Colocalization of leukotriene C synthase and microsomal glutathione S-transferase elucidated by indirect immunofluorescence analysis

Co-localization of and interaction between duck enteritis virus glycoprotein H and L

BACKGROUND

Duck Enteritis Virus (DEV), belonging to the α-herpesvirus subfamily, is a linear double-stranded DNA virus. Glycoprotein H and L (gH and gL), encoded by UL22 and UL1, are conserved in the family of herpesviruses. They play important roles as gH/gL dimers during viral entry into host cells through cell-cell fusion. The interaction between gH and gL has been confirmed in several human herpesviruses, such as Herpes Simplex Virus (HSV), Epstein-Barr virus (EBV) and Human Cytomegalovirus (HCMV). In this paper, we studied the interaction between DEV gH and gL.

RESULTS

Recombinant plasmids pEGFP-N-gH and pDsRED-N-gL were constructed successfully. Expressions of both DEV gH and gL were observed after incubation of COS-7 cells transfected with pEGFP-N-gH and pDsRED-N-gL plasmids after 12 h, respectively. Also, the co-localization of a proportion of the gH and gL was detected in the cytoplasm of COS-7 cells after co-transfection for 24 h. Then, pCMV-Flag-gL and pCMV-Myc-gH recombinant plasmids were constructed and co-transfected into COS-7 cells. It was showed that both gH and gL were tested with positive results through co-immunoprecipitation and Western-blotting.

CONCLUSIONS

Our results demonstrated not only the co-localization of DEV gH and gL in COS-7 cells, but also the interaction between them. It will provide an insight for the further studies in terms of protein-protein interaction in DEV.

Comparative analysis of an experimental subcellular protein localization assay and in silico prediction methods

The subcellular localization of a protein can provide important information about its function within the cell. As eukaryotic cells and particularly mammalian cells are characterized by a high degree of compartmentalization, most protein activities can be assigned to particular cellular compartments. The categorization of proteins by their subcellular localization is therefore one of the essential goals of the functional annotation of the human genome. We previously performed a subcellular localization screen of 52 proteins encoded on human chromosome 21. In the current study, we compared the experimental localization data to the in silico results generated by nine leading software packages with different prediction resolutions. The comparison revealed striking differences between the programs in the accuracy of their subcellular protein localization predictions. Our results strongly suggest that the recently developed predictors utilizing multiple prediction methods tend to provide significantly better performance over purely sequence-based or homology-based predictions.

Location of Substrate Binding Sites within the Integral Membrane Protein Microsomal Glutathione Transferase-1†

Microsomal glutathione transferase-1 (MGST1) is a trimeric, membrane-bound enzyme with both glutathione (GSH) transferase and hydroperoxidase activities. As a member of the MAPEG superfamily, MGST1 aids in the detoxication of numerous xenobiotic substrates and in cellular protection from oxidative stress through the GSH-dependent reduction of phospholipid hydroperoxides. However, little is known about the location of the different substrate binding sites, including whether the transferase and peroxidase activities overlap structurally. Although molecular density attributed to GSH has been observed in the 3.2 A resolution electron crystallographic structure of MGST1, the electrophilic and phospholipid hydroperoxide substrate binding sites remain elusive. Amide H-D exchange kinetics and H-D ligand footprinting experiments indicate that GSH and hydrophobic substrates bind within similar, but distinct, regions of MGST1. Site-directed mutagenesis, guided by the H-D exchange results, demonstrates that specific residues within the GSH footprint effect transferase activity toward 1-chloro-2,4-dinitrobenzene. In addition, cytosolic residues surrounding the chemical stress sensor C49 but not modeled in the crystal structure appear to play an important role in the formation of the binding site for hydrophobic substrates. Although the fatty acid/phospholipid binding site structurally overlaps that for GSH, it does not appear to be localized to the same region as other hydrophobic substrates. Finally, H-D exchange mass spectrometry reveals a specific conformational transition that may mediate substrate binding and/or product release. Such structural changes in MGST1 are essential for activation of the enzyme and are important for its biological function.

Identification of regions of leukotriene C4synthase which direct the enzyme to its nuclear envelope localization

