Transient receptor potential (TRP) melastatin 4 (TRPM4) is a widely expressed cation channel associated with a variety of cardiovascular disorders. TRPM4 is activated by increased intracellular ...calcium in a voltage-dependent manner but, unlike many other TRP channels, is permeable to monovalent cations only. Here we present two structures of full-length human TRPM4 embedded in lipid nanodiscs at ~3-angstrom resolution, as determined by single-particle cryo-electron microscopy. These structures, with and without calcium bound, reveal a general architecture for this major subfamily of TRP channels and a well-defined calcium-binding site within the intracellular side of the S1-S4 domain. The structures correspond to two distinct closed states. Calcium binding induces conformational changes that likely prime the channel for voltage-dependent opening.
The integrated stress response (ISR) tunes the rate of protein synthesis. Control is exerted by phosphorylation of the general translation initiation factor eIF2. eIF2 is a guanosine triphosphatase ...that becomes activated by eIF2B, a two-fold symmetric and heterodecameric complex that functions as eIF2's dedicated nucleotide exchange factor. Phosphorylation converts eIF2 from a substrate into an inhibitor of eIF2B. We report cryo-electron microscopy structures of eIF2 bound to eIF2B in the dephosphorylated state. The structures reveal that the eIF2B decamer is a static platform upon which one or two flexible eIF2 trimers bind and align with eIF2B's bipartite catalytic centers to catalyze nucleotide exchange. Phosphorylation refolds eIF2α, allowing it to contact eIF2B at a different interface and, we surmise, thereby sequestering it into a nonproductive complex.
Structure of the human 80S ribosome Khatter, Heena; Myasnikov, Alexander G; Natchiar, S Kundhavai ...
Nature (London),
04/2015, Letnik:
520, Številka:
7549
Journal Article
Recenzirano
Ribosomes are translational machineries that catalyse protein synthesis. Ribosome structures from various species are known at the atomic level, but obtaining the structure of the human ribosome has ...remained a challenge; efforts to address this would be highly relevant with regard to human diseases. Here we report the near-atomic structure of the human ribosome derived from high-resolution single-particle cryo-electron microscopy and atomic model building. The structure has an average resolution of 3.6 Å, reaching 2.9 Å resolution in the most stable regions. It provides unprecedented insights into ribosomal RNA entities and amino acid side chains, notably of the transfer RNA binding sites and specific molecular interactions with the exit site tRNA. It reveals atomic details of the subunit interface, which is seen to remodel strongly upon rotational movements of the ribosomal subunits. Furthermore, the structure paves the way for analysing antibiotic side effects and diseases associated with deregulated protein synthesis.
Sphingolipids are essential lipids in eukaryotic membranes. In humans, the first and rate-limiting step of sphingolipid synthesis is catalyzed by the serine palmitoyltransferase holocomplex, which ...consists of catalytic components (SPTLC1 and SPTLC2) and regulatory components (ssSPTa and ORMDL3). However, the assembly, substrate processing and regulation of the complex are unclear. Here, we present 8 cryo-electron microscopy structures of the human serine palmitoyltransferase holocomplex in various functional states at resolutions of 2.6-3.4 Å. The structures reveal not only how catalytic components recognize the substrate, but also how regulatory components modulate the substrate-binding tunnel to control enzyme activity: ssSPTa engages SPTLC2 and shapes the tunnel to determine substrate specificity. ORMDL3 blocks the tunnel and competes with substrate binding through its amino terminus. These findings provide mechanistic insights into sphingolipid biogenesis governed by the serine palmitoyltransferase complex.
Transcriptional pausing by RNA polymerases (RNAPs) is a key mechanism to regulate gene expression in all kingdoms of life and is a prerequisite for transcription termination. The essential bacterial ...transcription factor NusA stimulates both pausing and termination of transcription, thus playing a central role. Here, we report single-particle electron cryo-microscopy reconstructions of NusA bound to paused E. coli RNAP elongation complexes with and without a pause-enhancing hairpin in the RNA exit channel. The structures reveal four interactions between NusA and RNAP that suggest how NusA stimulates RNA folding, pausing, and termination. An asymmetric translocation intermediate of RNA and DNA converts the active site of the enzyme into an inactive state, providing a structural explanation for the inhibition of catalysis. Comparing RNAP at different stages of pausing provides insights on the dynamic nature of the process and the role of NusA as a regulatory factor.
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•Two cryo-EM reconstructions of paused RNAP elongation complexes bound by NusA•NusA provides positively charged cavity for RNA structures and stabilizes pause•Asymmetric translocation intermediate explains transcriptional pausing•Dynamic process of pausing reflected by RNAP global conformational changes
Guo et al. present cryo-EM structures of paused RNAP bound to NusA. NusA interacts through four points with RNAP and forms a positively charged cavity above the pause-stabilizing RNA hairpin. An asymmetric, half-translocated RNA-DNA hybrid (RNA post-translocated, template DNA pre-translocated) explains transcriptional pausing. NusA further stabilizes the paused RNAP.
