Phytochrome Kinase Substrate 4 is phosphorylated by the phototropin 1 photoreceptor

Phototropism allows plants to redirect their growth towards the light to optimize photosynthesis under reduced light conditions. Phototropin 1 (phot1) is the primary low blue light-sensing receptor triggering phototropism in Arabidopsis. Light-induced autophosphorylation of phot1, an AGC-class protein kinase, constitutes an essential step for phototropism. However, apart from the receptor itself, substrates of phot1 kinase activity are less clearly established. Phototropism is also influenced by the cryptochromes and phytochromes photoreceptors that do not provide directional information but influence the process through incompletely characterized mechanisms. Here, we show that Phytochrome Kinase Substrate 4 (PKS4), a known element of phot1 signalling, is a substrate of phot1 kinase activity in vitro that is phosphorylated in a phot1-dependent manner in vivo. PKS4 phosphorylation is transient and regulated by a type 2-protein phosphatase. Moreover, phytochromes repress the accumulation of the light-induced phosphorylated form of PKS4 showing a convergence of photoreceptor activity on this signalling element. Our physiological analyses suggest that PKS4 phosphorylation is not essential for phototropism but is part of a negative feedback mechanism.     

Kinetic and structural studies on the catalytic role of the aspartic acid residue conserved in copper amine oxidase

Copper amine oxidase contains a post-translationally generated quinone cofactor, topa quinone (TPQ), which mediates electron transfer from the amine substrate to molecular oxygen. The overall catalytic reaction is divided into the former reductive and the latter oxidative half-reactions based on the redox state of TPQ. In the reductive half-reaction, substrate amine reacts with the C5 carbonyl group of the oxidized TPQ, forming the substrate Schiff base (TPQ(ssb)), which is then converted to the product Schiff base (TPQ(psb)). During this step, an invariant Asp residue with an elevated pKa is presumed to serve as a general base accepting the alpha proton of the substrate. When Asp298, the putative active-site base in the recombinant enzyme from Arthrobacter globiformis, was mutated into Ala, the catalytic efficiency dropped to a level of about 10(6) orders of magnitude smaller than the wild-type (WT) enzyme, consistent with the essentiality of Asp298. Global analysis of the slow UV/vis spectral changes observed during the reductive half-reaction of the D298A mutant with 2-phenylethylamine provided apparent rate constants for the formation and decay of TPQ(ssb) (k(obs) = 4.7 and 4.8 x 10(-4) s(-1), respectively), both of which are markedly smaller than those of the WT enzyme determined by rapid-scan stopped-flow analysis (k(obs) = 699 and 411 s(-1), respectively). Thus, Asp298 plays important roles not only in the alpha-proton abstraction from TPQ(ssb) but also in other steps in the reductive half-reaction. X-ray diffraction analyses of D298A crystals soaked with the substrate for 1 h and 1 week revealed the structures of TPQ(ssb) and TPQ(psb), respectively, as pre-assigned by single-crystal microspectrophotometry. Consistent with the stereospecificity of alpha-proton abstraction, the pro-S alpha-proton of TPQ(ssb) to be abstracted is positioned nearly perpendicularly to the plane formed by the Schiff-base imine double bond conjugating with the quinone ring of TPQ, so that the orbitals of sigma and pi electrons maximally overlap in the conjugate system. More intriguingly, the pro-S alpha proton of the substrate is released stereospecifically even in the reaction catalyzed by the base-lacking D298A mutant. On the basis of these results, we propose that the stereospecificity of alpha-proton abstraction is primarily determined by the conformation of TPQ(ssb), rather than the relative geometry of TPQ and the catalytic base.     

