Crystal Structure of the Human Ribosome in Complex with DENR-MCT-1.
ABSTRACT: The repertoire of the density-regulated protein (DENR) and the malignant T cell-amplified sequence 1 (MCT-1/MCTS1) oncoprotein was recently expanded to include translational control of a specific set of cancer-related mRNAs. DENR and MCT-1 form the heterodimer, which binds to the ribosome and operates at both translation initiation and reinitiation steps, though by a mechanism that is yet unclear. Here, we determined the crystal structure of the human small ribosomal subunit in complex with DENR-MCT-1. The structure reveals the location of the DENR-MCT-1 dimer bound to the small ribosomal subunit. The binding site of the C-terminal domain of DENR on the ribosome has a striking similarity with those of canonical initiation factor 1 (eIF1), which controls the fidelity of translation initiation and scanning. Our findings elucidate how the DENR-MCT-1 dimer interacts with the ribosome and have functional implications for the mechanism of unconventional translation initiation and reinitiation.
Project description:The density-regulated protein (DENR) and the malignant T cell-amplified sequence 1 (MCT-1/MCTS1) oncoprotein support noncanonical translation initiation, promote translation reinitiation on a specific set of mRNAs with short upstream reading frames, and regulate ribosome recycling. DENR and MCT-1 form a heterodimer, which binds to the ribosome. We determined the crystal structure of the heterodimer formed by human MCT-1 and the N-terminal domain of DENR at 2.0-Å resolution. The structure of the heterodimer reveals atomic details of the mechanism of DENR and MCT-1 interaction. Four conserved cysteine residues of DENR (C34, C37, C44, C53) form a classical tetrahedral zinc ion-binding site, which preserves the structure of the DENR's MCT-1-binding interface that is essential for the dimerization. Substitution of all four cysteines by alanine abolished a heterodimer formation. Our findings elucidate further the mechanism of regulation of DENR-MCT-1 activities in unconventional translation initiation, reinitiation, and recycling.
Project description:The succession of molecular events leading to eukaryotic translation reinitiation-whereby ribosomes terminate translation of a short open reading frame (ORF), resume scanning, and then translate a second ORF on the same mRNA-is not well understood. Density-regulated reinitiation and release factor (DENR) and multiple copies in T-cell lymphoma-1 (MCTS1) are implicated in promoting translation reinitiation both in vitro in translation extracts and in vivo. We present here the crystal structure of MCTS1 bound to a fragment of DENR. Based on this structure, we identify and experimentally validate that DENR residues Glu42, Tyr43, and Tyr46 are important for MCTS1 binding and that MCTS1 residue Phe104 is important for tRNA binding. Mutation of these residues reveals that DENR-MCTS1 dimerization and tRNA binding are both necessary for DENR and MCTS1 to promote translation reinitiation in human cells. These findings thereby link individual residues of DENR and MCTS1 to specific molecular functions of the complex. Since DENR-MCTS1 can bind tRNA in the absence of the ribosome, this suggests the DENR-MCTS1 complex could recruit tRNA to the ribosome during reinitiation analogously to the eukaryotic initiation factor 2 (eIF2) complex in cap-dependent translation.
Project description:During cap-dependent eukaryotic translation initiation, ribosomes scan messenger RNA from the 5' end to the first AUG start codon with favourable sequence context. For many mRNAs this AUG belongs to a short upstream open reading frame (uORF), and translation of the main downstream ORF requires re-initiation, an incompletely understood process. Re-initiation is thought to involve the same factors as standard initiation. It is unknown whether any factors specifically affect translation re-initiation without affecting standard cap-dependent translation. Here we uncover the non-canonical initiation factors density regulated protein (DENR) and multiple copies in T-cell lymphoma-1 (MCT-1; also called MCTS1 in humans) as the first selective regulators of eukaryotic re-initiation. mRNAs containing upstream ORFs with strong Kozak sequences selectively require DENR-MCT-1 for their proper translation, yielding a novel class of mRNAs that can be co-regulated and that is enriched for regulatory proteins such as oncogenic kinases. Collectively, our data reveal that cells have a previously unappreciated translational control system with a key role in supporting proliferation and tissue growth.
Project description:Translation efficiency varies considerably between different mRNAs, thereby impacting protein expression. Translation of the stress response master-regulator ATF4 increases upon stress, but the molecular mechanisms are not well understood. We discover here that translation factors DENR, MCTS1 and eIF2D are required to induce ATF4 translation upon stress by promoting translation reinitiation in the ATF4 5'UTR. We find DENR and MCTS1 are only needed for reinitiation after upstream Open Reading Frames (uORFs) containing certain penultimate codons, perhaps because DENR•MCTS1 are needed to evict only certain tRNAs from post-termination 40S ribosomes. This provides a model for how DENR and MCTS1 promote translation reinitiation. Cancer cells, which are exposed to many stresses, require ATF4 for survival and proliferation. We find a strong correlation between DENR•MCTS1 expression and ATF4 activity across cancers. Furthermore, additional oncogenes including a-Raf, c-Raf and Cdk4 have long uORFs and are translated in a DENR•MCTS1 dependent manner.
Project description:The recycling of ribosomal subunits after translation termination is critical for efficient gene expression. Tma64 (eIF2D), Tma20 (MCT-1), and Tma22 (DENR) function as 40S recycling factors in vitro, but it is unknown whether they perform this function in vivo. Ribosome profiling of tma deletion strains revealed 80S ribosomes queued behind the stop codon, consistent with a block in 40S recycling. We found that unrecycled ribosomes could reinitiate translation at AUG codons in the 3' UTR, as evidenced by peaks in the footprint data and 3' UTR reporter analysis. In vitro translation experiments using reporter mRNAs containing upstream open reading frames (uORFs) further established that reinitiation increased in the absence of these proteins. In some cases, 40S ribosomes appeared to rejoin with 60S subunits and undergo an 80S reinitiation process in 3' UTRs. These results support a crucial role for Tma64, Tma20, and Tma22 in recycling 40S ribosomal subunits at stop codons and translation reinitiation.
