Pretomanid and delamanid are prodrugs, whose active derivatives have been reported to target decaprenylphosphoribose-2'-reductase, DprE2, while quabodepistat is a noncovalent inhibitor of decaprenylphosphoribose-2'-oxidase, DprE1. Both enzymes are involved in Mycobacterium tuberculosis cell wall synthesis, but the mechanism of the DprE1-DprE2 epimerase complex and its inhibition by these compounds remain unclear. We report cryo-EM structures of the M. tuberculosis DprE1-DprE2 complex bound with either substrate or quabodepistat in DprE1, and with either activated pretomanid or delamanid in DprE2, respectively. DprE1-DprE2 assembles as a membrane-associated tetramer of a DprE2 dimer flanked on each side by a DprE1 subunit. Both pretomanid and delamanid bind to DprE2 in an NADH-adduct form and within a conserved binding pocket that extends from the NADH-binding site to the substrate-binding site. Quabodepistat binds to DprE1 with a unique mode. Our data reveal the mode of action of these drugs, allowing rational design of new derivatives for improved tuberculosis treatments.

MFSD6 is a newly identified receptor that mediates the invasion of respiratory cells by enterovirus D68 (EV-D68), a non-polio enterovirus that causes severe respiratory disease and poliomyelitis-like illness in children. Here, we report near-atomic-resolution cryo-electron microscopy (cryo-EM) structures of historical and contemporary AFM-associated EV-D68 strains, together with their complexes bound to the third extracellular loop of MFSD6 (MFSD6-L3). These structures uncover a previously unrecognized "binary gating switch" mechanism of virus-receptor engagement that differs from the reported model. In this mechanism, the N200-V208 segment of MFSD6, carrying a glycosylated Asn207, inserts into one capsomer, whereas the sialyl-Gal-terminated glycan of MFSD6 engages an adjacent capsomer. Neu5Ac binding induces conformational rearrangements that expel the pocket factor, destabilize the virion, and prime infection. Functional analyses further define the contributions of the receptor-contacting residues and glycans to viral attachment and entry. Together, our findings refine the molecular basis of EV-D68 recognition of MFSD6 and reveal a glycan-mediated dual-lock mechanism that may enhance receptor specificity, prevent premature uncoating, and ensure productive infection only upon engagement of the correct host receptor. These results provide broader insight into enterovirus tropism and establish a framework for structure-guided antiviral design.

Tuberculosis (TB) remains a major global health threat, exacerbated by the emergence of drug-resistant strains. The mycobacterial enzyme Pks13 has emerged as a promising drug target for novel anti-TB agents. We herein report the design, synthesis, and biological evaluation of a series of pyrimido[1,2-a]imidazole derivatives as potent Pks13-TE inhibitors. An integrated virtual and biological screening of 10.5 million commercially available compounds identified TJA-31 as a hit compound, which showed moderate Pks13-TE inhibitory activity (IC50 = 1.34 μM). The systematic optimization of TJA-31 based on its physicochemical properties, docking scores, and MM/GBSA binding free energy estimates led to the synthesis of 50 analogues, among which 20 compounds exhibited submicromolar inhibition. The most promising derivative, compound 34, demonstrated significantly enhanced potency with an IC50 value of 0.23 μM, representing a sixfold improvement over the hit. Molecular docking studies indicated that the high activity of compound 34 could be attributed to a halogen bond between its bromine substituent and the nitrogen atom of residue His1664, a water-mediated hydrogen bond between the Ala1564 nitrogen and the 3-methoxy oxygen, and π-π stacking interactions with residues within the Pks13-TE binding pocket. These results underscore the pyrimido[1,2-a]imidazole scaffold as a promising lead series for the development of Pks13-TE inhibitors.