Leukotrienes (LTs) are fatty acid derivatives formed by oxygenation of arachidonic acid via the 5-lipoxygenase (5-LO) pathway. Upon activation of inflammatory cells 5-LO is translocated to the nuclear envelope (NE) where it converts arachidonic acid to the unstable epoxide LTA4. LTA4 is further converted to LTC4 by conjugation with glutathione, a reaction catalyzed by the integral membrane protein LTC4 synthase (LTC4S), which is localized on the NE and endoplasmic reticulum (ER). We now report the mapping of regions of LTC4S that are important for its subcellular localization. Multiple constructs encoding fusion proteins of green fluorescent protein (GFP) as the N-terminal part and various truncated variants of human LTC4S as C-terminal part were prepared and transfected into HEK 293/T or COS-7 cells. Constructs encoding hydrophobic region 1 of LTC4S (amino acids 6-27) did not give distinct membrane localized fluorescence. In contrast hydrophobic region 2 (amino acids 60-89) gave a localization pattern similar to that of full length LTC4S. Hydrophobic region 3 (amino acids 114-135) directed GFP to a localization indistinguishable from that of full length LTC4S. A minimal directing sequence, amino acids 117-132, was identified by further truncation. The involvement of the hydrophobic regions in the homo-oligomerization of LTC4S was investigated using bioluminescence resonance energy transfer (BRET) analysis in living cells. BRET data showed that hydrophobic regions 1 and 3 each allowed oligomerization to occur. These regions most likely form transmembrane helices, suggesting that homo-oligomerization of LTC4S is due to helix-helix interactions in the membrane.

Mutations in tap uncouple RNA export activity from translocation through the nuclear pore complex

Interactions between transport receptors and phenylalanine-glycine (FG) repeats on nucleoporins drive the translocation of receptor-cargo complexes through nuclear pores. Tap, a transport receptor that mediates nuclear export of cellular mRNAs, contains a UBA-like and NTF2-like folds that can associate directly with FG repeats. In addition, two nuclear export sequences (NESs) within the NTF2-like region can also interact with nucleoporins. The Tap-RNA complex was shown to bind to three nucleoporins, Nup98, p62, and RanBP2, and these interactions were enhanced by Nxt1. Mutations in the Tap-UBA region abolished interactions with all three nucleoporins, whereas the effect of point mutations within the NTF2-like domain of Tap known to disrupt Nxt1 binding or nucleoporin binding were nucleoporin dependent. A mutation in any of these Tap domains was sufficient to reduce RNA export but was not sufficient to disrupt Tap interaction with the NPC in vivo or its nucleocytoplasmic shuttling. However, shuttling activity was reduced or abolished by combined mutations within the UBA and either the Nxt1-binding domain or NESs. These data suggest that Tap requires both the UBA- and NTF2-like domains to mediate the export of RNA cargo, but can move through the pores independently of these domains when free of RNA cargo.

Human microsomal prostaglandin E synthase 1: a member of the MAPEG protein superfamily

In this chapter, we briefly review the MAPEG superfamily (membrane associated proteins in eicosanoid and glutathione metabolism), a family of proteins in which all human members except one possess glutathione conjugating capacity. Recent findings regarding the biological functions of MAPEG proteins are highlighted. More extensively, the characterization of human microsomal prostaglandin E synthase 1 is presented, including results and applied methodology.

Molecular and catalytic properties of three rat leukotriene C4 synthase homologs

The committed step in the biosynthesis of cysteinyl-leukotrienes is catalyzed by leukotriene C(4) synthase as well as microsomal glutathione S-transferase (MGST) type 2 and type 3, which belong to a family of membrane-associated proteins in eicosanoid and glutathione metabolism (MAPEG). We cloned and characterized these three enzymes from the rat to allow a side-by-side comparison of structural and catalytic properties. The proteins are 79.6-86.7% identical to the human orthologs. Rat MGST3 fails to convert leukotriene A(4) into leukotriene C(4), which in turn challenges the proposed catalytic role of a conserved Arg and Tyr residue for the leukotriene C(4) synthase reaction. Comparative inhibitor studies of all three enzymes, using MK-886 and cysteinyl-leukotrienes, indicate that their catalytic centers originate from structurally related and overlapping active sites. Hence, it seems feasible to design enzyme inhibitors, which simultaneously target several members of this protein family to yield compounds with increased anti-inflammatory action.

Bronchial mast cells are the dominating LTC4S-expressing cells in aspirin-tolerant asthma

The increased bronchial production of leukotriene C4 (LTC4) in asthma is assumed to derive from infiltrating eosinophils expressing LTC4-synthase (LTC4S). Multicolor immunohistofluorescence examination of bronchial cryosections from 30 treated, untreated, or bronchial antigen-provoked aspirin-tolerant individuals with asthma and nine control subjects revealed that the dominating LTC4S-expressing cells were mast cells (> 80%), and not eosinophils. Whereas 95% of the mast cells expressed high levels of LTC4S, only 8-27% of the eosinophils expressed low levels. Image analysis revealed a significantly higher LTC4S expression levels in mast cells than in eosinophils. The bronchial mRNA levels for LTC4S did not correlate with the densities of LTC4S-positive eosinophils or mast cells. Treated individuals with asthma with more than 12% reversibility had significantly higher density of LTC4S-positive mast cells than those with less reversibility, and it correlated significantly with reduction in lung function (FEV1-predicted), both before and after salbutamol inhalation. Thus, mucosal mast cells, and not eosinophils, were the dominating LTC4S-containing cells in both untreated and treated aspirin-tolerant asthma. The density of LTC4S-positive mast cells correlated, moreover, with both the reduction in lung function and the degree of reversibility in treated asthma.