assays confirmed direct PKR activation by the shRNA. These findings emphasize the need for rigorous controls and alternative methods to validate gene function and minimize unintended immune pathway activation.

molecular biology↗

bioRxiv2023

C. elegans Dicer acts with the RIG-I-like helicase DRH-1 and RDE-4 to cleave dsRNA

Abstract/SummaryInvertebrates use the endoribonuclease Dicer to cleave viral dsRNA during antiviral defense, while vertebrates use RIG-I-like Receptors (RLRs), which bind viral dsRNA to trigger an interferon response. While some invertebrate Dicers act alone during antiviral defense, C. elegans Dicer acts in a complex with a dsRNA binding protein called RDE-4, and an RLR ortholog called DRH-1. We used biochemical and structural techniques to provide mechanistic insight into how these proteins function together. We found RDE-4 is important for ATP-independent and ATP-dependent cleavage reactions, while helicase domains of both DCR-1 and DRH-1 contribute to ATP-dependent cleavage. DRH-1 plays the dominant role in ATP hydrolysis, and like mammalian RLRs, has an N-terminal domain that functions in autoinhibition. A cryo-EM structure indicates DRH-1 interacts with DCR-1s helicase domain, suggesting this interaction relieves autoinhibition. Our study unravels the mechanistic basis of the collaboration between two helicases from typically distinct innate immune defense pathways.

biochemistry↗

bioRxiv2022

Ancestral protein reconstruction reveals evolutionary events governing variation in Dicer helicase function

Antiviral defense in ecdysozoan invertebrates requires Dicer with a helicase domain capable of ATP hydrolysis. But despite well-conserved ATPase motifs, human Dicer is incapable of ATP hydrolysis, consistent with a muted role in antiviral defense. To investigate this enigma, we used ancestral protein reconstruction to resurrect Dicers helicase in animals and trace the evolutionary trajectory of ATP hydrolysis. Biochemical assays indicated ancient Dicer possessed ATPase function, that like extant invertebrate Dicers, is stimulated by dsRNA. Analyses revealed that dsRNA stimulates ATPase activity by increasing ATP affinity, reflected in Michaelis constants. Deuterostome Dicer-1 ancestor, while exhibiting lower dsRNA affinity, retained ATPase activity; importantly, ATPase activity was undetectable in the vertebrate Dicer-1 ancestor, which had even lower dsRNA affinity. Reverting residues in the ATP hydrolysis pocket was insufficient to rescue hydrolysis, but including additional substitutions distant from the ATPase pocket rescued vertebrate Dicer-1s ATPase function. Our work suggests Dicer lost ATPase function in the vertebrate ancestor due to loss of ATP affinity, involving motifs distant from the active site, important for coupling dsRNA binding to the active conformation. RLRs important for interferon signaling, and their competition with Dicer for viral dsRNAs, possibly provided incentive to jettison an active helicase in vertebrate Dicer.

biochemistry↗

bioRxiv2020

A real-time, transient kinetic study of Drosophila melanogaster Dicer-2 elucidates mechanism of termini-dependent cleavage of dsRNA.

Drosophila melanogaster Dicer-2 (dmDcr-2) differentially processes dsRNA with blunt or 2 nucleotide 3-overhanging termini. We investigated the transient kinetic mechanism of these reactions using a rapid reaction stopped-flow technique and time-resolved fluorescence spectroscopy. We found that ATP binding to dmDcr-2s helicase domain impacts the kinetics of dsRNA binding and dissociation in a termini-dependent manner, emphasizing the termini-dependent discrimination of dsRNA on a biologically-relevant time-scale. ATP-hydrolysis mediates local unwinding of dsRNA, and directional translocation on unwound single-stranded RNA, which is concurrent with a slow rewinding prior to dsRNA cleavage. Time-resolved fluorescence anisotropy reveals a nucleotide-dependent change in conformational dynamics of the helicase and Platform*PAZ domains in the nanosecond timescale that is correlated with termini-dependent dsRNA cleavage. Our study delineates kinetic events and transient intermediates for a Dicer-catalyzed reaction, thus establishing a framework for understanding other Dicers and how accessory factors modulate the reaction.

biophysics↗

bioRxiv2019

DUX4-induced bidirectional HSATII satellite repeat transcripts form intranuclear double stranded RNA foci in human cell models of FSHD

The DUX4 transcription factor is normally expressed in the cleavage stage embryo and regulates genes involved in embryonic genome activation. Mis-expression of DUX4 in skeletal muscle, however, is toxic and causes facioscapulohumeral muscular dystrophy (FSHD). We recently showed DUX4-induced toxicity is due, in part, to the activation of the double-stranded RNA (dsRNA) response pathway and the accumulation of intranuclear dsRNA foci. Here, we determined the composition of DUX4-induced dsRNAs. We found that a subset of DUX4-induced dsRNAs originate from inverted Alu repeats embedded within the introns of DUX4-induced transcripts and from DUX4-induced dsRNA-forming intergenic transcripts enriched for endogenous retroviruses, Alu and LINE-1 elements. However, these repeat classes were also represented in dsRNAs from cells not expressing DUX4. In contrast, pericentric human satellite II (HSATII) repeats formed a class of dsRNA specific to the DUX4 expressing cells. Further investigation revealed that DUX4 can initiate the bidirectional transcription of normally heterochromatin-silenced HSATII repeats. DUX4 induced HSATII RNAs co-localized with DUX4-induced nuclear dsRNA foci and with intranuclear aggregation of EIF4A3 and ADAR1. Finally, gapmer-mediated knockdown of HSATII transcripts depleted DUX4-induced intranuclear ribonucleoprotein aggregates and decreased DUX4-induced cell death, suggesting that HSATII formed dsRNAs contribute to DUX4 toxicity.

molecular biology↗