Pks13 is a promising target for tuberculosis (TB) treatment, offering a new pathway for anti-TB drug development. Although benzofuran derivatives such as TAM16 have demonstrated significant efficacy in vitro and in vivo, their development was discontinued due to concerns about hERG inhibition. Herein, we designed and synthesized a series of novel furo[2,3-b]isoquinoline derivatives using ring fusion and basicity-reduction strategies. Through SAR studies, compound B23 was identified as a potent Pks13 inhibitor (IC50 = 1.12 μM) with significantly reduced hERG inhibition (IC50 > 10 μM). The markedly improved hERG selectivity not only validates our structural strategy for mitigating cardiotoxicity risks, but also provides a solid foundation for further development of Pks13-TE inhibitors that combine potent anti-TB activity with an improved safety profile.

Poxviruses are large enveloped DNA viruses that cause severe human infectious diseases. The mature virion of poxvirus is covered with dense surface tubular elements (STEs), which play a role in assembly progress of mature virions (MVs) and inhibit host cell protein synthesis. However, the composition and assembly of STEs remain unclear. Cryo-electron microscopy (cryo-EM) has proven to be a powerful technique for determining the structure of proteins from complex biological samples. By integrating high-resolution cryo-EM maps with mass spectrometry, we reveal that STEs are helically assembled from two transmembrane proteins, A14 and A17, which bind to phospholipid molecules and form the tubular scaffold along the poxviral membrane. Extensive intermolecular interactions, including A14 dimers and A14-A17 complexes, drive the remarkable structural stability of STEs. Structural analysis further emphasizes the reticulon-like properties of A17, which promote membrane curvature and stabilize the tubular architecture. These results provide novel insights into the STE assembly, morphogenesis, and surface organization of poxviruses, offering valuable information for the development of vaccines and antiviral strategies against poxvirus infections.IMPORTANCESurface tubular elements (STEs) are critical components of poxvirus mature virions and play a role in suppressing host cell protein synthesis. In this study, we isolated and purified STEs from native poxvirus virions and subsequently determined their core composition and high-resolution architecture. We identified that STE is mainly composed of membrane proteins A14 and A17, along with phospholipid molecules. Within the repeat structural unit of STE, A14 proteins form two homodimers within the repeating unit, with A17 monomers flanking either side. Phospholipid molecules are distributed within the A14-A14 and A14-A17 interfaces. Our study not only revealed the molecular structures of A14 and A17 but also further emphasized that the reticulon-like and highly oligomerized characteristics of A17 provide membrane curvature, while the A14-A17-phospholipid network stabilizes the tubular structure. We proposed a hypothetical model that A17 drives changes in viral membrane curvature during maturation. These findings enhance our understanding of poxvirus biology and may guide therapeutic strategies against poxvirus infections.

The hepatitis B virus (HBV) X protein (HBx) is a multifunctional regulator essential for HBV replication and HBV-associated hepatocarcinogenesis. Despite its central role, the molecular basis of HBx function has remained elusive. Here, we present the first cryo-electron microscopy structure of the human HBx-CRL4-Smc5/6 complex at 3.1 Å resolution. In this reconstituted ten-subunit assembly, HBx adopts a Zn2 □-stabilized Y-shaped architecture that simultaneously engages the DDB1 and the Smc6 subunit. A composite helix-turn-helix (HTH) pocket in HBx accommodates a conserved “Leucine Key” motif ( LRCKL ) on Smc6, forming a critical interface essential for complex stability and function. Molecular docking and biochemical validation reveal that the compound Tranilast binds this HTH pocket, disrupts the HBx-Smc6 interaction, and suppresses HBV replication. These findings define the structural mechanism by which HBx counteracts host restriction and establish the HBx-Smc6 interface as a previously unrecognized and druggable target for antiviral intervention. ### Competing Interest Statement The structure and potential applications of the HBx composite HTH pocket have been granted patent protection (#: 2025115355230). the National Key R&D Program of China, 2022YFA1303600 the Strategic Priority Research Program of the Chinese Academy of Sciences, XDB29010205 the Shanghai Municipal Science and Technology Major Project, 2019SHZDZX02

