Mutation at tyrosine in AMLRY (GILRY like) motif of yeast eRF1 on nonsense codons suppression and binding affinity to eRF3

Ribosomal A-site interactions with near-cognate tRNAs drive stop codon readthrough

Transfer RNAs (tRNAs) serve as a dictionary for the ribosome translating the genetic message from mRNA into a polypeptide chain. In addition to this canonical role, tRNAs are involved in other processes such as programmed stop codon readthrough (SC-RT). There, tRNAs with near-cognate anticodons to stop codons must outcompete release factors and incorporate into the ribosomal decoding center to prevent termination and allow translation to continue. However, not all near-cognate tRNAs promote efficient SC-RT. Here, with the help of Saccharomyces cerevisiae and Trypanosoma brucei, we demonstrate that those tRNAs that promote efficient SC-RT establish critical contacts between their anticodon stem (AS) and ribosomal proteins Rps30/eS30 and Rps25/eS25 forming the decoding site. Unexpectedly, the length and well-defined nature of the AS determine the strength of these contacts, which is reflected in organisms with reassigned stop codons. These findings open an unexplored direction in tRNA biology that should facilitate the design of artificial tRNAs with specifically altered decoding abilities.

Destabilization of vitelline membrane outer layer protein 1 homolog (VMO1) by C-mannosylation

C-mannosylation is a rare type of protein glycosylation whereby a single mannose is added to the first tryptophan in the consensus sequence Trp-Xaa-Xaa-Trp/Cys (in which Xaa represents any amino acid). Its consensus sequence is mainly found in proteins containing a thrombospondin type-1 repeat (TSR1) domain and in type I cytokine receptors. In these proteins, C-mannosylation affects protein secretion, intracellular localization, and protein stability; however, the role of C-mannosylation in proteins that are not type I cytokine receptors and/or do not contain a TSR1 domain is less well explored. In this study, we focused on human vitelline membrane outer layer protein 1 homolog (VMO1). VMO1, which possesses two putative C-mannosylation sites, is a 21-kDa secreted protein that does not contain a TSR1 domain and is not a type I cytokine receptor. Mass spectrometry analyses revealed that VMO1 is C-mannosylated at Trp¹⁰⁵ but not at Trp⁴⁴ . Although C-mannosylation does not affect the extracellular secretion of VMO1, it destabilizes the intracellular VMO1. In addition, a structural comparison between VMO1 and C-mannosylated VMO1 showed that the modification of the mannose changes the conformation of three loops in VMO1. Taken together, our results demonstrate the first example of C-mannosylation for protein destabilization of VMO1.

Targeting Translation Termination Machinery with Antisense Oligonucleotides for Diseases Caused by Nonsense Mutations

Efforts to develop treatments for diseases caused by nonsense mutations have focused on identification of small molecules that promote translational read-through of messenger RNAs (mRNAs) harboring nonsense stop codons to produce full-length proteins. However, to date, no small molecule read-through drug has received FDA approval, probably because of a lack of balance between efficacy and safety. Depletion of translation termination factors eukaryotic release factor (eRF) 1 and eRF3a in cells was shown to promote translational read-through of a luciferase reporter gene harboring a nonsense mutation. In this study, we identified antisense oligonucleotides (ASOs) targeting translation termination factors and determined if ASO-mediated depletion of these factors could be a potentially effective and safe therapeutic approach for diseases caused by nonsense mutations. We found that ASO-mediated reduction of either eRF1 or eRF3a to 30%–40% of normal levels in the mouse liver is well tolerated. Hemophilia mice that express a mutant allele of human coagulation factor IX (FIX) containing nonsense mutation R338X were treated with eRF1- or eRF3a-ASO. We found that although eRF1- or eRF3a-ASO alone only elicited a moderate read-through effect on hFIX-R338X mRNA, both worked in synergy with geneticin, a small molecule read-through drug, demonstrating significantly increased production of functional full-length hFIX protein to levels that would rescue disease phenotypes in these mice. Overall our results indicate that modulating the translation termination pathway in the liver by ASOs may provide a novel approach to improving the efficacy of small molecule read-through drugs to treat human genetic diseases caused by nonsense mutations.

Rules of UGA-N decoding by near-cognate tRNAs and analysis of readthrough on short uORFs in yeast

The molecular mechanism of stop codon recognition by the release factor eRF1 in complex with eRF3 has been described in great detail; however, our understanding of what determines the difference in termination efficiencies among various stop codon tetranucleotides and how near-cognate (nc) tRNAs recode stop codons during programmed readthrough in Saccharomyces cerevisiae is still poor. Here, we show that UGA-C as the only tetranucleotide of all four possible combinations dramatically exacerbated the readthrough phenotype of the stop codon recognition-deficient mutants in eRF1. Since the same is true also for UAA-C and UAG-C, we propose that the exceptionally high readthrough levels that all three stop codons display when followed by cytosine are partially caused by the compromised sampling ability of eRF1, which specifically senses cytosine at the +4 position. The difference in termination efficiencies among the remaining three UGA-N tetranucleotides is then given by their varying preferences for nc-tRNAs. In particular, UGA-A allows increased incorporation of Trp-tRNA whereas UGA-G and UGA-C favor Cys-tRNA. Our findings thus expand the repertoire of general decoding rules by showing that the +4 base determines the preferred selection of nc-tRNAs and, in the case of cytosine, it also genetically interacts with eRF1. Finally, using an example of the GCN4 translational control governed by four short uORFs, we also show how the evolution of this mechanism dealt with undesirable readthrough on those uORFs that serve as the key translation reinitiation promoting features of the GCN4 regulation, as both of these otherwise counteracting activities, readthrough versus reinitiation, are mediated by eIF3.

