Activation of the insulin receptor by insulin-like growth factor 2

Insulin receptor (IR) controls growth and metabolism. Insulin-like growth factor 2 (IGF2) has different binding properties on two IR isoforms, mimicking insulin's function. However, the molecular mechanism underlying IGF2-induced IR activation remains unclear. Here, we present cryo-EM structures of full-length human long isoform IR (IR-B) in both the inactive and IGF2-bound active states, and short isoform IR (IR-A) in the IGF2-bound active state. Under saturated IGF2 concentrations, both the IR-A and IR-B adopt predominantly asymmetric conformations with two or three IGF2s bound at site-1 and site-2, which differs from that insulin saturated IR forms an exclusively T-shaped symmetric conformation. IGF2 exhibits a relatively weak binding to IR site-2 compared to insulin, making it less potent in promoting full IR activation. Cell-based experiments validated the functional importance of IGF2 binding to two distinct binding sites in optimal IR signaling and trafficking. In the inactive state, the C-terminus of α-CT of IR-B contacts FnIII-2 domain of the same protomer, hindering its threading into the C-loop of IGF2, thus reducing the association rate of IGF2 with IR-B. Collectively, our studies demonstrate the activation mechanism of IR by IGF2 and reveal the molecular basis underlying the different affinity of IGF2 to IR-A and IR-B.

Activation of human STING by a molecular glue-like compound

Stimulator of interferon genes (STING) is a dimeric transmembrane adapter protein that plays a key role in the human innate immune response to infection and has been therapeutically exploited for its antitumor activity. The activation of STING requires its high-order oligomerization, which could be induced by binding of the endogenous ligand, cGAMP, to the cytosolic ligand-binding domain. Here we report the discovery through functional screens of a class of compounds, named NVS-STGs, that activate human STING. Our cryo-EM structures show that NVS-STG2 induces the high-order oligomerization of human STING by binding to a pocket between the transmembrane domains of the neighboring STING dimers, effectively acting as a molecular glue. Our functional assays showed that NVS-STG2 could elicit potent STING-mediated immune responses in cells and antitumor activities in animal models.

Applications of cryo-EM in drug development for STING

STING is a critical adaptor protein in the cGAS-mediated DNA-sensing innate immune pathway. Binding of the second messenger cGAMP generated by cGAS to STING induces the high-order oligomerization and activation of the STING dimer. STING is a promising target for diseases associated with the cGAS/STING pathway such as cancer and autoimmune diseases. Recent applications of cryo-EM to STING have led to exciting progress in the understanding of its regulatory mechanism. Cryo-EM structures of STING bound to either cGAMP mimetics or novel small molecule ligands not only revealed the action mechanisms of these ligands but also suggested new ways to modulate the activity of STING for therapeutic purposes. Some of these recent studies are highlighted here.

CTCF and R-loops are boundaries of cohesin-mediated DNA looping

Cohesin and CCCTC-binding factor (CTCF) are key regulatory proteins of three-dimensional (3D) genome organization. Cohesin extrudes DNA loops that are anchored by CTCF in a polar orientation. Here, we present direct evidence that CTCF binding polarity controls cohesin-mediated DNA looping. Using single-molecule imaging, we demonstrate that a critical N-terminal motif of CTCF blocks cohesin translocation and DNA looping. The cryo-EM structure of the cohesin-CTCF complex reveals that this CTCF motif ahead of zinc fingers can only reach its binding site on the STAG1 cohesin subunit when the N terminus of CTCF faces cohesin. Remarkably, a C-terminally oriented CTCF accelerates DNA compaction by cohesin. DNA-bound Cas9 and Cas12a ribonucleoproteins are also polar cohesin barriers, indicating that stalling may be intrinsic to cohesin itself. Finally, we show that RNA-DNA hybrids (R-loops) block cohesin-mediated DNA compaction in vitro and are enriched with cohesin subunits in vivo, likely forming TAD boundaries.

The Activation Mechanism of the Insulin Receptor: A Structural Perspective

The insulin receptor (IR) is a type II receptor tyrosine kinase that plays essential roles in metabolism, growth, and proliferation. Dysregulation of IR signaling is linked to many human diseases, such as diabetes and cancers. The resolution revolution in cryo-electron microscopy has led to the determination of several structures of IR with different numbers of bound insulin molecules in recent years, which have tremendously improved our understanding of how IR is activated by insulin. Here, we review the insulin-induced activation mechanism of IR, including (a) the detailed binding modes and functions of insulin at site 1 and site 2 and (b) the insulin-induced structural transitions that are required for IR activation. We highlight several other key aspects of the activation and regulation of IR signaling and discuss the remaining gaps in our understanding of the IR activation mechanism and potential avenues of future research.

Structural basis of the alkaline pH-dependent activation of insulin receptor-related receptor

The insulin receptor (IR) family is a subfamily of receptor tyrosine kinases that controls metabolic homeostasis and cell growth. Distinct from IR and insulin-like growth factor 1 receptor, whose activation requires ligand binding, insulin receptor-related receptor (IRR)-the third member of the IR family-is activated by alkaline pH. However, the molecular mechanism underlying alkaline pH-induced IRR activation remains unclear. Here, we present cryo-EM structures of human IRR in both neutral pH inactive and alkaline pH active states. Combined with mutagenesis and cellular assays, we show that, upon pH increase, electrostatic repulsion of the pH-sensitive motifs of IRR disrupts its autoinhibited state and promotes a scissor-like rotation between two protomers, leading to a T-shaped active conformation. Together, our study reveals an unprecedented alkaline pH-dependent activation mechanism of IRR, opening up opportunities to understand the structure-function relationship of this important receptor.

Association between Respiratory Function and Motor Function in Different Stages of Parkinson's Disease

INTRODUCTION

Respiratory dysfunction in patients with Parkinson's disease (PD) could present in the early stage and worsen in the late stages. These changes could be a factor affecting the ability of daily living and quality of life of patients with PD. The primary objective of this study was to assess the respiratory function and its association with motor function in patients with different stages of PD.

METHODS

This was a cross-sectional study conducted at the Huashan Hospital of Fudan University in Shanghai, China. The study included 65 patients diagnosed with PD (the Hoehn and Yahr scale between 1 and 4) and 20 healthy individuals of similar age, gender, weight, and height. The ventilatory function was assessed using the spirometry. Motor function was evaluated using subscale III of the United Parkinson's disease rating scale (UPDRS-III). After confirming the normality of data distribution, we performed one-way ANOVA with a Tukey's post hoc test.

