Identification of the GGPS1 genes encoding geranylgeranyl diphosphate synthases from mouse and human

E,E,E-Geranylgeranyl diphosphate (GGPP) is an important precursor of carotenoids and geranylgeranylated proteins such as small G proteins. In this study, we have identified mouse and human GGPP synthase genes. Sequence analysis showed that mouse and human GGPP synthases share a high level of amino acid identity (94%) with each other, and share a high level of similarity (45-50%) with GGPP synthases of lower eukaryotes, but only weak similarity (22-31%) to plant and prokaryotic GGPP synthases. Both of the newly identified GGPP synthase genes from mouse and human were expressed in Escherichia coli, and their gene products displayed GGPP synthase activity when isopentenyl diphosphate and farnesyl diphosphate were used as substrates. The GGPP synthase activity of these genes was also confirmed by demonstrating carotenoid synthesis after co-transformation of E. coli with a plasmid expressing the crt genes derived from Erwinia uredovora, and a plasmid expressing either the mouse or human GGPS1 gene. Southern blot analysis suggests that the human GGPS1 gene is a single copy gene.     

Substrate specificity of N-acetylglucosaminyl(diphosphodolichol) N-acetylglucosaminyl transferase, a key enzyme in the dolichol pathway

N-Acetylglucosaminyl(diphosphodolichol) N-acetylglucosaminyl transferase, also known as Enzyme II, is the second enzyme in the dolichol pathway. This pathway is responsible for the assembly of the tetradecasaccharide pyrophosphate dolichol, which is the substrate for oligosaccharyl transferase. In order to study the specificity of Enzyme II, four unnatural dolichol diphosphate monosaccharides were synthesized, with the C-2 acetamido group in the natural substrate Dol-PP-GlcNAc 1a replaced by fluoro, ethoxy, trifluoroacetamido, and amino functionalities. These analogues 1b-e were evaluated as glycosyl acceptors for Enzyme II, which catalyzes the formation of dolichol diphosphate chitobiose (Dol-PP-GlcNAc(2)) from UDP-GlcNAc and Dol-PP-GlcNAc. Enzyme II from pig liver was found to be highly specific for its glycosyl acceptor and the acetamido group shown to be a key functional determinant for this glycosylation reaction.     

The Escherichia coli homologue of yeast RER2, a key enzyme of dolichol synthesis, is essential for carrier lipid formation in bacterial cell wall synthesis

We found in the Escherichia coli genome sequence a homologue of RER2, a Saccharomyces cerevisiae gene required for proper localization of an endoplasmic reticulum protein, and designated it rth (RER2 homologue). The disruption of this gene was lethal for E. coli. To reveal its biological function, we isolated temperature-sensitive mutants of the rth gene. The mutant cells became swollen and burst at the nonpermissive temperature, indicating that their cell wall integrity was defective. Further analysis showed that the mutant cells were deficient in the activity of cis-prenyltransferase, namely, undecaprenyl diphosphate synthase, a key enzyme of the carrier lipid formation of peptidoglycan synthesis. The cellular level of undecaprenyl phosphate was in fact markedly decreased in the mutants. These results are consistent with the fact that the Rer2 homologue of Micrococcus luteus shows undecaprenyl diphosphate synthase activity (N. Shimizu, T. Koyama, and K. Ogura, J. Biol. Chem. 273:19476-19481, 1998) and demonstrate that E. coli Rth is indeed responsible for the maintenance of cell wall rigidity. Our work on the yeast rer2 mutants shows that they are defective in the activity of cis-prenyltransferase, namely, dehydrodolichyl diphosphate synthase, a key enzyme of dolichol synthesis. Taking these data together, we conclude that the RER2 gene family encodes cis-prenyltransferase, which plays an essential role in cell wall biosynthesis in bacteria and in dolichol synthesis in eukaryotic cells and has been well conserved during evolution.     

