The coat protein of the yeast double-stranded RNA virus L-A attaches covalently to the cap structure of eukaryotic mRNA

Delineating organizational principles of the endogenous L-A virus by cryo-EM and computational analysis of native cell extracts

The high abundance of most viruses in infected host cells benefits their structural characterization. However, endogenous viruses are present in low copy numbers and are therefore challenging to investigate. Here, we retrieve cell extracts enriched with an endogenous virus, the yeast L-A virus. The determined cryo-EM structure discloses capsid-stabilizing cation-π stacking, widespread across viruses and within the Totiviridae, and an interplay of non-covalent interactions from ten distinct capsomere interfaces. The capsid-embedded mRNA decapping active site trench is supported by a constricting movement of two flexible opposite-facing loops. tRNA-loaded polysomes and other biomacromolecules, presumably mRNA, are found in virus proximity within the cell extract. Mature viruses participate in larger viral communities resembling their rare in-cell equivalents in terms of size, composition, and inter-virus distances. Our results collectively describe a 3D-architecture of a viral milieu, opening the door to cell-extract-based high-resolution structural virology.

Capsid Structure of Leishmania RNA Virus 1

Twelve million people worldwide suffer from leishmaniasis, resulting in more than 30 thousand deaths annually. The disease has several variants that differ in their symptoms.

Leishmania parasites cause a variety of symptoms, including mucocutaneous leishmaniasis, which results in the destruction of the mucous membranes of the nose, mouth, and throat. The species of Leishmania carrying Leishmania RNA virus 1 (LRV1), from the family Totiviridae, are more likely to cause severe disease and are less sensitive to treatment than those that do not contain the virus. Although the importance of LRV1 for the severity of leishmaniasis was discovered a long time ago, the structure of the virus remained unknown. Here, we present a cryo-electron microscopy reconstruction of the virus-like particle of LRV1 determined to a resolution of 3.65 Å. The capsid has icosahedral symmetry and is formed by 120 copies of a capsid protein assembled in asymmetric dimers. RNA genomes of viruses from the family Totiviridae are synthetized, but not capped at the 5′ end, by virus RNA polymerases. To protect viral RNAs from degradation, capsid proteins of the L-A totivirus cleave the 5′ caps of host mRNAs, creating decoys to overload the cellular RNA quality control system. Capsid proteins of LRV1 form positively charged clefts, which may be the cleavage sites for the 5′ cap of Leishmania mRNAs. The putative RNA binding site of LRV1 is distinct from that of the related L-A virus. The structure of the LRV1 capsid enables the rational design of compounds targeting the putative decapping site. Such inhibitors may be developed into a treatment for mucocutaneous leishmaniasis caused by LRV1-positive species of Leishmania.

IMPORTANCE Twelve million people worldwide suffer from leishmaniasis, resulting in more than 30 thousand deaths annually. The disease has several variants that differ in their symptoms. The mucocutaneous form, which leads to disintegration of the nasal septum, lips, and palate, is caused predominantly by Leishmania parasites carrying Leishmania RNA virus 1 (LRV1). Here, we present the structure of the LRV1 capsid determined using cryo-electron microscopy. Capsid proteins of a related totivirus, L-A virus, protect viral RNAs from degradation by cleaving the 5′ caps of host mRNAs. Capsid proteins of LRV1 may have the same function. We show that the LRV1 capsid contains positively charged clefts that may be sites for the cleavage of mRNAs of Leishmania cells. The structure of the LRV1 capsid enables the rational design of compounds targeting the putative mRNA cleavage site. Such inhibitors may be used as treatments for mucocutaneous leishmaniasis.

