A protein associated with toll-like receptor 4 (PRAT4A) regulates cell surface expression of TLR4

TLRs recognize microbial products. Their subcellular distribution is optimized for microbial recognition. Little is known, however, about mechanisms regulating the subcellular distribution of TLRs. LPS is recognized by the receptor complex consisting of TLR4 and MD-2. Although MD-2, a coreceptor for TLR4, enhances cell surface expression of TLR4, an additional mechanism regulating TLR4 distribution has been suggested. We show here that PRAT4A, a novel protein associated with TLR4, regulates cell surface expression of TLR4. PRAT4A is associated with the immature form of TLR4 but not with MD-2 or TLR2. PRAT4A knockdown abolished LPS responsiveness in a cell line expressing TLR4/MD-2, probably due to the lack of cell surface TLR4. PRAT4A knockdown down-regulated cell surface TLR4/MD-2 on dendritic cells. These results demonstrate a novel mechanism regulating TLR4/MD-2 expression on the cell surface.     

Crystal structure of SUMO-3-modified thymine-DNA glycosylase

Modification of cellular proteins by the small ubiquitin-like modifier SUMO is important in regulating various cellular events. Many different nuclear proteins are targeted by SUMO, and the functional consequences of this modification are diverse. For most proteins, however, the functional and structural consequences of modification by specific SUMO isomers are unclear. Conjugation of SUMO to thymine-DNA glycosylase (TDG) induces the dissociation of TDG from its product DNA. Structure determination of the TDG central region conjugated to SUMO-1 previously suggested a mechanism in which the SUMOylation-induced conformational change in the C-terminal region of TDG releases TDG from tight binding to its product DNA. Here, we have determined the crystal structure of the central region of TDG conjugated to SUMO-3. The overall structure of SUMO-3-conjugated TDG is similar to the previously reported structure of TDG conjugated to SUMO-1, despite the relatively low level of amino acid sequence similarity between SUMO-3 and SUMO-1. The two structures revealed that the sequence of TDG that resembles the SUMO-binding motif (SBM) can form an intermolecular beta-sheet with either SUMO-1 or SUMO-3. Structural comparison with the canonical SBM shows that this SBM-like sequence of TDG retains all of the characteristic interactions of the SBM, indicating sequence diversity in the SBM.     

Binding of AlF-C, an Orc1-binding transcriptional regulator, enhances replicator activity of the rat aldolase B origin

A region encompassing the rat aldolase B gene (aldB) promoter acts as a chromosomal origin of DNA replication (origin) in rat aldolase B-nonexpressing hepatoma cells. To examine replicator function of the aldB origin, we constructed recombinant mouse cell lines in which the rat aldB origin and the mutant derivatives were inserted into the same position at the mouse chromosome 8 by cre-mediated recombination. Nascent strand abundance assays revealed that the rat origin acts as a replicator at the ectopic mouse locus. Mutation of site C in the rat origin, which binds an Orc1-binding protein AlF-C in vitro, resulted in a significant reduction of the replicator activity in the mouse cells. Chromatin immunoprecipitation (ChIP) assays indicated that the reduction of replicator activity was paralleled with the reduced binding of AlF-C and Orc1, suggesting that sequence-specific binding of AlF-C to the ectopic rat origin leads to enhanced replicator activity in cooperation with Orc1. Involvement of AlF-C in replication in vivo was further examined for the aldB origin at its original rat locus and for a different rat origin identified in the present study, which contained an AlF-C-binding site. ChIP assays revealed that both replication origins bind AlF-C and Orc1. We think that the results presented here may represent one mode of origin recognition in mammalian cells.     

Functional differentiation in the leucine-rich repeat domains of closely related plant virus-resistance proteins that recognize common avr proteins

The N' gene of Nicotiana sylvestris and L genes of Capsicum plants confer the resistance response accompanying the hypersensitive response (HR) elicited by tobamovirus coat proteins (CP) but with different viral specificities. Here, we report the identification of the N' gene. We amplified and cloned an N' candidate using polymerase chain reaction primers designed from L gene sequences. The N' candidate gene was a single 4143 base pairs fragment encoding a coiled-coil nucleotide-binding leucine-rich repeat (LRR)-type resistance protein of 1,380 amino acids. The candidate gene induced the HR in response to the coexpression of tobamovirus CP with the identical specificity as reported for N'. Analysis of N'-containing and tobamovirus-susceptible N. tabacum accessions supported the hypothesis that the candidate is the N' gene itself. Chimera analysis between N' and L(3) revealed that their LRR domains determine the spectrum of their tobamovirus CP recognition. Deletion and mutation analyses of N' and L(3) revealed that the conserved sequences in their C-terminal regions have important roles but contribute differentially to the recognition of common avirulence proteins. The results collectively suggest that Nicotiana N' and Capsicum L genes, which most likely evolved from a common ancestor, differentiated in their recognition specificity through changes in the structural requirements for LRR function.     

