The complete assembly of human LAT1-4F2hc complex provides insights into its regulation, function and localisation.

The LAT1-4F2hc complex (SLC7A5-SLC3A2) facilitates uptake of essential amino acids, hormones and drugs. Its dysfunction is associated with many cancers and immune/neurological disorders. Here, we apply native mass spectrometry (MS)-based approaches to provide evidence of super-dimer formation (LAT1-4F2hc)2. When combined with lipidomics, and site-directed mutagenesis, we discover four endogenous phosphatidylethanolamine (PE) molecules at the interface and C-terminus of both LAT1 subunits. We find that interfacial PE binding is regulated by 4F2hc-R183 and is critical for regulation of palmitoylation on neighbouring LAT1-C187. Combining native MS with mass photometry (MP), we reveal that super-dimerization is sensitive to pH, and modulated by complex N-glycans on the 4F2hc subunit. We further validate the dynamic assemblies of LAT1-4F2hc on plasma membrane and in the lysosome. Together our results link PTM and lipid binding with regulation and localisation of the LAT1-4F2hc super-dimer.

Cryo-EM structure of the human Asc-1 transporter complex.

The Alanine-Serine-Cysteine transporter 1 (Asc-1 or SLC7A10) forms a crucial heterodimeric transporter complex with 4F2hc (SLC3A2) through a covalent disulfide bridge. This complex enables the sodium-independent transport of small neutral amino acids, including L-Alanine (L-Ala), Glycine (Gly), and D-Serine (D-Ser), within the central nervous system (CNS). D-Ser and Gly are two key endogenous glutamate co-agonists that activate N-methyl-d-aspartate (NMDA) receptors by binding to the allosteric site. Mice deficient in Asc-1 display severe symptoms such as tremors, ataxia, and seizures, leading to early postnatal death. Despite its physiological importance, the functional mechanism of the Asc-1-4F2hc complex has remained elusive. Here, we present cryo-electron microscopy (cryo-EM) structures of the human Asc-1-4F2hc complex in its apo state, D-Ser bound state, and L-Ala bound state, resolved at 3.6 Å, 3.5 Å, and 3.4 Å, respectively. Through detailed structural analysis and transport assays, we uncover a comprehensive alternating access mechanism that underlies conformational changes in the complex. In summary, our findings reveal the architecture of the Asc-1 and 4F2hc complex and provide valuable insights into substrate recognition and the functional cycle of this essential transporter complex.

Structural basis for the inhibition mechanism of the DNA polymerase holoenzyme from mpox virus.

There are three key components at the core of the mpox virus (MPXV) DNA polymerase holoenzyme: DNA polymerase F8, processivity factors A22, and the Uracil-DNA glycosylase E4. The holoenzyme is recognized as a vital antiviral target because MPXV replicates in the cytoplasm of host cells. Nucleotide analogs such as cidofovir and cytarabine (Ara-C) have shown potential in curbing MPXV replication and they also display promise against other poxviruses. However, the mechanism behind their inhibitory effects remains unclear. Here, we present the cryo-EM structure of the DNA polymerase holoenzyme F8/A22/E4 bound with its competitive inhibitor Ara-C-derived cytarabine triphosphate (Ara-CTP) at an overall resolution of 3.0 Å and reveal its inhibition mechanism. Ara-CTP functions as a direct chain terminator in proximity to the deoxycytidine triphosphate (dCTP)-binding site. The extra hydrogen bond formed with Asn665 makes it more potent in binding than dCTP. Asn665 is conserved among eukaryotic B-family polymerases.

Cryo-EM structures of the human NaS1 and NaDC1 transporters revealed the elevator transport and allosteric regulation mechanism.

