Ribosome inhibition by C9ORF72-ALS/FTD-associated poly-PR and poly-GR proteins revealed by cryo-EM.

Toxic dipeptide-repeat (DPR) proteins are produced from expanded G4C2 repeats in the C9ORF72 gene, the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Two DPR proteins, poly-PR and poly-GR, repress cellular translation but the molecular mechanism remains unknown. Here we show that poly-PR and poly-GR of ≥20 repeats inhibit the ribosome's peptidyl-transferase activity at nanomolar concentrations, comparable to specific translation inhibitors. High-resolution cryogenic electron microscopy (cryo-EM) reveals that poly-PR and poly-GR block the polypeptide tunnel of the ribosome, extending into the peptidyl-transferase center (PTC). Consistent with these findings, the macrolide erythromycin, which binds in the tunnel, competes with poly-PR and restores peptidyl-transferase activity. Our results demonstrate that strong and specific binding of poly-PR and poly-GR in the ribosomal tunnel blocks translation, revealing the structural basis of their toxicity in C9ORF72-ALS/FTD.

Termi-Luc: a versatile assay to monitor full-protein release from ribosomes.

Termination of protein biosynthesis is an essential step of gene expression, during which a complete functional protein is released from the ribosome. Premature or inefficient termination results in truncated, nonfunctional, or toxic proteins that may cause disease. Indeed, more than 10% of human genetic diseases are caused by nonsense mutations leading to premature termination. Efficient and sensitive approaches are required to study eukaryotic termination mechanisms and to identify potential therapeutics that modulate termination. Canonical radioactivity-based termination assays are complex, report on a short peptide release, and are incompatible with high-throughput screening. Here we describe a robust and simple in vitro assay to study the kinetics of full-protein release. The assay monitors luminescence upon release of nanoluciferase from a mammalian pretermination complex. The assay can be used to record time-progress curves of protein release in a high-throughput format, making it optimal for studying release kinetics and for high-throughput screening for small molecules that modulate the efficiency of termination.

Structural and functional properties of αß-heterodimers of tropomyosin with myopathic mutations Q147P and K49del in the ß-chain.

Tropomyosin (Tpm) is an α-helical coiled-coil actin-binding protein that plays a key role in the Ca2+-regulated contraction of striated muscles. Two Tpm isoforms, α (Tpm 1.1) and ß (Tpm 2.2), are expressed in fast skeletal muscles. These Tpm isoforms can form either αα and ßß homodimers, or αß heterodimers. However, only αα-Tpm and αß-Tpm dimers are usually present in most of fast skeletal muscles, because ßß-homodimers are relatively unstable and cannot exist under physiologic conditions. Nevertheless, the most of previous studies of myopathy-causing mutations in the Tpm ß-chains were performed on the ßß-homodimers. In the present work, we applied different methods to investigate the effects of two myopathic mutations in the ß-chain, Q147P and K49del (i.e. deletion of Lys49), on structural and functional properties of Tpm αß-heterodimers and to compare them with the properties of ßß-homodimers carrying these mutations in both ß-chains. The results show that the properties of αß-Tpm heterodimers with these mutations in the ß-chain differ significantly from the properties of ßß-homodimers with the same substitutions in both ß-chains. This indicates that the αß-heterodimer is a more appropriate model for studying the effects of myopathic mutations in the ß-chain of Tpm than the ßß-homodimer which virtually does not exist in human skeletal muscles.

Thermal unfolding of homodimers and heterodimers of different skeletal-muscle isoforms of tropomyosin.

We applied differential scanning calorimetry (DSC) to investigate the structural properties of three isoforms of tropomyosin (Tpm), α, ß, and γ, expressed from different genes in human skeletal muscles. We compared specific features of the thermal unfolding of αα, ßß, and γγ Tpm homodimers, as well as of αß and Î³ß Tpm heterodimers. The results show that the thermal stability of γγ homodimer is much higher than that of αα homodimer which, in turn, is much more thermostable than the ßß homodimer. The stability of the Î³ß Tpm heterodimer is much lower than that of the γγ homodimer, and its thermal unfolding is quite different from that for γγ and ßß homodimers, whereas the unfolding of the αß heterodimer is roughly similar to that of the αα homodimer.

Eukaryotic translation elongation factor 2 (eEF2) catalyzes reverse translocation of the eukaryotic ribosome.

During protein synthesis, a ribosome moves along the mRNA template and, using aminoacyl-tRNAs, decodes the template nucleotide triplets to assemble a protein amino acid sequence. This movement is accompanied by shifting of mRNA-tRNA complexes within the ribosome in a process called translocation. In living cells, this process proceeds in a unidirectional manner, bringing the ribosome to the 3' end of mRNA, and is catalyzed by the GTPase translation elongation factor 2 (EF-G in prokaryotes and eEF2 in eukaryotes). Interestingly, the possibility of spontaneous backward translocation has been shown in vitro for bacterial ribosomes, suggesting a potential reversibility of this reaction. However, this possibility has not yet been tested for eukaryotic ribosomes. Here, using a reconstituted mammalian translation system, we show that the eukaryotic elongation factor eEF2 catalyzes ribosomal reverse translocation at one mRNA triplet. We found that this process requires a cognate tRNA in the ribosomal E-site and cannot occur spontaneously without eEF2. The efficiency of this reaction depended on the concentrations of eEF2 and cognate tRNAs and increased in the presence of nonhydrolyzable GTP analogues. Of note, ADP-ribosylation of eEF2 domain IV blocked reverse translocation, suggesting a crucial role of interactions of this domain with the ribosome for the catalysis of the reaction. In summary, our findings indicate that eEF2 is able to induce ribosomal translocation in forward and backward directions, highlighting the universal mechanism of tRNA-mRNA movements within the ribosome.

