Reactive Oxygen Species Production Induced by Pore Opening in Cardiac Mitochondria: The Role of Complex II [Bioenergetics]

April 27th, 2017 by Paavo Korge, Scott A John, Guillaume Calmettes, James N Weiss

Succinate-driven reverse electron transport (RET) through complex I is hypothesized be a major source of ROS that induce permeability transition pore (PTP) opening and damage the heart during ischemia/reperfusion. Since RET can only generate ROS when mitochondria are fully polarized, however, this mechanism is self-limiting once PTP open during reperfusion. In the companion manuscript, we showed that ROS production after PTP opening can be sustained when complex III is damaged (simulated by antimycin). Here we show that complex II can also contribute to sustained ROS production in isolated rabbit cardiac mitochondria following inner membrane pore formation induced by either alamethicin or Ca-induced PTP opening. Two conditions are required to maximize malonate-sensitive ROS production by complex II in isolated mitochondria: a) complex II inhibition by atpenin A5 or complex III inhibition by stigmatellin that results in succinate-dependent reduction of the dicarboxylate binding site of complex II (site IIf ); b) pore opening in the inner membrane resulting in rapid efflux of succinate/fumarate and other dicarboxylates capable of competitively binding to site IIf. The decrease in matrix [dicarboxylates] allows O2 access to reduced site IIf, thereby making electron donation to O2 possible, explaining the rapid increase in ROS production provided that site IIf is reduced. Because ischemia is known to inhibit complexes II and III and increase matrix succinate/fumarate levels, we hypothesize that by allowing dicarboxylate efflux from the matrix, PTP opening during reperfusion may activate sustained ROS production by this mechanism after RET-driven ROS production has ceased.

Disease-Associated Extracellular Loop Mutations in the Adhesion G Protein-Coupled Receptor G1 (ADGRG1; GPR56) Differentially Regulate Downstream Signaling [Molecular Bases of Disease]

April 19th, 2017 by Ayush Kishore, Randy A. Hall

Mutations to the adhesion G protein-coupled receptor ADGRG1 (G1; also known as GPR56) underlie the neurological disorder bilateral frontoparietal polymicrogyria (BFPP). Disease-associated mutations in G1 studied to date are believed to induce complete loss of receptor function, either through disruption of receptor trafficking or signaling activity. Given that N-terminal truncation of G1 and other adhesion G protein-coupled receptors has been shown to significantly increase the receptors' constitutive signaling, we examined two different BFPP-inducing extracellular loop mutations (R565W and L640R) in the context of both full-length and N-terminally truncated (deltaNT) G1. Interestingly, we found that these mutations reduced surface expression of full-length G1 but not G1-deltaNT in HEK-293 cells. Moreover, the mutations ablated receptor-mediated activation of serum response factor luciferase, a classic measure of Galpha12/13-mediated signaling, but had no effect on G1-mediated signaling to nuclear factor of activated T cells (NFAT) luciferase. Given these differential signaling results, we sought to further elucidate the pathway by which G1 can activate NFAT luciferase. We found no evidence that deltaNT activation of NFAT is dependent on Galphaq/11-mediated or beta-arrestin-mediated signaling, but rather involves liberation of G beta gamma subunits and activation of calcium channels. These findings reveal that disease-associated mutations to the extracellular loops of G1 differentially alter receptor trafficking, depending on the presence of the N-terminus, and also differentially alter signaling to distinct downstream pathways.
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pH regulation in early endosomes and interferon-inducible transmembrane proteins control avian retrovirus fusion [Membrane Biology]

March 24th, 2017 by Tanay M Desai, Mariana Marin, Caleb Mason, Gregory B Melikyan

