Palmitoylation is a prerequisite for dimerization-dependent raftophilicity of rhodopsin [Membrane Biology]

July 26th, 2017 by Keiji Seno, Fumio Hayashi

The visual photopigment rhodopsin (Rh) is a prototypical G protein-coupled receptor (GPCR) responsible for initiation of the phototransduction cascade in rod photoreceptors. Similar to other GPCRs, Rh can form dimers or even higher oligomers, and tends to have a supramolecular organization that is likely important in the dim light response. Rh also exhibits high affinity for lipid rafts (raftophilicity) upon light-dependent binding with cognate G protein transducin (Gt), suggesting the presence of lipid raft-like domains in the retinal disk membrane and their importance in phototransduction. However, the relationship between Rh oligomerization and lipid rafts in the disk membrane remains to be explored. Given previous findings that Gt binds to dimeric Rh and Rh is post-translationally modified with two highly raftophilic palmitoyl moieties, we hypothesized that Rh becomes raftophilic upon dimerization. Here, we tested this hypothesis biochemically. First, we found that Rh*-Gt complexes in the detergent-resistant membrane (DRM) are partially resistant to cholesterol depletion by methyl-β-cyclodextrin (MCD), and the stoichiometry of Rh to Gt in this MCD-resistant complex is 2:1. We then found that IgG-crosslinking renders Rh highly raftophilic, supporting the premise that Rh becomes raftophilic upon dimerization. Depalmitoylation of Rh with the reduction of thioester linkages by dithiothreitol blocked the translocation of IgG-crosslinked Rh to the DRM, highlighting the importance of the two palmitoyl moieties in the dimerization-dependent raftophilicity of Rh. These results indicate that palmitoylated GPCRs, such as Rh, may acquire raftophilicity upon G protein-stabilized dimerization and thereby organize receptor-cluster rafts by recruiting raftophilic lipids.

Iron Transport Proteins: Gateways of Cellular and Systemic Iron Homeostasis [Cell Biology]

June 16th, 2017 by Mitchell D. Knutson

Cellular iron homeostasis is maintained by iron and heme transport proteins that work in concert with ferrireductases, ferroxidases, and chaperones to direct the movement of iron into, within, and out of cells. Systemic iron homeostasis is regulated by the liver-derived peptide hormone, hepcidin. The interface between cellular and systemic iron homeostasis is readily observed in the highly dynamic iron handling of four main cell types: duodenal enterocytes, erythrocyte precursors, macrophages, and hepatocytes. This review provides an overview of how these cell types handle iron, highlighting how iron and heme transporters mediate the exchange and distribution of body iron in health and disease.

A ciliary opsin in the brain of a marine annelid zooplankton is UV-sensitive and the sensitivity is tuned by a single amino acid residue [Molecular Biophysics]

June 16th, 2017 by Hisao Tsukamoto, I-Shan Chen, Yoshihiro Kubo, Yuji Furutani

Ciliary opsins were classically thought to function only in vertebrates for vision, but they have been recently identified also in invertebrates for non-visual photoreception. Larvae of the annelid Platynereis dumerilii are used as a zooplankton model, and this zooplankton species possesses a "vertebrate-type" ciliary opsin (named c-opsin) in the brain. Platynereis c-opsin is suggested to relay light signals to melatonin production and circadian behaviors. Thus, the spectral and biochemical characteristics of this c-opsin would be directly related to non-visual photoreception in this zooplankton model. Here, we demonstrate that the c-opsin can sense UV to activate intracellular signaling cascades, and that it can directly bind exogenous all-trans-retinal. These results suggest that this c-opsin regulates circadian signaling in a UV-dependent manner and that it does not require supply of 11-cis-retinal for photoreception. Avoidance of damaging UV irradiation is a major cause of a large-scale daily zooplankton movement, and the observed capability of the c-opsin to transmit UV signals and bind all-trans-retinal is ideally suited for sensing UV radiation in the brain, which presumably lacks enzymes producing 11-cis-retinal. Mutagenesis analyses indicated that a unique amino acid residue (Lys-94) is responsible for c-opsin-mediated UV sensing in the Platynereis brain. We therefore propose that acquisition of the lysine residue in the c-opsin would be a critical event in the evolution of Platynereis to enable detection of ambient UV. In summary, our findings indicate that the c-opsin possesses spectral and biochemical properties suitable for UV sensing by the zooplankton model.
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Endoplasmic Reticulum Stress-induced Degradation of DNAJB12 Stimulates BOK Accumulation and Primes Cancer Cells for Apoptosis [Cell Biology]

