Molecular Identification of D-Ribulokinase in Budding Yeast and Mammals

in Journal of Biological Chemistry (2017), 292(3), 1005-1028

Proteomes of even well characterized organisms still contain a high percentage of proteins with unknown or uncertain molecular and/or biological function. A significant fraction of those proteins are ... [more ▼]

Proteomes of even well characterized organisms still contain a high percentage of proteins with unknown or uncertain molecular and/or biological function. A significant fraction of those proteins are predicted to have catalytic properties. Here we aimed at identifying the function of the Saccharomyces cerevisiae Ydr109c protein and of its human homolog FGGY, both of which belong to the broadly conserved FGGY family of carbohydrate kinases. Functionally identified members of this family phosphorylate 3- to 7-carbon sugars or sugar derivatives, but the endogenous substrate of S. cerevisiae Ydr109c and human FGGY has remained unknown. Untargeted metabolomics analysis of an S. cerevisiae deletion mutant of YDR109C revealed ribulose as one of the metabolites with the most significantly changed intracellular concentration as compared to a wild-type strain. In human HEK293 cells, ribulose could only be detected when ribitol was added to the cultivation medium and under this condition, FGGY silencing led to ribulose accumulation. Biochemical characterization of the recombinant purified Ydr109c and FGGY proteins showed a clear substrate preference of both kinases for D-ribulose over a range of other sugars and sugar derivatives tested, including L-ribulose. Detailed sequence and structural analyses of Ydr109c and FGGY as well as homologs thereof furthermore allowed the definition of a 5-residue D-ribulokinase signature motif (TCSLV). The physiological role of the herein identified eukaryotic D-ribulokinase remains unclear, but we speculate that S. cerevisiae Ydr109c and human FGGY could act as metabolite repair enzymes, serving to re-phosphorylate free D-ribulose generated by promiscuous phosphatases from D-ribulose-5-phosphate. In human cells, FGGY can additionally participate in ribitol metabolism. [less ▲]

Impact of oxidative stress on ascorbate biosynthesis in Chlamydomonas via regulation of the VTC2 gene encoding a GDP-L-galactose phosphorylase

in Journal of Biological Chemistry (2012), 287(17), 14234-45

The L-galactose (Smirnoff-Wheeler) pathway represents the major route to L-ascorbic acid (vitamin C) biosynthesis in higher plants. Arabidopsis thaliana VTC2 and its paralogue VTC5 function as GDP-L ... [more ▼]

The L-galactose (Smirnoff-Wheeler) pathway represents the major route to L-ascorbic acid (vitamin C) biosynthesis in higher plants. Arabidopsis thaliana VTC2 and its paralogue VTC5 function as GDP-L-galactose phosphorylases converting GDP-L-galactose to L-galactose-1-P, thus catalyzing the first committed step in the biosynthesis of L-ascorbate. Here we report that the L-galactose pathway of ascorbate biosynthesis described in higher plants is conserved in green algae. The Chlamydomonas reinhardtii genome encodes all the enzymes required for vitamin C biosynthesis via the L-galactose pathway. We have characterized recombinant C. reinhardtii VTC2 as an active GDP-L-galactose phosphorylase. C. reinhardtii cells exposed to oxidative stress show increased VTC2 mRNA and L-ascorbate levels. Genes encoding enzymatic components of the ascorbate-glutathione system (e.g. ascorbate peroxidase, Mn superoxide dismutase, dehydroascorbate reductase) are also up-regulated in response to increased oxidative stress. These results indicate that C. reinhardtii VTC2, like its plant homologs, is a highly regulated enzyme in ascorbate biosynthesis in green algae and that, together with the ascorbate recycling system, the L-galactose pathway represents the major route for providing protective levels of ascorbate in oxidatively stressed algal cells. [less ▲]

Extremely conserved ATP- or ADP-dependent enzymatic system for nicotinamide nucleotide repair

in Journal of Biological Chemistry (2011), 286(48), 41246-52

The reduced forms of NAD and NADP, two major nucleotides playing a central role in metabolism, are continuously damaged by enzymatic or heat-dependent hydration. We report the molecular identification of ... [more ▼]