RESULTS

Compared with the healthy individuals, there was no statistical significance in forced vital capacity (FVC), forced expiratory volume in 1 s (FEV1), and forced expiratory volume in 1 s/forced vital capacity (FEV1/FVC) in the H&Y 1 group and H&Y 2 group (p > 0.05) but reduced peak expiratory flow (PEF) in the H&Y 2 group (p = 0.002). Reduced FVC, FEV1, and PEF was seen in the H&Y 3 group (p = 0.002, p = 0.001, and p = 0.0001, respectively). Reduced FVC, FEV1, PEF, and FEF25-75% was seen in the H&Y 4 group (p = 0.001, p = 0.0001, p = 0.0001, and p = 0.025, respectively). The correlation analysis revealed that there was a significant negative correlation between FVC and UPDRS-III scores (r = -0.248, p = 0.046), disease duration (r = -0.276, p = 0.026), H&Y scale (r = -0.415, p = 0.001). FEV1 was negatively correlated with UPDRS-III scores (r = -0.277, p = 0.025), disease duration (r = -0.291, p = 0.019), H&Y scale (r = -0.434, p = 0.0001). FEF25-75% was negatively correlated with disease duration (r = -0.247, p = 0.047), H&Y scale (r = -0.278, p = 0.025).

CONCLUSION

Our findings revealed that respiratory impairment is present in moderate and advanced PD patients, and directly related to the severity of the disease. It is important to conduct respiratory function test in the clinical practice.

Inhibition of FAM46/TENT5 activity by BCCIPα adopting a unique fold

The FAM46 (also known as TENT5) proteins are noncanonical poly(A) polymerases (PAPs) implicated in regulating RNA stability. The regulatory mechanisms of FAM46 are poorly understood. Here, we report that the nuclear protein BCCIPα, but not the alternatively spliced isoform BCCIPβ, binds FAM46 and inhibits their PAP activity. Unexpectedly, our structures of the FAM46A/BCCIPα and FAM46C/BCCIPα complexes show that, despite sharing most of the sequence and differing only at the C-terminal portion, BCCIPα adopts a unique structure completely different from BCCIPβ. The distinct C-terminal segment of BCCIPα supports the adoption of the unique fold but does not directly interact with FAM46. The β sheets in BCCIPα and FAM46 pack side by side to form an extended β sheet. A helix-loop-helix segment in BCCIPα inserts into the active site cleft of FAM46, thereby inhibiting the PAP activity. Our results together show that the unique fold of BCCIPα underlies its interaction with and functional regulation of FAM46.

Impact of the COVID-19 pandemic on congenital diaphragmatic hernia patients: a single-center retrospective study

PURPOSE

To investigate the impact of COVID-19 on the treatment of children with congenital diaphragmatic hernia (CDH).

METHODS

We retrospectively collected and compared the data of patients with CDH admitted between January 1, 2020 and December 31, 2021(study group) with the CDH patients admitted before the pandemic between January 1, 2018 and December 31, 2019 (control group).

RESULTS

During the pandemic, 41 patients with CDH diagnosed prenatally were transferred to our hospital, and 40 underwent surgical repair. The number of patients treated in our hospital increased by 24.2% compared with the 33 patients before the pandemic. During the pandemic, the overall survival rate, postoperative survival rate and recurrence rate were 85.4%, 87.5% and 7.3%, respectively, and there were no significant differences compared with the control group (75.8%, 83.3% and 9.1%, respectively). The average length of hospital stay in patients admitted during the pandemic was longer than that in the control group (31 days vs. 16 days, P < 0.001), and the incidence of nosocomial infection was higher than that in the control group (19.5% vs. 3%, P = 0.037).

CONCLUSIONS

CDH patients confirmed to be SARS-CoV-2 infection-free can receive routine treatment. Our data indicate that the implementation of protective measures during the COVID-19 pandemic, along with appropriate screening and case evaluation, do not have a negative impact on the prognosis of children.

SNARE assembly enlightened by cryo-EM structures of a synaptobrevin-Munc18-1-syntaxin-1 complex

Munc18-1 forms a template to organize assembly of the neuronal SNARE complex that triggers neurotransmitter release, binding first to a closed conformation of syntaxin-1 where its amino-terminal region interacts with the SNARE motif, and later binding to synaptobrevin. However, the mechanism of SNARE complex assembly remains unclear. Here, we report two cryo-EM structures of Munc18-1 bound to cross-linked syntaxin-1 and synaptobrevin. The structures allow visualization of how syntaxin-1 opens and reveal how part of the syntaxin-1 amino-terminal region can help nucleate interactions between the amino termini of the syntaxin-1 and synaptobrevin SNARE motifs, while their carboxyl termini bind to distal sites of Munc18-1. These observations, together with mutagenesis, SNARE complex assembly experiments, and fusion assays with reconstituted proteoliposomes, support a model whereby these interactions are critical to initiate SNARE complex assembly and multiple energy barriers enable diverse mechanisms for exquisite regulation of neurotransmitter release.

Activation of STING by targeting a pocket in the transmembrane domain

Stimulator of interferon genes (STING) is an adaptor protein in innate immunity against DNA viruses or bacteria1-5. STING-mediated immunity could be exploited in the development of vaccines or cancer immunotherapies. STING is a transmembrane dimeric protein that is located in the endoplasmic reticulum or in the Golgi apparatus. STING is activated by the binding of its cytoplasmic ligand-binding domain to cyclic dinucleotides that are produced by the DNA sensor cyclic GMP-AMP (cGAMP) synthase or by invading bacteria1,6,7. Cyclic dinucleotides induce a conformational change in the STING ligand-binding domain, which leads to a high-order oligomerization of STING that is essential for triggering the downstream signalling pathways8,9. However, the cGAMP-induced STING oligomers tend to dissociate in solution and have not been resolved to high resolution, which limits our understanding of the activation mechanism. Here we show that a small-molecule agonist, compound 53 (C53)10, promotes the oligomerization and activation of human STING through a mechanism orthogonal to that of cGAMP. We determined a cryo-electron microscopy structure of STING bound to both C53 and cGAMP, revealing a stable oligomer that is formed by side-by-side packing and has a curled overall shape. Notably, C53 binds to a cryptic pocket in the STING transmembrane domain, between the two subunits of the STING dimer. This binding triggers outward shifts of transmembrane helices in the dimer, and induces inter-dimer interactions between these helices to mediate the formation of the high-order oligomer. Our functional analyses show that cGAMP and C53 together induce stronger activation of STING than either ligand alone.

