Characterization and localization of a long-chain isoprenyltransferase activity in porcine brain: proposed role in the biosynthesis of dolichyl phosphate

Identification of individual components of a commercial wheat germ acid phosphatase preparation

Wheat germ acid phosphatase (WGAP) is a commercial preparation of partially purified protein commonly used in laboratory settings for non-specific enzymatic dephosphorylation. It is known that these preparations contain multiple phosphatase isozymes and are still relatively crude. This study therefore aimed to identify the protein components of a commercial preparation of wheat germ acid phosphatase using mass spectroscopy and comparative genomics. After one post-purchase purification step, the most prevalent fifteen proteins in the mixture included heat shock proteins, beta-amylases, glucoseribitol dehydrogenases, enolases, and an aminopeptidase. While not among the most abundant components, eight unique dephosphorylation enzymes were also present including three purple acid phosphatases. Furthermore, it is shown that some of these correspond to previously isolated isozymes; one of which has been also previously shown by transcriptome data to be overexpressed in wheat seeds. In summary, this study identified the major components of WGAP including phosphatases and hypothesizes the most active components towards a better understanding of this commonly used laboratory tool.

The enigmatic ATP supply of the endoplasmic reticulum

The endoplasmic reticulum (ER) is a functionally and morphologically complex cellular organelle largely responsible for a variety of crucial functions, including protein folding, maturation and degradation. Furthermore, the ER plays an essential role in lipid biosynthesis, dynamic Ca²⁺ storage, and detoxification. Malfunctions in ER‐related processes are responsible for the genesis and progression of many diseases, such as heart failure, cancer, neurodegeneration and metabolic disorders. To fulfill many of its vital functions, the ER relies on a sufficient energy supply in the form of adenosine‐5′‐triphosphate (ATP), the main cellular energy source. Despite landmark discoveries and clarification of the functional principles of ER‐resident proteins and key ER‐related processes, the mechanism underlying ER ATP transport remains somewhat enigmatic. Here we summarize ER‐related ATP‐consuming processes and outline our knowledge about the nature and function of the ER energy supply.

cis-Prenyltransferase atCPT6 produces a family of very short-chain polyisoprenoids in planta

cis-Prenyltransferases (CPTs) comprise numerous enzymes synthesizing isoprenoid hydrocarbon skeleton with isoprenoid units in the cis (Z) configuration. The chain-length specificity of a particular plant CPT is in most cases unknown despite thecomposition of the accumulated isoprenoids in the tissue of interest being well established. In this report AtCPT6, one of the nine Arabidopsis thaliana CPTs, is shown to catalyze the synthesis of a family of very short-chain polyisoprenoid alcohols of six, seven, and eight isoprenoid units, those of seven units dominating The product specificity of AtCPT6 was established in vivo following its expression in the heterologous system of the yeast Saccharomyces cerevisiae and was confirmed by the absence of specific products in AtCPT6 T-DNA insertion mutants and their overaccumulation in AtCPT6-overexpressing plants. These observations are additionally validated in silico using an AtCPT6 model obtained by homology modeling. AtCPT6 only partially complements the function of the yeast homologue of CPT-Rer2 since it restores the growth but not protein glycosylation in rer2delta yeast.This is the first in planta characterization of specific products of a plant CPT producing polyisoprenoids. Their distribution suggests that a joint activity of several CPTs is required to produce the complex mixture of polyisoprenoid alcohols found in Arabidopsis roots.

Expression of functional bacterial undecaprenyl pyrophosphate synthase in the yeast rer2{Delta} mutant and CHO cells