The reduced forms of NAD and NADP, two major nucleotides playing a central role in metabolism, are continuously damaged by enzymatic or heat-dependent hydration. We report the molecular identification of the eukaryotic dehydratase that repairs these nucleotides and show that this enzyme (Carkd in mammals, YKL151C in yeast) catalyzes the dehydration of the S form of NADHX and NADPHX, at the expense of ATP, which is converted to ADP. Surprisingly, the Escherichia coli homolog, YjeF, a bidomain protein, catalyzes a similar reaction, but using ADP instead of ATP. The latter reaction is ascribable to the C-terminal domain of YjeF. This represents an unprecedented example of orthologous enzymes using either ADP or ATP as phosphoryl donor. We also show that eukaryotic proteins homologous to the N-terminal domain of YjeF (apolipoprotein A-1-binding protein (AIBP) in mammals, YNL200C in yeast) catalyze the epimerization of the S and R forms of NAD(P)HX, thereby allowing, in conjunction with the energy-dependent dehydratase, the repair of both epimers of NAD(P)HX. Both enzymes are very widespread in eukaryotes, prokaryotes, and archaea, which together with the ADP dependence of the dehydratase in some species indicates the ancient origin of this repair system. [less ▲]

De Novo Lipogenesis Maintains Vascular Homeostasis through Endothelial Nitric-oxide Synthase (eNOS) Palmitoylation

in Journal of Biological Chemistry (2011), 286(4), 2933-2945

Endothelial dysfunction leads to lethal vascular complications in diabetes and related metabolic disorders. Here, we demonstrate that de novo lipogenesis, an insulin-dependent process driven by the ... [more ▼]

Endothelial dysfunction leads to lethal vascular complications in diabetes and related metabolic disorders. Here, we demonstrate that de novo lipogenesis, an insulin-dependent process driven by the multifunctional enzyme fatty-acid synthase (FAS), maintains endothelial function by targeting endothelial nitric-oxide synthase (eNOS) to the plasma membrane. In mice with endothelial inactivation of FAS (FASTie mice), eNOS membrane content and activity were decreased. eNOS and FAS were physically associated; eNOS palmitoylation was decreased in FAS-deficient cells, and incorporation of labeled carbon into eNOS-associated palmitate was FAS-dependent. FASTie mice manifested a proinflammatory state reflected as increases in vascular permeability, endothelial inflammatory markers, leukocyte migration, and susceptibility to LPS-induced death that was reversed with an NO donor. FAS-deficient endothelial cells showed deficient migratory capacity, and angiogenesis was decreased in FASTie mice subjected to hindlimb ischemia. Insulin induced FAS in endothelial cells freshly isolated from humans, and eNOS palmitoylation was decreased in mice with insulin-deficient or insulin-resistant diabetes. Thus, disrupting eNOS bioavailability through impaired lipogenesis identifies a novel mechanism coordinating nutritional status and tissue repair that may contribute to diabetic vascular disease. [less ▲]

Arabidopsis VTC2 encodes a GDP-L-galactose phosphorylase, the last unknown enzyme in the Smirnoff-Wheeler pathway to ascorbic acid in plants

in Journal of Biological Chemistry (2007), 282(26), 18879-85

The first committed step in the biosynthesis of L-ascorbate from D-glucose in plants requires conversion of GDP-L-galactose to L-galactose 1-phosphate by a previously unidentified enzyme. Here we show ... [more ▼]

The first committed step in the biosynthesis of L-ascorbate from D-glucose in plants requires conversion of GDP-L-galactose to L-galactose 1-phosphate by a previously unidentified enzyme. Here we show that the protein encoded by VTC2, a gene mutated in vitamin C-deficient Arabidopsis thaliana strains, is a member of the GalT/Apa1 branch of the histidine triad protein superfamily that catalyzes the conversion of GDP-L-galactose to L-galactose 1-phosphate in a reaction that consumes inorganic phosphate and produces GDP. In characterizing recombinant VTC2 from A. thaliana as a specific GDP-L-galactose/GDP-D-glucose phosphorylase, we conclude that enzymes catalyzing each of the ten steps of the Smirnoff-Wheeler pathway from glucose to ascorbate have been identified. Finally, we identify VTC2 homologs in plants, invertebrates, and vertebrates, suggesting that a similar reaction is used widely in nature. [less ▲]