Characterization of the subunit composition and structure of adult human glycine receptors

The strychnine-sensitive pentameric glycine receptor (GlyR) mediates fast inhibitory neurotransmission in the mammalian nervous system. Only heteromeric GlyRs mediate synaptic transmission, as they contain the β subunit that permits clustering at the synapse through its interaction with scaffolding proteins. Here, we show that α2 and β subunits assemble with an unexpected 4:1 stoichiometry to produce GlyR with native electrophysiological properties. We determined structures in multiple functional states at 3.6-3.8 Å resolutions and show how 4:1 stoichiometry is consistent with the structural features of α2β GlyR. Furthermore, we show that one single β subunit in each GlyR gives rise to the characteristic electrophysiological properties of heteromeric GlyR, while more β subunits render GlyR non-conductive. A single β subunit ensures a univalent GlyR-scaffold linkage, which means the scaffold alone regulates the cluster properties.

Scap structures highlight key role for rotation of intertwined luminal loops in cholesterol sensing

The cholesterol-sensing protein Scap induces cholesterol synthesis by transporting membrane-bound transcription factors called sterol regulatory element-binding proteins (SREBPs) from the endoplasmic reticulum (ER) to the Golgi apparatus for proteolytic activation. Transport requires interaction between Scap's two ER luminal loops (L1 and L7), which flank an intramembrane sterol-sensing domain (SSD). Cholesterol inhibits Scap transport by binding to L1, which triggers Scap's binding to Insig, an ER retention protein. Here we used cryoelectron microscopy (cryo-EM) to elucidate two structures of full-length chicken Scap: (1) a wild-type free of Insigs and (2) mutant Scap bound to chicken Insig without cholesterol. Strikingly, L1 and L7 intertwine tightly to form a globular domain that acts as a luminal platform connecting the SSD to the rest of Scap. In the presence of Insig, this platform undergoes a large rotation accompanied by rearrangement of Scap's transmembrane helices. We postulate that this conformational change halts Scap transport of SREBPs and inhibits cholesterol synthesis.

Structural basis of the activation of c-MET receptor

The c-MET receptor is a receptor tyrosine kinase (RTK) that plays essential roles in normal cell development and motility. Aberrant activation of c-MET can lead to both tumors growth and metastatic progression of cancer cells. C-MET can be activated by either hepatocyte growth factor (HGF), or its natural isoform NK1. Here, we report the cryo-EM structures of c-MET/HGF and c-MET/NK1 complexes in the active state. The c-MET/HGF complex structure reveals that, by utilizing two distinct interfaces, one HGF molecule is sufficient to induce a specific dimerization mode of c-MET for receptor activation. The binding of heparin as well as a second HGF to the 2:1 c-MET:HGF complex further stabilize this active conformation. Distinct to HGF, NK1 forms a stable dimer, and bridges two c-METs in a symmetrical manner for activation. Collectively, our studies provide structural insights into the activation mechanisms of c-MET, and reveal how two isoforms of the same ligand use dramatically different mechanisms to activate the receptor.

Activation of c-MET receptor tyrosine kinase involves hepatocyte growth factor (HGF) and glycosaminoglycans, but the molecular mechanism is still under debate. Here, the authors present cryoEM structures of c-MET bound to two HGF splice variants and heparin, revealing the structural basis for c-MET activation.

Architecture of the Sema3A/PlexinA4/Neuropilin tripartite complex

Secreted class 3 semaphorins (Sema3s) form tripartite complexes with the plexin receptor and neuropilin coreceptor, which are both transmembrane proteins that together mediate semaphorin signal for neuronal axon guidance and other processes. Despite extensive investigations, the overall architecture of and the molecular interactions in the Sema3/plexin/neuropilin complex are incompletely understood. Here we present the cryo-EM structure of a near intact extracellular region complex of Sema3A, PlexinA4 and Neuropilin 1 (Nrp1) at 3.7 Å resolution. The structure shows a large symmetric 2:2:2 assembly in which each subunit makes multiple interactions with others. The two PlexinA4 molecules in the complex do not interact directly, but their membrane proximal regions are close to each other and poised to promote the formation of the intracellular active dimer for signaling. The structure reveals a previously unknown interface between the a2b1b2 module in Nrp1 and the Sema domain of Sema3A. This interaction places the a2b1b2 module at the top of the complex, far away from the plasma membrane where the transmembrane regions of Nrp1 and PlexinA4 embed. As a result, the region following the a2b1b2 module in Nrp1 must span a large distance to allow the connection to the transmembrane region, suggesting an essential role for the long non-conserved linkers and the MAM domain in neuropilin in the semaphorin/plexin/neuropilin complex.

Seeing Atoms by Single-Particle Cryo-EM

Nakane et al. and Yip et al., for the first time, demonstrate that, with recent technological advances, atomic-resolution structure determination can be achieved by single-particle cryo-electron microscopy (cryo-EM). This breakthrough opens the door for researchers to apply single-particle cryo-EM to obtain atomic structural information for a wide range of protein complexes.

Structures and Mechanisms in the cGAS-STING Innate Immunity Pathway

Besides its role as the blueprint of life, DNA can also alert the cell to the presence of microbial pathogens as well as damaged or malignant cells. A major sensor of DNA that triggers the innate immune response is cyclic guanosine monophosphate (GMP)-adenosine monophosphate (AMP) (cGAMP) synthase (cGAS), which produces the second messenger cGAMP. cGAMP activates stimulator of interferon genes (STING), which activates a signaling cascade leading to the production of type I interferons and other immune mediators. Recent research has demonstrated an expanding role of the cGAS-cGAMP-STING pathway in many physiological and pathological processes, including host defense against microbial infections, anti-tumor immunity, cellular senescence, autophagy, and autoimmune and inflammatory diseases. Biochemical and structural studies have elucidated the mechanism of signal transduction in the cGAS pathway at the atomic resolution. This review focuses on the structural and mechanistic insights into the roles of cGAS and STING in immunity and diseases revealed by these recent studies.

Cryo-EM structure of the human cohesin-NIPBL-DNA complex

As a ring-shaped adenosine triphosphatase (ATPase) machine, cohesin organizes the eukaryotic genome by extruding DNA loops and mediates sister chromatid cohesion by topologically entrapping DNA. How cohesin executes these fundamental DNA transactions is not understood. Using cryo-electron microscopy (cryo-EM), we determined the structure of human cohesin bound to its loader NIPBL and DNA at medium resolution. Cohesin and NIPBL interact extensively and together form a central tunnel to entrap a 72-base pair DNA. NIPBL and DNA promote the engagement of cohesin's ATPase head domains and ATP binding. The hinge domains of cohesin adopt an "open washer" conformation and dock onto the STAG1 subunit. Our structure explains the synergistic activation of cohesin by NIPBL and DNA and provides insight into DNA entrapment by cohesin.