PCDH10 is a newly identified general receptor for Western equine encephalitis virus (WEEV) members, a group of encephalitic alphaviruses that cause severe diseases in humans and equids. While WEEV typically binds PCDH10 as a receptor, nonpathogenic strains have evolved to lose mammalian PCDH10 binding, retaining only avian PCDH10 affinity. Virulent strains also engage VLDLR and ApoER2 as alternative receptors. Here, we determine the structure of WEEV strain 71V1658 virus-like particles (VLPs) in complex with human PCDH10 extracellular cadherin repeats 1-2 (EC1-EC2) by cryo-electron microscopy at 2.99 Å resolution. EC1 inserts into a cleft clamped by two adjacent E2-E1 heterodimers within a single trimeric spike, whereas EC2 maintains no contact with the WEEV VLP. Mutagenesis studies elucidate the impacts of the interacting residues on PCDH10. And residue 153 of E2 is crucial for PCDH10 binding, and the E2Q153L mutation observes in the nonpathogenic strain Imperial-181 restores its ability to bind to PCDH10. Moreover, the arginine residue at position 89 on avian PCDH10 is essential for its interaction with strain Imperial-181. These results advance our understanding of receptor recognition by alphaviruses and the shift in receptor usage, providing insights for the development of antiviral therapies.

HCoV-HKU1, one of seven human coronaviruses (HCoVs) that have harmful effects on human health, accounts for a substantial portion of common cold cases and can cause severe respiratory diseases in certain populations. Currently, effective antiviral treatments against this virus are limited. Recently, TMPRSS2, a host protease long acknowledged for its role in priming the spike proteins of various CoVs and promoting viral entry, was identified as a functional receptor for HCoV-HKU1, opening an avenue for anti-HCoV-HKU1 therapy development. In this study, we elucidate the detailed molecular mechanism underlying the interaction between the HCoV-HKU1 receptor-binding domain (RBD) and TMPRSS2 via crystallography. Guided by these structural insights, we successfully develop two types of therapeutic antibodies against HCoV-HKU1. The first type neutralizes the RBD, potently disrupting its interaction with TMPRSS2 and preventing viral infection. The second type targets TMPRSS2, inhibiting its enzymatic activity and/or interfering with its binding to the RBD. The latter demonstrates broad-spectrum anti-CoV activity, as the enzymatic activity of TMPRSS2 is crucial for both HCoV-HKU1 infection and other CoV infections. Our findings provide crucial structural insights into the recognition of TMPRSS2 by HCoV-HKU1 and offer promising antibody-based strategies for combating HCoV-HKU1 and other CoV infections.

Heterodimeric ATP-binding cassette (ABC) transporters containing one catalytically impaired degenerate nucleotide-binding site (NBS) have a mechanism different from those with two active NBSs. However, the structural basis of their transport mechanism remains to be explained. Here, we determine mycobacterial MsRv1273c/72c to be an isoniazid efflux pump and determine several structures by cryo-electron microscopy showing specific asymmetrical features including an N-terminal extending loop and a periplasmic helical hairpin only found in MsRv1272c. In addition, we capture three distinct asymmetric states where the nucleotide-binding domains are partially dimerized at the degenerate site. Using these intermediate states, the D-WalkerB loop and X-signature loop of MsRv1272c modulate and couple the function of both NBSs through conformational changes. Thus, these data provide insights into the mechanism of this heterodimeric ABC transporter containing a degenerate NBS. The structures also provide a framework for the rational design of anti-tuberculosis drugs targeting this drug-efflux pump.