The cap-snatching reaction of yeast L-A double-stranded RNA virus is reversible and the catalytic sites on both Gag and the Gag domain of Gag-Pol are active

The yeast L-A double-stranded RNA virus synthesizes capped transcripts by a unique cap-snatching mechanism in which the m⁷ Gp moiety of host mRNA (donor) is transferred to the diphosphorylated 5' end of the viral transcript (acceptor). This reaction is activated by viral transcription. Here, we show that cap snatching can be reversible. Because only m⁷ Gp is transferred during the reaction, the resulting decapped donor, as expected, retained diphosphates at the 5' end. We also found that the 5' terminal nucleotide of the acceptor needs to be G but not A. Interestingly, the A-initiated molecule when equipped with a cap structure (m⁷ GpppA…) could work as cap donor. Because the majority of host mRNAs in yeast have A after the cap structures at the 5' ends, this finding implies that cap-snatching in vivo is virtually a one-way reaction, in favor of furnishing the viral transcript with a cap. The cap-snatching sites are located on the coat protein Gag and also the Gag domain of Gag-Pol. Here, we demonstrate that both sites are functional, indicating that activation of cap snatching by transcription is not transmitted through the peptide bonding between the Gag and Pol domains of Gag-Pol.

Diphosphates at the 5' end of the positive strand of yeast L-A double-stranded RNA virus as a molecular self-identity tag

The 5'end of RNA conveys important information on self-identity. In mammalian cells, double-stranded RNA (dsRNA) with 5'di- or triphosphates generated during virus infection is recognized as foreign and elicits the host innate immune response. Here, we analyze the 5' ends of the dsRNA genome of the yeast L-A virus. The positive strand has largely diphosphates with a minor amount of triphosphates, while the negative strand has only diphosphates. Although the virus can produce capped transcripts by cap snatching, neither strand carried a cap structure, suggesting that only non-capped transcripts serve as genomic RNA for encapsidation. We also found that the 5' diphosphates of the positive but not the negative strand within the dsRNA genome are crucial for transcription in vitro. Furthermore, the presence of a cap structure in the dsRNA abrogated its template activity. Given that the 5' diphosphates of the transcripts are also essential for cap acquisition and that host cytosolic RNAs (mRNA, rRNA, and tRNA) are uniformly devoid of 5' pp-structures, the L-A virus takes advantage of its 5' terminal diphosphates, using them as a self-identity tag to propagate in the host cytoplasm.

mRNA capping: biological functions and applications

The 5′ m7G cap is an evolutionarily conserved modification of eukaryotic mRNA. Decades of research have established that the m7G cap serves as a unique molecular module that recruits cellular proteins and mediates cap-related biological functions such as pre-mRNA processing, nuclear export and cap-dependent protein synthesis. Only recently has the role of the cap 2′O methylation as an identifier of self RNA in the innate immune system against foreign RNA has become clear. The discovery of the cytoplasmic capping machinery suggests a novel level of control network. These new findings underscore the importance of a proper cap structure in the synthesis of functional messenger RNA. In this review, we will summarize the current knowledge of the biological roles of mRNA caps in eukaryotic cells. We will also discuss different means that viruses and their host cells use to cap their RNA and the application of these capping machineries to synthesize functional mRNA. Novel applications of RNA capping enzymes in the discovery of new RNA species and sequencing the microbiome transcriptome will also be discussed. We will end with a summary of novel findings in RNA capping and the questions these findings pose.

Rapid, optimized interactomic screening

We must reliably map the interactomes of cellular macromolecular complexes in order to fully explore and understand biological systems. However, there are no methods to accurately predict how to capture a given macromolecular complex with its physiological binding partners. Here, we present a screen that comprehensively explores the parameters affecting the stability of interactions in affinity-captured complexes, enabling the discovery of physiological binding partners and the elucidation of their functional interactions in unparalleled detail. We have implemented this screen on several macromolecular complexes from a variety of organisms, revealing novel profiles even for well-studied proteins. Our approach is robust, economical and automatable, providing an inroad to the rigorous, systematic dissection of cellular interactomes.