Structure of a D2 dopamine receptor-G-protein complex in a lipid membrane

The D₂ dopamine receptor (DRD2) is a therapeutic target for Parkinson’s disease¹ and antipsychotic drugs². DRD2 is activated by the endogenous neurotransmitter dopamine and synthetic agonist drugs such as bromocriptine³, leading to stimulation of G_i and inhibition of adenylyl cyclase. We used cryo-electron microscopy to elucidate the structure of an agonist-bound activated DRD2-G_i complex reconstituted into a phospholipid membrane. The extracellular ligand binding site of DRD2 is remodeled in response to agonist binding, with conformational changes in extracellular loop 2 (ECL2), transmembrane domain 5 (TM5), TM6, and TM7 propagating to opening of the intracellular G_i binding site. The DRD2-G_i structure represents the first experimental model of a GPCR-G protein complex embedded in a phospholipid bilayer, which serves as a benchmark to validate the interactions seen in previous detergent-bound structures. The structure also reveals interactions that are unique to the membrane-embedded complex, including helix 8 burial in the inner leaflet, ordered lysine and arginine sidechains in the membrane interfacial regions, and lipid anchoring of the G protein in the membrane. Our model of the activated DRD2 will help inform the design of subtype-selective DRD2 ligands for multiple human CNS disorders.

Cryo-EM structure of the PlexinC1/A39R complex reveals inter-domain interactions critical for ligand-induced activation

Plexins are receptors for semaphorins that transduce signals for regulating neuronal development and other processes. Plexins are single-pass transmembrane proteins with multiple domains in both the extracellular and intracellular regions. Semaphorin activates plexin by binding to its extracellular N-terminal Sema domain, inducing the active dimer of the plexin intracellular region. The mechanism underlying this activation process of plexin is incompletely understood. We present cryo-electron microscopic structure of full-length human PlexinC1 in complex with the viral semaphorin mimic A39R. The structure shows that A39R induces a specific dimer of PlexinC1 where the membrane-proximal domains from the two PlexinC1 protomers are placed close to each other, poised to promote the active dimer of the intracellular region. This configuration is imposed by a distinct conformation of the PlexinC1 extracellular region, stabilized by inter-domain interactions among the Sema and membrane-proximal domains. Our mutational analyses support the critical role of this conformation in PlexinC1 activation.

Plexins are the receptors for the guidance molecules semaphorins and regulate immunity and the development of the nervous and cardiovascular systems. Here authors present a structure of full-length human PlexinC1 in complex with its ligand A39R, which reveals how inter-domain interactions couple extracellular ligand binding to receptor activation and signaling.

Cryo-EM structures of the human cation-chloride cotransporter KCC1

Cation-chloride cotransporters (CCCs) mediate the coupled, electroneutral symport of cations with chloride across the plasma membrane and are vital for cell volume regulation, salt reabsorption in the kidney, and γ-aminobutyric acid (GABA)-mediated modulation in neurons. Here we present cryo-electron microscopy (cryo-EM) structures of human potassium-chloride cotransporter KCC1 in potassium chloride or sodium chloride at 2.9- to 3.5-angstrom resolution. KCC1 exists as a dimer, with both extracellular and transmembrane domains involved in dimerization. The structural and functional analyses, along with computational studies, reveal one potassium site and two chloride sites in KCC1, which are all required for the ion transport activity. KCC1 adopts an inward-facing conformation, with the extracellular gate occluded. The KCC1 structures allow us to model a potential ion transport mechanism in KCCs and provide a blueprint for drug design.

Structural basis of the activation of type 1 insulin-like growth factor receptor

Type 1 insulin-like growth factor receptor (IGF1R) is a receptor tyrosine kinase that regulates cell growth and proliferation, and can be activated by IGF1, IGF2, and insulin. Here, we report the cryo-EM structure of full-length IGF1R–IGF1 complex in the active state. This structure reveals that only one IGF1 molecule binds the Γ-shaped asymmetric IGF1R dimer. The IGF1-binding site is formed by the L1 and CR domains of one IGF1R protomer and the α-CT and FnIII-1 domains of the other. The liganded α-CT forms a rigid beam-like structure with the unliganded α-CT, which hinders the conformational change of the unliganded α-CT required for binding of a second IGF1 molecule. We further identify an L1–FnIII-2 interaction that mediates the dimerization of membrane-proximal domains of IGF1R. This interaction is required for optimal receptor activation. Our study identifies a source of the negative cooperativity in IGF1 binding to IGF1R and reveals the structural basis of IGF1R activation.

The activation mechanism of type 1 insulin-like growth factor receptor (IGF1R) is not fully understood. Here, the authors determine the cryo-EM structure of full-length, IGF1-bound IGF1R in the active conformation, providing insights into how IGF1 triggers receptor activation.

Structural Mechanism of EMRE-Dependent Gating of the Human Mitochondrial Calcium Uniporter

Mitochondrial calcium uptake is crucial to the regulation of eukaryotic Ca²⁺ homeostasis and is mediated by the mitochondrial calcium uniporter (MCU). While MCU alone can transport Ca²⁺ in primitive eukaryotes, metazoans require an essential single membrane-spanning auxiliary component called EMRE to form functional channels; however, the molecular mechanism of EMRE regulation remains elusive. Here, we present the cryo-EM structure of the human MCU-EMRE complex, which defines the interactions between MCU and EMRE as well as pinpoints the juxtamembrane loop of MCU and extended linker of EMRE as the crucial elements in the EMRE-dependent gating mechanism among metazoan MCUs. The structure also features the dimerization of two MCU-EMRE complexes along an interface at the N-terminal domain (NTD) of human MCU that is a hotspot for post-translational modifications. Thus, the human MCU-EMRE complex, which constitutes the minimal channel components among metazoans, provides a framework for future mechanistic studies on MCU.