May 23rd, 2017 by Pattarawut Sopha, Hong Yu Ren, Diane E. Grove, Douglas M Cyr

DNAJB12 (JB12) is an endoplasmic reticulum (ER)-associated Hsp40 family protein that recruits Hsp70 to the ER surface to coordinate the function of ER-associated and cytosolic chaperone systems in protein quality control. Hsp70 is stress inducible, but paradoxically, we report here that JB12 was degraded by the proteasome during severe ER stress. Destabilized JB12 was degraded by ER-associated degradation (ERAD) complexes that contained HERP, Sel1L, and gp78. JB12 was the only ER-associated chaperone that was destabilized by reductive stress. JB12 knockdown by siRNA led to the induction of Caspase processing, but not the unfolded protein response. ER stress-induced apoptosis is regulated by the highly labile and ER associated BCL-2 family member BOK, which is controlled at the level of protein stability by ERAD components. We found that JB12 was required in Huh-7 liver cancer cells to maintain BOK at low levels and BOK was detected in complexes with JB12 and gp78. Depletion of JB12 during reductive stress or by shRNA from Huh-7 cells was associated with accumulation of BOK, and activation of Caspase 3, 7, and 9. Absence of JB12 sensitized Huh-7 to death caused by proteotoxic agents and the proapoptotic chemotherapeutic LCL-161. In summary, JB12 is a stress sensitive Hsp40 whose degradation during severe ER stress provides a mechanism to promote BOK accumulation and induction of apoptosis.

The sialate O-acetylesterase EstA from gut Bacteroidetes species enables sialidase-mediated cross-species foraging of 9-O-acetylated sialoglycans [Microbiology]

May 19th, 2017 by Lloyd S Robinson, Warren G Lewis, Amanda L Lewis

The gut harbors many symbiotic, commensal, and pathogenic microbes that engage in the breakdown and metabolism of host carbohydrates. Sialic acids are prominent outermost carbohydrates on mucins and protect underlying glycan chains from enzymatic degradation. Sialidases produced by some members of the colonic microbiota have been shown to promote the expansion of several potential pathogens (e.g. Clostridium difficile, Salmonella, Escherichia coli) that do not produce sialidases. O-acetyl ester modifications of sialic acids help resist the action of many sialidases and are found at high levels in the mammalian colon. However, some gut bacteria, in turn, produce sialylate-O-acetyl esterases to remove them. Here we investigated O-acetylation as a shield against the release of sialic acids by Bacteroidetes sialidases and the subsequent utilization of host sialic acids by commensal and pathogenic strains of E. coli. In vitro foraging studies demonstrated that sialidase-dependent E. coli outgrowth on mucin is enabled by Bacteroides EstA, which acts on glycosidically-linked sialylate-O-acety-esterase substrates, particularly at neutral pH. Biochemical studies suggest that spontaneous migration of O-acetyl esters on the side chain of sialic acid, which can occur at colonic pH, may serve as a switch controlling EstA-assisted sialic acid liberation. Specifically, EstA does not act on O-acetyl esters in their initial 7-position. But, following migration to the 9-position, glycans with O-acetyl esters become susceptible to the sequential enzyme action of bacterial esterases and sialidases. Thus, EstA specifically unlocks the nutritive potential of 9-O-acetylated mucus sialic acids for foraging by bacteria that otherwise lack the means to access this potential carbon source.
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Functional regions of the peroxin Pex19 necessary for peroxisome biogenesis [Membrane Biology]