The solute carrier 13 (SLC13) family comprises electrogenic sodium ion-coupled anion cotransporters, segregating into sodium ion-sulfate cotransporters (NaSs) and sodium ion-di- and-tricarboxylate cotransporters (NaDCs). NaS1 and NaDC1 regulate sulfate homeostasis and oxidative metabolism, respectively. NaS1 deficiency affects murine growth and fertility, while NaDC1 affects urinary citrate and calcium nephrolithiasis. Despite their importance, the mechanisms of substrate recognition and transport remain insufficiently characterized. In this study, we determined the cryo-electron microscopy structures of human NaS1, capturing inward-facing and combined inward-facing/outward-facing conformations within a dimer both in apo and sulfate-bound states. In addition, we elucidated NaDC1's outward-facing conformation, encompassing apo, citrate-bound, and N-(p-amylcinnamoyl) anthranilic acid (ACA) inhibitor-bound states. Structural scrutiny illuminates a detailed elevator mechanism driving conformational changes. Notably, the ACA inhibitor unexpectedly binds primarily anchored by transmembrane 2 (TM2), Loop 10, TM11, and TM6a proximate to the cytosolic membrane. Our findings provide crucial insights into SLC13 transport mechanisms, paving the way for future drug design.

Structural insight into the assembly and working mechanism of helicase-primase D5 from Mpox virus.

The Mpox pandemic, caused by the Mpox virus (or monkeypox virus, MPXV), has gained global attention. The D5 protein, a putative helicase-primase found in MPXV, plays a vital role in viral replication and genome uncoating. Here we determined multiple cryo-EM structures of full-length hexameric D5 in diverse states. These states were captured during ATP hydrolysis while moving along the single-stranded DNA (ssDNA) track. Through comprehensive structural analysis combined with the helicase activity system, we revealed that when the primase domain is truncated or the interaction between the primase and helicase domains is disrupted, the double-stranded DNA (dsDNA) unwinds into ssDNA, suggesting a critical regulatory role of the primase domain. Two transition states bound with ssDNA substrate during unwinding reveals that two ATP molecules were consumed to drive DNA moving forward two nucleotides. Collectively, our findings shed light on the molecular mechanism that links ATP hydrolysis to the DNA unwinding in poxviruses.

The cryo-EM structure of homotetrameric attachment glycoprotein from langya henipavirus.

Langya Henipavirus (LayV) infection is an emerging zoonotic disease that has been causing respiratory symptoms in China since 2019. For virus entry, LayV's genome encodes the fusion protein F and the attachment glycoprotein G. However, the structural and functional information regarding LayV-G remains unclear. In this study, we revealed that LayV-G cannot bind to the receptors found in other HNVs, such as ephrin B2/B3, and it shows different antigenicity from HeV-G and NiV-G. Furthermore, we determined the near full-length structure of LayV-G, which displays a distinct mushroom-shaped configuration, distinguishing it from other attachment glycoproteins of HNV. The stalk and transmembrane regions resemble the stem and root of mushroom and four downward-tilted head domains as mushroom cap potentially interact with the F protein and influence membrane fusion process. Our findings enhance the understanding of emerging HNVs that cause human diseases through zoonotic transmission and provide implication for LayV related vaccine development.

Structural Basis for the Enhanced Infectivity and Immune Evasion of Omicron Subvariants.

The Omicron variants of SARS-CoV-2 have emerged as the dominant strains worldwide, causing the COVID-19 pandemic. Each Omicron subvariant contains at least 30 mutations on the spike protein (S protein) compared to the original wild-type (WT) strain. Here we report the cryo-EM structures of the trimeric S proteins from the BA.1, BA.2, BA.3, and BA.4/BA.5 subvariants, with BA.4 and BA.5 sharing the same S protein mutations, each in complex with the surface receptor ACE2. All three receptor-binding domains of the S protein from BA.2 and BA.4/BA.5 are "up", while the BA.1 S protein has two "up" and one "down". The BA.3 S protein displays increased heterogeneity, with the majority in the all "up" RBD state. The different conformations preferences of the S protein are consistent with their varied transmissibility. By analyzing the position of the glycan modification on Asn343, which is located at the S309 epitopes, we have uncovered the underlying immune evasion mechanism of the Omicron subvariants. Our findings provide a molecular basis of high infectivity and immune evasion of Omicron subvariants, thereby offering insights into potential therapeutic interventions against SARS-CoV-2 variants.

Structural basis for the assembly of the DNA polymerase holoenzyme from a monkeypox virus variant.