Enveloped viruses infect host cells by fusing their membranes with those of the host cell, a process mediated by viral glycoproteins upon binding to cognate host receptors or entering into acidic intracellular compartments. Whereas the effect of receptor density on viral infection has been well studied, the role of cell type-specific factors/processes, such as pH regulation, has not been characterized in sufficient detail. Here, we examined the effects of cell-extrinsic factors (buffer environment) and cell-intrinsic factors (interferon-inducible transmembrane proteins, IFITMs), on the pH regulation in early endosomes and on the efficiency of acid-dependent fusion of the Avian Sarcoma and Leukosis Virus, ASLV, with endosomes. First, we found that a modest elevation of external pH can raise the pH in early endosomes in a cell type-dependent manner and thereby delay the acid-induced fusion of endocytosed ASLV. Second, we observed a cell type-dependent delay between the low pH-dependent and temperature-dependent steps of viral fusion, consistent with the delayed enlargement of the fusion pore. Third, ectopic expression of IFITMs, known to potently block influenza virus fusion with late compartments, was found to only partially inhibit ASLV fusion with early endosomes. Interestingly, IFITM expression promoted virus uptake and the acidification of endosomal compartments, resulting in an accelerated fusion rate, when driven by the glycosylphosphatidylinositol-anchored, but not by the transmembrane isoform of the ASLV receptor. Collectively, these results highlight the role of cell-extrinsic and cell-intrinsic factors in regulating the efficiency and kinetics of virus entry and fusion with target cells.

The RecJ2 Protein in the Thermophilic Archaeon Thermoplasma acidophilum Is a 3′ 5′ Exonuclease and Associates with a DNA Replication Complex [Microbiology]

March 16th, 2017 by Hiromi Ogino, Sonoko Ishino, Daisuke Kohda, Yoshizumi Ishino

RecJ/cell division cycle 45 (Cdc45) proteins are widely conserved in the three domains of life, i.e., in Bacteria, Eukarya and Archaea. Bacterial RecJ is a 5′ 3′ exonuclease and functions in DNA repair pathways, while using its 5′ 3′ exonuclease activity. Eukaryotic Cdc45 has no identified enzymatic activity, but participates in the CMG complex so named because it is composed of Cdc45, minichromosome maintenance protein complex (MCM) proteins 2-7, and GINS complex proteins (Sld5, Psf11 to 3). Eukaryotic Cdc45 and bacterial/archaeal RecJ share similar amino acid sequences and are considered functional counterparts. In Archaea, a RecJ homolog in Thermococcus kodakarensis was shown to associate with GINS and accelerate its nuclease activity and was therefore designated GAN (GINS-associated nuclease); however, to date, no archaeal RecJ MCM GINS complex has been isolated. The thermophilic archaeon Thermoplasma acidophilum has two RecJ like proteins, designated TaRecJ1 and TaRecJ2. TaRecJ1 exhibited DNA-specific 5′ 3′exonuclease activity, while TaRecJ2 had 3′ 5′ exonuclease activity and preferred RNA over DNA. TaRecJ2, but not TaRecJ1, formed a stable complex with TaGINS in a 2:1 molar ratio. Furthermore, the TaRecJ2-TaGINS complex stimulated activity of TaMCM helicase in vitro, and the TaRecJ2-TaMCM-TaGINS complex was also observed in vivo. However, TaRecJ2 did not interact with TaMCM directly and was not required for the helicase activation in vitro. These findings suggest that the function of archaeal RecJ in DNA replication evolved divergently from Cdc45 despite conservation of the CMG-like complex formation between Archaea and Eukarya.
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Adjacent channelrhodopsin-2 residues within transmembranes 2 and 7 regulate cation selectivity and distribution of the two open states [Membrane Biology]

March 16th, 2017 by Ryan Richards, Robert E. Dempski

Channelrhodopsin-2 (ChR2) is a light-activated channel that can conduct cations of multiple valencies down the electrochemical gradient. Under continuous light exposure, ChR2 transitions from a high conducting open state (O1) to a low conducting open state (O2) with differing ion selectivity. The molecular basis for the O1 to O2 transition and how ChR2 modulates selectivity between states is currently unresolved. To this end, we used steered molecular dynamics, electrophysiology, and kinetic modeling to identify residues that contribute to gating and selectivity in discrete open states. Analysis of steered molecular dynamics experiments identified three transmembrane residues (V86, K93 and N258) that form a putative barrier to ion translocation. Kinetic modeling of photocurrents generated from ChR2 proteins with conservative mutations at these positions demonstrated that these residues contribute to cation selectivity (V86 and N258), the transition between the two open states (V86), open channel stability, and the hydrogen-bonding network (K93I and K93N). These results suggest that this approach can be used to identify residues that contribute to the open state transitions and the discrete ion selectivity within these states. With the rise of ChR2 use in optogenetics, it will be critical to identify residues that contribute to O1 or O2 selectivity and gating to minimize undesirable effects.
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Two alternative binding mechanisms connect the protein translocation Sec71/Sec72 complex with heat shock proteins [Protein Structure and Folding]