RNA helicase DDX19 stabilizes ribosomal elongation and termination complexes.

The human DEAD-box RNA-helicase DDX19 functions in mRNA export through the nuclear pore complex. The yeast homolog of this protein, Dbp5, has been reported to participate in translation termination. Using a reconstituted mammalian in vitro translation system, we show that the human protein DDX19 is also important for translation termination. It is associated with the fraction of translating ribosomes. We show that DDX19 interacts with pre-termination complexes (preTCs) in a nucleotide-dependent manner. Furthermore, DDX19 increases the efficiency of termination complex (TC) formation and the peptide release in the presence of eukaryotic release factors. Using the eRF1(AGQ) mutant protein or a non-hydrolysable analog of GTP to inhibit subsequent peptidyl-tRNA hydrolysis, we reveal that the activation of translation termination by DDX19 occurs during the stop codon recognition. This activation is a result of DDX19 binding to preTC and a concomitant stabilization of terminating ribosomes. Moreover, we show that DDX19 stabilizes ribosome complexes with translation elongation factors eEF1 and eEF2. Taken together, our findings reveal that the human RNA helicase DDX19 actively participates in protein biosynthesis.

Upstream Open Reading Frames Located in the Leader of Protein Kinase Mζ mRNA Regulate Its Translation.

For protein synthesis that occurs locally in dendrites, the translational control mechanisms are much more important for neuronal functioning than the transcription levels. Here, we show that uORFs (upstream open reading frames) in the 5' untranslated region (5'UTR) play a critical role in regulation of the translation of protein kinase Mζ (PKMζ). Elimination of these uORFs activates translation of the reporter protein in vitro and in primary cultures of rat hippocampal neurons. Using cell-free translation systems, we demonstrate that translational initiation complexes are formed only on uORFs. Further, we address the mechanism of translational repression of PKMζ translation, by uORFs. We observed an increase in translation of the reporter protein under the control of PKMζ leader in neuronal culture during non-specific activation by picrotoxin. We also show that such a mechanism is similar to the mechanism seen in cell stress, as application of sodium arsenite to neuron cultures induced translation of mRNA carrying PKMζ 5'UTR similarly to picrotoxin activation. Therefore, we suppose that phosphorylation of eIF2a, like in cell stress, is a main regulator of PKMζ translation. Altogether, our findings considerably extend our understanding of the role of uORF in regulation of PKMζ translation in activated neurons, important at early stages of LTP.

PABP enhances release factor recruitment and stop codon recognition during translation termination.

Poly(A)-binding protein (PABP) is a major component of the messenger RNA-protein complex. PABP is able to bind the poly(A) tail of mRNA, as well as translation initiation factor 4G and eukaryotic release factor 3a (eRF3a). PABP has been found to stimulate translation initiation and to inhibit nonsense-mediated mRNA decay. Using a reconstituted mammalian in vitro translation system, we show that PABP directly stimulates translation termination. PABP increases the efficiency of translation termination by recruitment of eRF3a and eRF1 to the ribosome. PABP's function in translation termination depends on its C-terminal domain and its interaction with the N-terminus of eRF3a. Interestingly, we discover that full-length eRF3a exerts a different mode of function compared to its truncated form eRF3c, which lacks the N-terminal domain. Pre-association of eRF3a, but not of eRF3c, with pre-termination complexes (preTCs) significantly increases the efficiency of peptidyl-tRNA hydrolysis by eRF1. This implicates new, additional interactions of full-length eRF3a with the ribosomal preTC. Based on our findings, we suggest that PABP enhances the productive binding of the eRF1-eRF3 complex to the ribosome, via interactions with the N-terminal domain of eRF3a which itself has an active role in translation termination.

Stabilization of eukaryotic ribosomal termination complexes by deacylated tRNA.

Stabilization of the ribosomal complexes plays an important role in translational control. Mechanisms of ribosome stabilization have been studied in detail for initiation and elongation of eukaryotic translation, but almost nothing is known about stabilization of eukaryotic termination ribosomal complexes. Here, we present one of the mechanisms of fine-tuning of the translation termination process in eukaryotes. We show that certain deacylated tRNAs, remaining in the E site of the ribosome at the end of the elongation cycle, increase the stability of the termination and posttermination complexes. Moreover, only the part of eRF1 recognizing the stop codon is stabilized in the A site of the ribosome, and the stabilization is not dependent on the hydrolysis of peptidyl-tRNA. The determinants, defining this property of the tRNA, reside in the acceptor stem. It was demonstrated by site-directed mutagenesis of tRNA(Val) and construction of a mini-helix structure identical to the acceptor stem of tRNA. The mechanism of this stabilization is different from the fixation of the unrotated state of the ribosome by CCA end of tRNA or by cycloheximide in the E site. Our data allow to reveal the possible functions of the isodecoder tRNAs in eukaryotes.