The ongoing global pandemic caused by a variant of the monkeypox (or mpox) virus (MPXV) has prompted widespread concern. The MPXV DNA polymerase holoenzyme, consisting of F8, A22, and E4, is vital for replicating the viral genome and represents a crucial target for the development of antiviral drugs. However, the assembly and working mechanism for the DNA polymerase holoenzyme of MPXV remains elusive. Here, we present the cryo-electron microscopy (cryo-EM) structure of the DNA polymerase holoenzyme at an overall resolution of 3.5 Å. Unexpectedly, the holoenzyme is assembled as a dimer of heterotrimers, of which the extra interface between the thumb domain of F8 and A22 shows a clash between A22 and substrate DNA, suggesting an autoinhibition state. Addition of exogenous double-stranded DNA shifts the hexamer into trimer exposing DNA binding sites, potentially representing a more active state. Our findings provide crucial steps toward developing targeted antiviral therapies for MPXV and related viruses.

A high-throughput fully automatic biosensing platform for efficient COVID-19 detection.

We propose a label-free biosensor based on a porous silicon resonant microcavity and localized surface plasmon resonance. The biosensor detects SARS-CoV-2 antigen based on engineered trimeric angiotensin converting enzyme-2 binding protein, which is conserved across different variants. Robotic arms run the detection process including sample loading, incubation, sensor surface rinsing, and optical measurements using a portable spectrometer. Both the biosensor and the optical measurement system are readily scalable to accommodate testing a wide range of sample numbers. The limit of detection is 100 TCID50/ml. The detection time is 5 min, and the throughput of one single robotic site is up to 384 specimens in 30 min. The measurement interface requires little training, has standard operation, and therefore is suitable for widespread use in rapid and onsite COVID-19 screening or surveillance.

Cryo-EM structures of recombinant human sodium-potassium pump determined in three different states.

Sodium-Potassium Pump (Na+/K+-ATPase, NKA) is an ion pump that generates an electrochemical gradient of sodium and potassium ions across the plasma membrane by hydrolyzing ATP. During each Post-Albers cycle, NKA exchanges three cytoplasmic sodium ions for two extracellular potassium ions through alternating changes between the E1 and E2 states. Hitherto, several steps remained unknown during the complete working cycle of NKA. Here, we report cryo-electron microscopy (cryo-EM) structures of recombinant human NKA (hNKA) in three distinct states at 2.7-3.2 Å resolution, representing the E1·3Na and E1·3Na·ATP states with cytosolic gates open and the basic E2·[2K] state, respectively. This work provides the insights into the cytoplasmic Na+ entrance pathway and the mechanism of cytoplasmic gate closure coupled with ATP hydrolysis, filling crucial gaps in the structural elucidation of the Post-Albers cycle of NKA.

Neutralization mechanism of a human antibody with pan-coronavirus reactivity including SARS-CoV-2.

Frequent outbreaks of coronaviruses underscore the need for antivirals and vaccines that can counter a broad range of coronavirus types. We isolated a human antibody named 76E1 from a COVID-19 convalescent patient, and report that it has broad-range neutralizing activity against multiple α- and ß-coronaviruses, including the SARS-CoV-2 variants. 76E1 also binds its epitope in peptides from γ- and δ-coronaviruses. 76E1 cross-protects against SARS-CoV-2 and HCoV-OC43 infection in both prophylactic and therapeutic murine animal models. Structural and functional studies revealed that 76E1 targets a unique epitope within the spike protein that comprises the highly conserved S2' site and the fusion peptide. The epitope that 76E1 binds is partially buried in the structure of the SARS-CoV-2 spike trimer in the prefusion state, but is exposed when the spike protein binds to ACE2. This observation suggests that 76E1 binds to the epitope at an intermediate state of the spike trimer during the transition from the prefusion to the postfusion state, thereby blocking membrane fusion and viral entry. We hope that the identification of this crucial epitope, which can be recognized by 76E1, will guide epitope-based design of next-generation pan-coronavirus vaccines and antivirals.

ACE2, B0AT1, and SARS-CoV-2 spike protein: Structural and functional implications.

The coronavirus disease 2019 (COVID-19) pandemic, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has emerged as a public health crisis and led to tremendous economic devastation. The spike protein (S) of SARS-CoV-2 hijacks the angiotensin converting enzyme 2 (ACE2) as a receptor for virus entry, representing the initial step of viral infection. S is one of the major targets for development of the antiviral drugs, antibodies, and vaccines. ACE2 is a peptidase that plays a physiologically important role in the renin-angiotensin system. Concurrently, it also forms dimer of heterodimer with the neutral amino acid transporter B0AT1 to regulate intestinal amino acid metabolism. The symptoms of COVID-19 are closely correlated with the physiological functions of ACE2. In this review, we summarize the functional and structural studies on ACE2, B0AT1, and their complex with S of SARS-CoV-2, providing insights into the various symptoms caused by viral infection and the development of therapeutic strategies.