Crystal structures of cytochrome c(L) and methanol dehydrogenase from Hyphomicrobium denitrificans: structural and mechanistic insights into interactions between the two proteins

Methanol dehydrogenase (Hd-MDH) and its physiological electron acceptor, cytochrome c(L) (Hd-Cyt c(L)), isolated from a methylotrophic denitrifying bacterium, Hyphomicrobium denitrificans A3151, have been kinetically and structurally characterized; the X-ray structures of Hd-MDH and Hd-Cyt c(L) have been determined using molecular replacement at 2.5 and 2.0 A resolution, respectively. To explain the mechanism for electron transfer between these proteins, the dependence of MDH activity on the concentration of Hd-Cyt c(L) has been investigated at pH 4.5-7.0. The Michaelis constant for Hd-Cyt c(L) shows the smallest value (approximately 0.3 microM) at pH 5.5. The pseudo-first-order rate constant (k(obs)) of the reduction of Hd-Cyt c(L) exhibits a hyperbolic concentration dependence of Hd-MDH at pH 5.5, although k(obs) linearly increases at pH 6.5. These findings indicate formation of a transient complex between these proteins during an electron transfer event. Hd-MDH (148 kDa) is a large tetrameric protein with an alpha(2)beta(2) subunit composition, showing a high degree of structural similarity with other MDHs. Hd-Cyt c(L) (19 kDa) exhibiting the alpha-band at 550.7 nm has a unique C-terminal region involving a disulfide bond between Cys47 and Cys165. Moreover, there is a pair of Hd-Cyt c(L) monomers related with a pseudo-2-fold axis of symmetry in the asymmetric unit, and the two monomers tightly interact with each other through three hydrogen bonds. This configuration is the first example in the studies of cytochrome c as the physiological electron acceptor for MDH. The docking simulation between the coupled Hd-Cyt c(L) molecules and the heterotetrameric Hd-MDH molecule has been carried out.     

Sphingosine 1-phosphate receptors mediate stimulatory and inhibitory signalings for expression of adhesion molecules in endothelial cells

Sphingosine 1-phosphate (S1P) stimulates expression of vascular cell adhesion molecule-1 and intercellular adhesion molecule-1 in human umbilical vein endothelial cells. S1P-induced actions were associated with nuclear factor kappa-B activation and inhibited by pertussis toxin as well as by antisense oligonucleotides specific to S1P receptors, especially, S1P(3). S1P also stimulated endothelial nitric oxide synthase (eNOS) and its activation was markedly inhibited by the antisense oligonucleotide for the S1P(1) receptor rather than that for the S1P(3) receptor. The dose-response curve of S1P to stimulate adhesion molecule expression was shifted to the left in the presence of the phosphatidylinositol 3-kinase inhibitor wortmannin and the NOS inhibitor Nomega-nitro-l-arginine methyl ester. NO donor S-nitroso-N-acetylpenicillamine inhibited S1P-induced adhesion molecule expression. Moreover, tumor necrosis factor-alpha-induced adhesion molecule expression was markedly inhibited by S1P in a manner sensitive to inhibitors for PI3-K and NOS. These results suggest that S1P receptors are coupled to both stimulatory and inhibitory pathways for adhesion molecule expression. The stimulatory pathway involves nuclear factor kappa-B and inhibitory one does phosphatidylinositol 3-kinase and NOS.     

Intron-dependent accumulation of mRNA in Coriolus hirsutus of lignin peroxidase gene the product of which is involved in conversion/degradation of polychlorinated aromatic hydrocarbons

The homobasidiomycete Coriolus hirsutus coding sequences of a lignin peroxidase (LiP) gene (lip, containing six (I-VI) introns), a lip cDNA (lipc), and three lipc derivatives containing one (I), three (I-III), or five (I-V) introns were inserted into chromosome-integrating expression vector. These recombinant plasmids were introduced into C. hirustus monokaryotic strain. The transformant carrying the promoter-lipc-terminator cassette did not contain enough mRNA molecules to be detectable by Northern-blot analysis. On the other hand, all the transformants carrying cassettes of genomic lip and intron(s)-containing lipc sequences contained sufficient amounts of mRNAs to be easily detected by Northern-blot analysis. LiP activities in the culture supernatants of these transformants were found to be about five times as high as those of transformants carrying the lipc cassette (or no cassette). The culture supernatants of the transformants with high LiP activity showed remarkably high conversion activity toward pentachlorophenol (PCP) and degradation activity toward 2,7-dichlorodibenzo-p-dioxin (2,7-DCDD). These results indicate that at least one intron (intron I) is required for accumulation of lip mRNA and its subsequent translational expression in C. hirsutus.     