March 12th, 2017 by Arati Tripathi, Elisabet C Mandon, Reid Gilmore, Tom A Rapoport

The biosynthesis of many eukaryotic proteins requires accurate targeting to and translocation across the endoplasmic reticulum (ER) membrane. Post-translational protein translocation in yeast requires both the Sec61 translocation channel, and a complex of four additional proteins: Sec63, Sec62, Sec71, and Sec72. The structure and function of these proteins are largely unknown. This pathway also requires the cytosolic Hsp70 protein Ssa1, but whether Ssa1 associates with the translocation machinery to target protein substrates to the membrane is unclear. Here, we use a combined structural and biochemical approach to explore the role of Sec71/Sec72 subcomplex in post-translational protein translocation. To this end, we report a crystal structure of the Sec71/Sec72 complex, which revealed that Sec72 contains a tetratricopeptide repeat (TPR) domain that is anchored to the ER membrane by Sec71. We also determined the crystal structure of this TPR domain with a C-terminal peptide derived from Ssa1, which suggests how Sec72 interacts with full-length Ssa1. Surprisingly, Ssb1, a cytoplasmic Hsp70 that binds ribosome- associated nascent polypeptide chains also binds to the TPR domain of Sec72, even though it lacks the TPR-binding C-terminal residues of Ssa1. We demonstrate that Ssb1 binds through its ATPase domain to the TPR domain, an interaction that leads to inhibition of nucleotide exchange. Taken together, our results suggest that translocation substrates can be recruited to the Sec71/72 complex either post-translationally through Ssa1 or co-translationally through Ssb1.
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A New General Method for Simultaneous Fitting of Temperature- and Concentration-Dependence of Reaction Rates Yields Kinetic and Thermodynamic Parameters for HIV Reverse Transcriptase Specificity [DNA and Chromosomes]

March 2nd, 2017 by An Li, Jessica L. Ziehr, Kenneth A. Johnson

Recent studies have demonstrated the dominant role of induced-fit in enzyme specificity of HIV reverse transcriptase and many other enzymes. However, relevant thermodynamic parameters are lacking and equilibrium thermodynamic methods are of no avail because the key parameters can only determined by kinetic measurement. By modifying KinTek Explorer software, we present a new general method for globally fitting data collected over a range of substrate concentrations and temperatures and apply it to HIV reverse transcriptase. Fluorescence stopped-flow methods were used to record the kinetics of enzyme conformational changes that monitor nucleotide binding and incorporation. The nucleotide concentration dependence was measured at temperatures ranging from 5 to 37C and the raw data were fit globally to derive a single set of rate constants at 37C and a set of activation enthalpy terms to account for the kinetics at all other temperatures. This comprehensive analysis afforded thermodynamic parameters for nucleotide binding (Kd, ΔG, ΔH, ΔS at 37C), and kinetic parameters for enzyme conformational changes and chemistry (rate constants and activation enthalpy). Comparisons between wild-type enzyme and a mutant resistant to nucleoside analogs used to treat HIV infections reveal that the ground state binding is weaker and the activation enthalpy for the conformational change step is significantly larger for the mutant. Further studies to explore the structural underpinnings of the observed thermodynamics and kinetics of the conformational change step may help to design better analogs to treat HIV infections and other diseases. Our new method is generally applicable to enzyme and chemical kinetics.
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Structural Basis for the Lesion-scanning Mechanism of the Bacterial MutY DNA Glycosylase [Enzymology]