Structures of nitrogenases, dark-operative protochlorophyllide oxidoreductases, and light-dependent protochlorophyllide oxidoreductases (LPOR) have been resolved. However, their evolutionary relatedness remains elusive. Here, we show, through structural alignment, that all subunits of nitrogenase-like proteins originated from a co-ancestral archaic one-subdomain precursor. LPOR evolved from the BchX/BchY subunits of nitrogenase-like chlorophyllide a oxidoreductase (COR), and the intermediary retinol dehydrogenase through possible genetic recombination. We thus establish previously unknown structural links among key enzymes involved in biological nitrogen-fixation (BNF) and photosynthesis, unraveling structure-guided functional evolution from a single-subunit iron protein to multi-subunit nitrogenase-like COR, and to the single-subunit LPOR for phototrophic metabolism via bacteriochlorophyll, retinal, and chlorophyll. This work also demonstrates structural similarities are imperative for inferring distant origins of functionally divergent proteins, particularly those lacking primary amino-acid sequence identity. Moreover, our findings coupled with AI may be exploited to design innovative light-driven CORs and/or light-utilizing nitrogenases with enhanced efficacy of photosynthesis and BNF. ### Competing Interest Statement The authors have declared no competing interest.

Influenza virus ribonucleoprotein complexes (RNPs), composed of the polymerase complex (FluPol), nucleoprotein (NP), and RNA, are essential for replication and transcription. We report atomic-resolution cryo-EM structures of mini-vRNPs in two states: FluPol located inside (State-In) or at the outer rim (State-Out) of the NP-RNA ring. In both states, the 5' and 3' termini of vRNA are bound to FluPol as previously reported. One NP (NP-0) contacts PA/PB1 of FluPol and binds the distal double-stranded vRNA promoter, with its D72-K90 loop inserting into the RNA fork; separated strands occupy NP-0 RNA-binding grooves. Grooves from other NPs form a continuous RNA-protective path, consistent with negative-strand RNA virus mechanisms. In State-In, interfaces for FluPol dimerization or Pol II interaction are blocked, but fully exposed in State-Out. These structures reveal detailed FluPol-NP-RNA coupling and suggest a conformational shift in RNPs during the viral life cycle.

The MmpL5/MmpS5 efflux system in Mycobacterium tuberculosis plays crucial roles in extruding therapeutic drugs (e.g., bedaquiline), and exporting siderophores (i.e., (carboxy)mycobactins). However, the molecular basis underlying these processes remains unknown due to the lack of structural information. Here, we report the cryo-electron microscopy structures of Mycobacterium tuberculosis MmpL5/MmpS5 at resolutions ranging from 2.64 to 3.31 Å, revealing it to be a trimer. The core of this complex is formed by three MmpL5 subunits assembled in a unique shoulder-to-shoulder ring-like configuration, with each MmpS5 subunit positioned between the two adjacent MmpL5 subunits. A remarkable feature of this system is the extracellular stalk, which spans approximately 130 Å in length and is composed of three intertwined anti-parallel coiled-coil portions of MmpL5. The stalk secures the tight association of the three MmpL5 subunits and exhibits intrinsic structural flexibility. Additionally, an unexpected MmpL5 binder, AcpM, a mycobacterial acyl carrier protein, has also been identified. Collectively, the study provides insights into the biological assembly and molecular function of MmpL5/MmpS5, which will facilitate the discovery of innovative inhibitors for this system.

In the SARS-CoV-2 replication-transcription complex (RTC), the nascent template-product duplex is unwound into a template strand for recycling and a product strand that needs to be capped. Here, we determined structures of the SARS-CoV-2 RTC in the pre- and post-capping initiation (CI) states. In the pre-CI state, the RTC has a dimer-of-dimeric architecture (ddRTC). The upstream RNA duplex in one RTC is reciprocally unwound by a helicase in a head-to-head-positioned RTC in the 3'-5' direction. The helicases bind either ADP or ADP⋅Pi in their ATP-binding pockets, suggesting a mechanism for ATP-hydrolysis-driven unwinding. In the post-CI state, the binding of nsp9 to the nsp12 nidovirus RdRp-associated nucleotidyltransferase (NiRAN) disrupts the ddRTC. The N terminus of nsp9 and the triphosphorylated 5' end of the product strand co-localize in NiRAN's catalytic site, exhibiting the state prior to nsp9 RNAylation for capping. These results provide an insight into the concurrence of template recycling and RNA capping in the SARS-CoV-2 RTC.