Structural and functional characterization of an otopetrin family proton channel

The otopetrin (OTOP) proteins were recently characterized as proton channels. Here we present the cryo-EM structure of OTOP3 from Xenopus tropicalis (XtOTOP3) along with functional characterization of the channel. XtOTOP3 forms a homodimer with each subunit containing 12 transmembrane helices that can be divided into two structurally homologous halves; each half assembles as an α-helical barrel that could potentially serve as a proton conduction pore. Both pores open from the extracellular half before becoming occluded at a central constriction point consisting of three highly conserved residues - Gln_232/585-Asp₂₆₂/Asn₆₂₃-Tyr_322/666 (the constriction triads). Mutagenesis shows that the constriction triad from the second pore is less amenable to perturbation than that of the first pore, suggesting an unequal contribution between the two pores to proton transport. We also identified several key residues at the interface between the two pores that are functionally important, particularly Asp509, which confers intracellular pH-dependent desensitization to OTOP channels.

Structural mechanisms of phospholipid activation of the human TPC2 channel

Mammalian two-pore channels (TPCs) regulate the physiological functions of the endolysosome. Here we present cryo-EM structures of human TPC2 (HsTPC2), a phosphatidylinositol 3,5-bisphosphate (PI(3,5)P₂)-activated, Na⁺ selective channel, in the ligand-bound and apo states. The apo structure captures the closed conformation, while the ligand-bound form features the channel in both open and closed conformations. Combined with functional analysis, these structures provide insights into the mechanism of PI(3,5)P₂-regulated gating of TPC2, which is distinct from that of TPC1. Specifically, the endolysosome-specific PI(3,5)P₂ binds at the first 6-TM and activates the channel – independently of the membrane potential – by inducing a structural change at the pore-lining inner helix (IS6), which forms a continuous helix in the open state but breaks into two segments at Gly317 in the closed state. Additionally, structural comparison to the voltage-dependent TPC1 structure allowed us to identify Ile551 as being responsible for the loss of voltage dependence in TPC2.

Structural insights into the voltage and phospholipid activation of the mammalian TPC1 channel

Organellar two-pore channels (TPCs) function as a homodimer with each subunit containing two homologous Shaker-like 6-TM repeats¹. They belong to the voltage-gated ion channel superfamily² and are ubiquitously expressed in animals and plants^3,4. Mammalian TPC1 and TPC2 are localized to the endolysosomal membrane and play critical roles in regulating the physiological functions of these acidic organelles^5–7. Here we present the cryo-EM structures of mouse TPC1 (MmTPC1), a voltage-dependent, phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P₂) activated Na⁺ selective channel, in both the apo closed and ligand-bound open states which, combined with functional analysis, provide comprehensive structural insights into the selectivity and gating mechanisms of mammalian TPC channels. The channel has a coin slot-shaped ion pathway in the filter that defines the selectivity of mammalian TPCs. Only the voltage sensing domain from the second 6-TM domain confers voltage dependence to MmTPC1. Endolysosome-specific PtdIns(3,5)P₂ binds to the first 6-TM domain and activates the channel under depolarizing membrane potential. Structural comparison between the apo and PtdIns(3,5)P₂-bound structures elucidates the interplay between voltage and ligand in channel activation. In light of the emerging importance of phosphoinositide regulation of ion channels, the MmTPC1 structures exemplify the lipid binding and regulation in a 6-TM voltage-gated channel.

Structures of the calcium-activated, non-selective cation channel TRPM4

TRPM4 is a calcium-activated, phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P₂) -modulated, non-selective cation channel that belongs to the family of melastatin-related transient receptor potential (TRPM) channels. Here we present the electron cryo-microscopy structures of the mouse TRPM4 channel with and without ATP. TRPM4 consists of multiple transmembrane and cytosolic domains, which assemble into a three-tiered architecture. The N-terminal nucleotide-binding domain and the C-terminal coiled-coil participate in the tetrameric assembly of the channel; ATP binds at the nucleotide-binding domain and inhibits channel activity. TRPM4 has an exceptionally wide filter but is only permeable to monovalent cations; filter residue Gln973 is essential in defining monovalent selectivity. The S1-S4 domain and the post-S6 TRP domain form the central gating apparatus that probably houses the Ca²⁺- and PtdIns(4,5)P₂-binding sites. These structures provide an essential starting point for elucidating the complex gating mechanisms of TRPM4 and reveal the molecular architecture of the TRPM family.

Structure of mammalian endolysosomal TRPML1 channel in nanodiscs

Transient receptor potential mucolipin 1 (TRPML1) is an endo/lysosomal cation channel ubiquitously expressed in mammalian cells^1,2 and its loss-of-function mutations are the direct cause of Type IV mucolipidosis (MLIV), an autosomal recessive lysosomal storage disease^3-6. Here we present the single particle cryo-electron microscopy (cryo-EM) structure of the mouse TRPML1 channel embedded in nanodiscs. Combined with mutagenesis, the TRPML1 structure reveals that phosphatidylinositol bisphosphate (PIP₂) binds to the N-terminus of the channel – distal from the pore – and the helix-turn-helix extension between S2 and S3 likely couples ligand binding to pore opening. The tightly packed selectivity filter contains multiple ion binding sites and the conserved acidic residues form the luminal Ca²⁺ blocking site that confers luminal pH and Ca²⁺ modulation on channel conductance. A luminal linker domain forms a fenestrated canopy atop the channel, providing multiple luminal ion passages to the pore and also creating a negative electrostatic trap – preferably for divalent cations at the luminal entrance. The structure also reveals two equally distributed S4-S5 linker conformations in the closed channel, providing structural implication for the S4-S5 linker-mediated PIP₂ gating mechanism among TRPML channels^7,8.

Cryo-EM structure of a metazoan separase-securin complex at near-atomic resolution

Separase is a caspase-family protease that initiates chromatid segregation by cleaving the kleisin subunits (Scc1 and Rec8) of cohesin, and regulates centrosome duplication and mitotic spindle function through cleavage of kendrin and Slk19. To understand the mechanisms of securin regulation of separase, we used single-particle cryo-electron microscopy (cryo-EM) to determine a near-atomic-resolution structure of the Caenorhabditis elegans separase-securin complex. Separase adopts a triangular-shaped bilobal architecture comprising an N-terminal tetratricopeptide repeat (TPR)-like α-solenoid domain docked onto the conserved C-terminal protease domain. Securin engages separase in an extended antiparallel conformation, interacting with both lobes. It inhibits separase by interacting with the catalytic site through a pseudosubstrate mechanism, thus revealing that in the inhibited separase-securin complex, the catalytic site adopts a conformation compatible with substrate binding. Securin is protected from cleavage because an aliphatic side chain at the P1 position represses protease activity by disrupting the organization of catalytic site residues.