During evolution the average chain length of polyisoprenoid glycosyl carrier lipids increased from C55 (prokaryotes) to C75 (yeast) to C95 (mammalian cells). In this study, the ability of the E. coli enzyme, undecaprenyl pyrophosphate synthase (UPPS), to complement the loss of the yeast cis-isoprenyltransferase in the rer2Δ mutant was tested to determine if (55)dolichyl phosphate (Dol-P) could functionally substitute in the protein N-glycosylation pathway for (75)Dol-P, the normal isoprenologue synthesized in S. cerevisiae. First, expression of UPPS in the yeast mutant was found to complement the growth and the hypoglycosylation of carboxypeptidase Y defects suggesting that the (55)polyprenyl-P-P intermediate was converted to (55)Dol-P and that (55)Dol-P could effectively substitute for (75)Dol-P in the biosynthesis and function of Man-P-Dol, Glc-P-Dol and Glc(3)Man(9)GlcNAc(2)-P-P-Dol (mature DLO) in the protein N-glycosylation pathway and glycosylphosphatidylinositol anchor assembly. In support of this conclusion, mutant cells expressing UPPS (1) synthesized (55)Dol-P based on MS analysis, (2) utilized (55)Dol-P to form Man-P-(55)Dol in vitro and in vivo, and (3) synthesized N-linked glycoproteins at virtually normal rates as assessed by metabolic labeling with [(3)H]mannose. In addition, an N-terminal GFP-tagged construct of UPPS was shown to localize to the endoplasmic reticulum of Chinese hamster ovary cells. Consistent with the synthesis of (55)Dol-P by the transfected cells, microsomes from the transfected cells synthesized the [(14)C](55)polyprenyl-P-P intermediate when incubated with [(14)C]isopentenyl pyrophosphate and [(3)H]Man-P-(55)Dol when incubated with GDP-[(3)H]Man. These results indicate that (C55)polyisoprenoid chains, significantly shorter than the natural glycosyl carrier lipid, can function in the transbilayer movement of DLOs in the endoplasmic reticulum of yeast and mammalian cells, and that conserved sequences in the cis-isoprenyltransferases are recognized by, yet to be identified, binding partners in the endoplasmic reticulum of mammalian cells.

Structure and synthesis of polyisoprenoids used in N-glycosylation across the three domains of life

N-linked protein glycosylation was originally thought to be specific to eukaryotes, but evidence of this post-translational modification has now been discovered across all domains of life: Eucarya, Bacteria, and Archaea. In all cases, the glycans are first assembled in a step-wise manner on a polyisoprenoid carrier lipid. At some stage of lipid-linked oligosaccharide synthesis, the glycan is flipped across a membrane. Subsequently, the completed glycan is transferred to specific asparagine residues on the protein of interest. Interestingly, though the N-glycosylation pathway seems to be conserved, the biosynthetic pathways of the polyisoprenoid carriers, the specific structures of the carriers, and the glycan residues added to the carriers vary widely. In this review we will elucidate how organisms in each basic domain of life synthesize the polyisoprenoids that they utilize for N-linked glycosylation and briefly discuss the subsequent modifications of the lipid to generate a lipid-linked oligosaccharide.

A defect in dolichol phosphate biosynthesis causes a new inherited disorder with death in early infancy

The following study describes the discovery of a new inherited metabolic disorder, dolichol kinase (DK1) deficiency. DK1 is responsible for the final step of the de novo biosynthesis of dolichol phosphate. Dolichol phosphate is involved in several glycosylation reactions, such as N-glycosylation, glycosylphosphatidylinositol (GPI)-anchor biosynthesis, and C- and O-mannosylation. We identified four patients who were homozygous for one of two mutations (c.295T-->A [99Cys-->Ser] or c.1322A-->C [441Tyr-->Ser]) in the corresponding hDK1 gene. The residual activity of mutant DK1 was 2%-4% when compared with control cells. The mutated alleles failed to complement the temperature-sensitive phenotype of DK1-deficient yeast cells, whereas the wild-type allele restored the normal growth phenotype. Affected patients present with a very severe clinical phenotype, with death in early infancy. Two of the patients died from dilative cardiomyopathy.

