Nature structural & molecular biology最新文献

DNA double-strand breaks occur daily in all human cells and must be repaired with high fidelity to minimize genomic instability. Deficiencies in high-fidelity DNA repair by homologous recombination lead to dependence on DNA polymerase θ, which identifies DNA microhomologies in 3′ single-stranded DNA overhangs and anneals them to initiate error-prone double-strand break repair. The resulting genomic instability is associated with numerous cancers, thereby making this polymerase an attractive therapeutic target. However, despite the biomedical importance of polymerase θ, the molecular details of how it initiates DNA break repair remain unclear. Here, we present cryo-electron microscopy structures of the polymerase θ helicase domain bound to microhomology-containing DNA, revealing DNA-induced rearrangements of the helicase that enable DNA repair. Our structures show that DNA-bound helicase dimers facilitate a microhomology search that positions 3′ single-stranded DNA ends in proximity to align complementary bases and anneal DNA microhomology. We characterize the molecular determinants that enable the helicase domain of polymerase θ to identify and pair DNA microhomologies to initiate mutagenic DNA repair, thereby providing insight into potentially targetable interactions for therapeutic interventions.

Circularly permuted (CP) group II introns, identified in various bacteria phyla, swap domains D5 and D6 near the 5′ end and have reversed splice sites (SSs), leading to backsplicing and circular RNA formation. In this study, we present multiple high-resolution cryo-electron microscopy structures of a natural CP group II intron from Comamonas testosteroni KF-1 (Cte 1), elucidating the molecular mechanisms of branching and backsplicing. During branching, the 5′ SS is positioned by an auxiliary sequence (AUX)-enhanced interaction between the exon-binding site and intron-binding site (IBS) and stacks on the branch-site adenosine within D6, allowing the attacking 2′-OH group to coordinate with a metal ion in the active center. In backsplicing, the 3′ SS is aligned with the branching step, leaving IBS in the active center, stabilized by base pairing with the AUX, which enables the free 3′-end hydroxyl group to directly attack the scissile phosphate of 3′ SS. Furthermore, a groove in Cte 1 may stabilize the circular RNA. These findings highlight a conserved catalytic mechanism for canonical group II introns, albeit facilitated by the versatile AUX, opening avenues for designing potent ribozymes producing circular RNAs.

Many naturally occurring protein assemblies have dynamic structures that allow them to perform specialized functions. Although computational methods for designing novel self-assembling proteins have advanced substantially over the past decade, they primarily focus on designing static structures. Here we characterize three distinct computationally designed protein assemblies that exhibit unanticipated structural diversity arising from flexibility in their subunits. Cryo-EM single-particle reconstructions and native mass spectrometry reveal two distinct architectures for two assemblies, while six cryo-EM reconstructions for the third likely represent a subset of its solution-phase structures. Structural modeling and molecular dynamics simulations indicate that constrained flexibility within the subunits of each assembly promotes a defined range of architectures rather than nonspecific aggregation. Redesigning the flexible region in one building block rescues the intended monomorphic assembly. These findings highlight structural flexibility as a powerful design principle, enabling exploration of new structural and functional spaces in protein assembly design.

Polyunsaturated fatty acid (PUFA) lipids modulate the neuronal and microglial leak potassium channel K_2P13.1 (THIK1) and other voltage-gated ion channel (VGIC) superfamily members through poorly understood mechanisms. Here we present cryo-electron microscopy structures of human THIK1 and mutants, revealing a unique two-chamber aqueous inner cavity obstructed by a hydrophilic barrier important for gating, the flow restrictor, and a P1–M4 intersubunit interface lipid at a site, the PUFA site, corresponding to the K_2P small-molecule modulator pocket. This overlap, together with functional studies, indicates that PUFA site lipids are THIK1 cofactors. Comparison with a PUFA-responsive VGIC, K_v7.1, reveals a shared modulatory role for the pore domain intersubunit interface, providing a framework for understanding PUFA action on the VGIC superfamily. Our findings reveal the distinct THIK1 architecture, highlight the importance of the P1–M4 interface for K_2P control by natural and synthetic ligands and should aid in the development of THIK subfamily modulators for neuroinflammation and autism.