Affinity resins containing enzymatically resistant mRNA cap analogs--a new tool for the analysis of cap-binding proteins

Cap-binding proteins have been routinely isolated using m⁷GTP-Sepharose; however, this resin is inefficient for proteins such as DcpS (scavenger decapping enzyme), which interacts not only with the 7-methylguanosine, but also with the second cap base. In addition, DcpS purification may be hindered by the reduced resin capacity due to the ability of DcpS to hydrolyze m⁷GTP. Here, we report the synthesis of new affinity resins, m⁷GpCH₂pp- and m⁷GpCH₂ppA-Sepharoses, with attached cap analogs resistant to hydrolysis by DcpS. Biochemical tests showed that these matrices, as well as a hydrolyzable m⁷GpppA-Sepharose, bind recombinant mouse eIF4E²⁸⁻²¹⁷ specifically and at high capacity. In addition, purification of cap-binding proteins from yeast extracts confirmed the presence of all expected cap-binding proteins, including DcpS in the case of m⁷GpCH₂pp- and m⁷GpCH₂ppA-Sepharoses. In contrast, binding studies in vitro demonstrated that recombinant human DcpS efficiently bound only m⁷GpCH₂ppA-Sepharose. Our data prove the applicability of these novel resins, especially m⁷GpCH₂ppA-Sepharose, in biochemical studies such as the isolation and identification of cap-binding proteins from different organisms.

Conventional and unconventional mechanisms for capping viral mRNA

mRNAs are protected at their 5′ ends by a cap structure consisting of an N7-methylated GTP molecule linked to the first transcribed nucleotide by a 5′–5′ triphosphate bond.

The cap structure is essential for RNA splicing, export and stability, and allows the ribosomal complex to recognize mRNAs and ensure their efficient translation.

Uncapped RNA molecules are degraded in cytoplasmic granular compartments called processing bodies and may be detected as 'non-self' by the host cell, triggering antiviral innate immune responses through the production of interferons.

Conventional RNA capping (that is, of mRNAs from the host cell and from DNA viruses) requires hydrolysis of the 5′ γ-phosphate of RNA by an RNA triphosphatase, transfer of a GMP molecule onto the 5′-end of RNA by a guanylyltransferase, and methylation of this guanosine by an (guanine-N7)-methyltransferase. Subsequent methylations on the first and second transcribed nucleotides by (nucleoside-2′-O)-methyltransferases form cap-1 and cap-2 structures.

Viruses have evolved highly diverse capping mechanisms to acquire cap structures using their own or cellular capping machineries, or by stealing cap structures from cellular mRNAs.

Virally encoded RNA-capping machineries are diverse in terms of their genetic components, protein domain organization, enzyme structures, and reaction mechanisms and pathways, making viral RNA capping an attractive target for antiviral-drug design.

Capping the 5′ end of eukaryotic mRNAs with a 7-methylguanosine moiety enables efficient splicing, nuclear export and translation of mRNAs, and also limits their degradation by cellular exonucleases. Here, Canard and colleagues describe how viruses synthesize their own mRNA cap structures or steal them from host mRNAs, allowing efficient synthesis of viral proteins and avoidance of host innate immune responses.

In the eukaryotic cell, capping of mRNA 5′ ends is an essential structural modification that allows efficient mRNA translation, directs pre-mRNA splicing and mRNA export from the nucleus, limits mRNA degradation by cellular 5′–3′ exonucleases and allows recognition of foreign RNAs (including viral transcripts) as 'non-self'. However, viruses have evolved mechanisms to protect their RNA 5′ ends with either a covalently attached peptide or a cap moiety (7-methyl-Gppp, in which p is a phosphate group) that is indistinguishable from cellular mRNA cap structures. Viral RNA caps can be stolen from cellular mRNAs or synthesized using either a host- or virus-encoded capping apparatus, and these capping assemblies exhibit a wide diversity in organization, structure and mechanism. Here, we review the strategies used by viruses of eukaryotic cells to produce functional mRNA 5′-caps and escape innate immunity.