Decaprenyl diphosphate synthesis in Mycobacterium tuberculosis

Z-prenyl diphosphate synthases catalyze the sequential condensation of isopentenyl diphosphate with allylic diphosphates to synthesize polyprenyl diphosphates. In mycobacteria, these are precursors of decaprenyl phosphate, a molecule which plays a central role in the biosynthesis of essential mycobacterial cell wall components, such as the mycolyl-arabinogalactan-peptidoglycan complex and lipoarabinomannan. Recently, it was demonstrated that open reading frame Rv2361c of the Mycobacterium tuberculosis H37Rv genome encodes a unique prenyl diphosphate synthase (M. C. Schulbach, P. J. Brennan, and D. C. Crick, J. Biol. Chem. 275:22876-22881, 2000). We have now purified the enzyme to near homogeneity by using an Escherichia coli expression system and have shown that the product of this enzyme is decaprenyl diphosphate. Rv2361c has an absolute requirement for divalent cations and an optimal pH range of 7.5 to 8.5, and the activity is stimulated by both detergent and dithiothreitol. The enzyme catalyzes the addition of isopentenyl diphosphate to geranyl diphosphate, neryl diphosphate, omega,E,E-farnesyl diphosphate, omega,E,Z-farnesyl diphosphate, or omega,E,E,E-geranylgeranyl diphosphate, with Km values for the allylic substrates of 490, 29, 84, 290, and 40 microM, respectively. The Km value for isopentenyl diphosphate is 89 microM. The catalytic efficiency is greatest when omega,E,Z-farnesyl diphosphate is used as the allylic acceptor, suggesting that this is the natural substrate in vivo, a conclusion that is supported by previous structural studies of decaprenyl phosphoryl mannose isolated from M. tuberculosis. This is the first report of a bacterial Z-prenyl diphosphate synthase that preferentially utilizes an allylic diphosphate primer having the alpha-isoprene unit in the Z configuration, indicating that Rv1086 (omega,E,Z-farnesyl diphosphate synthase) and Rv2361c act sequentially in the biosynthetic pathway that leads to the formation of decaprenyl phosphate in M. tuberculosis.

Identification and characterization of a cDNA encoding a long-chain cis-isoprenyltranferase involved in dolichyl monophosphate biosynthesis in the ER of brain cells

A long-chain cis-isoprenyltransferase (cis-IPTase) located in the endoplasmic reticulum (ER) catalyzes the chain elongation stage in the pathway for the de novo biosynthesis of dolichyl monophosphate (Dol-P) in eukaryotic cells. In Saccharomyces cerevisiae, the ER-associated cis-IPTase is encoded by the RER2 gene. Mutations in the RER2 gene result in defects in growth and protein N-glycosylation. In this study a cDNA isolated from human brain (Accession No. AK023164.1), which has substantial homology to cis-IPTases from bacteria, Arabidopsis, and S. cerevisiae, has been shown to: (1) complement the growth defect; (2) restore cis-IPTase activity; dolichol and Dol-P synthesis; and (3) restore normal N-glycosylation of carboxypeptidase Y (CPY) in the yeast rer2Delta mutant. Consistent with a role in Dol-P biosynthesis, overexpression of the human cis-isoprenyltransferase (hCIT) cDNA also suppresses the temperature-sensitive growth and CPY hypoglycosylation phenotypes in sec59-1 cells which are defective in Dol-P biosynthesis due to a temperature-sensitive mutation in dolichol kinase. Overexpression of hCIT in Chinese hamster ovary (CHO) cells results in a modest increase in cis-IPTase activity associated with microsomal fractions and the appearance of a new 38kDa polypeptide that co-localizes with calnexin in the ER, the site of Dol-P biosynthesis, even though no transmembrane domains are predicted by a hydropathy plot.

The CWH8 gene encodes a dolichyl pyrophosphate phosphatase with a luminally oriented active site in the endoplasmic reticulum of Saccharomyces cerevisiae

Mutations in the CWH8 gene, which encodes an ER transmembrane protein with a phosphate binding pocket in Saccharomyces cerevisiae, result in a deficiency in dolichyl pyrophosphate (Dol-P-P)-linked oligosaccharide intermediate synthesis and protein N-glycosylation (van Berkel, M. A., Rieger, M., te Heesen, S., Ram, A. F., van den Ende, H., Aebi, M., and Klis, F. M. (1999) Glycobiology 9, 243-253). Genetic, enzymological, and topological approaches were taken to investigate the potential role of Cwh8p in Dol-P-P/Dol-P metabolism. Overexpression of Cwh8p in the yeast double mutant strain, lacking LPP1/DPP1, resulted in an impressive increase in Dol-P-P phosphatase activity, a relatively small increase in Dol-P phosphatase activity, but no change in phosphatidate (PA) phosphatase activity in microsomal fractions. The Dol-P-P phosphatase encoded by CWH8 is optimally active in the presence of 0.5% octyl glucoside and relatively unstable in Triton X-100, distinguishing this activity from the lipid phosphatases encoded by LPP1 and DPP1. Stoichiometric amounts of P(i) and Dol-P are formed during the enzymatic reaction indicating that Cwh8p cleaves the anhydride linkage in Dol-P-P. Membrane fractions from Sf-9 cells expressing Cwh8p contained a 30-fold higher level of Dol-P-P phosphatase activity, a slight increase in Dol-P phosphatase activity, but no increase in PA phosphatase relative to controls. This is the first report of a lipid phosphatase that hydrolyzes Dol-P-P/Dol-P but not PA. In accord with this enzymatic function, Dol-P-P accumulated in cells lacking the Dol-P-P phosphatase. Topological studies using different approaches indicate that Cwh8p is a transmembrane protein with a luminally oriented active site. The specificity, subcellular location, and topological orientation of this novel enzyme are consistent with a role in the re-utilization of the glycosyl carrier lipid for additional rounds of lipid intermediate biosynthesis after its release during protein N-glycosylation reactions.