Broad-range thermal receptor 1 (BRTNaC1), activated by heat at low extracellular pH, was recently identified in myriapods. Although the overexpression of BRTNaC1 leads to robust heat-activated current with a cation selectivity profile, the structure of this receptor and how it is gated by proton and heat remain to be investigated. Here we determine cryogenic electron microscopy structures of BRTNaC1 in the apo, proton-induced and heated states. Based on these structures, patch-clamp recordings and molecular dynamic simulations, we found that a ‘twist the wrist’ mechanism is used for proton activation of BRTNaC1, while heat induces broad conformational changes in BRTNaC1, including rotation and shift in the transmembrane helices to open this channel. Moreover, as testosterone inhibited BRTNaC1 activation, we identified four clustered residues important for such inhibition. Therefore, our study has established the structural basis for ligand and temperature gating in the BRTNaC1 ion channel.

THIK-1 (KCNK13) is a halothane-inhibited and anionic-lipid-activated two-pore domain (K2P) K⁺ channel implicated in microglial activation and neuroinflammation, and a current target for the treatment of neurodegenerative disorders, for example Alzheimer’s disease and amyothropic lateral sclerosis (ALS). However, compared to other K2P channels, little is known about the structural and functional properties of THIK-1. Here we present a 3.16-Å-resolution cryo-EM structure of human THIK-1 that reveals several distinct features, in particular, a tyrosine in M4 that contributes to a lower ‘Y gate’ that opens upon activation by physiologically relevant G-protein-coupled receptor and lipid signaling pathways. We demonstrate that linoleic acid bound within a modulatory pocket adjacent to the filter influences channel activity, and that halothane inhibition involves a binding site within the inner cavity, both resulting in conformational changes to the Y gate. Finally, the extracellular cap domain contains positively charged residues that line the ion exit pathway and contribute to the distinct biophysical properties of this channel. Overall, our results provide structural insights into THIK-1 function and identify distinct regulatory sites that expand its potential as a drug target for the modulation of microglial function.

Recent findings indicate that nuclear speckles, a distinct type of nuclear body, interact with certain chromatin regions in a ground state. Here, we report that the chromatin structural factors CTCF and cohesin are required for full ground-state association between DNA and nuclear speckles. We identified a putative speckle-targeting motif (STM) within cohesin subunit RAD21 and demonstrated that the STM is required for chromatin–nuclear speckle association, disruption of which also impaired induction of speckle-associated genes. Depletion of the cohesin-releasing factor WAPL, which stabilizes cohesin on chromatin, resulted in reinforcement of DNA–speckle contacts and enhanced inducibility of speckle-associated genes. Additionally, we observed disruption of chromatin–nuclear speckle association in patient-derived cells with Cornelia de Lange syndrome, a congenital neurodevelopmental disorder involving defective cohesin pathways. In summary, our findings reveal a mechanism for establishing the ground state of chromatin–speckle association and promoting gene inducibility, with relevance to human disease.

Structural diversity of the nucleosome affects chromatin conformations and regulates eukaryotic genome functions. Here we identify DEK, whose function is unknown, as a nucleosome-binding protein. In embryonic neural progenitor cells, DEK colocalizes with H3 K27 trimethylation (H3K27me3), the facultative heterochromatin mark. DEK stimulates the methyltransferase activity of Polycomb repressive complex 2 (PRC2), which is responsible for H3K27me3 deposition in vitro. Cryo-electron microscopy structures of the DEK–nucleosome complexes reveal that DEK binds the nucleosome by its tripartite DNA-binding mode on the dyad and linker DNAs and interacts with the nucleosomal acidic patch by its newly identified histone-binding region. The DEK–nucleosome interaction mediates linker DNA reorientation and induces chromatin compaction, which may facilitate PRC2 activation. These findings provide mechanistic insights into chromatin structure-mediated gene regulation by DEK.