Endogenous ribosomal frameshift signals operate as mRNA destabilizing elements through at least two molecular pathways in yeast

Although first discovered in viruses, previous studies have identified operational −1 ribosomal frameshifting (−1 RF) signals in eukaryotic genomic sequences, and suggested a role in mRNA stability. Here, four yeast −1 RF signals are shown to promote significant mRNA destabilization through the nonsense mediated mRNA decay pathway (NMD), and genetic evidence is presented suggesting that they may also operate through the no-go decay pathway (NGD) as well. Yeast EST2 mRNA is highly unstable and contains up to five −1 RF signals. Ablation of the −1 RF signals or of NMD stabilizes this mRNA, and changes in −1 RF efficiency have opposing effects on the steady-state abundance of the EST2 mRNA. These results demonstrate that endogenous −1 RF signals function as mRNA destabilizing elements through at least two molecular pathways in yeast. Consistent with current evolutionary theory, phylogenetic analyses suggest that −1 RF signals are rapidly evolving cis-acting regulatory elements. Identification of high confidence −1 RF signals in ∼10% of genes in all eukaryotic genomes surveyed suggests that −1 RF is a broadly used post-transcriptional regulator of gene expression.

The control of mRNA decapping and P-body formation

mRNA decapping is a critical step in eukaryotic cytoplasmic mRNA turnover. Cytoplasmic mRNA decapping is catalyzed by Dcp2 in conjunction with its coactivator Dcp1 and is stimulated by decapping enhancer proteins. mRNAs associated with the decapping machinery can assemble into cytoplasmic mRNP granules called processing bodies (PBs). Evidence suggests that PB-associated mRNPs are translationally repressed and can be degraded or stored for subsequent translation. However, whether mRNP assembly into a PB is important for translational repression, decapping, or decay has remained controversial. Here, we discuss the regulation of decapping machinery recruitment to specific mRNPs and how their assembly into PBs is governed by the relative rates of translational repression, mRNP multimerization, and mRNA decay.

RNA decapping inside and outside of processing bodies

Decapping is a central step in eukaryotic mRNA turnover. Recent studies have identified several factors involved in catalysis and regulation of decapping. These include the following: an mRNA decapping complex containing the proteins Dcp1 and Dcp2; a nucleolar decapping enzyme, X29, involved in the degradation of U8 snoRNA and perhaps of other capped nuclear RNAs; and a decapping 'scavenger' enzyme, DcpS, that hydrolyzes the cap structure resulting from complete 3'-to-5' degradation of mRNAs by the exosome. Several proteins that stimulate mRNA decapping by the Dcp1:Dcp2 complex co-localize with Dcp1 and Dcp2, together with Xrn1, a 5'-to-3' exonuclease, to structures in the cytoplasm called processing bodies. Recent evidence suggests that the processing bodies may constitute specialized cellular compartments of mRNA turnover, which suggests that mRNA and protein localization may be integral to mRNA decay.

The structural basis of recognition and removal of cellular mRNA 7-methyl G 'caps' by a viral capsid protein: a unique viral response to host defense

The single segment, double-stranded RNA genome of the L-A virus (L-A) of yeast encodes two proteins: the major coat protein Gag (76 kDa) and the Gag-Pol fusion protein (180 kDa). The icosahedral L-A capsid is formed by 120 copies of Gag and has architecture similar to that seen in the reovirus, blue tongue virus and rice dwarf virus inner protein shells. Gag chemically removes the m7GMP caps from host cellular mRNAs. Previously we identified a trench on the outer surface of Gag that included His154, to which caps are covalently attached. Here we report the refined L-A coordinates at 3.4 angstroms resolution with additional structural features and the structure of L-A with bound m7GDP at 6.5 angstroms resolution, which shows the conformational change of the virus upon ligand binding. Based on site-directed mutations, residues in or adjacent to the trench that are essential (or dispensable) for the decapping reaction are described here. Along with His154, the reaction requires a cluster of positive charge adjoining the trench and residues Tyr 452, Tyr150 and either Tyr or Phe at position 538. A tentative mechanism for decapping is proposed.

'Cap-tabolism'

distinctive feature of eukaryotic mRNA and small nuclear RNA (snRNA) that are transcribed by RNA polymerase II (Pol II) is the presence of a cap structure at their 5' end. This essential modification serves as an inviting 'landing pad' for factors that are involved in various cellular processes such as pre-mRNA splicing, nucleocytoplasmic RNA export and localization, and translation initiation. Because of the important functions mediated by the mRNA cap, this structure needs to be modified and/or degraded in a tightly controlled manner. Several cellular and viral systems implicated in cap metabolism have been uncovered recently; their analyses provide interesting new information on cell structure and function.