The ins(ide) and out(side) of dolichyl phosphate biosynthesis and recycling in the endoplasmic reticulum

The precursor oligosaccharide donor for protein N-glycosylation in eukaryotes, Glc3Man9GlcNAc(2)-P-P-dolichol, is synthesized in two stages on both leaflets of the rough endoplasmic reticulum (ER). There is good evidence that the level of dolichyl monophosphate (Dol-P) is one rate-controlling factor in the first stage of the assembly process. In the current topological model it is proposed that ER proteins (flippases) then mediate the transbilayer movement of Man-P-Dol, Glc-P-Dol, and Man5GlcNAc(2)-P-P-Dol from the cytoplasmic leaflet to the lumenal leaflet. The rate of flipping of the three intermediates could plausibly influence the conversion of Man5GlcNAc(2)-P-P-Dol to Glc3Man(9)GlcNAc(2)-P-P-Dol in the second stage on the lumenal side of the rough ER. This article reviews the current understanding of the enzymes involved in the de novo biosynthesis of Dol-P and other polyisoprenoid glycosyl carrier lipids and speculates about the role of membrane proteins and enzymes that could be involved in the transbilayer movement of the lipid intermediates and the recycling of Dol-P and Dol-P-P discharged during glycosylphosphatidylinositol anchor biosynthesis, N-glycosylation, and O- and C-mannosylation reactions on the lumenal surface of the rough ER.

Purification, enzymatic characterization, and inhibition of the Z-farnesyl diphosphate synthase from Mycobacterium tuberculosis

We have recently shown that open reading frame Rv1086 of the Mycobacterium tuberculosis H37Rv genome sequence encodes a unique isoprenyl diphosphate synthase. The product of this enzyme, omega,E,Z-farnesyl diphosphate, is an intermediate for the synthesis of decaprenyl phosphate, which has a central role in the biosynthesis of most features of the mycobacterial cell wall, including peptidoglycan, arabinan, linker unit galactan, and lipoarabinomannan. We have now purified Z-farnesyl diphosphate synthase to near homogeneity using a novel mycobacterial expression system. Z-Farnesyl diphosphate synthase catalyzed the addition of isopentenyl diphosphate to omega,E-geranyl diphosphate or omega,Z-neryl diphosphate yielding omega,E,Z-farnesyl diphosphate and omega,Z,Z-farnesyl diphosphate, respectively. The enzyme has an absolute requirement for a divalent cation, an optimal pH range of 7-8, and K(m) values of 124 micrometer for isopentenyl diphosphate, 38 micrometer for geranyl diphosphate, and 16 micrometer for neryl diphosphate. Inhibitors of the Z-farnesyl diphosphate synthase were designed and chemically synthesized as stable analogs of omega,E-geranyl diphosphate in which the labile diphosphate moiety was replaced with stable moieties. Studies with these compounds revealed that the active site of Z-farnesyl diphosphate synthase differs substantially from E-farnesyl diphosphate synthase from pig brain (Sus scrofa).

