An Evolutionarily Conserved Strategy for Ribosome Binding and Host Translation Inhibition by β-coronavirus Non-structural Protein 1

An important pathogenicity factor of SARS-CoV-2 and related coronaviruses is Non-structural protein 1 (Nsp1), which suppresses host gene expression and stunts antiviral signaling. SARS-CoV-2 Nsp1 binds the ribosome to inhibit translation through mRNA displacement and induces degradation of host mRNAs. Here we show that Nsp1-dependent host shutoff is conserved in diverse coronaviruses, but only Nsp1 from β-Coronaviruses (β-CoV) inhibits translation through ribosome binding. The C-terminal domain (CTD) of all β-CoV Nsp1s confers high-affinity ribosome binding despite low sequence conservation. Modeling of interactions of four Nsp1s with the ribosome identified the few absolutely conserved amino acids that, together with an overall conservation in surface charge, form the β-CoV Nsp1 ribosome-binding domain. Contrary to previous models, the Nsp1 ribosome-binding domain is an inefficient translation inhibitor. Instead, the Nsp1-CTD likely functions by recruiting Nsp1's N-terminal "effector" domain. Finally, we show that a cis-acting viral RNA element has co-evolved to fine-tune SARS-CoV-2 Nsp1 function, but does not provide similar protection against Nsp1 from related viruses. Together, our work provides new insight into the diversity and conservation of ribosome-dependent host-shutoff functions of Nsp1, knowledge that could aid future efforts in pharmacological targeting of Nsp1 from SARS-CoV-2 and related human-pathogenic β-CoVs. Our study also exemplifies how comparing highly divergent Nsp1 variants can help to dissect the different modalities of this multi-functional viral protein.

An evolutionarily conserved strategy for ribosome binding and inhibition by β-coronavirus non-structural protein 1

An important pathogenicity factor of SARS-CoV-2 and related coronaviruses is Nsp1, which suppresses host gene expression and stunts antiviral signaling. SARS-CoV-2 Nsp1 binds the ribosome to inhibit translation through mRNA displacement and induces degradation of host mRNAs through an unknown mechanism. Here we show that Nsp1-dependent host shutoff is conserved in diverse coronaviruses, but only Nsp1 from β-CoV inhibits translation through ribosome binding. The C-terminal domain of all β-CoV Nsp1s confers high-affinity ribosome-binding despite low sequence conservation. Modeling of interactions of four Nsp1s to the ribosome identified few absolutely conserved amino acids that, together with an overall conservation in surface charge, form the β-CoV Nsp1 ribosome-binding domain. Contrary to previous models, the Nsp1 ribosome-binding domain is an inefficient translation inhibitor. Instead, the Nsp1-CTD likely functions by recruiting Nsp1's N-terminal "effector" domain. Finally, we show that a viral cis -acting RNA element has co-evolved to fine-tune SARS-CoV-2 Nsp1 function, but does not provide similar protection against Nsp1 from related viruses. Together, our work provides new insight into the diversity and conservation of ribosome-dependent host-shutoff functions of Nsp1, knowledge that could aide future efforts in pharmacological targeting of Nsp1 from SARS-CoV-2, but also related human-pathogenic β-coronaviruses. Our study also exemplifies how comparing highly divergent Nsp1 variants can help to dissect the different modalities of this multi-functional viral protein.

Nanopore sequencing of internal 2'-PO4 modifications installed by RNA repair

Ligation by plant and fungal RNA ligases yields an internal 2'-phosphate group on each RNA ligation product. In budding yeast, this covalent mark occurs at the splice junction of two targets of ligation: intron-containing tRNAs and the messenger RNA HAC1 The repertoire of RNA molecules repaired by RNA ligation has not been explored due to a lack of unbiased approaches for identifying RNA ligation products. Here, we define several unique signals produced by 2'-phosphorylated RNAs during nanopore sequencing. A 2'-phosphate at the splice junction of HAC1 mRNA inhibits 5' → 3' degradation, enabling detection of decay intermediates in yeast RNA repair mutants by nanopore sequencing. During direct RNA sequencing, intact 2'-phosphorylated RNAs on HAC1 and tRNAs produce diagnostic changes in nanopore current properties and base calling features, including stalls produced as the modified RNA translocates through the nanopore motor protein. These approaches enable directed studies to identify novel RNA repair events in other contexts.

A viral RNA hijacks host machinery using dynamic conformational changes of a tRNA-like structure

Viruses require multifunctional structured RNAs to hijack their host’s biochemistry, but their mechanisms can be obscured by the difficulty of solving conformationally dynamic RNA structures. Using cryo–electron microscopy (cryo-EM), we visualized the structure of the mysterious viral transfer RNA (tRNA)–like structure (TLS) from the brome mosaic virus, which affects replication, translation, and genome encapsidation. Structures in isolation and those bound to tyrosyl-tRNA synthetase (TyrRS) show that this ~55-kilodalton purported tRNA mimic undergoes large conformational rearrangements to bind TyrRS in a form that differs substantially from that of tRNA. Our study reveals how viral RNAs can use a combination of static and dynamic RNA structures to bind host machinery through highly noncanonical interactions, and we highlight the utility of cryo-EM for visualizing small, conformationally dynamic structured RNAs.