Growth differentiation factor 9 is antiapoptotic during follicular development from preantral to early antral stage

Ovarian follicular atresia represents a selection process that ensures the release of only healthy and viable oocytes during ovulation. The transition from preantral to early antral stage is the penultimate stage of development in terms of gonadotropin dependence and follicle destiny (survival/growth vs. atresia). We have examined whether and how oocyte-derived growth differentiation factor 9 (GDF-9) and FSH regulate follicular development and atresia during the preantral to early antral transition, by a novel combination of in vitro gene manipulation (i.e. intraoocyte injection of GDF-9 antisense oligos) and preantral follicle culture. Injection of GDF-9 antisense suppressed basal and FSH-induced preantral follicle growth in vitro, whereas addition of GDF-9 enhanced basal and FSH-induced follicular development. GDF-9 antisense activated caspase-3 and induced apoptosis in cultured preantral follicles, a response attenuated by exogenous GDF-9. GDF-9 increased phospho-Akt content in granulosa cells of early antral follicles. Although granulosa cell apoptosis induced by ceramide was attenuated by the presence of GDF-9, this protective effect of GDF-9 was prevented by the phosphatidylinositol 3-kinase inhibitor LY294002 and a dominant negative form of Akt. Injection of GDF-9 antisense decreased FSH receptor mRNA levels in cultured follicles, a response preventable by the presence of exogenous GDF-9. The data suggest that GDF-9 is antiapoptotic in preantral follicles and protects granulosa cells from undergoing apoptosis via activation of the phosphatidylinositol 3-kinase/Akt pathway. An adequate level of GDF-9 is required for follicular FSH receptor mRNA expression. GDF-9 promotes follicular survival and growth during the preantral to early antral transition by suppressing granulosa cell apoptosis and follicular atresia.     

The ATR-Chk1 pathway plays a role in the generation of centrosome aberrations induced by Rad51C dysfunction

Rad51C is a central component of two complexes formed by five Rad51 paralogs in vertebrates. These complexes are involved in repairing DNA double-strand breaks through homologous recombination. Despite accumulating evidence suggesting that the paralogs may prevent aneuploidy by controlling centrosome integrity, Rad51C's role in maintaining chromosome stability remains unclear. Here we demonstrate that Rad51C deficiency leads to both centrosome aberrations in an ATR-Chk1-dependent manner and increased aneuploidy in human cells. While it was reported that Rad51C deficiency did not cause centrosome aberrations in interphase in hamster cells, such aberrations were observed in interphase in HCT116 cells with Rad51C dysfunction. Caffeine treatment and down-regulation of ATR, but not that of ATM, reduced the frequency of centrosome aberrations in the mutant cells. Silencing of Rad51C by RNA interference in HT1080 cells resulted in similar aberrations. Treatment with a Chk1 inhibitor and silencing of Chk1 also reduced the frequency in HCT116 mutants. Accumulation of Chk1 at the centrosome and nuclear foci of γH2AX were increased in the mutants. Moreover, the mutant cells had a higher frequency of aneuploidy. These findings indicate that the ATR-Chk1 pathway plays a role in increased centrosome aberrations induced by Rad51C dysfunction.     

Inhibition of superoxide anion production by extracellular acidification in neutrophils

Extracellular acidification inhibited formyl-Met-Leu-Phe- or C5a-induced superoxide anion (O₂⁻) production in differentiated HL-60 neutrophil-like cells and human neutrophils. A cAMP-increasing agonist, prostaglandin E₁, also inhibited the formyl peptide-induced O₂⁻ production. The inhibitory action on the O₂⁻ production by extracellular acidic pH was associated with cAMP accumulation and partly attenuated by H89, a protein kinase A inhibitor. A significant amount of mRNAs for T-cell death-associated gene 8 (TDAG8) and other proton-sensing ovarian cancer G-protein-coupled receptor 1 (OGR1)-family receptors is expressed in these cells. These results suggest that cAMP/protein kinase A, possibly through proton-sensing G-protein-coupled receptors, may be involved in extracellular acidic pH-induced inhibition of O₂⁻ production.