ResearchPad - 501 https://www.researchpad.co Default RSS Feed en-us © 2020 Newgen KnowledgeWorks <![CDATA[Structural and functional insights into transmembrane AMPA receptor regulatory protein complexes]]> https://www.researchpad.co/article/elastic_article_15288 Fast excitatory neurotransmission is mediated by the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) subtype of ionotropic glutamate receptor (AMPAR). AMPARs initiate depolarization of the postsynaptic neuron by allowing cations to enter through their ion channel pores in response to binding of the neurotransmitter glutamate. AMPAR function is dramatically affected by auxiliary subunits, which are regulatory proteins that form various complexes with AMPARs throughout the brain. The most well-studied auxiliary subunits are the transmembrane AMPAR regulatory proteins (TARPs), which alter the assembly, trafficking, localization, kinetics, and pharmacology of AMPARs. Recent structural and functional studies of TARPs and the TARP-fold germ cell-specific gene 1-like (GSG1L) subunit have provided important glimpses into how auxiliary subunits regulate the function of synaptic complexes. In this review, we put these recent structures in the context of new functional findings in order to gain insight into the determinants of AMPAR regulation by TARPs. We thus reveal why TARPs display a broad range of effects despite their conserved modular architecture.

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<![CDATA[A channel profile report of the unusual K<sup>+</sup> channel KtrB]]> https://www.researchpad.co/article/elastic_article_15282 KtrAB is a key player in bacterial K+ uptake required for K+ homeostasis and osmoadaptation. The system is unique in structure and function. It consists of the K+-translocating channel subunit KtrB, which forms a dimer in the membrane, and the soluble regulatory subunit KtrA, which attaches to the cytoplasmic side of the dimer as an octameric ring conferring Na+ and ATP dependency to the system. Unlike most K+ channels, KtrB lacks the highly conserved T(X)GYG selectivity filter sequence. Instead, only a single glycine residue is found in each pore loop, which raises the question of how selective the ion channel is. Here, we characterized the KtrB subunit from the Gram-negative pathogen Vibrio alginolyticus by isothermal titration calorimetry, solid-supported membrane–based electrophysiology, whole-cell K+ uptake, and ACMA-based transport assays. We found that, despite its simple selectivity filter, KtrB selectively binds K+ with micromolar affinity. Rb+ and Cs+ bind with millimolar affinities. However, only K+ and the poorly binding Na+ are efficiently translocated, based on size exclusion by the gating loop. Importantly, the physiologically required K+ over Na+ selectivity is provided by the channel’s high affinity for potassium, which interestingly results from the presence of the sodium ions themselves. In the presence of the KtrA subunit, sodium ions further decrease the Michaelis–Menten constant for K+ uptake from milli- to micromolar concentrations and increase the Vmax, suggesting that Na+ also facilitates channel gating. In conclusion, high binding affinity and facilitated K+ gating allow KtrAB to function as a selective K+ channel.

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<![CDATA[Lattice arrangement of myosin filaments correlates with fiber type in rat skeletal muscle]]> https://www.researchpad.co/article/elastic_article_15281 The thick (myosin-containing) filaments of vertebrate skeletal muscle are arranged in a hexagonal lattice, interleaved with an array of thin (actin-containing) filaments with which they interact to produce contraction. X-ray diffraction and EM have shown that there are two types of thick filament lattice. In the simple lattice, all filaments have the same orientation about their long axis, while in the superlattice, nearest neighbors have rotations differing by 0° or 60°. Tetrapods (amphibians, reptiles, birds, and mammals) typically have only a superlattice, while the simple lattice is confined to fish. We have performed x-ray diffraction and electron microscopy of the soleus (SOL) and extensor digitorum longus (EDL) muscles of the rat and found that while the EDL has a superlattice as expected, the SOL has a simple lattice. The EDL and SOL of the rat are unusual in being essentially pure fast and slow muscles, respectively. The mixed fiber content of most tetrapod muscles and/or lattice disorder may explain why the simple lattice has not been apparent in these vertebrates before. This is supported by only weak simple lattice diffraction in the x-ray pattern of mouse SOL, which has a greater mix of fiber types than rat SOL. We conclude that the simple lattice might be common in tetrapods. The correlation between fiber type and filament lattice arrangement suggests that the lattice arrangement may contribute to the functional properties of a muscle.

