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High-conductance voltage-and Ca 2+ -activated K + (BK) channels encode negative feedback regulation of membrane voltage and Ca 2+ signaling, playing a central role in numerous physiological processes. We determined the x-ray structure of the human BK Ca 2+ gating apparatus at a resolution of 3.0 angstroms and deduced its tetrameric assembly by solving a 6-angstrom resolution structure of a Na + -activated homolog. Two tandem C-terminal regulator of K + conductance (RCK) domains from each of four channel subunits form a 350-kilodalton gating ring at the intracellular membrane surface. A sequence of aspartic amino acids that is known as the Ca 2+ bowl, and is located within the second of the tandem RCK domains, creates four Ca 2+ binding sites on the outer perimeter of the gating ring at the "assembly interface" between RCK domains. Functionally important mutations cluster near the Ca 2+ bowl, near the "flexible interface" between RCK domains, and on the surface of the gating ring that faces the voltage sensors. The structure suggests that the Ca 2+ gating ring, in addition to regulating the pore directly, may also modulate the voltage sensor.High-conductance voltage-and calcium-activated K + channels (BK or Slo1 channels) participate in numerous physiological processes, including neuronal excitability, smooth muscle contractility, and hair cell tuning (1-6). BK channels have an unusually high singlechannel conductance, but their most important physiological property is dual regulation through membrane voltage and intracellular Ca 2+ (7)(8)(9). Depolarization of the membrane voltage and increased intracellular Ca 2+ levels both cause BK channels to open, which hyperpolarizes the membrane and closes voltage-dependent channels, including Ca 2+ channels, reducing Ca 2+ influx into the cell. Thus, BK channels are negative-feedback regulators of electrical excitation (membrane depolarization) as well as the numerous biochemical pathways that are stimulated through Ca 2+ acting as a second messenger.The complexity of BK channel function mirrors the complexity of its protein structure. The amino acid sequence includes the integral membrane pore shared by all K + channels, the integral membrane voltage sensor domains present in voltage-dependent channels, and also a cytoplasmic domain (CTD) consisting of approximately 800 amino acids per subunit, which accounts for the C-terminal two thirds of the entire channel. The CTD structure † To whom correspondence should be addressed. confers upon the BK channel its ability to respond to changes in intracellular Ca 2+ . It is also the source of functional heterogeneity through alternate splicing, polymorphisms, phosphorylation, and protein interactions, which modulate BK channel activity (10-12).The only information currently available on BK channel structure is either low resolution from cryogenic electron microscopy (cryo-EM) (13) or indirect through homology models. The pore and voltage sensors of the BK channel will undoubtedly resemble the corresponding regions of oth...

High-conductance voltage- and Ca(2+)-activated K(+) channels function in many physiological processes that link cell membrane voltage and intracellular Ca(2+) concentration, including neuronal electrical activity, skeletal and smooth muscle contraction, and hair cell tuning. Like other voltage-dependent K(+) channels, Ca(2+)-activated K(+) channels open when the cell membrane depolarizes, but in contrast to other voltage-dependent K(+) channels, they also open when intracellular Ca(2+) concentrations rise. Channel opening by Ca(2+) is made possible by a structure called the gating ring, which is located in the cytoplasm. Recent structural studies have defined the Ca(2+)-free, closed, conformation of the gating ring, but the Ca(2+)-bound, open, conformation is not yet known. Here we present the Ca(2+)-bound conformation of the gating ring. This structure shows how one layer of the gating ring, in response to the binding of Ca(2+), opens like the petals of a flower. The degree to which it opens explains how Ca(2+) binding can open the transmembrane pore. These findings present a molecular basis for Ca(2+) activation of K(+) channels and suggest new possibilities for targeting the gating ring to treat conditions such as asthma and hypertension.

The transient receptor potential (TRP) channel TRPV4 participates in multiple biological processes, and numerous TRPV4 mutations underlie several distinct and devastating diseases. Here we present the structure of Xenopus tropicalis TRPV4 at 3.8 Å resolution by cryo-electron microscopy (cryo-EM). The ion conduction pore contains an intracellular gate formed by the inner helices, but lacks any extracellular gate in the selectivity filter, as is detected in other TRPV channels. Anomalous X-ray diffraction analyses identify a single ion-binding site in the selectivity filter, explaining non-selectivity. Structural comparison with other TRP channels and distantly related voltage-gated cation channels reveals an unprecedented, unique packing interface between the voltage sensor-like domain and the pore domain, suggesting distinct gating mechanisms. Moreover, our structure begins to provide mechanistic insights to the large set of pathogenic mutations and to offer new opportunities for drug development.