Pokeweed antiviral protein binds to the cap structure of eukaryotic mRNA and depurinates the mRNA downstream of the cap

Several cap-binding proteins from both the nucleus and cytosol have been identified that mediate processes such as pre-mRNA splicing, translation initiation, and mRNA turnover. Here we describe a novel cap-binding protein, pokeweed antiviral protein (PAP), a 29-kDa type I ribosome-inactivating protein (RIP) isolated from Phytolacca americana. In addition to depurinating the sarcin/ricin loop of the large rRNA, an activity common to all RIPs, we have reported recently that PAP depurinates capped, but not uncapped RNAs in vitro. Here we characterize this activity further and, using affinity chromatography, show that PAP binds to the m7Gppp cap structure. PAP UV-crosslinks to m7GpppG-capped luciferase mRNA more efficiently than GpppG-capped luciferase mRNA, indicating specificity for the methylated guanosine. We present evidence that PAP does not remove the cap structure or depurinate the m7Gppp as shown by primer extension of capped and uncapped luciferase transcripts incubated with PAP. Modeling studies of cap interaction with PAP predict that the cap structure would bind to the active site of PAP in a similar manner to guanine. We map the depurination sites on the capped luciferase RNA and illustrate that depurination occurs at specific adenine and guanine residues throughout the RNA sequence. Incubation of isolated ribosomes with PAP and increasing molar concentrations of m7GpppG relative to PAP resulted in a decrease in the level of rRNA depurination. Therefore, at elevated concentrations, the methylated cap structure competes with the adenine or guanine for binding to PAP, even though the affinity of PAP for capped message is almost fourfold lower than for rRNA. These results demonstrate that the activity of PAP is not limited to rRNA depurination, but that PAP binds to the cap structure and depurinates mRNAs downstream of the cap in vitro. These findings may have implications for understanding PAP activity in vivo.

Comparisons among the larger genome segments of six nodaviruses and their encoded RNA replicases

The Nodaviridae are a family of isometric RNA viruses that infect insects and fish. Their genomes, which are among the smallest known for animal viruses, consist of two co-encapsidated positive-sense RNA segments: RNA1 encodes the viral contribution to the RNA-dependent RNA polymerase (RdRp) which replicates the viral genome, whereas RNA2 encodes the capsid protein precursor. In this study, the RNA1 sequences of two insect nodaviruses - Nodamura virus (the prototype of the genus) and Boolarra virus - are reported as well as detailed comparisons of their encoded RdRps with those of three other nodaviruses of insects and one of fish. Although the 5' and 3' untranslated regions did not reveal common features of RNA sequence or secondary structure, these divergent viruses showed similar genome organizations and encoded RdRps that had from 26 to 99% amino acid sequence identity. All six RdRp amino acid sequences contained canonical RNA polymerase motifs in their C-terminal halves and conserved elements of predicted secondary structure throughout. A search for structural homologues in the protein structure database identified the poliovirus RdRp, 3D(pol), as the best template for homology modelling of the RNA polymerase domain of Pariacoto virus and allowed the construction of a congruent three-dimensional model. These results extend our understanding of the relationships among the RNA1 segments of nodaviruses and the predicted structures of their encoded RdRps.

Translation-competent extracts from Saccharomyces cerevisiae: effects of L-A RNA, 5' cap, and 3' poly(A) tail on translational efficiency of mRNAs

Yeast genetics has proven fruitful in the identification of key players that are involved in translational initiation. However, the exact roles of many translation initiation factors in translation initiation remain unknown. This has been due to lack of a suitable in vitro translation system in which the mode of action of certain translation factors can be studied. This report describes the preparation of cell-free Saccharomyces cerevisiae lysates that can mediate the translation of exogenously added mRNAs. Optimal translation required the absence of viral L-A RNA in the lysate and the presence of both a 5' cap and a 3' poly(A) tail on the mRNAs. A cooperative effect of cap and poly(A) tail on translation initiation was observed, a property that has been found to operate in intact yeast cells as well. In addition, the yeast lysates mediated translational initiation through several viral internal ribosome entry sites, demonstrating that the yeast translation apparatus can perform internal initiation. Thus, these lysates may be useful in the biochemical analysis of cap-dependent and cap-independent translation events.