May 19th, 2017 by Gaurav Agrawal, Helen H. Shang, Zhi-Jie Xia, Suresh Subramani

The peroxins Pex19 and Pex3 play an indispensable role in peroxisomal membrane protein (PMP) biogenesis, peroxisome division and inheritance. Pex19 plays multiple roles in these processes, but how these functions relate to the structural organization of the Pex19 domains is unresolved. To this end, using deletion mutants, we mapped the Pex19 regions required for peroxisome biogenesis in the yeast Pichia pastoris. Surprisingly, import-competent peroxisomes still formed when Pex19 domains previously believed to be required for biogenesis were deleted, although the peroxisome size was larger than that in wild-type cells. Moreover, these mutants exhibited a delay of 14-24 h in peroxisome biogenesis. The shortest functional N-terminal (NTCs) and C-terminal constructs (CTCs) were Pex19 (aa 1-150) and Pex19 (aa 89-300), respectively. Deletions of the N-terminal Pex3-binding site disrupted direct interactions of Pex19 with Pex3, but preserved interactions with a membrane peroxisomal targeting signal (mPTS)-binding PMP, Pex10. In contrast, deletion of the C-terminal mPTS-binding domain of Pex19 disrupted its interaction with Pex10, while leaving the Pex19-Pex3 interactions intact. However, Pex11 and Pex25 retained their interactions with both N- and C-terminal deletion mutants. NTC-CTC co-expression improved growth and reverted the larger-than-normal peroxisome size observed with the single deletions. Pex25 was critical for peroxisome formation with the CTC variants, and its overexpression enhanced their interactions with Pex3 and aided the growth of both NTC and CTC Pex19 variants. In conclusion, physical segregation of the Pex3 and PMP-binding domains of Pex19 has provided novel insights into the modular architecture of Pex19. We define the minimum region of Pex19 required for peroxisome biogenesis and a unique role for Pex25 in this process.

H2S oxidation by nanodisc-embedded human sulfide quinone oxidoreductase [Enzymology]

May 16th, 2017 by Aaron P Landry, David P Ballou, Ruma Banerjee

Buildup of hydrogen sulfide (H2S), which functions as a signaling molecule but is toxic at high concentrations, is averted by its efficient oxidation by the mitochondrial sulfide oxidation pathway. The first step in this pathway is catalyzed by a flavoprotein, sulfide quinone oxidoreductase (SQR), which converts H2S to a persulfide and transfers electrons to coenzyme Q via a flavin cofactor. All previous studies on human SQR have used detergent-solubilized protein. Here, we embedded human SQR in nanodiscs (ndSQR) and studied highly homogenous preparations by steady-state and rapid kinetics techniques. ndSQR exhibited higher catalytic rates in its membranous environment than in its solubilized state. Stopped-flow spectroscopic data revealed that transfer of the sulfane sulfur from an SQR-bound cysteine persulfide intermediate to a small-molecule acceptor is the rate-limiting step. The physiological acceptor of sulfane sulfur from SQR has been the subject of controversy; we report that the kinetic analysis of ndSQR is consistent with glutathione rather than sulfite being the predominant acceptor at physiologically relevant concentrations of the respective metabolites. The identity of the acceptor has an important bearing on how the sulfide oxidation pathway is organized. Our data are more consistent with the following reaction sequence for sulfide oxidation: H2S→glutathione persulfide→sulfite→sulfate, than with a more convoluted route that would result if sulfite were the primary acceptor of sulfane sulfur. In summary, nanodisc-incorporated human SQR exhibits enhanced catalytic performance, and pre-steady state kinetic characterization of the complete SQR catalytic cycle indicates that GSH serves as the physiologically relevant sulfur acceptor.

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.