All coronaviruses (CoVs) encode an exoribonuclease in nonstructural protein nsp14 (nsp14 ExoN), which is required for the excision of mismatched nucleotides or nucleotide analogues (NAs) that are incorporated into nascent RNA. Here, we investigated the mechanism by which NAs evade SARS-CoV-2 nsp14 ExoN cleavage using chemically synthesized RNA with NAs incorporated at the 3' end. Nsp14 ExoN exhibited significantly attenuated activity on RNA with sofosbuvir monophosphate (SMP) compared with natural nucleotides, remdesivir/molnupiravir monophosphate, and, in particular, AT-9010 monophosphate (ATMP), which has the same chemically modified ribose moiety as SMP, incorporated at the 3' end. Cryo-electron microscopy structures of nsp10/14 bound to RNA-SMP/-ATMP and mutagenesis studies revealed the essential roles of H95/Q145/F146 in recognizing the base moiety and thus pulling the NAs into a favored conformation for cleavage. Therefore, NAs may evade nsp14 ExoN cleavage by having (1) a base that does not interact with H95, Q145, or F146 and (2) a chemically modified ribose. Guided by this hypothesis, two NAs were designed to effectively resist nsp14 ExoN cleavage. These results inform the rational design of anti-CoV NAs.

The coronavirus disease 2019 (COVID-19) pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has resulted in millions of deaths and continues to pose serious threats to global public health. The main protease (Mpro) of SARS-CoV-2 is crucial for viral replication and its conservation, making it an attractive drug target. Here, we employed a structure-based drug design strategy to develop and optimize novel inhibitors targeting SARS-CoV-2 Mpro. By fully exploring occupation of the S1, S2, and S3/S4 binding pockets, we identified eight promising inhibitors with half-maximal inhibitory concentration (IC50) values below 20 nM. The cocrystal structure of Mpro with compound 10 highlighted the crucial roles of the interactions within the S3/S4 pockets in inhibitor potency enhancement. These findings demonstrated that expanding the utilization of these binding pockets was an effective strategy for developing noncovalent small molecule inhibitors that target SARS-CoV-2 Mpro. Compound 4 demonstrated outstanding in vitro antiviral activity against wild-type SARS-CoV-2 with an EC50 of 9.4 nM. Moreover, oral treatment with compounds 1 and 9 exhibited excellent antiviral potency and substantially ameliorated virus-induced tissue damage in the lungs of Omicron BA.5-infected K18-human ACE2 (K18-hACE2) transgenic mice, indicating that these novel noncovalent inhibitors could be potential oral agents for the treatment of COVID-19.

SARS-CoV-2 and its emerging variants continue to pose a significant global public health threat. The SARS-CoV-2 main protease (Mpro) is a critical target for the development of antiviral agents that can inhibit viral replication and transcription. In this study, we identified chebulagic acid (CHLA), isolated from Terminalia chebula Retz., as a potent non-peptidomimetic and non-covalent Mpro inhibitor. CHLA exhibited intermolecular interactions and provided significant protection to Vero E6 cells against a range of SARS-CoV-2 variants, including the wild-type, Delta, Omicron BA.1.1, BA.2.3, BA.4, and BA.5, with EC50 values below 2 μmol/L. Moreover, in vivo studies confirmed the antiviral efficacy of CHLA in K18-hACE2 mice. Notably, CHLA bound to a unique groove at the interface between Mpro domains I and II, which was revealed by the high-resolution crystal structure (1.4 Å) of the Mpro–CHLA complex, shrinking the substrate binding pocket of Mpro and inducing Mpro aggregation. CHLA was proposed to act as an allosteric inhibitor. Pharmacokinetic profiling and safety assessments underscore CHLA's potential as a promising broad-spectrum antiviral candidate. These findings report a novel binding site on Mpro and identify antiviral activity of CHLA, providing a robust framework for lead compounds discovery and elucidating the underlying molecular mechanisms of inhibition.