Interplay between the cis-prenyltransferases and polyprenol reductase in the yeast Saccharomyces cerevisiae

Dolichol formation is examined in three Saccharomyces cerevisiae strains with mutations in the ERG20 gene encoding farnesyl diphosphate synthase (mevalonic acid pathway) and/or the ERG9 gene encoding squalene synthase (sterol synthesis pathway) differing in the amount and chain length of the polyisoprenoids synthesized. Our results suggest that the activities of two yeast cis-prenyltransferases Rer2p and Srt1p and polyprenol reductase are not co-regulated and that reductase may be the rate-limiting enzyme in dolichol synthesis if the amount of polyisoprenoids synthesized exceeds a certain level. We demonstrate that reductase preferentially acts on typical polyprenols with 13-18 isoprene residues but can reduce much longer polyprenols with even 32 isoprene residues.     

Depletion of essential isoprenoids and ER stress induction following acute liver-specific deletion of HMG-CoA reductase

HMG-CoA reductase (Hmgcr) is the rate-limiting enzyme in the mevalonate pathway and is inhibited by statins. In addition to cholesterol, Hmgcr activity is also required for synthesizing nonsterol isoprenoids, such as dolichol, ubiquinone, and farnesylated and geranylgeranylated proteins. Here, we investigated the effects of Hmgcr inhibition on nonsterol isoprenoids in the liver. We have generated new genetic models to acutely delete genes in the mevalonate pathway in the liver using AAV-mediated delivery of Cre-recombinase (AAV-Cre) or CRISPR/Cas9 (AAV-CRISPR). The genetic deletion of Hmgcr by AAV-Cre resulted in extensive hepatocyte apoptosis and compensatory liver regeneration. At the biochemical level, we observed decreased levels of sterols and depletion of the nonsterol isoprenoids, dolichol and ubiquinone. At the cellular level, Hmgcr-null hepatocytes showed ER stress and impaired N-glycosylation. We further hypothesized that the depletion of dolichol, essential for N-glycosylation, could be responsible for ER stress. Using AAV-CRISPR, we somatically disrupted dehydrodolichyl diphosphate synthase subunit (Dhdds), encoding a branch point enzyme required for dolichol biosynthesis. Dhdds-null livers showed ER stress and impaired N-glycosylation, along with apoptosis and regeneration. Finally, the combined deletion of Hmgcr and Dhdds synergistically exacerbated hepatocyte ER stress. Our data show a critical role for mevalonate-derived dolichol in the liver and suggest that dolichol depletion is at least partially responsible for ER stress and apoptosis upon potent Hmgcr inhibition.     

A two‐component enzyme complex is required for dolichol biosynthesis in tomato

Dolichol plays an indispensable role in the N‐glycosylation of eukaryotic proteins. As proteins enter the secretory pathway they are decorated by a ‘glycan’, which is preassembled onto a membrane‐anchored dolichol molecule embedded within the endoplasmic reticulum (ER). Genetic and biochemical evidence in yeast and animals indicate that a cis‐prenyltransferase (CPT) is required for dolichol synthesis, but also point to other factor(s) that could be involved. In this study, RNAi‐mediated suppression of one member of the tomato CPT family (SlCPT3) resulted in a ~60% decrease in dolichol content. We further show that the involvement of SlCPT3 in dolichol biosynthesis requires the participation of a distantly related partner protein, designated as CPT‐binding protein (SlCPTBP), which is a close homolog of the human Nogo‐B receptor. Yeast two‐hybrid and co‐immunoprecipitation assays demonstrate that SlCPT3 and its partner protein interact in vivo and that both SlCPT3 and SlCPTBP are required to complement the growth defects and dolichol deficiency of the yeast dolichol mutant, rer2?. Co‐expression of SlCPT3 and SlCPTBP in yeast and in E. coli confirmed that dolichol synthase activity strictly requires both proteins. Finally, organelle isolation and in vivo localization of fluorescent protein fusions showed that both SlCPT3 and SlCPTBP localize to the ER, the site of dolichol accumulation and synthesis in eukaryotes.     

