No‐go decay (NGD) and non‐stop decay (NSD) are eukaryotic surveillance mechanisms that target mRNAs on which elongation complexes (ECs) are stalled by, for example, stable secondary structures (NGD) ...or due to the absence of a stop codon (NSD). Two interacting proteins Dom34(yeast)/Pelota(mammals) and Hbs1, which are paralogues of eRF1 and eRF3, are implicated in these processes. Dom34/Hbs1 were shown to promote dissociation of stalled ECs and release of intact peptidyl‐tRNA. Using an in vitro reconstitution approach, we investigated the activities of mammalian Pelota/Hbs1 and report that Pelota/Hbs1 also induced dissociation of ECs and release of peptidyl‐tRNA, but only in the presence of ABCE1. Whereas Pelota and ABCE1 were essential, Hbs1 had a stimulatory effect. Importantly, ABCE1/Pelota/Hbs1 dissociated ECs containing only a limited number of mRNA nucleotides downstream of the P‐site, which suggests that ABCE1/Pelota/Hbs1 would disassemble NSD complexes stalled at the 3′‐end, but not pre‐cleavage NGD complexes stalled in the middle of mRNA. ABCE1/Pelota/Hbs1 also dissociated vacant 80S ribosomes, which stimulated 48S complex formation, suggesting that Pelota/Hbs1 have an additional role outside of NGD.
The mammalian Pelota/Hbs1 complex needs an additional protein, ABCE1, to dissociate stalled translation elongation complexes. ABCE1/Pelota/Hbs1 act specifically on elongation complexes stalled at the 3′‐end (non‐stop decay complexes) and play an additional role in the recycling of vacant 80S ribosome complexes.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
After translational termination, mRNA and P site deacylated tRNA remain associated with ribosomes in posttermination complexes (post-TCs), which must therefore be recycled by releasing mRNA and ...deacylated tRNA and by dissociating ribosomes into subunits. Recycling of bacterial post-TCs requires elongation factor EF-G and a ribosome recycling factor RRF. Eukaryotes do not encode a RRF homolog, and their mechanism of ribosomal recycling is unknown. We investigated eukaryotic recycling using post-TCs assembled on a model mRNA encoding a tetrapeptide followed by a UAA stop codon and report that initiation factors eIF3, eIF1, eIF1A, and eIF3j, a loosely associated subunit of eIF3, can promote recycling of eukaryotic post-TCs. eIF3 is the principal factor that promotes splitting of posttermination ribosomes into 60S subunits and tRNA- and mRNA-bound 40S subunits. Its activity is enhanced by eIFs 3j, 1, and 1A. eIF1 also mediates release of P site tRNA, whereas eIF3j ensures subsequent dissociation of mRNA.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
After termination, eukaryotic 80S ribosomes remain associated with mRNA, P-site deacylated tRNA, and release factor eRF1 and must be recycled by dissociating these ligands and separating ribosomes ...into subunits. Although recycling of eukaryotic posttermination complexes (post-TCs) can be mediated by initiation factors eIF3, eIF1, and eIF1A (Pisarev et al., 2007), this energy-free mechanism can function only in a narrow range of low Mg2+ concentrations. Here, we report that ABCE1, a conserved and essential member of the ATP-binding cassette (ABC) family of proteins, promotes eukaryotic ribosomal recycling over a wide range of Mg2+ concentrations. ABCE1 dissociates post-TCs into free 60S subunits and mRNA- and tRNA-bound 40S subunits. It can hydrolyze ATP, GTP, UTP, and CTP. NTP hydrolysis by ABCE1 is stimulated by post-TCs and is required for its recycling activity. Importantly, ABCE1 dissociates only post-TCs obtained with eRF1/eRF3 (or eRF1 alone), but not post-TCs obtained with puromycin in eRF1's absence.
► ABCE1 promotes eukaryotic ribosomal recycling over a wide range of Mg2+ concentrations ► ABCE1 dissociates posttermination complexes into free 60S and mRNA/tRNA-bound 40S subunits ► NTP hydrolysis by ABCE1 is required for its activity in ribosomal recycling ► eRF1 is essential for ribosomal recycling by ABCE1
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Co-opting the cellular machinery for protein production is a compulsory requirement for viruses. The Cricket Paralysis Virus employs an Internal Ribosomal Entry Site (CrPV-IRES) to express its ...structural genes in the late stage of infection. Ribosome hijacking is achieved by a sophisticated use of molecular mimicry to tRNA and mRNA, employed to manipulate intrinsically dynamic components of the ribosome. Binding and translocation through the ribosome is required for this IRES to initiate translation. We report two structures, solved by single particle electron cryo-microscopy (cryoEM), of a double translocated CrPV-IRES with aminoacyl-tRNA in the peptidyl site (P site) of the ribosome. CrPV-IRES adopts a previously unseen conformation, mimicking the acceptor stem of a canonical E site tRNA. The structures suggest a mechanism for the positioning of the first aminoacyl-tRNA shared with the distantly related Hepatitis C Virus IRES.
