ALG1-CDG: Clinical and Molecular Characterization of 39 Unreported Patients

Congenital disorders of glycosylation (CDG) arise from pathogenic mutations in over 100 genes leading to impaired protein or lipid glycosylation. ALG1 encodes a β1,4 mannosyltransferase that catalyzes the addition of the first of nine mannose moieties to form a dolichol-lipid linked oligosaccharide intermediate required for proper N-linked glycosylation. ALG1 mutations cause a rare autosomal recessive disorder termed ALG1-CDG. To date 13 mutations in 18 patients from 14 families have been described with varying degrees of clinical severity. We identified and characterized 39 previously unreported cases of ALG1-CDG from 32 families and add 26 new mutations. Pathogenicity of each mutation was confirmed based on its inability to rescue impaired growth or hypoglycosylation of a standard biomarker in an alg1-deficient yeast strain. Using this approach we could not establish a rank order comparison of biomarker glycosylation and patient phenotype, but we identified mutations with a lethal outcome in the first two years of life. The recently identified protein-linked xeno-tetrasaccharide biomarker, NeuAc-Gal-GlcNAc2 , was seen in all 27 patients tested. Our study triples the number of known patients and expands the molecular and clinical correlates of this disorder.

Novel Citronellyl-Based Photoprobes Designed to Identify ER Proteins Interacting with Dolichyl Phosphate in Yeast and Mammalian Cells

BACKGROUND

Dolichyl phosphate-linked mono- and oligosaccharides (DLO) are essential intermediates in protein N-glycosylation, C- and O-mannosylation and GPI anchor biosynthesis. While many membrane proteins in the endoplasmic reticulum (ER) involved in the assembly of DLOs are known, essential proteins believed to be required for the transbilayer movement (flip-flopping) and proteins potentially involved in the regulation of DLO synthesis remain to be identified.

METHODS

The synthesis of a series of Dol-P derivatives composed of citronellyl-based photoprobes with benzophenone groups equipped with alkyne moieties for Huisgen "click" chemistry is now described to utilize as tools for identifying ER proteins involved in regulating the biosynthesis and transbilayer movement of lipid intermediates. In vitro enzymatic assays were used to establish that the photoprobes contain the critical structural features recognized by pertinent enzymes in the dolichol pathway. ER proteins that photoreacted with the novel probes were identified by MS.

RESULTS

The potential of the newly designed photoprobes, m-PAL-Cit-P and p-PAL-Cit-P, for identifying previously unidentified Dol-P-interacting proteins is supported by the observation that they are enzymatically mannosylated by Man-P-Dol synthase (MPDS) from Chinese Hamster Ovary (CHO) cells at an enzymatic rate similar to that for Dol-P. MS analyses reveal that DPM1, ALG14 and several other yeast ER proteins involved in DLO biosynthesis and lipid-mediated protein O-mannosylation photoreacted with the novel probes.

CONCLUSION

The newly-designed photoprobes described in this paper provide promising new tools for the identification of yet to be identified Dol-P interacting ER proteins in yeast and mammalian cells, including the Dol-P flippase required for the "re-cycling" of the glycosyl carrier lipid from the lumenal monolayer of the ER to the cytoplasmic leaflet for new rounds of DLO synthesis.

Congenital disorder of glycosylation due to DPM1 mutations presenting with dystroglycanopathy-type congenital muscular dystrophy

Congenital disorders of glycosylation (CDG) are rare genetic defects mainly in the post-translational modification of proteins via attachment of carbohydrate chains. We describe an infant with the phenotype of a congenital muscular dystrophy, with borderline microcephaly, hypotonia, camptodactyly, severe motor delay, and elevated creatine kinase. Muscle biopsy showed muscular dystrophy and reduced α-dystroglycan immunostaining with glycoepitope-specific antibodies in a pattern diagnostic of dystroglycanopathy. Carbohydrate deficient transferrin testing showed a pattern pointing to a CDG type I. Sanger sequencing of DPM1 (dolichol-P-mannose synthase subunit 1) revealed a novel Gly > Val change c.455G > T missense mutation resulting in p.Gly152Val) of unknown pathogenicity and deletion/duplication analysis revealed an intragenic deletion from exons 3 to 7 on the other allele. DPM1 activity in fibroblasts was reduced by 80%, while affinity for the substrate was not depressed, suggesting a decrease in the amount of active enzyme. Transfected cells expressing tagged versions of wild type and the p.Gly152Val mutant displayed reduced binding to DPM3, an essential, non-catalytic subunit of the DPM complex, suggesting a mechanism for pathogenicity. The present case is the first individual described with DPM1-CDG (CDG-Ie) to also have clinical and muscle biopsy findings consistent with dystroglycanopathy.