Structural and functional analysis of an inter-Spike bivalent neutralizing antibody against SARS-CoV-2 variants.

The different variants of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have attracted most public concern because they caused "wave and wave" COVID-19 pandemic. The initial step of viral infection is mediated by the SARS-CoV-2 Spike (S) protein, which mediates the receptor recognition and membrane fusion between virus and host cells. Neutralizing antibodies (nAbs) targeting the S protein of SARS-CoV-2 have become promising candidates for clinical intervention strategy, while multiple studies have shown that different variants have enhanced infectivity and antibody resistance. Here, we explore the structure and function of STS165, a broadly inter-Spike bivalent nAb against SARS-CoV-2 variants and even SARS-CoV, contributing to further understanding of the working mechanism of nAbs.

Novel sarbecovirus bispecific neutralizing antibodies with exceptional breadth and potency against currently circulating SARS-CoV-2 variants and sarbecoviruses.

The emergence of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) variants of concern, including Alpha (B.1.1.7), Beta (B.1.351), Gamma (P.1), Delta (B.1.617.2), and Omicron (B.1.1.529) has aroused concerns over their increased infectivity and transmissibility, as well as decreased sensitivity to SARS-CoV-2-neutralizing antibodies (NAbs) and the current coronavirus disease 2019 (COVID-19) vaccines. Such exigencies call for the development of pan-sarbecovirus vaccines or inhibitors to combat the circulating SARS-CoV-2 NAb-escape variants and other sarbecoviruses. In this study, we isolated a broadly NAb against sarbecoviruses named GW01 from a donor who recovered from COVID-19. Cryo-EM structure and competition assay revealed that GW01 targets a highly conserved epitope in a wide spectrum of different sarbecoviruses. However, we found that GW01, the well-known sarbecovirus NAb S309, and the potent SARS-CoV-2 NAbs CC12.1 and REGN10989 only neutralize about 90% of the 56 tested currently circulating variants of SARS-CoV-2 including Omicron. Therefore, to improve efficacy, we engineered an IgG-like bispecific antibody GW01-REGN10989 (G9) consisting of single-chain antibody fragments (scFv) of GW01 and REGN10989. We found that G9 could neutralize 100% of NAb-escape mutants (23 out of 23), including Omicron variant, with a geometric mean (GM) 50% inhibitory concentration of 8.8 ng/mL. G9 showed prophylactic and therapeutic effects against SARS-CoV-2 infection of both the lung and brain in hACE2-transgenic mice. Site-directed mutagenesis analyses revealed that GW01 and REGN10989 bind to the receptor-binding domain in different epitopes and from different directions. Since G9 targets the epitopes for both GW01 and REGN10989, it was effective against variants with resistance to GW01 or REGN10989 alone and other NAb-escape variants. Therefore, this novel bispecific antibody, G9, is a strong candidate for the treatment and prevention of infection by SARS-CoV-2, NAb-escape variants, and other sarbecoviruses that may cause future emerging or re-emerging coronavirus diseases.

Structural advances in sterol-sensing domain-containing proteins.

The sterol-sensing domain (SSD) is present in several membrane proteins that function in cholesterol metabolism, transport, and signaling. Recent progress in structural studies of SSD-containing proteins, such as sterol regulatory element-binding protein (SREBP)-cleavage activating protein (Scap), Patched, Niemann-Pick disease type C1 (NPC1), and related proteins, reveals a conserved core that is essential for their sterol-dependent functions. This domain, by its name, 'senses' the presence of sterol substrates through interactions and may modulate protein behaviors with changing sterol levels. We summarize recent advances in structural and mechanistic investigations of these proteins and propose to divide them to two classes: M for 'moderator' proteins that regulate sterol metabolism in response to membrane sterol levels, and T for 'transporter' proteins that harbor inner tunnels for cargo trafficking across cellular membranes.