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<![CDATA[Outer hair cell electromotility is low-pass filtered relative to the molecular conformational changes that produce nonlinear capacitance]]> https://www.researchpad.co/article/elastic_article_15280 The outer hair cell (OHC) of the organ of Corti underlies a process that enhances hearing, termed cochlear amplification. The cell possesses a unique voltage-sensing protein, prestin, that changes conformation to cause cell length changes, a process termed electromotility (eM). The prestin voltage sensor generates a capacitance that is both voltage- and frequency-dependent, peaking at a characteristic membrane voltage (Vh), which can be greater than the linear capacitance of the OHC. Accordingly, the OHC membrane time constant depends upon resting potential and the frequency of AC stimulation. The confounding influence of this multifarious time constant on eM frequency response has never been addressed. After correcting for this influence on the whole-cell voltage clamp time constant, we find that both guinea pig and mouse OHC eM is low pass, substantially attenuating in magnitude within the frequency bandwidth of human speech. The frequency response is slowest at Vh, with a cut-off, approximated by single Lorentzian fits within that bandwidth, near 1.5 kHz for the guinea pig OHC and near 4.3 kHz for the mouse OHC, each increasing in a U-shaped manner as holding voltage deviates from Vh. Nonlinear capacitance (NLC) measurements follow this pattern, with cut-offs about double that for eM. Macro-patch experiments on OHC lateral membranes, where voltage delivery has high fidelity, confirms low pass roll-off for NLC. The U-shaped voltage dependence of the eM roll-off frequency is consistent with prestin’s voltage-dependent transition rates. Modeling indicates that the disparity in frequency cut-offs between eM and NLC may be attributed to viscoelastic coupling between prestin’s molecular conformations and nanoscale movements of the cell, possibly via the cytoskeleton, indicating that eM is limited by the OHC’s internal environment, as well as the external environment. Our data suggest that the influence of OHC eM on cochlear amplification at higher frequencies needs reassessment.

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<![CDATA[Phosphoinositides modulate the voltage dependence of two-pore channel 3]]> https://www.researchpad.co/article/N20f2aa01-0a02-4cb4-81d8-0e377fcc0916

Among the three two-pore channels (TPCs), TPC1 and TPC2 are selectively activated by PI(3,5)P2, while TPC3 has been considered not to respond to any PIP2s. Shimomura and Kubo find that TPC3 responds to both PI(3,5)P2 and PI(3,4)P2, but not to PI(4,5)P2.

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<![CDATA[A primer on resolving the nanoscale structure of the plasma membrane with light and electron microscopy]]> https://www.researchpad.co/article/Nacd486ea-ae63-420d-9feb-12677773bcf2

Taraska reviews the imaging methods that are being used to understand the structure of the plasma membrane at the molecular level.

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<![CDATA[Dendritic spine geometry and spine apparatus organization govern the spatiotemporal dynamics of calcium]]> https://www.researchpad.co/article/N5550f0e8-6951-45e5-8f22-2f4ac5fa0187

Dendritic spines can have numerous different shapes, but how this affects the function of these neuronal subcompartments is unclear. Bell et al. develop a mathematical model that reveals how the size and shape of both the spine head and spine apparatus impact local calcium dynamics.

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<![CDATA[Myosin lever arm orientation in muscle determined with high angular resolution using bifunctional spin labels]]> https://www.researchpad.co/article/Nde1cd7a3-65c6-48d3-b4f8-9f0851568715

High-resolution structural information is invaluable for understanding muscle function. Savich et al. use bifunctional spin labeling to determine the orientation of the myosin lever arm in muscle fibers at high resolution under ambient conditions, augmenting previous insights obtained from fluorescence and EM.

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<![CDATA[A comparison of the transport kinetics of glycine transporter 1 and glycine transporter 2]]> https://www.researchpad.co/article/N0ee40ae2-cfe1-46c5-af07-4b54a9419fc5

Erdem et al. compare the kinetics of the SLC6 family glycine transporters GlyT1 and GlyT2. Though the two transporters are rate-limited by distinct reaction steps, they both display high transport capacity, with the kinetics of GlyT1 sufficient to supply extracellular glycine to the NMDA receptor.

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<![CDATA[Regulation of heart rate and the pacemaker current by phosphoinositide 3-kinase signaling]]> https://www.researchpad.co/article/N8ad5f073-e5fd-4399-b131-3161328e48d9

Heart rate is set by the specialized tissue of the sinoatrial node. Lin et al. demonstrate a novel role for phosphoinositide 3-kinase in regulating cardiac pacemaking currents independently of the autonomic nervous system, a finding with relevance for diabetes, heart disease, and cancer.