Mechanism of inhibition of protein glycosylation by the antiviral sugar analogue 2-deoxy-2-fluoro-D-mannose: inhibition of synthesis of man(GlcNAc)2-PP-Dol by the guanosine diphosphate ester

2-Deoxy-2-fluoro-D-mannose (2FMan), an antiviral mannose analogue, inhibited the dolichol cycle of protein glycosylation. To specifically inhibit oligosaccharide-lipid synthesis, and not (viral) protein synthesis in influenza virus infected cells, the addition of guanosine to the 2FMan-treated cells was required. Under these conditions an early step in the assembly of the oligosaccharide-lipid was inhibited, and as a consequence, the glycosylation of proteins was strongly inhibited. Low-molecular-weight, lipid-linked oligosaccharides accumulated in cells treated with 2FMan plus guanosine, although dolichol phosphate (Dol-P) and GDP-Man were still present in the treated cells, and membranes from these cells were not defective in assembly of lipid-linked oligosaccharides. Thus, the presence of a soluble inhibitor of oligosaccharide-lipid assembly in these cells was postulated, and GDP-2FMan and UDP-2FMan, two metabolites found in 2FMan-treated cells, were synthesized and used to study in cell-free systems the inhibition of oligosaccharide-lipid assembly. GDP-2FMan inhibited the synthesis of Man(GlcNAc)2-PP-Dol from (GlcNAc)2-PP-Dol and GDP-Man, and in addition, it caused a trapping of Dol-P as 2FMan-P-Dol, whereas UDP-2FMan only inhibited Glc-P-Dol synthesis. However, it is probable that neither trapping of Dol-P nor inhibition of Glc-P-Dol synthesis by UDP-2FMan contributed to inhibition of protein glycosylation in cells treated with 2FMan. Incorporation of 2FMan from GDP-2FMan or UDP-2FMan into dolichol diphosphate linked oligosaccharides and interference of GDP-2FMan with the latter steps of assembly of the dolichol diphosphate linked oligosaccharide could not be shown.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of bis-(p-nitrophenyl) phosphate on the biosynthesis and the utilization of lipid-intermediates.

Incubations of rat spleen lymphocytes with the required labelled nucleotide sugars lead to the formation of the various lipid-intermediates involved in the N-glycosylation of proteins. The effect of bis-(p-nitrophenyl) phosphate on the different reactions involved in the dolichol pathway has been studied. Although dolichyl phosphate mannose, dolichyl phosphate glucose and dolichyl diphosphate N-acetylglucosamine synthesis is not affected at all by bis-(p-nitrophenyl) phosphate (20 mM), this product inhibits completely the addition of the second N-acetylglucosamine residue on the dolichyl diphosphate N-acetylglucosamine acceptor. The addition of the five innermost mannose residues from GDP-mannose as donor is also strongly abolished. However, the addition of the more distal sugars, i.e. the four mannose residues using dolichyl phosphate mannose as donors and the additional glucose residues are only slightly affected. The reactions involved in the utilization of dolichyl diphosphate oligosaccharide, i.e. transfer to the proteins or degradation into soluble phospho-oligosaccharides, are also strongly inhibited. Thus bis-(p-nitrophenyl) phosphate appears to affect only the reactions involving the presence of dolichyl diphosphate sugar as substrate.     