Prions and RNA viruses of Saccharomyces cerevisiae

Saccharomyces cerevisiae is host to the dsRNA viruses L-A (including its killer toxin-encoding satellite, M) and L-BC, the 20S and 23S ssRNA replicons, and the putative prions, [URE3] and [PSI]. review the genetic and biochemical evidence indicating that [URE3] and [PSI] are prion forms of Ure2p and Sup35p, respectively. Each has an N-terminal domain involved in propagation or generation of the prion state and a C-terminal domain responsible for the protein's normal function, nitrogen regulation, or translation termination, respectively. The L-A dsRNA virus expression, replication, and RNA packaging are reviewed. L-A uses a -1 ribosomal frameshift to produce a Gag-Pol fusion protein. The host SK12, SK13 and SK18 proteins block translation of nonpoly(A) mRNAs (such as viral mRNA). Mutants deficient in 60S ribosomal subunits replicate L-A poorly, but not if cells are also ski-. Interaction of 60S subunits with the 3' polyA is suggested. SKI1/XRN1 is a 5'--> 3' exoribonuclease that degrades uncapped mRNAs. The viral Gag protein decapitates cellular mRNAs apparently to decoy this enzyme from working on viral mRNA.

mRNA decapping activities and their biological roles

The 5' cap structure of eukaryotic mRNAs is significant for a variety of cellular events and also serves to protect mRNAs from premature degradation. Analysis of mRNA decay in Saccharomyces cerevisiae has shown that removal of the 5' cap structure is a key step in the turnover of many yeast mRNAs, and that this decapping is carried out by Dcp1p. In addition to the yeast decapping enzyme, other activities that can cleave the 5' cap structure have been described. These include two mammalian enzymes and two viral activities that cleave cellular mRNA cap structures as part of their life cycle. Here we review these various decapping activities and discuss their biological roles.

Saccharomyces cerevisiae L-BC double-stranded RNA virus replicase recognizes the L-A positive-strand RNA 3' end

L-A and L-BC are two double-stranded RNA viruses present in almost all strains of Saccharomyces cerevisiae. L-A, the major species, has been extensively characterized with in vitro systems established, but little is known about L-BC. Here we report in vitro template-dependent transcription, replication, and RNA recognition activities of L-BC. The L-BC replicase activity converts positive, single-stranded RNA to double-stranded RNA by synthesis of the complementary RNA strand. Although L-A and L-BC do not interact in vivo, in vitro L-BC virions can replicate the positive, single-stranded RNA of L-A and its satellite, M1, with the same 3' end sequence and stem-loop requirements shown by L-A virions for its own template. However, the L-BC virions do not recognize the internal replication enhancer of the L-A positive strand. In a direct comparison of L-A and L-BC virions, each preferentially recognizes its own RNA for binding, replication, and transcription. These results suggest a close evolutionary relation of these two viruses, consistent with their RNA-dependent RNA polymerase sequence similarities.

Ribosomal frameshifting in yeast viruses

Proper maintenance of translational reading frame by ribosomes is essential for cell growth and viability. In the last 10 years it has been shown that a number of viruses induce ribosomes to shift reading frame in order to regulate the expression of gene products having enzymatic functions. Studies on ribosomal frameshifting in viruses of yeast have been particularly enlightening. The roles of viral mRNA sequences and secondary structures have been elucidated and a picture of how these interact with host chromosomal gene products is beginning to emerge. The efficiency of ribosomal frameshifting is important for viral particle assembly, and has identified ribosomal frameshifting as a potential target for antiviral agents. The availability of mutants of host chromosomal gene products involved in maintaining the efficiency of ribosomal frameshifting bodes well for the use of yeast in future studies of ribosomal frameshifting.