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<![CDATA[Propofol inhibits the voltage-gated sodium channel NaChBac at multiple sites]]> https://www.researchpad.co/article/5c910771d5eed0c4841a4292

General anesthetics inhibit voltage-gated sodium channels by unknown molecular mechanisms. Using computation-guided NMR and electrophysiology analyses, Wang et al. show that propofol binds to the prokaryotic sodium channel NaChBac at multiple distinct sites.

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<![CDATA[Singlet oxygen modification abolishes voltage-dependent inactivation of the sea urchin spHCN channel]]> https://www.researchpad.co/article/5c910773d5eed0c4841a42c0

Singlet oxygen modifies several different proteins within cells. Idikuda et al. show that, in the case of the sea urchin hyperpolarization-activated cyclic nucleotide–gated channel, a histidine residue in S6 is essential for the abolition of voltage-dependent inactivation by singlet oxygen.

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<![CDATA[Propofol inhibits prokaryotic voltage-gated Na+ channels by promoting activation-coupled inactivation]]> https://www.researchpad.co/article/5c91076fd5eed0c4841a4276

Despite extensive use in clinical practice, the mechanisms of propofol action on sodium channels are not fully understood. Yang et al. incorporate complementary biophysical approaches (electrophysiology and molecular dynamics simulations) to demonstrate that propofol inhibits two prokaryotic voltage-gated sodium channels, NaChBac and NavMs, by modulating both activation and inactivation gating.

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<![CDATA[All four subunits of HCN2 channels contribute to the activation gating in an additive but intricate manner]]> https://www.researchpad.co/article/5c910775d5eed0c4841a42eb

HCN pacemaker channels are dually gated by hyperpolarizing voltages and cyclic nucleotide binding. Sunkara et al. show that each of the four binding sites promotes channel opening, most likely by exerting a turning momentum on the tetrameric intracellular gating ring.

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<![CDATA[Minimal molecular determinants of isoform-specific differences in efficacy in the HCN channel family]]> https://www.researchpad.co/article/5c69ec9ed5eed0c48414dfc9

HCN channels generate rhythmic firing patterns in the brain and heart. Alvarez-Baron et al. identify key amino acids responsible for functional differences between cAMP-sensitive and insensitive HCN isoforms, revealing their role in communication between the nucleotide-binding domain and the pore.

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<![CDATA[The N terminus of α-ENaC mediates ENaC cleavage and activation by furin]]> https://www.researchpad.co/article/5c69eca0d5eed0c48414e004

A natural splice deletion of rat α-ENaC was previously reported to produce reduced ENaC current that was not attributable to reduced surface expression. Kota et al. show that this ENaC variant resists furin cleavage, implicating α-ENaC residues 34–82 in ENaC posttranslational processing.

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<![CDATA[Heat-resistant action potentials require TTX-resistant sodium channels NaV1.8 and NaV1.9]]> https://www.researchpad.co/article/5c69ec9ad5eed0c48414df94

Nociceptors prevent damage by being able to detect and transmit noxious stimuli, such as hot temperatures. Touska et al. show that the TTX-resistant NaV channels, NaV1.8 and NaV1.9, are required for heat-resistant nociceptors to encode noxious heat and that the current through NaV1.9 increases at higher temperatures.

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<![CDATA[Duplex signaling by CaM and Stac3 enhances CaV1.1 function and provides insights into congenital myopathy]]> https://www.researchpad.co/article/5c69ec9cd5eed0c48414dfaf

CaV1.1 is essential for initiating skeletal muscle contraction. Niu et al. demonstrate that both CaM and stac3 enhance trafficking and gating of CaV1.1. Stac3 mutations associated with congenital myopathy, weaken its binding of CaV1.1, and thus reduce trafficking.

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<![CDATA[Gating currents]]> https://www.researchpad.co/article/5c368374d5eed0c4841f3d9c

A voltage change across a membrane protein moves charges or dipoles producing a gating current that is an electrical expression of a conformational change.

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<![CDATA[Molecular determination of claudin-15 organization and channel selectivity]]> https://www.researchpad.co/article/5c368377d5eed0c4841f3e48

Members of the claudin family form tight junctions between adjacent epithelial and endothelial cells. Samanta et al. build an atomic model of claudin-15 using molecular dynamics simulations and conclude that four claudin-15 molecules each contribute an aspartic acid residue to form a selectivity filter.

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