An alternative cis-isoprenyltransferase activity in yeast that produces polyisoprenols with chain lengths similar to mammalian dolichols

Dolichyl monophosphate (Dol-P) is a polyisoprenoid glycosyl carrier lipid essential for the assembly of a variety of glycoconjugates in the endoplasmic reticulum of eukaryotic cells. In yeast, dolichols with chain lengths of 14--17 isoprene units are predominant, whereas in mammalian cells they contain 19--22 isoprene units. In this biosynthetic pathway, t,t-farnesyl pyrophosphate is elongated to the appropriate long chain polyprenyl pyrophosphate by the sequential addition of cis-isoprene units donated by isopentenyl pyrophosphate with t,t,c-geranylgeranyl pyrophosphate being the initial intermediate formed. The condensation steps are catalyzed by cis-isoprenyltransferase (cis-IPTase). Genes encoding cis-IPTase activity have been identified in Micrococcus luteus, Escherichia coli, Arabidopsis thaliana, and Saccharomyces cerevisiae (RER2). Yeast cells deleted for the RER2 locus display a severe growth defect, but are still viable, possibly due to the activity of an homologous locus, SRT1. The dolichol and Dol-P content of exponentially growing revertants of RER2 deleted cells (Delta rer2) and of cells overexpressing SRT1 have been determined by HPLC analysis. Dolichols and Dol-Ps with 19--22 isoprene units, unusually long for yeast, were found, and shown to be utilized for the biosynthesis of lipid intermediates involved in protein N-glycosylation. In addition, cis-IPTase activity in microsomes from Delta rer2 cells overexpressing SRT1 was 7- to 17-fold higher than in microsomes from Delta rer2 cells. These results establish that yeast contains at least two cis-IPTases, and indicate that the chain length of dolichols is determined primarily by the enzyme catalyzing the chain elongation stage of the biosynthetic process.

Polyprenyl phosphate biosynthesis in Mycobacterium tuberculosis and Mycobacterium smegmatis

Mycobacterium smegmatis has been shown to contain two forms of polyprenyl phosphate (Pol-P), while Mycobacterium tuberculosis contains only one. Utilizing subcellular fractions from M. smegmatis and M. tuberculosis, we show that Pol-P synthesis is different in these species. The specific activities of the prenyl diphosphate synthases in M. tuberculosis are 10- to 100-fold lower than those in M. smegmatis. In M. smegmatis decaprenyl diphosphate and heptaprenyl diphosphate were the main products synthesized in vitro, whereas in M. tuberculosis only decaprenyl diphosphate was synthesized. The data from both organisms suggest that geranyl diphosphate is the allylic substrate for two distinct prenyl diphosphate synthases, one located in the cell membrane that synthesizes omega,E,Z-farnesyl diphosphate and the other present in the cytosol that synthesizes omega,E,E,E-geranylgeranyl diphosphate. In M. smegmatis, the omega,E, Z-farnesyl diphosphate is utilized by a membrane-associated prenyl diphosphate synthase activity to generate decaprenyl diphosphate, and the omega,E,E,E-geranylgeranyl diphosphate is utilized by a membrane-associated activity for the synthesis of the heptaprenyl diphosphate. In M. tuberculosis, however, omega,E,E,E-geranylgeranyl diphosphate is not utilized for the synthesis of heptaprenyl diphosphate. Thus, the difference in the compositions of the Pol-P of M. smegmatis and M. tuberculosis can be attributed to distinct enzymatic differences between these two organisms.

Identification of a short (C15) chain Z-isoprenyl diphosphate synthase and a homologous long (C50) chain isoprenyl diphosphate synthase in Mycobacterium tuberculosis

We report the cloning, overexpression, and partial characterization of two unique Z-isoprenyl diphosphate synthase homologs from Mycobacterium tuberculosis. The first enzyme, Rv1086, adds one isoprene unit to omega,E-geranyl diphosphate. The product, omega,E, Z-farnesyl diphosphate, is the putative substrate of the second enzyme, Rv2361c. This enzyme adds seven more isoprene units to omega, E,Z-farnesyl diphosphate and releases decaprenyl diphosphate. Both open reading frames were cloned from the M. tuberculosis H37Rv genome and overexpressed in M. smegmatis. Membrane and cytosol fractions from wild type and the two recombinant strains were assayed for [(14)C]isopentenyl diphosphate incorporation into isoprenyl diphosphates in the presence of various allylic isoprenyl diphosphate acceptors. Membrane fractions of recombinant cells overexpressing Rv2361c incubated with farnesyl diphosphate showed a 10-fold increase of [(14)C]isopentenyl diphosphate incorporation into decaprenyl diphosphate. Membrane fractions of recombinant cells overexpressing Rv1086 incubated with geranyl diphosphate showed a 5-fold increase of [(14)C]isopentenyl diphosphate incorporation into farnesyl diphosphate. Analysis of the stereochemistry revealed that all of the overexpressed farnesyl diphosphate was in the omega,E, Z-configuration. This is the first description of a short chain isoprenyl diphosphate synthase that generates products with Z-stereochemistry. Previously, all known short chain isoprenyl diphosphate synthases catalyze the synthesis of products with E-stereochemistry.