Mefloquine targets the Plasmodium falciparum 80S ribosome to inhibit protein synthesis

Malaria control is heavily dependent on chemotherapeutic agents for disease prevention and drug treatment. Defining the mechanism of action for licensed drugs, for which no target is characterized, is critical to the development of their second-generation derivatives to improve drug potency towards inhibition of their molecular targets. Mefloquine is a widely used antimalarial without a known mode of action. Here, we demonstrate that mefloquine is a protein synthesis inhibitor. We solved a 3.2 Å cryo-electron microscopy structure of the Plasmodium falciparum 80S ribosome with the (+)-mefloquine enantiomer bound to the ribosome GTPase-associated centre. Mutagenesis of mefloquine-binding residues generates parasites with increased resistance, confirming the parasite-killing mechanism. Furthermore, structure-guided derivatives with an altered piperidine group, predicted to improve binding, show enhanced parasiticidal effect. These data reveal one possible mode of action for mefloquine and demonstrate the vast potential of cryo-electron microscopy to guide the development of mefloquine derivatives to inhibit parasite protein synthesis.

Structure of a spliceosome remodelled for exon ligation

The spliceosome excises introns from pre-mRNAs in two sequential transesterifications – branching and exon ligation¹ – catalysed at a single catalytic metal site in U6 snRNA^2,3. The recent structures of the spliceosomal C complex^4,5 with the cleaved 5’-exon and lariat—3’-exon bound to the catalytic centre revealed that branching-specific factors such as Cwc25 lock the branch helix into position for nucleophilic attack of the branch adenosine at the 5’-splice site. Furthermore, the ATPase Prp16 is positioned to bind and translocate the intron downstream of the branch point to destabilize branching-specific factors and release the branch helix from the active site⁴. Here we present the 3.8Å cryo-EM structure of a Saccharomyces cerevisiae spliceosome stalled after Prp16-mediated remodelling but prior to exon ligation. While the U6 snRNA catalytic core remains firmly held in the active site cavity of Prp8 by proteins common to both steps, the branch helix has rotated by 75 degrees compared to complex C and is stabilized into a new position by Prp17, Cef1, and the reoriented Prp8 RNaseH domain. This rotation of the branch helix removes the branch adenosine from the catalytic core, creates a space for 3’-exon docking, and restructures the pairing of the 5’-splice site with the U6 snRNA ACAGAGA region. Slu7 and Prp18, which promote exon ligation, bind together to the Prp8 RNaseH domain. The ATPase Prp22, bound to Prp8 in place of Prp16, could interact with the 3’-exon, suggesting a possible basis for mRNA release after exon ligation^6,7. Together with the C complex structure⁴, our new C* complex structure reveals the two major conformations of the spliceosome during the catalytic stages of splicing.

Design of a molecular support for cryo-EM structure determination

Despite the recent rapid progress in cryo-electron microscopy (cryo-EM), there still exist ample opportunities for improvement in sample preparation. Macromolecular complexes may disassociate or adopt nonrandom orientations against the extended air-water interface that exists for a short time before the sample is frozen. We designed a hollow support structure using 3D DNA origami to protect complexes from the detrimental effects of cryo-EM sample preparation. For a first proof-of-principle, we concentrated on the transcription factor p53, which binds to specific DNA sequences on double-stranded DNA. The support structures spontaneously form monolayers of preoriented particles in a thin film of water, and offer advantages in particle picking and sorting. By controlling the position of the binding sequence on a single helix that spans the hollow support structure, we also sought to control the orientation of individual p53 complexes. Although the latter did not yet yield the desired results, the support structures did provide partial information about the relative orientations of individual p53 complexes. We used this information to calculate a tomographic 3D reconstruction, and refined this structure to a final resolution of ∼15 Å. This structure settles an ongoing debate about the symmetry of the p53 tetramer bound to DNA.

Cryo-EM structure of the yeast U4/U6.U5 tri-snRNP at 3.7 Å resolution

U4/U6.U5 tri-snRNP represents a substantial part of the spliceosome before activation. A cryoEM structure of Saccharomyces cerevisiae U4/U6.U5 tri-snRNP at 3.7Å resolution led to an essentially complete atomic model comprising 30 proteins plus U4/U6 and U5 snRNAs. The structure reveals striking interweaving interactions of the protein and RNA components including extended polypeptides penetrating into subunit interfaces. The invariant ACAGAGA sequence of U6 snRNA, which base-pairs with the 5′-splice site during catalytic activation, forms a hairpin stabilised by Dib1 and Prp8 while the adjacent nucleotides interact with the exon binding loop 1 of U5 snRNA. Snu114 harbours GTP but its putative catalytic histidine is held away from the γ-phosphate by hydrogen bonding to a tyrosine in Prp8’s N-terminal domain. Mutation of this histidine to alanine has no detectable effect on yeast growth. The structure provides important new insights into the spliceosome activation process leading to the formation of the catalytic centre.

Atomic structure of the apoptosome: mechanism of cytochrome c- and dATP-mediated activation of Apaf-1

In this study, Zhou et al. report the first atomic structure of the mammalian apoptosome, determined at 3.8 Å resolution by cryo-electron microscopy. These findings provide novel insight into how CytC relieves the autoinhibition of Apaf-1 and how dATP triggers Apaf-1 oligomerization.

The apoptotic protease-activating factor 1 (Apaf-1) controls the onset of many known forms of intrinsic apoptosis in mammals. Apaf-1 exists in normal cells as an autoinhibited monomer. Upon binding to cytochrome c and dATP, Apaf-1 oligomerizes into a heptameric complex known as the apoptosome, which recruits and activates cell-killing caspases. Here we present an atomic structure of an intact mammalian apoptosome at 3.8 Å resolution, determined by single-particle, cryo-electron microscopy (cryo-EM). Structural analysis, together with structure-guided biochemical characterization, uncovered how cytochrome c releases the autoinhibition of Apaf-1 through specific interactions with the WD40 repeats. Structural comparison with autoinhibited Apaf-1 revealed how dATP binding triggers a set of conformational changes that results in the formation of the apoptosome. Together, these results constitute the molecular mechanism of cytochrome c- and dATP-mediated activation of Apaf-1.

Ultrafast high-power microwave window breakdown: nonlinear and postpulse effects

The time- and space-dependent optical emissions of nanosecond high-power microwave discharges near a dielectric-air interface have been observed by nanosecond-response four-framing intensified-charged-coupled device cameras. The experimental observations indicate that plasma developed more intensely at the dielectric-air interface than at the free-space region with a higher electric-field amplitude. A thin layer of intense light emission above the dielectric was observed after the microwave pulse. The mechanisms of the breakdown phenomena are analyzed by a three-dimensional electromagnetic-field modeling and a two-dimensional electromagnetic particle-in-cell simulation, revealing the formation of a space-charge microwave sheath near the dielectric surface, accelerated by the normal components of the microwave field, significantly enhancing the local-field amplitude and hence ionization near the dielectric surface. The nonlinear positive feedback of ionization, higher electron mobility, and ultraviolet-driven photoemission due to the elevated electron temperature are crucial for achieving the ultrafast discharge. Following the high-power microwave pulse, the sheath sustains a glow discharge until the sheath collapses.