A three-dimensional FRET analysis to construct an atomic model of the actin-tropomyosin-troponin core domain complex on a muscle thin filament

It is essential to know the detailed structure of the thin filament to understand the regulation mechanism of striated muscle contraction. Fluorescence resonance energy transfer (FRET) was used to construct an atomic model of the actin-tropomyosin (Tm)-troponin (Tn) core domain complex. We generated single-cysteine mutants in the 167-195 region of Tm and in TnC, TnI, and the β-TnT 25-kDa fragment, and each was attached with an energy donor probe. An energy acceptor probe was located at actin Gln41, actin Cys374, or the actin nucleotide-binding site. From these donor-acceptor pairs, FRET efficiencies were determined with and without Ca(2+). Using the atomic coordinates for F-actin, Tm, and the Tn core domain, we searched all possible arrangements for Tm or the Tn core domain on F-actin to calculate the FRET efficiency for each donor-acceptor pair in each arrangement. By minimizing the squared sum of deviations for the calculated FRET efficiencies from the observed FRET efficiencies, we determined the location of Tm segment 167-195 and the Tn core domain on F-actin with and without Ca(2+). The bulk of the Tn core domain is located near actin subdomains 3 and 4. The central helix of TnC is nearly perpendicular to the F-actin axis, directing the N-terminal domain of TnC toward the actin outer domain. The C-terminal region in the I-T arm forms a four-helix-bundle structure with the Tm 175-185 region. After Ca(2+) release, the Tn core domain moves toward the actin outer domain and closer to the center of the F-actin axis.     

Polyhydroxylated fullerene C60(OH)44 suppresses intracellular lipid accumulation together with repression of intracellular superoxide anion radicals and subsequent PPARγ2 expression during spontaneous differentiation of OP9 preadipocytes into adipocytes

Reactive oxygen species has been suggested to be one of the key factors associated with the development of obesity. During spontaneous differentiation of mouse stromal preadipocytes OP9 into adipocytes, intracellular superoxide anion radicals (O (2) (-.) ) level markedly increases and is accompanied by a significant elevation of intracellular lipid accumulation. This differentiation-dependent increase in intracellular O (2) (-.) level positively correlated with the intracellular augmentation of the lipid level. Super-highly hydroxylated fullerene (SHH-F; C(60)(OH)(44)), a novel polyhydroxylated fullerene derivative, quenched intracellular O (2) (-.) , and lipid accumulation to 38.7 and 42.7 % of that in the control, respectively. By thin-layer chromatographic analysis of extracted cellular lipid components, SHH-F clearly decreased the triglycerides ratio in the whole lipid droplet fraction, but scarcely influenced other lipids components. PPARγ2 expression, which plays a key role in regulating adipogenic differentiation, was significantly suppressed by SHH-F at the late stage of differentiation, with unaltered PPARγ1 expression. The intracellular superoxide anion radical augmentation preceded expression of PPARγ2, strongly suggesting that the primary O (2) (-.) generation was closely associated with lipid accumulation and subsequent PPARγ2 induction. These results indicate that SHH-F suppresses intracellular lipid accumulation, particularly in lipid droplets, and decreases O (2) (-.) level and subsequent PPARγ2 upregulation during spontaneous differentiation of OP9 preadipocytes into adipocytes.     

Histochemical analyses and quantum dot imaging of microvascular blood flow with pulmonary edema in living mouse lungs by “in vivo cryotechnique”

Light microscopic imaging of blood vessels and distribution of serum proteins is essential to analyze hemodynamics in living animal lungs under normal respiration or respiratory diseases. In this study, to demonstrate dynamically changing morphology and immunohistochemical images of their living states, “in vivo cryotechnique” (IVCT) combined with freeze-substitution fixation was applied to anesthetized mouse lungs. By hematoxylin–eosin staining, morphological features, such as shapes of alveolar septum and sizes of alveolar lumen, reflected their respiratory conditions in vivo, and alveolar capillaries were filled with variously shaped erythrocytes. Albumin was usually immunolocalized in the capillaries, which was confirmed by double-immunostaining for aquaporin-1 of endothelium. To capture accurate time-courses of blood flow in peripheral pulmonary alveoli, glutathione-coated quantum dots (QDs) were injected into right ventricles, and then IVCT was performed at different time-points after the QD injection. QDs were localized in most arterioles and some alveolar capillaries at 1 s, and later in venules at 2 s, reflecting a typical blood flow direction in vivo. Three-dimensional QD images of microvascular networks were reconstructed by confocal laser scanning microscopy. It was also applied to lungs of acute pulmonary hypertension mouse model. Erythrocytes were crammed in blood vessels, and some serum components leaked into alveolar lumens, as confirmed by mouse albumin immunostaining. Some separated collagen fibers and connecting elastic fibers were still detected in edematous tunica adventitia near terminal bronchioles. Thus, IVCT combined with histochemical approaches enabled us to capture native images of dynamically changing structures and microvascular hemodynamics of living mouse lungs.     

Neutrophil extracellular traps mediate a host defense response to human immunodeficiency virus-1

Neutrophils contribute to pathogen clearance by producing neutrophil extracellular traps (NETs), which are genomic DNA-based net-like structures that capture bacteria and fungi. Although NETs also express antiviral factors, such as myeloperoxidase and α-defensin, the involvement of NETs in antiviral responses remains unclear. We show that NETs capture human immunodeficiency virus (HIV)-1 and promote HIV-1 elimination through myeloperoxidase and α-defensin. Neutrophils detect HIV-1 by Toll-like receptors (TLRs) TLR7 and TLR8, which recognize viral nucleic acids. Engagement of TLR7 and TLR8 induces the generation of reactive oxygen species that trigger NET formation, leading to NET-dependent HIV-1 elimination. However, HIV-1 counteracts this response by inducing C-type lectin CD209-dependent production of interleukin (IL)-10 by dendritic cells to inhibit NET formation. IL-10 suppresses the reactive oxygen species-dependent generation of NETs induced upon TLR7 and TLR8 engagement, resulting in disrupted NET-dependent HIV-1 elimination. Therefore, NET formation is an antiviral response that is counteracted by HIV-1.