The dolichol pathway of N-linked glycosylation

The oligosaccharide substrate for the N-linked protein glycosylation is assembled at the membrane of the endoplasmic reticulum. Dolichyl pyrophosphate serves as a carrier in this biosynthetic pathway. In this review, we discuss the function of the lipid carrier dolichol in oligosaccharide assembly and give an overview of the biosynthesis of the different sugar donors required for the building of the oligosaccharide. Yeast genetic techniques have made it possible to identify many different loci encoding specific glycosyltransferases required for the precise and ordered assembly of the dolichyl pyrophosphate-linked oligosaccharide. Based on the knowledge obtained from studying this pathway in yeast, we compare it to the process of N-linked protein glycosylation in archaea. We suggest that N-linked glycosylation in eukaryotes and in archaea share a common evolutionary origin.

Purification and characterization of a polyisoprenyl phosphate phosphatase from pig brain. Possible dual specificity

Microsomal fractions from pig and calf brain catalyze the enzymatic dephosphorylation of endogenous and exogenous dolichyl monophosphate (Dol-P) (Sumbilla, C. A., and Waechter, C. J. (1985) Methods Enzymol. 111, 471-482). The Dol-P phosphatase (EC 3.1.3.51) has been solubilized by extracting pig brain microsomes with the nonionic detergent Nonidet P-40 and purified approximately 1,107-fold by a combination of anion exchange chromatography, polyethylene glycol fractionation, dye-ligand chromatography, and wheat germ agglutinin affinity chromatography. Treatment of the enzyme with neuraminidase prevented binding to wheat germ agglutinin-Sepharose, indicating the presence of one or more N-acetylneuraminyl residues per molecule of enzyme. When the highly purified polyisoprenyl phosphate phosphatase was analyzed by SDS-polyacrylamide gel electrophoresis, a major 33-kDa polypeptide was observed. Enzymatic dephosphorylation of Dol-P by the purified phosphatase was 1) optimal at pH 7; 2) potently inhibited by F-, orthovanadate, and Zn2+ > Co2+ > Mn2+ but unaffected by Mg2+; 3) exhibited an approximate Km for C95-Dol-P of 45 microM; and 4) was sensitive to N-ethylmaleimide, phenylglyoxal, and diethylpyrocarbonate. The pig brain phosphatase did not dephosphorylate glucose 6-phosphate, mannose 6-phosphate, 5'-AMP, or p-nitrophenylphosphate, but it dephosphorylated dioleoyl-phosphatidic acid at initial rates similar to those determined for Dol-P. Based on the virtually identical sensitivity of Dol-P and phosphatidic acid dephosphorylation by the highly purified enzyme to N-ethylmaleimide, F-, phenylglyoxal, and diethylpyrocarbonate, both substrates appear to be hydrolyzed by a single enzyme with an apparent dual specificity. This is the first report of the purification of a neutral Dol-P phosphatase from mammalian tissues. Although the enzyme is Mg2+-independent and capable of dephosphorylating Dol-P and PA, several enzymological properties distinguish this lipid phosphomonoesterase from PAP2.

Synthesis of dolichol in a polyprenol reductase mutant is restored by elevation of cis-prenyl transferase activity

CHB11-1-3 is a glycosylation mutant of Chinese hamster ovary (CHO) cells, isolated by screening mutagenized cells for those with decreased intracellular lysosomal enzyme activity [C. W. Hall et al. (1986) Mol. Cell. Biochem. 72, 35-45]. CHB11-1-3 synthesizes the lipid polyprenol, the metabolic precursor of dolichol, rather than dolichol, indicating a defect in polyprenol reductase. This defect was demonstrated previously in Lec9 CHO mutants, and cell fusion experiments confirmed that CHB11-1-3 is a member of this complementation group. A revertant of CHB11-1-3, CHBREV, isolated for its ability to grow at 39 degrees C, synthesizes dolichol at near-normal levels. CHBREV is probably a second-site revertant, because it synthesizes three to four times as much polyprenol as CHB11-1-3 and exhibits a similar elevation in the specific activity of cis-prenyl transferase. This higher activity appears to reflect an increase in enzyme molecules rather than the presence of an activator or absence of an inhibitor. These results suggest that CHB11-1-3 is a "K(m)" mutant, because synthesis of higher amounts of the substrate of polyprenol reductase obviates the defect.