Alendronate is a specific, nanomolar inhibitor of farnesyl diphosphate synthase

Alendronate, a nitrogen-containing bisphosphonate, is a potent inhibitor of bone resorption used for the treatment and prevention of osteoporosis. Recent findings suggest that alendronate and other N-containing bisphosphonates inhibit the isoprenoid biosynthesis pathway and interfere with protein prenylation, as a result of reduced geranylgeranyl diphosphate levels. This study identified farnesyl disphosphate synthase as the mevalonate pathway enzyme inhibited by bisphosphonates. HPLC analysis of products from a liver cytosolic extract narrowed the potential targets for alendronate inhibition (IC(50) = 1700 nM) to isopentenyl diphosphate isomerase and farnesyl diphosphate synthase. Recombinant human farnesyl diphosphate synthase was inhibited by alendronate with an IC(50) of 460 nM (following 15 min preincubation). Alendronate did not inhibit isopentenyl diphosphate isomerase or GGPP synthase, partially purified from liver cytosol. Recombinant farnesyl diphosphate synthase was also inhibited by pamidronate (IC(50) = 500 nM) and risedronate (IC(50) = 3.9 nM), negligibly by etidronate (IC50 = 80 microM), and not at all by clodronate. In osteoclasts, alendronate inhibited the incorporation of [(3)H]mevalonolactone into proteins of 18-25 kDa and into nonsaponifiable lipids, including sterols. These findings (i) identify farnesyl diphosphate synthase as the selective target of alendronate in the mevalonate pathway, (ii) show that this enzyme is inhibited by other N-containing bisphosphonates, such as risendronate, but not by clodronate, supporting a different mechanism of action for different bisphosphonates, and (iii) document in purified osteoclasts alendronate inhibition of prenylation and sterol biosynthesis.     

A novel family of longer chain length dolichols present in oleate-induced yeast Saccharomyces cerevisiae

The typical size of the yeast dolichol family ranges from 14 to 19 isoprene units D((14-19)) with dolichol(16) being the dominating species. Induction of peroxisome proliferation by growing the cells in medium containing oleate as carbon source induces the synthesis of an additional family of longer dolichols D((19-24)) with D(21) being the most prominent. This phenomenon is abolished in the peroxisome biogenesis deficient strain in which the PEX1 gene (encoding Pex1p peroxin) has been disrupted. The total amount of dolichols in pex1Delta cells is lower than in the wild-type cells, as is the amount of phosphatidylcholine. Moreover, the levels of 3-hydroxy-3-methylglutaryl CoA reductase and farnesyl diphosphate synthase, two key enzymes in dolichol biosynthesis, are decreased in the absence of a functional PEX1 gene. The presence of longer dolichols in oleate-induced Saccharomyces cerevisiae cells, the absence of this additional family in peroxisome deficient cells, and a decrease of the total amount of dolichols in these cells indicate the involvement of peroxisomes in the biosynthesis of dolichols in this organism.     

鼻咽癌下调新基因NOR1相互作用蛋白的筛选和鉴定

NOR1基因是新的鼻咽癌相关基因,该基因在鼻咽癌细胞系HNE1和鼻咽癌组织中表达下调．在鼻咽癌细胞HNE1中恢复NOR1基因表达抑制了鼻咽癌细胞的生长和增殖能力．为了探讨NOR1基因的生物学功能,以NOR1基因为诱饵运用酵母双杂交技术在人胎脑文库中筛选其交互作用蛋白,挑选阳性克隆,进行DNA序列分析和同源检索,阳性克隆编码7个不同的蛋白质,其中一个阳性克隆编码线粒体ATP合成酶亚基OSCP蛋白．瞬时转染pCMV-myc-NOR1质粒进入鼻咽癌5-8F细胞,通过密度梯度离心法分离线粒体蛋白,Western blot检测表明myc-NOR1蛋白分布于线粒体与胞浆．免疫荧光检测表明在鼻咽正常上皮细胞NP69中内源性NOR1蛋白与线粒体存在明显共定位．随后采用特异性酵母双杂交、免疫荧光共定位、免疫共沉淀技术证实了NOR1与OSCP在线粒体内存在交互作用．提示,NOR1是一个新的线粒体蛋白,可能通过结合OSCP蛋白调控细胞能量代谢,为深入探讨其功能提供了重要线索．     

Biosynthesis of Glycoproteins in the Human Pathogenic Fungus Sporothrix Schenckii: Synthesis of Dolichol Phosphate Mannose and Mannoproteins by Membrane-Bound and Solubilized Mannosyl Transferases