Three dileucine-like motifs within the interbox1/2 region of the human oncostatin M receptor prevent efficient surface expression in the absence of an associated Janus kinase

in Journal of Biological Chemistry (2006), 281(7), 4024-34

The oncostatin M receptor (OSMR) is part of receptor complexes for oncostatin M and interleukin-31. Signaling events are triggered by Jaks (Janus kinases) that constitutively bind to membrane-proximal ... [more ▼]

The oncostatin M receptor (OSMR) is part of receptor complexes for oncostatin M and interleukin-31. Signaling events are triggered by Jaks (Janus kinases) that constitutively bind to membrane-proximal receptor regions. Besides their established role in signaling, Jaks are involved in the regulation of the surface expression of several cytokine receptors. Here, we analyzed the structural requirements within the human OSMR that underlie its limited surface expression in the absence of associated Jaks. We identified three dileucine-like motifs within the Jak-binding region of the OSMR that control receptor surface and overall expression. A receptor mutant in which all three motifs were mutated to alanine displayed markedly increased surface expression. Although the surface half-life of this mutant was increased compared with that of the wild-type receptor, no difference in the internalization rate was detectable, implying that these receptors differ in their post-endocytic fate. The protein stability of the wild-type receptor was markedly lower than that of mutant receptors, but could be strongly increased in the presence of the lysosomal inhibitor chloroquine. Our data are consistent with the dileucine motifs being involved in destabilization of receptors devoid of associated Jaks as part of a quality control ensuring signaling competence of OSMRs. [less ▲]

Are STATS arginine-methylated?

in Journal of Biological Chemistry (2005), 280(23), 21700-5

Transcription factors of the STAT (signal transducer and activator of transcription) family are important in signal transduction of cytokines. They are subject to post-translational modification by ... [more ▼]

Transcription factors of the STAT (signal transducer and activator of transcription) family are important in signal transduction of cytokines. They are subject to post-translational modification by phosphorylation on tyrosine and serine residues. Recent evidence suggested that STATs are methylated on a conserved arginine residue within the N-terminal region. STAT arginine methylation has been described to be important for STAT function and loss of arginine methylation was discussed to be involved in interferon resistance of cancer cells. Here we provide several independent lines of evidence indicating that the issue of arginine methylation of STATs has to be reassessed. First, we show that treatment of melanoma and fibrosarcoma cells with inhibitors used to suppress methylation (N-methyl-2-deoxyadenosine, adenosine, dl-homocysteine) had profound and rapid effects on phosphorylation of STAT1 and STAT3 but also on p38 and Erk signaling cascades which are known to cross-talk with the Jak/STAT pathway. Second, we show that anti-methylarginine antibodies did not precipitate specifically STAT1 or STAT3. Third, we show that mutation of Arg(31) to Lys led to destabilization of STAT1 and STAT3, implicating an important structural role of Arg(31). Finally, purified catalytically active protein arginine methyltransferases (PRMT1, -2, -3, -4, and -6) did not methylate STAT proteins, and cotransfection with PRMT1 did not affect STAT1-controlled reporter gene activity. Taken together, our data suggest the absence of arginine methylation of STAT1 and STAT3. [less ▲]

Cyclophilin a binds to linear peptide motifs containing a consensus that is present in many human proteins

in Journal of Biological Chemistry (2005), 280(25), 23668-23674

Cyclophilin A ( CypA) is a peptidyl-prolyl cis/trans-isomerase that is involved in multiple signaling events of eukaryotic cells. It might either act as a catalyst for prolyl bond isomerization, or it can ... [more ▼]

Cyclophilin A ( CypA) is a peptidyl-prolyl cis/trans-isomerase that is involved in multiple signaling events of eukaryotic cells. It might either act as a catalyst for prolyl bond isomerization, or it can form stoichiometric complexes with target proteins. We have investigated the linear sequence recognition code for CypA by phage display and found the consensus motif FGPXLp to be selected after five rounds of panning. The peptide FGP-DLPAGD showed inhibition of the isomerase reaction and NMR chemical shift mapping experiments highlight the CypA interaction epitope. Ligand docking suggests that the peptide was able to bind to CypA in the cis- and trans-conformation. Protein Data Bank searches reveal that many human proteins contain the consensus motif, and several of these protein motifs are shown to interact with CypA in vitro. These sequences represent putative target sites for binding of CypA to intracellular proteins. [less ▲]