Modulations in hepatic branch-point enzymes involved in isoprenoid biosynthesis upon dietary and drug treatments of rats

Three branch-point enzymes of the mevalonate pathway, farnesyl pyrophosphate synthase, cis-prenyltransferase and squalene synthase were characterized in rat hepatic cytosol, microsomes and peroxisomes isolated from rats after treatment with peroxisome proliferators, inducers of the endoplasmic reticulum or modulators of lipid metabolism. Cholestyramine and phenobarbital induced primarily the cytosolic farnesyl pyrophosphate synthase, whereas clofibrate and phthalates elevated the corresponding peroxisomal activity. cis-Prenyltransferase activities in microsomes were induced 4-5-fold after clofibrate, phthalate and phenobarbital administration, but these same treatments affected the peroxisomal activity to only a limited extent. Squalene synthase activity in microsomes was completely abolished, but the peroxisomal activity was unaffected after administration of cholesterol. On the other hand, clofibrate and phthalate induced only the microsomal activities. Mevinolin treatment greatly increased peroxisomal and cytosolic farnesyl pyrophosphate synthase activities, but not the mitochondrial activity, and the cis-prenyltransferase activities were elevated in peroxisomes, but not in microsomes. These results demonstrate that the branch-point enzymes in cholesterol and dolichol biosynthesis at various cellular locations are regulated differentially and that the capacities of peroxisomes and the endoplasmic reticulum to participate in the synthesis of polyisoprenoid lipids is affected profoundly by treatment with different xenobiotics.

Long-chain cis-isoprenyltransferase activity is induced early in the developmental program for protein N-glycosylation in embryonic rat brain cells

A large developmental increase in Glc3Man9-GlcNAc2-P-P-dolichol (Oligo-P-P-Dol) synthesis and protein N-glycosylation in primary cultures of embryonic rat brain cells has been reported previously. In vitro enzyme studies and metabolic labeling experiments now show that there is a coordinate induction of long-chain cis-isoprenyltransferase (IPTase) activity, an activity required for the chain-elongation stage of dolichyl monophosphate (Dol-P) biosynthesis de novo, and Oligo-P-P-Dol biosynthesis in embryonic rat brain. Different developmental patterns were observed for IPTase and beta-hydroxy-beta-methyl-glutaryl-CoA (HMG-CoA) reductase activity as well as Dol-P and cholesterol biosynthesis, indicating that these pathways are regulated independently in rat brain. Three separate experimental approaches provide evidence that the amount of Dol-P available in the rough endoplasmic reticulum (RER) is a rate-limiting factor in the expression of the lipid intermediate pathway. First, metabolic labeling experiments show that the biosynthesis of Dol-P is induced at the same time or just prior to the induction of Oligo-P-P-Dol biosynthesis. Second, the time of induction and rate of Oligo-P-P-Dol synthesis are accelerated when Dol-P is supplemented in the culture medium. Third, in vitro assays of mannosylphosphoryldolichol synthase and N-acetylglucosaminylpyrophosphoryldolichol synthase indicate that there are only minor increases in the levels of these enzymes during development, but the amount of endogenous Dol-P in the RER that is accessible to the glycosyltransferases increases when IPTase activity is induced. In summary, the current studies with embryonic rat brain cells document the coordinate induction of IPTase activity and Oligo-P-P-Dol synthesis, support the hypothesis that the availability of Dol-P in the RER is one rate-limiting factor in Oligo-P-P-Dol synthesis, and strongly suggest that increases in IPTase activity and the rate of de novo Dol-P biosynthesis enhance the capacity of embryonic rat brain cells for lipid intermediate synthesis early in the developmental program for N-linked glycoprotein biosynthesis.