Maintaining a healthy proteome is fundamental for the survival of all organisms
. Integral to this are Hsp90 and Hsp70, molecular chaperones that together facilitate the folding, remodelling and ...maturation of the many 'client proteins' of Hsp90
. The glucocorticoid receptor (GR) is a model client protein that is strictly dependent on Hsp90 and Hsp70 for activity
. Chaperoning GR involves a cycle of inactivation by Hsp70; formation of an inactive GR-Hsp90-Hsp70-Hop 'loading' complex; conversion to an active GR-Hsp90-p23 'maturation' complex; and subsequent GR release
. However, to our knowledge, a molecular understanding of this intricate chaperone cycle is lacking for any client protein. Here we report the cryo-electron microscopy structure of the GR-loading complex, in which Hsp70 loads GR onto Hsp90, uncovering the molecular basis of direct coordination by Hsp90 and Hsp70. The structure reveals two Hsp70 proteins, one of which delivers GR and the other scaffolds the Hop cochaperone. Hop interacts with all components of the complex, including GR, and poises Hsp90 for subsequent ATP hydrolysis. GR is partially unfolded and recognized through an extended binding pocket composed of Hsp90, Hsp70 and Hop, revealing the mechanism of GR loading and inactivation. Together with the GR-maturation complex structure
, we present a complete molecular mechanism of chaperone-dependent client remodelling, and establish general principles of client recognition, inhibition, transfer and activation.
mRNA translation initiation in eukaryotes requires the cooperation of a dozen eukaryotic initiation factors (eIFs) forming several complexes, which leads to mRNA attachment to the small ribosomal 40S ...subunit, mRNA scanning for start codon, and accommodation of initiator tRNA at the 40S P site. eIF3, composed of 13 subunits, 8 core (a, c, e, f, h, l, k, and m) and 5 peripheral (b, d, g, i, and j), plays a central role during this process. Here we report a cryo-electron microscopy structure of a mammalian 48S initiation complex at 5.8 Å resolution. It shows the relocation of subunits eIF3i and eIF3g to the 40S intersubunit face on the GTPase binding site, at a late stage in initiation. On the basis of a previous study, we demonstrate the relocation of eIF3b to the 40S intersubunit face, binding below the eIF2-Met-tRNAiMet ternary complex upon mRNA attachment. Our analysis reveals the deep rearrangement of eIF3 and unravels the molecular mechanism underlying eIF3 function in mRNA scanning and timing of ribosomal subunit joining.
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•Several subunits of eIF3 relocate upon mRNA binding and start-codon recognition•mRNA binding leads to the relocation of eIF3b to interact with eIF2 ternary complex•eIF3i+g relocate to the 40S intersubunit face after start-codon recognition•eIF3b and eIF1 reposition the eIF2 ternary complex and assist in scanning
Protein synthesis initiation in mammals requires the assembly of a dozen initiation factors, in which eIF3 has a pivotal role. Simonetti et al. report by cryo-EM the large-scale conformational changes of several subunits of eIF3 responsible for the regulation of several key steps, such as mRNA scanning and timing of ribosomal subunit joining.
Chemical modifications of human ribosomal RNA (rRNA) are introduced during biogenesis and have been implicated in the dysregulation of protein synthesis, as is found in cancer and other diseases. ...However, their role in this phenomenon is unknown. Here we visualize more than 130 individual rRNA modifications in the three-dimensional structure of the human ribosome, explaining their structural and functional roles. In addition to a small number of universally conserved sites, we identify many eukaryote- or human-specific modifications and unique sites that form an extended shell in comparison to bacterial ribosomes, and which stabilize the RNA. Several of the modifications are associated with the binding sites of three ribosome-targeting antibiotics, or are associated with degenerate states in cancer, such as keto alkylations on nucleotide bases reminiscent of specialized ribosomes. This high-resolution structure of the human 80S ribosome paves the way towards understanding the role of epigenetic rRNA modifications in human diseases and suggests new possibilities for designing selective inhibitors and therapeutic drugs.
Abstract
Proteasomal machinery performs essential regulated protein degradation in eukaryotes. Classic proteasomes are symmetric, with a regulatory ATPase docked at each end of the cylindrical 20S. ...Asymmetric complexes are also present in cells, either with a single ATPase or with an ATPase and non-ATPase at two opposite ends. The mechanism that populates these different proteasomal complexes is unknown. Using archaea homologs, we construct asymmetric forms of proteasomes. We demonstrate that the gate conformation of the two opposite ends of 20S are coupled: binding one ATPase opens a gate locally, and also opens the opposite gate allosterically. Such allosteric coupling leads to cooperative binding of proteasomal ATPases to 20S and promotes formation of proteasomes symmetrically configured with two identical ATPases. It may also promote formation of asymmetric complexes with an ATPase and a non-ATPase at opposite ends. We propose that in eukaryotes a similar mechanism regulates the composition of the proteasomal population.
Peptide-chain elongation during protein synthesis entails sequential aminoacyl-tRNA selection and translocation reactions that proceed rapidly (2-20 per second) and with a low error rate (around 10
... to 10
at each step) over thousands of cycles
. The cadence and fidelity of ribosome transit through mRNA templates in discrete codon increments is a paradigm for movement in biological systems that must hold for diverse mRNA and tRNA substrates across domains of life. Here we use single-molecule fluorescence methods to guide the capture of structures of early translocation events on the bacterial ribosome. Our findings reveal that the bacterial GTPase elongation factor G specifically engages spontaneously achieved ribosome conformations while in an active, GTP-bound conformation to unlock and initiate peptidyl-tRNA translocation. These findings suggest that processes intrinsic to the pre-translocation ribosome complex can regulate the rate of protein synthesis, and that energy expenditure is used later in the translocation mechanism than previously proposed.
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