Nogo-B receptor is necessary for cellular dolichol biosynthesis and protein N-glycosylation

Dolichol monophosphate (Dol-P) functions as an obligate glycosyl carrier lipid in protein glycosylation reactions. Dol-P is synthesized by the successive condensation of isopentenyl diphosphate (IPP), with farnesyl diphosphate catalysed by a cis-isoprenyltransferase (cis-IPTase) activity. Despite the recognition of cis-IPTase activity 40 years ago and the molecular cloning of the human cDNA encoding the mammalian enzyme, the molecular machinery responsible for regulating this activity remains incompletely understood. Here, we identify Nogo-B receptor (NgBR) as an essential component of the Dol-P biosynthetic machinery. Loss of NgBR results in a robust deficit in cis-IPTase activity and Dol-P production, leading to diminished levels of dolichol-linked oligosaccharides and a broad reduction in protein N-glycosylation. NgBR interacts with the previously identified cis-IPTase hCIT, enhances hCIT protein stability, and promotes Dol-P production. Identification of NgBR as a component of the cis-IPTase machinery yields insights into the regulation of dolichol biosynthesis.

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.

A novel epimerase that converts GlcNAc-P-P-undecaprenol to GalNAc-P-P-undecaprenol in Escherichia coli O157

Escherichia coli strain O157 produces an O-antigen with the repeating tetrasaccharide unit alpha-D-PerNAc-alpha-l-Fuc-beta-D-Glc-alpha-D-GalNAc, preassembled on undecaprenyl pyrophosphate (Und-P-P). These studies were conducted to determine whether the biosynthesis of the lipid-linked repeating tetrasaccharide was initiated by the formation of GalNAc-P-P-Und by WecA. When membrane fractions from E. coli strains K12, O157, and PR4019, a WecA-overexpressing strain, were incubated with UDP-[3H]GalNAc, neither the enzymatic synthesis of [3H]GlcNAc-P-P-Und nor [3H]GalNAc-P-P-Und was detected. However, when membrane fractions from strain O157 were incubated with UDP-[3H]GlcNAc, two enzymatically labeled products were observed with the chemical and chromatographic properties of [3H]GlcNAc-P-P-Und and [3H]GalNAc-P-P-Und, suggesting that strain O157 contained an epimerase capable of interconverting GlcNAc-P-P-Und and GalNAc-P-P-Und. The presence of a novel epimerase was demonstrated by showing that exogenous [3H]GlcNAc-P-P-Und was converted to [3H]GalNAc-P-P-Und when incubated with membranes from strain O157. When strain O157 was metabolically labeled with [3H]GlcNAc, both [3H]GlcNAc-P-P-Und and [3H]GalNAc-P-P-Und were detected. Transformation of E. coli strain 21546 with the Z3206 gene enabled these cells to synthesize GalNAc-P-P-Und in vivo and in vitro. The reversibility of the epimerase reaction was demonstrated by showing that [3H]GlcNAc-P-P-Und was reformed when membranes from strain O157 were incubated with exogenous [3H]GalNAc-P-P-Und. The inability of Z3206 to complement the loss of the gne gene in the expression of the Campylobacter jejuni N-glycosylation system in E. coli indicated that it does not function as a UDP-GlcNAc/UDP-GalNAc epimerase. Based on these results, GalNAc-P-P-Und is synthesized reversibly by a novel GlcNAc-P-P-Und epimerase after the formation of GlcNAc-P-P-Und by WecA in E. coli O157.