Project description:The non-canonical initiation factors DENR and MCTS1 have been linked to cancer and autism. We recently showed in Drosophila that DENR and MCTS1 regulate translation re-initiation on transcripts containing upstream Open Reading Frames (uORFs) with strong Kozak sequences (stuORFs). Due to the medical relevance of DENR and MCTS1, it is worthwhile identifying the transcripts in human cells that depend on DENR and MCTS1 for their translation. We show here that in humans, as in Drosophila, transcripts with short stuORFs require DENR and MCTS1 for their optimal expression. In contrast to Drosophila, however, the dependence on stuORF length in human cells is very strong, so that only transcripts with very short stuORFs coding for 1 amino acid are dependent on DENR and MCTS1. This identifies circa 100 genes as putative DENR and MCTS1 translational targets. These genes are enriched for neuronal genes and G protein-coupled receptors. The identification of DENR and MCTS1 target transcripts will serve as a basis for future studies aimed at understanding the mechanistic involvement of DENR and MCTS1 in cancer and autism.
Project description:The density regulated protein (DENR) forms a stable heterodimer with malignant T-cell-amplified sequence 1 (MCT-1). DENR-MCT-1 heterodimer then participates in regulation of non-canonical translation initiation and ribosomal recycling. The N-terminal domain of DENR interacts with MCT-1 and carries a classical tetrahedral zinc ion-binding site, which is crucial for the dimerization. DENR-MCT-1 binds the small (40S) ribosomal subunit through interactions between MCT-1 and helix h24 of the 18S rRNA, and through interactions between the C-terminal domain of DENR and helix h44 of the 18S rRNA. This later interaction occurs in the vicinity of the P site that is also the binding site for canonical translation initiation factor eIF1, which plays the key role in initiation codon selection and scanning. Sequence homology modeling and a low-resolution crystal structure of the DENR-MCT-1 complex with the human 40S subunit suggests that the C-terminal domain of DENR and eIF1 adopt a similar fold. Here we present the crystal structure of the C-terminal domain of DENR determined at 1.74 Å resolution, which confirms its resemblance to eIF1 and advances our understanding of the mechanism by which DENR-MCT-1 regulates non-canonical translation initiation and ribosomal recycling.
Project description:The recycling of ribosomal subunits after translation termination is critical for efficient gene expression. Tma64 (eIF2D), Tma20 (MCT-1), and Tma22 (DENR) function as 40S recycling factors in vitro, but it is unknown whether they perform this function in vivo or serve as alternative initiation factors. Ribosome profiling of strains missing these factors revealed 80S ribosomes queued behind the stop codon, consistent with a block in 40S recycling. We found that unrecycled ribosomes could reinitiate translation at AUG codons in the 3’UTR, as evidenced by peaks in the footprint data and 3’UTR reporter analysis. In vitro translation experiments using reporter mRNAs containing upstream ORFs (uORFs) further established that reinitiation increased in the absence of these proteins. In some cases, 40S ribosomes appeared to rejoin with 60S subunits and undergo an alternative 80S reinitiation process in 3’UTRs. These results support a crucial role for Tma64, Tma20, and Tma22 in the recycling of 40S ribosomal subunits at stop codons and translation reinitiation. Overall design: 14 biological samples are included in the study for ribosome footprinting. These include wild-type and combination knockouts of TMA64, TMA20, and TMA22, and mutation of SUI1.
Project description:The non-canonical initiation factor DENR promotes translation reinitiation on mRNAs harbouring upstream open reading frames (uORFs). Moreover, DENR depletion shortens circadian period in mouse fibroblasts, suggesting involvement of uORF usage and reinitiation in clock regulation. To identify DENR-regulated translation events transcriptome-wide and, in particular, specific core clock transcripts affected by this mechanism, we have used ribosome profiling in DENR-deficient NIH3T3 cells. We uncovered 240 transcripts with altered translation rate, and used linear regression analysis to extract 5' UTR features predictive of DENR dependence. Among core clock genes, we identified Clock as a DENR target. Using Clock 5' UTR mutants, we mapped the specific uORF through which DENR acts to regulate CLOCK protein biosynthesis. Notably, these experiments revealed an alternative downstream start codon, likely representing the bona fide CLOCK N-terminus. Our findings provide insights into uORF-mediated translational regulation that can regulate the mammalian circadian clock and gene expression at large.
Project description:Disruptions to neuronal mRNA translation are hypothesized to underlie human neurodevelopmental syndromes. Notably, the mRNA translation re-initiation factor DENR is a regulator of eukaryotic translation and cell growth, but its mammalian functions are unknown. Here, we report that Denr influences the migration of murine cerebral cortical neurons in vivo with its binding partner Mcts1, whereas perturbations to Denr impair the long-term positioning, dendritic arborization, and dendritic spine characteristics of postnatal projection neurons. We characterized de novo missense mutations in DENR (p.C37Y and p.P121L) detected in two unrelated human subjects diagnosed with brain developmental disorder to find that each variant impairs the function of DENR in mRNA translation re-initiation and disrupts the migration and terminal branching of cortical neurons in different ways. Thus, our findings link human brain disorders to impaired mRNA translation re-initiation through perturbations in DENR (OMIM: 604550) function in neurons.