A membrane fraction obtained from the filamentous form of Sporothrix schenckii was able to transfer mannose from GDP-Mannose into dolichol phosphate mannose and from this inTermediate into mannoproteins in coupled reactions catalyzed by dolichol phosphate mannose synthase and protein mannosyl transferase(s), respectively. Although the transfer reaction depended on exogenous dolichol monophosphate, membranes failed to use exogenous dolichol phosphate mannose for protein mannosylation to a substantial extent. Over 95% of the sugar was transferred to proteins via dolichol phosphate mannose and the reaction was stimulated several fold by Mg²⁺ and Mn²⁺. Incubation of membranes with detergents such as Brij 35 and Lubrol PX released soluble fractions that transferred the sugar from GDP-Mannose mostly into mannoproteins, which were separated by affinity chromatography on Concanavilin A–Sepharose 4B into lectin-reacting and non-reacting fractions. All proteins mannosylated in vitro eluted with the lectin-reacting proteins and analytical electrophoresis of this fraction revealed the presence of at least nine putative mannoproteins with molecular masses in the range of 26–112 kDa. The experimental approach described here can be used to identify and isolate specific glycoproteins mannosylated in vitro in studies of O-glycosylation.     

Quantitative proteomic analysis of differential proteins in the stroma of nasopharyngeal carcinoma and normal nasopharyngeal epithelial tissue

The importance of stromal cells and the factors that they expressed during cancer initiation and progression have been highlighted by recent literature. To identify the stromal proteins involved in nasopharyngeal carcinoma (NPC) carcinogenesis, we assessed differences in protein expression of the stroma from NPC and normal nasopharyngeal epithelium tissues (NNET) using a quantitative proteomic approach combined with laser capture microdissection (LCM). LCM was performed to purify stromal cells from the NPC and NNET, respectively. The differential proteins between the pooled microdissected tumor and normal stroma were analyzed by two‐dimensional difference gel electrophoresis (2D‐DIGE) combined with mass spectrometry (MS). Twenty differential proteins were identified, and the expression and location of two differential proteins (L ‐plastin and S100A9) were further confirmed by Western blotting and immunohistochemical analysis. Our results will be helpful to study the role of stroma in the NPC carcinogenesis, as well as discover the interaction between NPC cells and their surrounding microenvironment. J. Cell. Biochem. 106: 570–579, 2009. © 2009 Wiley‐Liss, Inc.     

The YTA7 gene is involved in the regulation of the isoprenoid pathway in the yeast Saccharomyces cerevisiae

The isoprenoid pathway in yeasts is important not only for sterol biosynthesis but also for the production of nonsterol molecules, deriving from farnesyl diphosphate (FPP), implicated in N -glycosylation and biosynthesis of heme and ubiquinones. FPP formed from mevalonate in a reaction catalyzed by FPP synthase (Erg20p). In order to investigate the regulation of Erg20p in Saccharomyces cerevisiae , we searched for its protein partners using a two-hybrid screen, and identified five interacting proteins, among them Yta7p. Subsequently, we showed that Yta7p was a membrane-associated protein localized both to the nucleus and to the endoplasmic reticulum. Deletion of YTA7 affected the enzymatic activity of cis -prenyltransferase (the enzyme that utilizes FPP for dolichol biosynthesis) and the cellular levels of isoprenoid compounds. Additionally, it rendered cells hypersensitive to lovastatin, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) that acts upstream of FPP synthase in the isoprenoid pathway. While HMGR is encoded by two genes, HMG1 and HMG2 , only HMG2 overexpression was able to restore growth of the yta7 Δ cells in the presence of lovastatin. Moreover, the expression level of the S. cerevisiae YTA7 gene was altered upon impairment of the isoprenoid pathway not only by lovastatin but also by zaragozic acid, an inhibitor of squalene synthase. Altogether, these results provide substantial evidence of Yta7p involvement in the regulation of isoprenoid biosynthesis.     

Identification of novel mammalian squalene synthase inhibitors using a three-dimensional pharmacophore

Squalene synthase (E.C. 2.5.1.21) catalyses the reductive dimerisation of farnesyl diphosphate in a [1-4] head to head fashion to form squalene, and is the first committed step in cholesterol biosynthesis. Specific inhibitors of squalene synthase would inhibit cholesterol formation and allow production of other important compounds derived from the cholesterol biosynthetic pathway, namely the ubiquinones (co-enzyme Q(10)), dolichol, and would also allow the isoprenylation process of ras by farnesyl-protein transferase. The construction of a hypothetical squalene synthase three-dimensional pharmacophore is presented. It serves as a template for the identification of several new potential classes of inhibitors. The synthesis, anti-microbial and mammalian pig liver squalene synthase activities of analogues based on the bicyclo[3.2.0]heptane and bicyclo[3.3.0]octane ring systems are reported. Analogues of the latter system are pro-drug type inhibitors and exhibit promising biological activity.     