A new functional role of the fibrinogen RGD motif as the molecular switch that selectively triggers integrin alphaIIbbeta3-dependent RhoA activation during cell spreading

in Journal of Biological Chemistry (2005), 280(39), 33610-33619

A number of RGD-type integrins rely on a synergistic site in addition to the canonical RGD site for ligand binding and signaling, although it is still unclear whether these two recognition sites function ... [more ▼]

A number of RGD-type integrins rely on a synergistic site in addition to the canonical RGD site for ligand binding and signaling, although it is still unclear whether these two recognition sites function independently, synergistically, or competitively. Experimental evidence has suggested that fibrinogen binding to the RGD-type integrin alphaIIbbeta3 occurs exclusively through the synergistic gamma(400-411) sequence, thus questioning the functional role of the RGD recognition site. Here we have investigated the respective role of the fibrinogen gamma(400-411) sequence and the RGD motif in the molecular events leading to ligand-induced alphaIIbbeta3-dependent Chinese hamster ovary (CHO) cell or platelet spreading, by using intact fibrinogen and well characterized plasmin-generated fibrinogen fragments containing either the RGD motif (fragment C) or the gamma(400-411) sequence (fragment D), and CHO cells expressing resting wild type (alphaIIbbeta3wt), constitutively active (alphaIIbbeta3T562N), or non-functional (alphaIIbbeta3D119Y) receptors. Our data provide evidence that the gamma(400-411) site by itself is able to initiate alphaIIbbeta3 clustering and recruitment of intracellular proteins to early focal complexes, mediating cell attachment, FAK phosphorylation, and Rac1 activation, while the RGD motif subsequently acts as a molecular switch on the beta3 subunit to trigger cell spreading. More importantly, we show that the premier functional role of the RGD site is not to reinforce cell attachment but, rather, to imprint a conformational change on the beta3 subunit leading to maximal RhoA activation and actin cytoskeleton organization in CHO cells as well as in platelets. Finally, alphaIIbbeta3-dependent RhoA stimulation and cell spreading, but not cell attachment, are Src-dependent and phosphoinositide 3-kinase-independent and are inhibited by the Src antagonist PP2. [less ▲]

A fluorescence cell biology approach to map the second integrin-binding site of talin to a 130-amino acid sequence within the rod domain

in Journal of Biological Chemistry (2004), 279(21), 22258-22266

The cytoskeletal protein talin, which provides a direct link between integrins and actin filaments, has been shown to contain two distinct binding sites for integrin β subunits. Here, we report the ... [more ▼]

The cytoskeletal protein talin, which provides a direct link between integrins and actin filaments, has been shown to contain two distinct binding sites for integrin β subunits. Here, we report the precise delimitation and a first functional analysis of the talin rod domain integrin-binding site. Partially overlapping cDNAs covering the entire human talin gene were transiently expressed as DsRed fusion proteins in Chinese hamster ovary cells expressing α IIbβ 3, linked to green fluorescent protein (GFP). Two-color fluorescence analysis of the transfected cells, spread on fibrinogen, revealed distinct subcellular staining patterns including focal adhesion, actin filament, and granular labeling for different talin fragments. The rod domain fragment G (residues 1984-2344), devoid of any known actin- or vinculin-binding sites, colocalized with β 3-GFP in focal adhesions. Direct in vitro interaction of fragment G with native platelet integrin α IIbβ 3 or with the recombinant wild type, but not the Y747A mutant β 3 cytoplasmic tail, linked to glutathione S-transferase, was demonstrated by surface plasmon resonance analysis and pull-down assays, respectively. Here, we demonstrate for the first time the in vivo relevance of this interaction by fluorescence resonance energy transfer between β 3-GFP and DsRed-talin fragment G. Further in vitro pull-down studies allowed us to map out the integrin-binding site within fragment G to a stretch of 130 residues (fragment J, residues 1984-2113) that also localized to focal adhesions. Finally, we show by a cell biology approach that this integrin- binding site within the talin rod domain is important for β 3-cytoskeletal interactions but does not participate in α IIbβ 3 activation. [less ▲]