Structure of the large ribosomal subunit from human mitochondria

Human mitochondrial ribosomes are highly divergent from all other known ribosomes and are specialized to exclusively translate membrane proteins. They are linked with hereditary mitochondrial diseases and are often the unintended targets of various clinically useful antibiotics. Using single-particle cryogenic electron microscopy, we have determined the structure of its large subunit to 3.4 angstrom resolution, revealing 48 proteins, 21 of which are specific to mitochondria. The structure unveils an adaptation of the exit tunnel for hydrophobic nascent peptides, extensive remodeling of the central protuberance, including recruitment of mitochondrial valine transfer RNA (tRNA(Val)) to play an integral structural role, and changes in the tRNA binding sites related to the unusual characteristics of mitochondrial tRNAs.

Three-dimensional structure of human γ-secretase

The γ-secretase complex, comprising presenilin 1 (PS1), PEN-2, APH-1 and nicastrin, is a membrane-embedded protease that controls a number of important cellular functions through substrate cleavage. Aberrant cleavage of the amyloid precursor protein (APP) results in aggregation of amyloid-β, which accumulates in the brain and consequently causes Alzheimer's disease. Here we report the three-dimensional structure of an intact human γ-secretase complex at 4.5 Å resolution, determined by cryo-electron-microscopy single-particle analysis. The γ-secretase complex comprises a horseshoe-shaped transmembrane domain, which contains 19 transmembrane segments (TMs), and a large extracellular domain (ECD) from nicastrin, which sits immediately above the hollow space formed by the TM horseshoe. Intriguingly, nicastrin ECD is structurally similar to a large family of peptidases exemplified by the glutamate carboxypeptidase PSMA. This structure serves as an important basis for understanding the functional mechanisms of the γ-secretase complex.

Initiation of translation by cricket paralysis virus IRES requires its translocation in the ribosome

The cricket paralysis virus internal ribosome entry site (CrPV-IRES) is a folded structure in a viral mRNA that allows initiation of translation in the absence of any host initiation factors. By using recent advances in single-particle electron cryomicroscopy, we have solved the structure of CrPV-IRES bound to the ribosome of the yeast Kluyveromyces lactis in both the canonical and rotated states at overall resolutions of 3.7 and 3.8 Å, respectively. In both states, the pseudoknot PKI of the CrPV-IRES mimics a tRNA/mRNA interaction in the decoding center of the A site of the 40S ribosomal subunit. The structure and accompanying factor-binding data show that CrPV-IRES binding mimics a pretranslocation rather than initiation state of the ribosome. Translocation of the IRES by elongation factor 2 (eEF2) is required to bring the first codon of the mRNA into the A site and to allow the start of translation.

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The high-resolution structure of CrPV-IRES bound to the ribosome was solved by cryoEM

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Pseudoknot I of CrPV-IRES binds in the decoding center, thus blocking the A site

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CrPV-IRES mimics a pretranslocation rather than initiation complex of the ribosome

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Translocation of CrPV-IRES by eEF2 is required for the start of translation

Structure of the yeast mitochondrial large ribosomal subunit

Mitochondria have specialized ribosomes that have diverged from their bacterial and cytoplasmic counterparts. We have solved the structure of the yeast mitoribosomal large subunit using single-particle cryo-electron microscopy. The resolution of 3.2 angstroms enabled a nearly complete atomic model to be built de novo and refined, including 39 proteins, 13 of which are unique to mitochondria, as well as expansion segments of mitoribosomal RNA. The structure reveals a new exit tunnel path and architecture, unique elements of the E site, and a putative membrane docking site.

Molecular architecture of a eukaryotic translational initiation complex

The last step in eukaryotic translational initiation involves the joining of the large and small subunits of the ribosome, with initiator transfer RNA (Met-tRNA(i)(Met)) positioned over the start codon of messenger RNA in the P site. This step is catalyzed by initiation factor eIF5B. We used recent advances in cryo-electron microscopy (cryo-EM) to determine a structure of the eIF5B initiation complex to 6.6 angstrom resolution from <3% of the population, comprising just 5143 particles. The structure reveals conformational changes in eIF5B, initiator tRNA, and the ribosome that provide insights into the role of eIF5B in translational initiation. The relatively high resolution obtained from such a small fraction of a heterogeneous sample suggests a general approach for characterizing the structure of other dynamic or transient biological complexes.

Ribosome structures to near-atomic resolution from thirty thousand cryo-EM particles

Although electron cryo-microscopy (cryo-EM) single-particle analysis has become an important tool for structural biology of large and flexible macro-molecular assemblies, the technique has not yet reached its full potential. Besides fundamental limits imposed by radiation damage, poor detectors and beam-induced sample movement have been shown to degrade attainable resolutions. A new generation of direct electron detectors may ameliorate both effects. Apart from exhibiting improved signal-to-noise performance, these cameras are also fast enough to follow particle movements during electron irradiation. Here, we assess the potentials of this technology for cryo-EM structure determination. Using a newly developed statistical movie processing approach to compensate for beam-induced movement, we show that ribosome reconstructions with unprecedented resolutions may be calculated from almost two orders of magnitude fewer particles than used previously. Therefore, this methodology may expand the scope of high-resolution cryo-EM to a broad range of biological specimens.DOI:http://dx.doi.org/10.7554/eLife.00461.001.

Structural characterization of full-length NSF and 20S particles

The 20S particle, which is composed of the N-ethylmaleimide-sensitive factor (NSF), soluble NSF attachment proteins (SNAPs) and the SNAP receptor (SNARE) complex, has an essential role in intracellular vesicle fusion events. Using single-particle cryo-EM and negative stain EM, we reconstructed four related three-dimensional structures: Chinese hamster NSF hexamer in the ATPγS, ADP-AlFx and ADP states, and the 20S particle. These structures reveal a parallel arrangement between the D1 and D2 domains of the hexameric NSF and characterize the nucleotide-dependent conformational changes in NSF. The structure of the 20S particle shows that it holds the SNARE complex at two interaction interfaces around the C terminus and N-terminal half of the SNARE complex, respectively. These findings provide insight into the molecular mechanism underlying disassembly of the SNARE complex by NSF.