Overexpression of the Saccharomyces cerevisiae RER2 gene in Trichoderma reesei affects dolichol dependent enzymes and protein glycosylation

Protein secretion in Trichoderma reesei could be stimulated by overexpression of the yeast Saccharomyces cerevisiae DPM1 gene encoding dolichyl phosphate mannose synthase (DPMS) a key enzyme in the O-glycosylation pathway. The secreted proteins were glycosylated to the wild type level. On the other hand, the elevated concentration of GDP-mannose, a direct substrate for DPMS, resulting from overexpression in T. reesei of the mpg1 gene coding for guanyltransferase, did not affect secretion of proteins but did affect the degree of their O- and N-glycosylation. In this paper, we examined the effects of dolichol, an indispensable carrier of sugar residues in protein glycosylation, on the synthesis of glycosylated proteins. An increase in dolichol synthesis was obtained by overexpression of the yeast gene encoding cis-prenyltransferase, the first enzyme of the mevalonate pathway committed to dolichol biosynthesis. We observed that, an increased concentration of dolichol resulted in an increased expression of the dpm1 gene and DPMS activity and in overglycosylation of secreted proteins.     

Molecular analysis of prenyl chain elongating enzymes

Multiple alignments of primary structures of many kinds of prenyltransferases that participate in the most fundamental prenyl-chain backbone synthesizing process in isoprenoid biosynthesis showed seven conserved regions in the primary structures of (E)-prenyl diphosphate synthases. However, no information has been available about the structures of (Z)-prenyl diphosphate synthases until our recent isolation of the gene for the undecaprenyl diphosphate synthase of Micrococcus luteus B-P 26. The amino acid sequence of the (Z)-prenyl diphosphate synthase is totally different from those of (E)-prenyl chain elongating enzymes. Protein data base searches for sequences similar to that of the undecaprenyl diphosphate synthase yielded many unknown proteins which have not yet been characterized. Two of the proteins have recently been identified as the undecaprenyl diphosphate synthase of Escherichia coli and the dehydrodolichyl diphosphate synthase of Saccharomyces cerevisiae, indicating that there are three highly conserved regions in the primary structure of (Z)-prenyl chain elongating enzymes.     

Structure of 2-methylisoborneol synthase from Streptomyces coelicolor and implications for the cyclization of a noncanonical C-methylated monoterpenoid substrate

The crystal structure of 2-methylisoborneol synthase (MIBS) from Streptomyces coelicolor A3(2) has been determined in complex with substrate analogues geranyl-S-thiolodiphosphate and 2-fluorogeranyl diphosphate at 1.80 and 1.95 ? resolution, respectively. This terpenoid cyclase catalyzes the cyclization of the naturally occurring, noncanonical C-methylated isoprenoid substrate, 2-methylgeranyl diphosphate, to form the bicyclic product 2-methylisoborneol, a volatile C(11) homoterpene alcohol with an earthy, musty odor. While MIBS adopts the tertiary structure of a class I terpenoid cyclase, its dimeric quaternary structure differs from that previously observed in dimeric terpenoid cyclases from plants and fungi. The quaternary structure of MIBS is nonetheless similar in some respects to that of dimeric farnesyl diphosphate synthase, which is not a cyclase. The structures of MIBS complexed with substrate analogues provide insights regarding differences in the catalytic mechanism of MIBS and the mechanisms of (+)-bornyl diphosphate synthase and endo-fenchol synthase, plant cyclases that convert geranyl diphosphate into products with closely related bicyclic bornyl skeletons, but distinct structures and stereochemistries.