The pre-dimerization of endosome-localized RNA sensor Toll-like receptor 3 (TLR3) is required for its innate recognition, yet how TLR3 pre-dimers are formed and precisely primed for innate activation remains unclear. Here, we demonstrate that endosome-localized self RNA Rmrp directly binds to TLR3 and induces TLR3 dimerization in the early endosome but does not interact with endosome-localized TLR7, TLR8, TLR9 or cytoplasmic RNA sensor RIG-I under homeostatic conditions. Cryo-EM structure of Rmrp–TLR3 complex reveals a novel lapped conformation of TLR3 dimer engaged by Rmrp, which is distinct from the activation mechanism by dsRNA and the specific structural feature at the 3’-end of Rmrp is critical for its functional interaction with TLR3. Furthermore, K42 residue of TLR3 is essential for binding to Rmrp and subsequent dimerization. Rmrp dissociates from TLR3 following endosomal acidification, generating a matured TLR3 dimer which is primed for innate recognition and activation. Myeloid-cell deficiency of Rmrp reduces TLR3 dimerization and attenuates TLR3-mediated antiviral responses against influenza A both in vitro and in vivo. These findings elucidate the structural mode of self RNA Rmrp-primed TLR3 dimerization and ready for efficient innate recognition on endosomal membrane, extending our knowledge of how membrane-associated TLRs pre-dimerize and suggesting a new function of subcellular localized self RNAs in empowering innate activation.

F-type ATP synthase (F1FO) catalyzes proton motive force-driven ATP synthesis in mitochondria, chloroplasts, and bacteria. Different from the mitochondrial and bacterial enzymes, F1FO from photosynthetic organisms have evolved diverse structural and mechanistic details to adapt to the light-dependent reactions. Although complete structure of chloroplast F1FO has been reported, no high-resolution structure of an F1FO from photosynthetic bacteria has been available. Here, we report cryo-EM structures of an intact and functionally competent F1FO from Chloroflexus aurantiacus (CaF1FO), a filamentous anoxygenic phototrophic bacterium from the earliest branch of photosynthetic organisms. The structures of CaF1FO in its ADP-free and ADP-bound forms for three rotational states reveal a previously unrecognized architecture of ATP synthases. A pair of peripheral stalks connect to the CaF1 head through a dimer of δ-subunits, and associate with two membrane-embedded a-subunits that are asymmetrically positioned outside and clamp CaFO's c10-ring. The two a-subunits constitute two proton inlets on the periplasmic side and two proton outlets on the cytoplasmic side, endowing CaF1FO with unique proton translocation pathways that allow more protons being translocated relative to single a-subunit F1FO. Our findings deepen understanding of the architecture and proton translocation mechanisms of F1FO synthases and suggest innovative strategies for modulating their activities by altering the number of a-subunit.

Poxviruses cause severe diseases, including smallpox and mpox, that pose major threats to human health. The poxvirus core protease (CorePro) is essential for viral maturation and is highly conserved in poxviruses, making it an attractive antiviral target1. However, the structure of CorePro remains unknown, hampering antiviral development. Here we determined the apo structure of monkeypox virus (MPXV) CorePro and the structure of CorePro in a complex with the inhibitor aloxistatin, a drug candidate for muscular dystrophy2. These structures show that CorePro forms a homodimer that features a unique 'dancing couple' fold. The catalytic intermediate state of CorePro was characterized by an aldehyde derivative from a natural substrate (I-G18). This derivative binds covalently to the catalytic Cys328, shifting the active site of the viral protease from a closed conformation in the apo form to a favourable open conformation upon substrate binding. On the basis of the CorePro-I-G18 complex, we designed a series of peptidomimetic inhibitors with a nitrile warhead, which could covalently anchor with the catalytic Cys328. These compounds inhibit CorePro with half-maximal inhibitory concentrations of 44.9-100.3 nM, and exhibit potent and broad anti-poxvirus activity. Our studies provide a basis for designing wide-spectrum inhibitors against poxvirus infections.