Cellular internalization of exosomes occurs through phagocytosis

Exosomes play important roles in many physiological and pathological processes. However, the exosome-cell interaction mode and the intracellular trafficking pathway of exosomes in their recipient cells remain unclear. Here, we report that exosomes derived from K562 or MT4 cells are internalized more efficiently by phagocytes than by non-phagocytic cells. Most exosomes were observed attached to the plasma membrane of non-phagocytic cells, while in phagocytic cells these exosomes were found to enter via phagocytosis. Specifically, they moved to phagosomes together with phagocytic polystyrene carboxylate-modified latex beads (biospheres) and were further sorted into phagolysosomes. Moreover, exosome internalization was dependent on the actin cytoskeleton and phosphatidylinositol 3-kinase, and could be inhibited by the knockdown of dynamin2 or overexpression of a dominant-negative form of dynamin2. Further, antibody pretreatment assays demonstrated that tim4 but not tim1 was involved in exosomes uptake. We also found that exosomes did not enter the internalization pathway involving caveolae, macropinocytosis and clathrin-coated vesicles. Our observation that the cellular uptake of exosomes occurs through phagocytosis has important implications for exosome-cell interactions and the exosome intracellular trafficking pathway.

Traditional Chinese medicine Bao Gan Ning increase phosphorylation of CREB in liver fibrosis in vivo and in vitro

Previous studies have demonstrated that traditional Chinese medicine Bao Gan Ning, which contains six different drugs: Trionyx sinensis Wiegmann shell, Prunus persica (L.) Batsch seed, Salvia miltiorrhiza Bge. root, Mallotus opelta (Lour.) Muell-Arg root, Astragalus membranaceus (Fisch.) Bge. var. mongho-licus (Bge.) Hsiao root and Scutellaria baicalensis Georgi root, was able to protect liver against fibrosis in CCL4 models. In an effort to elucidate molecular mechanisms by which Bao Gan Ning exerts its anti-fibrosis activity, effects of Bao Gan Ning on liver fibrosis and cAMP response element binding protein (CREB), an important transcription factor involved in liver fibrosis, were evaluated in animal and cell models in this work. Results showed that Bao Gan Ning (2.16 or 4.32 g/kg/day) significantly decreased alanine aminotransferase (ALT) and hyaluronidase levels and reversed liver fibrosis in rat liver fibrosis models. The proliferation of HSC-T6, a hepatic stellate cell line, was also significantly inhibited by incubation with serums that were prepared from rats fed with Bao Gan Ning. Most interestingly, results from Western blot, immunohistochemistry and electrophoretic mobility shift assay (EMSA) showed that Bao Gan Ning up-regulated CREB phosphorylation both in rat liver fibrosis models and in HSC-T6 cells, but did not affect protein level of CREB and the DNA binding activity of CREB. These results suggested that up-regulation of CREB phosphorylation may be involved in anti-fibrosis activity of Chinese medicine Bao Gan Ning.

Up-regulation of Phosphatidylinositol-3 Kinase Signaling in plcg1 Gene Null Fibroblasts

Phospholipase C-gamma1(PLC-gamma1) and phosphatidylinositol-3 kinase(PI-3K) play crucial role in growth factor-induced cell growth and proliferation. To investigate the complementary mechanism of PLC-gamma1 in cell growth and epidermal growth factor (EGF)-induced mitogenic signaling, PLC-gamma1 deficient mouse embryonic fibroblasts(PLC-gamma1(-/-)) and its wild type(PLC-gamma1(+/+)) were exposed to U73122, a phospholipase C-specific inhibitor, or wortmannin, a PI-3K inhibitor then the clonogenicity, viability, EGF-induced DNA synthesis of the two cell lines were determined by cloning formation, MTT method and (3)H -thymidine incorporation assay. Results showed that either U73122 or wortmannin inhibited PLC-gamma1(-/-) and PLC-gamma1(+/+) cells in terms of EGF-induced DNA synthesis, cloning formation and cell viability, but PLC-gamma1(-/-) cells were more dependent on PI-3K and less dependent on PLC compared with wild types. The PLC-gamma1 signaling pathway of PLC-gamma1(-/-) cells might be complemented by PI-3K pathway, because after EGF stimulation, the tyrosine phospholation of p85alpha PI-3K increased significantly in PLC-gamma1(-/-), but not in PLC-gamma2(-/-), as Western blotting showed that there was neither complementary PLC-gamma2 expression in PLC-gamma1(-/-) cells, nor other PLC isozymes such as PLC-beta and PLC-delta. These results suggest the redundancy of EGF-mediated signaling and the complementary mechanism of PLC-gamma1 pathway.

[Zinc modulation of GABA receptors expressed in Xenopus oocytes injected with carp brain poly(A)+ RNA]

Two types of GABA receptors, GABAA receptors (approximately 85%) and GABAC-like receptors (approximately 15%), were expressed in Xenopus oocytes after injection of Poly(A)+ RNA with carp (Carassius carassius) brain. Using voltage-clamp technique and pharmacological methods, the effects of zinc on the two receptors were studied. It was found that zinc modulation of both GABAA and GABAC-like receptors by zinc was inhibitory and reversible with the former being stronger than the latter as indicated by a value of IC50 of 48.4 +/- 10.1 mumol/L against 255.6 +/- 21.5 mumol/L.

[Blocking effects of benzyltetrahydropalmatine on delayed rectified K+ currents expressed in Xenopus oocytes and in toad oocytes]

The effects of benzyltetrahydropalmatine (BTHP) on delayed rectified K+ currents (Ik) expressed in Xenopus oocytes and Ik of toad (Bufo bufo gargarizans) oocytes were studied. The Ik expressed in Xenopus oocytes was measured after microinjection of mRNA isolated from carp fish (C anratus L.) brains with double -microelectrode voltage clamp technique. The maximum and mean value of Ik expressed in Xenopus oocytes were 600 nA and 360 +/- 104 nA, respectively. BTHP reduced the current amplitude of Ik expressed in Xenopus oocytes in 10-1000 mumol.L-1 dose-dependently, EC50 was 29 mumol.L-1. Also, the reduction of Ik of toad oocytes was 9.1%, 29.1%, 54.7% and 68.6% by BTHP 10, 30, 100 and 1000 mumol.L-1, respectively, EC50 was 33 mumol.L-1. The results showed that BTHP possesses an inhibitory effect on Ik, the main ion mechanism of antiarrhythmic action of BTHP.