January 27th, 2017 by Lan Wang, Srinivas Chakravarthy, Gregory L Verdine

The highly mutagenic A:oxoG (8-oxoguanine) base-pair is generated mainly by misreplication of the C:oxoG base-pair, the oxidation product of the C:G base-pair. A:oxoG base-pair is particularly insidious because neither base in it carries faithful information to direct the repair of the other. The bacterial MutY (MUTYH in humans) adenine DNA glycosylase is able to initiate the repair of A:oxoG by selectively cleaving the A base from the A:oxoG base-pair. The difference between faithful repair and wreaking mutagenic havoc on the genome lies in the accurate discrimination between two structurally similar base-pairs: A:oxoG and A:T. Here we present two crystal structures of the MutY N-terminal domain in complex with either undamaged DNA or DNA containing an intrahelical lesion. These structures have captured for the first time, a DNA glycosylase scanning the genome for a damaged base in the very first stage of lesion-recognition and the base-extrusion pathway. The mode of interaction observed here has suggested a common lesion-scanning mechanism across the entire helix-hairpin-helix superfamily to which MutY belongs. In addition, small-angle X-ray scattering (SAXS) studies together with accompanying biochemical assays have suggested a possible role played by the C-terminal oxoG-recognition domain of MutY in lesion-scanning.

Super-resolution visualization of caveolae deformation in response to osmotic stress [Signal Transduction]

January 17th, 2017 by Lu Yang, Suzanne Scarlata

Caveolae are protein dense plasma membrane domains structurally composed of caveolin -1 or -3 along with other proteins. Our previous studies have shown that caveolae enhance calcium signals generated through the Gαq/phospholipase Cβ signaling pathway, and that subjecting cells to hypo-osmotic stress reverses this enhancement. In this study, we have used super-resolution fluorescence microscopy supplemented by fluorescence correlation studies to determine the structural factors that underlie this behavior. We find similar and significant populations of Gαq and one of its receptors, bradykinin type 2 receptor (β2R), as well as Gαi and its coupled 2-adrenergic receptor (βAR), localize to caveolae domains. While mild osmotic stress deforms caveolae altering interactions between caveolae and these proteins, it does not affect the general structure and the localization of caveolae components remain largely unchanged. Additionally, in contrast to calcium signals mediated through Gαq-B2R, osmotic stress does not affect cAMP signals mediated through Gαi and βAR. Structurally, we find that mild osmotic stress corresponding roughly to a pressure of 3.82 N/m2 increases the domain diameter by ~30% and increases the fluorescence intensity in the center of the domain mouth suggesting a flattening of the invagination. Approximate calculations show that caveolae in muscle tissue have the strength to handle the stress of muscle movement.

Autoinhibition of the Nuclease ARTEMIS is Mediated by a Physical Interaction between Its Catalytic and C-terminal Domains [Immunology]

January 12th, 2017 by Doris Niewolik, Ingrid Peter, Carmen Butscher, Klaus Schwarz

The nuclease ARTEMIS is essential for the development of B and T lymphocytes. It is required for opening DNA hairpins generated during antigen receptor gene assembly from variable (V), diversity (D) and joining (J) subgenic elements (V(D)J recombination). As a member of the non-homologous end joining pathway it is also involved in repairing a subset of pathological DNA double-strand breaks. Loss of ARTEMIS function therefore results in radiosensitive severe combined immunodeficiency (RS-SCID). The hairpin-opening activity is dependent on the DNA-dependent protein kinase catalytic subunit (DNA-PKcs), which can bind to and phosphorylate ARTEMIS. The ARTEMIS C-terminus is dispensable for cellular V(D)J recombination and in vitro nuclease assays with C-terminally truncated ARTEMIS show DNA PKcs-independent hairpin-opening activity. Therefore it has been postulated that ARTEMIS is regulated via autoinhibition by its C-terminus. To obtain evidence for the autoinhibition model, we performed co-immunoprecipitation experiments with combinations of ARTEMIS mutants. We show that an N-terminal fragment comprising the catalytic domain can interact both with itself and with a C-terminal fragment. Amino acid exchanges N456A+S457A+E458Q in the C-terminus of full-length ARTEMIS resulted in unmasking of the N-terminus and in increased ARTEMIS activity in cellular V(D)J recombination assays. Mutations in ARTEMIS-deficient patients impaired the interaction with the C-terminus and also affected protein stability. The interaction between the N- and C-terminal domains was not DNA-PKcs dependent and phosphomimetic mutations in the C-terminal domain did not result in unmasking of the catalytic domain. Our experiments provide strong evidence that a physical interaction between the C-terminal and catalytic domains mediates ARTEMIS autoinhibition.