Suppression of Rft1 expression does not impair the transbilayer movement of Man5GlcNAc2-P-P-dolichol in sealed microsomes from yeast

To further evaluate the role of Rft1 in the transbilayer movement of Man(5)GlcNAc(2)-P-P-dolichol (M5-DLO), a series of experiments was conducted with intact cells and sealed microsomal vesicles. First, an unexpectedly large accumulation (37-fold) of M5-DLO was observed in Rft1-depleted cells (YG1137) relative to Glc(3)Man(9)GlcNAc(2)-P-P-Dol in wild type (SS328) cells when glycolipid levels were compared by fluorophore-assisted carbohydrate electrophoresis analysis. When sealed microsomes from wild type cells and cells depleted of Rft1 were incubated with GDP-[(3)H]mannose or UDP-[(3)H]GlcNAc in the presence of unlabeled GDP-Man, no difference was observed in the rate of synthesis of [(3)H]Man(9)GlcNAc(2)-P-P-dolichol or Man(9)[(3)H]GlcNAc(2)-P-P-dolichol, respectively. In addition, no difference was seen in the level of M5-DLO flippase activity in sealed wild type and Rft1-depleted microsomal vesicles when the activity was assessed by the transport of GlcNAc(2)-P-P-Dol(15), a water-soluble analogue. The entry of the analogue into the lumenal compartment was confirmed by demonstrating that [(3)H]chitobiosyl units were transferred to endogenous peptide acceptors via the yeast oligosaccharyltransferase when sealed vesicles were incubated with [(3)H]GlcNAc(2)-P-P-Dol(15) in the presence of an exogenously supplied acceptor peptide. In addition, several enzymes involved in Dol-P and lipid intermediate biosynthesis were found to be up-regulated in Rft1-depleted cells. All of these results indicate that although Rft1 may play a critical role in vivo, depletion of this protein does not impair the transbilayer movement of M5-DLO in sealed microsomal fractions prepared from disrupted cells.

Recycling of dolichyl monophosphate to the cytoplasmic leaflet of the endoplasmic reticulum after the cleavage of dolichyl pyrophosphate on the lumenal monolayer

During protein N-glycosylation, dolichyl pyrophosphate (Dol-P-P) is discharged in the lumenal monolayer of the endoplasmic reticulum (ER). Dol-P-P is then cleaved to Dol-P by Dol-P-P phosphatase (DPPase). Studies with the yeast mutant cwh8Delta, lacking DPPase activity, indicate that recycling of Dol-P produced by DPPase contributes significantly to the pool of Dol-P utilized for lipid intermediate biosynthesis on the cytoplasmic leaflet. Whether Dol-P formed in the lumen diffuses directly back to the cytoplasmic leaflet or is first dephosphorylated to dolichol has not been determined. Incubation of sealed ER vesicles from calf brain with acetyl-Asn-Tyr-Thr-NH(2), an N-glycosylatable peptide, to generate Dol-P-P in the lumenal monolayer produced corresponding increases in the rates of Man-P-Dol, Glc-P-Dol, and GlcNAc-P-P-Dol synthesis in the absence of CTP. No changes in dolichol kinase activity were observed. When streptolysin-O permeabilized CHO cells were incubated with an acceptor peptide, N-glycopeptide synthesis, requiring multiple cycles of the dolichol pathway, occurred in the absence of CTP. The results obtained with sealed microsomes and CHO cells indicate that Dol-P, formed from Dol-P-P, returns to the cytoplasmic leaflet where it can be reutilized for lipid intermediate biosynthesis, and dolichol kinase is not required for recycling. It is possible that the flip-flopping of the carrier lipid is mediated by a flippase, which would provide a mechanism for the recycling of Dol-P derived from Man-P-Dol-mediated reactions in N-, O-, and C-mannosylation of proteins, GPI anchor assembly, and the three Glc-P-Dol-mediated reactions in Glc(3)Man(9)GlcNAc(2)-P-P-Dol (DLO) biosynthesis.

Glycosyltransferase mechanisms: impact of a 5-fluoro substituent in acceptor and donor substrates on catalysis

In glycosyltransferase-catalyzed reactions a new carbohydrate-carbohydrate bond is formed between a carbohydrate acceptor and the carbohydrate moiety of either a sugar nucleotide donor or lipid-linked saccharide donor. It is currently believed that most glycosyltransferase-catalyzed reactions occur via an electrophilic activation mechanism with the formation of an oxocarbenium ion-like transition state, a hypothesis that makes clear predictions regarding the charge development on the donor (strong positive charge) and acceptor (minimal negative charge) substrates. To better understand the mechanism of these enzyme-catalyzed reactions, we have introduced a strongly electron-withdrawing group (fluorine) at C-5 of both donor and acceptor substrates in order to explore its effect on catalysis. In particular, we have investigated the effects of the 5-fluoro analogues on the kinetics of two glycosyltransferase-catalyzed reactions mediated by UDP-GlcNAc:GlcNAc-P-P-Dol N-acetylglucosaminyltransferase (chitobiosyl-P-P-lipid synthase, CLS) and beta-N-acetylglucosaminyl-beta-1,4 galactosyltransferase (GalT). The 5-fluoro group has a marked effect on catalysis when inserted into the UDP-GlcNAc donor, with the UDP(5-F)-GlcNAc serving as a competitive inhibitor of CLS rather than a substrate. The (5-F)-GlcNAc beta-octyl glycoside acceptor, however, is an excellent substrate for GalT. Both of these results support a weakly associative transition state for glycosyltransferase-catalyzed reactions that proceed with inversion of configuration.