An expanded class of histidine-accepting viral tRNA-like structures

Structured RNA elements are common in the genomes of RNA viruses, often playing critical roles during viral infection. Some viral RNA elements use forms of tRNA mimicry, but the diverse ways this mimicry can be achieved are poorly understood. Histidine-accepting tRNA-like structures (TLS^His) are examples found at the 3' termini of some positive-sense single-stranded RNA (+ssRNA) viruses where they interact with several host proteins, induce histidylation of the RNA genome, and facilitate processes important for infection, to include genome replication. As only five TLS^His examples had been reported, we explored the possible larger phylogenetic distribution and diversity of this TLS class using bioinformatic approaches. We identified many new examples of TLS^His, yielding a rigorous consensus sequence and secondary structure model that we validated by chemical probing of representative TLS^His RNAs. We confirmed new examples as authentic TLS^His by demonstrating their ability to be histidylated in vitro, then used mutational analyses to imply a tertiary interaction that is likely analogous to the D- and T-loop interaction found in canonical tRNAs. These results expand our understanding of how diverse RNA sequences achieve tRNA-like structure and function in the context of viral RNA genomes and lay the groundwork for high-resolution structural studies of tRNA mimicry by histidine-accepting TLSs.

Structural diversity and phylogenetic distribution of valyl tRNA-like structures in viruses

Viruses commonly use specifically folded RNA elements that interact with both host and viral proteins to perform functions important for diverse viral processes. Examples are found at the 3' termini of certain positive-sense ssRNA virus genomes where they partially mimic tRNAs, including being aminoacylated by host cell enzymes. Valine-accepting tRNA-like structures (TLS^Val) are an example that share some clear homology with canonical tRNAs but have several important structural differences. Although many examples of TLS^Val have been identified, we lacked a full understanding of their structural diversity and phylogenetic distribution. To address this, we undertook an in-depth bioinformatic and biochemical investigation of these RNAs, guided by recent high-resolution structures of a TLS^Val We cataloged many new examples in plant-infecting viruses but also in unrelated insect-specific viruses. Using biochemical and structural approaches, we verified the secondary structure of representative TLS^Val substrates and tested their ability to be valylated, confirming previous observations of structural heterogeneity within this class. In a few cases, large stem-loop structures are inserted within variable regions located in an area of the TLS distal to known host cell factor binding sites. In addition, we identified one virus whose TLS has switched its anticodon away from valine, causing a loss of valylation activity; the implications of this remain unclear. These results refine our understanding of the structural and functional mechanistic details of tRNA mimicry and how this may be used in viral infection.

Ribosome-induced RNA conformational changes in a viral 3'-UTR sense and regulate translation levels

Structured RNA elements, programmed RNA conformational changes, and interactions between different RNA domains underlie many modes of regulating gene expression, mandating studies to understand the foundational principles that govern these phenomena. Exploring the structured 3' untranslated region (UTR) of a viral RNA, we discovered that different contexts of the 3'-UTR confer different abilities to enhance translation of an associated open reading frame. In one context, ribosome-induced conformational changes in a 'sensor' RNA domain affect a separate RNA 'functional' domain, altering translation efficiency. The structure of the entire 3'-UTR reveals that structurally distinct domains use a spine of continuously stacked bases and a strut-like linker to create a conduit for communication within the higher-order architecture. Thus, this 3'-UTR RNA illustrates how RNA can use programmed conformational changes to sense the translation status of an upstream open reading frame, then create a tuned functional response by communicating that information to other RNA elements.

Autoregulation of the 26S proteasome by in situ ubiquitination

The 26S proteasome degrades ubiquitinated proteins, and proteasomal degradation controls various cellular events. Here we report that the human 26S proteasome is ubiquitinated, by which the ubiquitin receptors Adrm1 and S5a, the ATPase subunit Rpt5, and the deubiquitinating enzyme Uch37 are ubiquitinated in situ by proteasome-associating ubiquitination enzymes. Ubiquitination of these subunits significantly impairs the 26S proteasome's ability to bind, deubiquitinate, and degrade ubiquitinated proteins. Moreover, ubiquitination of the 26S proteasome can be antagonized by proteasome-residing deubiquitinating enzymes, by the binding of polyubiquitin chains, and by certain cellular stress, indicating that proteasome ubiquitination is dynamic and regulated in cells. We propose that in situ ubiquitination of the 26S proteasome regulates its activity, which could function to adjust proteasomal activity in response to the alteration of cellular ubiquitination levels.