Taking control of the cellular apparatus for protein production is a requirement for virus progression. To ensure this control, diverse strategies of cellular mimicry and/or ribosome hijacking have ...evolved. The initiation stage of translation is especially targeted as it involves multiple steps and the engagement of numerous initiation factors. The use of structured RNA sequences, called
nternal
ibosomal
ntry
ites (IRES), in viral RNAs is a widespread strategy for the exploitation of eukaryotic initiation. Using a combination of electron cryo-microscopy (cryo-EM) and reconstituted translation initiation assays with native components, we characterized how a novel IRES at the 5'-UTR of a viral RNA assembles a functional initiation complex via an uAUG intermediate. The IRES features a novel extended, multi-domain architecture, that circles the 40S head. The structures and accompanying functional data illustrate the importance of 5'-UTR regions in translation regulation and underline the relevance of the untapped diversity of viral IRESs.
The position of mRNA on 40S ribosomal subunits in eukaryotic initiation complexes was determined by UV crosslinking using mRNAs containing uniquely positioned 4‐thiouridines. Crosslinking of mRNA ...positions +11 to ribosomal protein (rp) rpS2(S5p) and rpS3(S3p), and +9–+11 and +8–+9 to h18 and h34 of 18S rRNA, respectively, indicated that mRNA enters the mRNA‐binding channel through the same layers of rRNA and proteins as in prokaryotes. Upstream of the P‐site, the proximity of positions −3/−4 to rpS5(S7p) and h23b, −6/−7 to rpS14(S11p), and −8–−11 to the 3′‐terminus of 18S rRNA (mRNA/rRNA elements forming the bacterial Shine–Dalgarno duplex) also resembles elements of the bacterial mRNA path. In addition to these striking parallels, differences between mRNA paths included the proximity in eukaryotic initiation complexes of positions +7/+8 to the central region of h28, +4/+5 to rpS15(S19p), and −6 and −7/−10 to eukaryote‐specific rpS26 and rpS28, respectively. Moreover, we previously determined that eukaryotic initiation factor2α (eIF2α) contacts position −3, and now report that eIF3 interacts with positions −8–−17, forming an extension of the mRNA‐binding channel that likely contributes to unique aspects of eukaryotic initiation.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
48S initiation complex (48S IC) formation is the first stage in the eukaryotic translation process. According to the canonical mechanism, 40S ribosomal subunit binds to the 5'-end of messenger RNA ...(mRNA) and scans its 5'-untranslated region (5'-UTR) to the initiation codon where it forms the 48S IC. Entire process is mediated by initiation factors. Here we show that eIF5 and eIF5B together stimulate 48S IC formation influencing initiation codon selection during ribosomal scanning. Initiation on non-optimal start codons--following structured 5'-UTRs, in bad AUG context, within few nucleotides from 5'-end of mRNA and CUG start codon--is the most affected. eIF5-induced hydrolysis of eIF2-bound GTP is essential for stimulation. GTP hydrolysis increases the probability that scanning ribosomal complexes will recognize and arrest scanning at a non-optimal initiation codon. Such 48S ICs are less stable owing to dissociation of eIF2*GDP from initiator tRNA, and eIF5B is then required to stabilize the initiator tRNA in the P site of 40S subunit. Alternative model that eIF5 and eIF5B cause 43S pre-initiation complex rearrangement favoring more efficient initiation codon recognition during ribosomal scanning is equally possible. Mutational analysis of eIF1A and eIF5B revealed distinct functions of eIF5B in 48S IC formation and subunit joining.
Eukaryotic translation termination is triggered by peptide release factors eRF1 and eRF3. Whereas eRF1 recognizes all three termination codons and induces hydrolysis of peptidyl tRNA, eRF3's function ...remains obscure. Here, we reconstituted all steps of eukaryotic translation in vitro using purified ribosomal subunits; initiation, elongation, and termination factors; and aminoacyl tRNAs. This allowed us to investigate termination using pretermination complexes assembled on mRNA encoding a tetrapeptide and to propose a model for translation termination that accounts for the cooperative action of eRF1 and eRF3 in ensuring fast release of nascent polypeptide. In this model, binding of eRF1, eRF3, and GTP to pretermination complexes first induces a structural rearrangement that is manifested as a 2 nucleotide forward shift of the toeprint attributed to pretermination complexes that leads to GTP hydrolysis followed by rapid hydrolysis of peptidyl tRNA. Cooperativity between eRF1 and eRF3 required the eRF3 binding C-terminal domain of eRF1.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
During eukaryotic translation initiation, the 43S preinitiation complex (43S PIC), consisting of the 40S ribosomal subunit, eukaryotic initiation factors (eIFs) and initiator tRNA scans mRNA to find ...an appropriate start codon. Key roles in the accuracy of initiation codon selection belong to eIF1 and eIF1A, whereas the mammalian-specific DHX29 helicase substantially contributes to ribosomal scanning of structured mRNAs. Here, we show that DHX29 stimulates the recognition of the AUG codon but not the near-cognate CUG codon regardless of its nucleotide context during ribosomal scanning. The stimulatory effect depends on the contact between DHX29 and eIF1A. The unique DHX29 N-terminal domain binds to the ribosomal site near the mRNA entrance, where it contacts the eIF1A OB domain. UV crosslinking assays revealed that DHX29 may rearrange eIF1A and eIF2α in key nucleotide context positions of ribosomal complexes. Interestingly, DHX29 impedes the 48S initiation complex formation in the absence of eIF1A perhaps due to forming a physical barrier that prevents the 43S PIC from loading onto mRNA. Mutational analysis allowed us to split the mRNA unwinding and codon selection activities of DHX29. Thus, DHX29 is another example of an initiation factor contributing to start codon selection.
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