Partial purification of mannosylphosphorylundecaprenol synthase from Micrococcus luteus: a useful enzyme for the biosynthesis of a variety of mannosylphosphorylpolyisoprenol products

Membrane fractions from Micrococcus luteus catalyze the transfer of mannose from GDP-mannose to mono- and dimannosyldiacylglycerol, mannosylphosphorylundecaprenol (Man-P-Undec), and a membrane-associated lipomannan. This chapter describes the detergent solubilization, partial purification, and properties of Man-P-Undec synthase. The mobility of the mannosyltransferase activity on sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicates that the enzyme is a polypeptide with a molecular weight of approx 30.7 kDa. Utilizing the broad specificity of the bacterial mannosyltransferase provides a useful approach for the enzymatic synthesis of a wide variety of Man-P-polyisoprenol products.

Human dolichol kinase, a polytopic endoplasmic reticulum membrane protein with a cytoplasmically oriented CTP-binding site

Dolichol kinase (DK) catalyzes the CTP-dependent phosphorylation of dolichol in the biosynthesis de novo and possibly the recycling of dolichyl monophosphate in yeast and mammals. A cDNA clone from human brain encoding the mammalian homologue, hDKp, of the yeast enzyme has recently been identified. In this study hDK has been overexpressed in Chinese hamster ovary cells and shown to be a polytopic membrane protein localized in the endoplasmic reticulum with an N terminus extended into the lumen and a cytoplasmically oriented C terminus. A conserved sequence, DXXAXXXGXXXGX(8)KKTXEG, found in several enzymes utilizing CTP as substrate including DKs, phytol kinases, and several CDP-diacylglycerol synthetases has been identified, and the possibility that it is part of the CTP-binding domain of hDKp has been investigated. Topological studies indicate that the loop between transmembrane domains (TMD) 11 and TMD12 of hDKp, containing the putative CTP binding domain, faces the cytoplasm. Deletion of the loop between TMD11-12, hDK(Delta459-474), or mutation of selected conserved residues within the cytoplasmic loop results in either a partial or total loss of activity and significant reductions in the affinity for CTP. In addition, the SEC59 gene in the yeast DK mutant was sequenced, and a G420D substitution was found. Conversion of the corresponding residue Gly-443 in hDKp to aspartic acid resulted in inactivation of the mammalian enzyme. These results extend the information on the topological arrangement of hDKp and indicate that the cytoplasmic loop between TMDs 11-12, containing the critical conserved residues, lysine 470 and lysine 471 in the (470)KKTXEG(475) motif, is part of the CTP-binding site in hDK.

Assay for the transbilayer movement of polyisoprenoid-linked saccharides based on the transport of water-soluble analogues

Flippases are a class of membrane proteins that are proposed to facilitate the transbilayer movement of amphipathic polar lipids that are required for membrane biogenesis and the assembly of many diverse complex glycoconjugates in eukaryotic and prokaryotic cells. Despite their crucial roles in membrane biology, very little is known about their structures and the precise mechanism(s) by which they overcome the biophysical barriers of the hydrophobic core, and allow polar head groups to traverse membrane bilayers. This chapter presents methods based on the transport of water-soluble analogues that can be applied to investigate membrane proteins mediating the transverse diffusion of polyisoprenoid-linked glycolipid intermediates involved in the biosynthesis of N-linked glycoproteins, glycosylphosphatidylinositol anchors and bacterial polysaccharides.