Translation initiation factors eIF3 and HCR1 control translation termination and stop codon read-through in yeast cells

Translation is divided into initiation, elongation, termination and ribosome recycling. Earlier work implicated several eukaryotic initiation factors (eIFs) in ribosomal recycling in vitro. Here, we uncover roles for HCR1 and eIF3 in translation termination in vivo. A substantial proportion of eIF3, HCR1 and eukaryotic release factor 3 (eRF3) but not eIF5 (a well-defined "initiation-specific" binding partner of eIF3) specifically co-sediments with 80S couples isolated from RNase-treated heavy polysomes in an eRF1-dependent manner, indicating the presence of eIF3 and HCR1 on terminating ribosomes. eIF3 and HCR1 also occur in ribosome- and RNA-free complexes with both eRFs and the recycling factor ABCE1/RLI1. Several eIF3 mutations reduce rates of stop codon read-through and genetically interact with mutant eRFs. In contrast, a slow growing deletion of hcr1 increases read-through and accumulates eRF3 in heavy polysomes in a manner suppressible by overexpressed ABCE1/RLI1. Based on these and other findings we propose that upon stop codon recognition, HCR1 promotes eRF3·GDP ejection from the post-termination complexes to allow binding of its interacting partner ABCE1/RLI1. Furthermore, the fact that high dosage of ABCE1/RLI1 fully suppresses the slow growth phenotype of hcr1Δ as well as its termination but not initiation defects implies that the termination function of HCR1 is more critical for optimal proliferation than its function in translation initiation. Based on these and other observations we suggest that the assignment of HCR1 as a bona fide eIF3 subunit should be reconsidered. Together our work characterizes novel roles of eIF3 and HCR1 in stop codon recognition, defining a communication bridge between the initiation and termination/recycling phases of translation.

NMR solution structure and function of the C-terminal domain of eukaryotic class 1 polypeptide chain release factor

Termination of translation in eukaryotes is triggered by two polypeptide chain release factors, eukaryotic class 1 polypeptide chain release factor (eRF1) and eukaryotic class 2 polypeptide chain release factor 3. eRF1 is a three-domain protein that interacts with eukaryotic class 2 polypeptide chain release factor 3 via its C-terminal domain (C-domain). The high-resolution NMR structure of the human C-domain (residues 277–437) has been determined in solution. The overall fold and the structure of the β-strand core of the protein in solution are similar to those found in the crystal structure. The structure of the minidomain (residues 329–372), which was ill-defined in the crystal structure, has been determined in solution. The protein backbone dynamics, studied using ¹⁵N-relaxation experiments, showed that the C-terminal tail 414–437 and the minidomain are the most flexible parts of the human C-domain. The minidomain exists in solution in two conformational states, slowly interconverting on the NMR timescale. Superposition of this NMR solution structure of the human C-domain onto the available crystal structure of full-length human eRF1 shows that the minidomain is close to the stop codon-recognizing N-terminal domain. Mutations in the tip of the minidomain were found to affect the stop codon specificity of the factor. The results provide new insights into the possible role of the C-domain in the process of translation termination.

NMR solution structure and function of the C-terminal domain of eukaryotic class 1 polypeptide chain release factor

Termination of translation in eukaryotes is triggered by two polypeptide chain release factors, eukaryotic class 1 polypeptide chain release factor (eRF1) and eukaryotic class 2 polypeptide chain release factor 3. eRF1 is a three-domain protein that interacts with eukaryotic class 2 polypeptide chain release factor 3 via its C-terminal domain (C-domain). The high-resolution NMR structure of the human C-domain (residues 277-437) has been determined in solution. The overall fold and the structure of the beta-strand core of the protein in solution are similar to those found in the crystal structure. The structure of the minidomain (residues 329-372), which was ill-defined in the crystal structure, has been determined in solution. The protein backbone dynamics, studied using (15)N-relaxation experiments, showed that the C-terminal tail 414-437 and the minidomain are the most flexible parts of the human C-domain. The minidomain exists in solution in two conformational states, slowly interconverting on the NMR timescale. Superposition of this NMR solution structure of the human C-domain onto the available crystal structure of full-length human eRF1 shows that the minidomain is close to the stop codon-recognizing N-terminal domain. Mutations in the tip of the minidomain were found to affect the stop codon specificity of the factor. The results provide new insights into the possible role of the C-domain in the process of translation termination.