Serine integrases catalyze the integration of bacteriophage DNA into a host genome by site-specific recombination between ‘attachment sites’ in the phage (attP) and the host (attB). The reaction is highly directional; the reverse excision reaction between the product attL and attR sites does not occur in the absence of a phage-encoded factor, nor does recombination occur between other pairings of attachment sites. A mechanistic understanding of how these enzymes achieve site-selectivity and directionality has been limited by a lack of structural models. Here, we report the structure of the C-terminal domains of a serine integrase bound to an attP DNA half-site. The structure leads directly to models for understanding how the integrase-bound attP and attB sites differ, why these enzymes preferentially form attP × attB synaptic complexes to initiate recombination, and how attL × attR recombination is prevented. In these models, different domain organizations on attP vs. attB half-sites allow attachment-site specific interactions to form between integrase subunits via an unusual protruding coiled-coil motif. These interactions are used to preferentially synapse integrase-bound attP and attB and inhibit synapsis of integrase-bound attL and attR. The results provide a structural framework for understanding, testing and engineering serine integrase function.

Copper–hydrides are known catalysts for several technologically important reactions such as hydrogenation of CO, hydroamination of alkenes and alkynes, and chemoselective hydrogenation of unsaturated ketones to unsaturated alcohols. Stabilizing copper-based particles by ligand chemistry to nanometer scale is an appealing route to make active catalysts with optimized material economy; however, it has been long believed that the ligand–metal interface, particularly if sulfur-containing thiols are used as stabilizing agent, may poison the catalyst. We report here a discovery of an ambient-stable thiolate-protected copper–hydride nanocluster [Cu25H10(SPhCl2)18]3– that readily catalyzes hydrogenation of ketones to alcohols in mild conditions. A full experimental and theoretical characterization of its atomic and electronic structure shows that the 10 hydrides are instrumental for the stability of the nanocluster and are in an active role being continuously consumed and replenished in the hydrogenation reaction. Density functional theory computations suggest, backed up by the experimental evidence, that the hydrogenation takes place only around a single site of the 10 hydride locations, rendering the [Cu25H10(SPhCl2)18]3– one of the first nanocatalysts whose structure and catalytic functions are characterized fully to atomic precision. Understanding of a working catalyst at the atomistic level helps to optimize its properties and provides fundamental insights into the controversial issue of how a stable, ligand-passivated, metal-containing nanocluster can be at the same time an active catalyst.

Na+-activated K+ channels are members of the Slo family of large conductance K+ channels that are widely expressed in the brain, where their opening regulates neuronal excitability. These channels are fascinating for the biological roles they fulfill as well as for their intriguing biophysical properties, including conductance levels ten times most other K+ channels and gating sensitivity to intracellular Na+. Here we present the structure a complete Na+-activated K+ channel, Slo2.2, in the Na+-free state, determined by cryo-electron microscopy at a nominal resolution of 4.5 Å. The channel is composed of a large cytoplasmic gating ring within which resides the Na+-binding site and a transmembrane domain that closely resembles voltage-gated K+ channels. In the structure, the cytoplasmic domain adopts a closed conformation and the ion conduction pore is also closed. The structure provides a first view of a member of the Slo K+ channel family, which reveals features explaining their high conductance and gating mechanism.

Bottom-up design of functional device components based on nanometer-sized building blocks relies on accurate control of their self-assembly behavior. Atom-precise metal nanoclusters are well-characterizable building blocks for designing tunable nanomaterials, but it has been challenging to achieve directed assembly to macroscopic functional cluster-based materials with highly anisotropic properties. Here, we discover a solvent-mediated assembly of 34atom intermetallic gold-silver clusters protected by 20 1-ethynyladamantanes into 1D polymers with Ag-Au-Ag bonds between neighboring clusters as shown directly by the atomic structure from single-crystal X-ray diffraction analysis. Density functional theory calculations predict that the single crystals of cluster polymers have a band gap of about 1.3 eV. Fieldeffect transistors fabricated with single crystals of cluster polymers feature highly anisotropic p-type semiconductor properties with ≈1800-fold conductivity in the direction of the polymer as compared to cross directions, hole mobility of ≈0.02 cm 2 V −1 s −1 , and an ON/OFF ratio up to ≈4000. This performance holds promise for further design of functional cluster-based materials with highly anisotropic semiconducting properties.