Dimannosyldiacylglycerol serves as a lipid anchor precursor in the assembly of the membrane-associated lipomannan in Micrococcus luteus

Based on recent analytical and enzymological studies, a topological model for the role of alpha-D-mannosyl-(1-->3)-alpha-D-mannosyl-(1-->3)-diacylglycerol (Man(2)-DAG) as a lipid anchor precursor and mannosylphosphorylundecaprenol (Man-P-Und) as a mannosyl donor in the assembly of a membrane-associated lipomannan (LM) in Micrococcus luteus has been proposed. In this study, a [(3)H]mannose-suicide selection procedure has been used to identify temperature-sensitive (ts) mutants defective in LM assembly. Two micrococcal mutants with abnormal levels of Man(2)-DAG and LM at the nonpermissive temperature (37 degrees C), mms1 and mms2, have been isolated and characterized. In vivo and in vitro biochemical assays indicate that mms1 cells have a defect in the mannosyltransferase catalyzing the conversion of Man-DAG to Man(2)-DAG, and mms2 has a temperature-sensitive defect in the synthesis of Man-P-Und. Because mms1 cells are depleted of endogenous Man(2)-DAG, membranes from this mutant efficiently converted purified, exogenous [(3)H]Man(2)-DAG to [(3)H]LM by a Man-P-Und-dependent process. An obligatory role for Man-P-Und as a mannosyl donor in the elongation process was also demonstrated by showing that the conversion of exogenous [(3)H]Man(2)-DAG to [(3)H]LM by membranes from mms1 cells in the presence of GDP-Man was inhibited by amphomycin. In addition, consistent with Man(2)-DAG serving as a lipid anchor precursor for LM assembly, endogenous, prelabeled [(3)H]Man(2)-DAG was converted to [(3)H]LM when membranes from mms2 cells were incubated with purified, exogenous Man-P-Und. These studies provide the first direct proof for the role of Man(2)-DAG as the lipid anchor precursor for LM, and suggest that Man(2)-DAG may be essential for the normal growth of M. luteus cells.

Functional reconstitution into proteoliposomes and partial purification of a rat liver ER transport system for a water-soluble analogue of mannosylphosphoryldolichol

Mannosylphosphoryldolichol (Man-P-Dol) is synthesized on the cytosolic leaflet of the rough endoplasmic reticulum (ER), and functions as a mannosyl donor in the biosynthesis of Glc(3)Man(9)GlcNAc(2)-P-P-Dol after being translocated to the lumenal leaflet. An assay, based on the transport of Man-P-citronellol (Man-P-Dol(10)), a water-soluble analogue of Man-P-Dol(95), into sealed microsomal vesicles, has been devised to identify protein(s) (flippases) that could mediate the thermodynamically unfavorable movement of Man-P-Dol between the two leaflets of the ER. To develop a defined system for the systematic investigation of the properties of the Man-P-Dol(10) transporter, and as an initial step toward purification of the protein(s) involved in the transport of Man-P-Dol(10), the activity has been solubilized from rat liver microsomes with n-octyl-beta-D-glucoside and reconstituted into proteoliposomes (approximately 0.1 microm in diameter). The properties of the reconstituted Man-P-Dol(10) transport system are similar to the Man-P-Dol(10) uptake activity in microsomal vesicles from rat liver. Man-P-Dol(10) transport into reconstituted proteoliposomes is time-dependent, reversible, saturable, and stereoselective. The direct role of ER proteins in the functionally reconstituted transport system is supported by the inhibitory effects of trypsin treatment, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), or diethylpyrocarbonate (DEPC). Solubilization and functional reconstitution are shown to provide an experimental approach to the partial purification of the protein(s) mediating the transport process.

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.

Structural and topological studies on the lipid-mediated assembly of a membrane-associated lipomannan in Micrococcus luteus

The biosynthesis of three mannolipids and the presence of a membrane-associated lipomannan in Micrococcus luteus (formerly Micrococcus lysodeikticus) were documented over 30 years ago. Structural and topological studies have been conducted to learn more about the possible role of the mannolipids in the assembly of the lipomannan. The major mannolipid has been purified and characterized as alpha-D-mannosyl-(1 --> 3)-alpha-D-mannosyl-(1 --> 3)-diacylglycerol (Man2-DAG) by negative-ion electrospray-ionization multistage mass spectrometry (ESI-MSn). Analysis of the fragmentation patterns indicates that the sn-1 position is predominantly acylated with a 12-methyltetradecanoyl group and the sn-2 position is acylated with a myristoyl group. The lipomannan is shown to be located on the exterior face of the cytoplasmic membrane, and not exposed on the surface of intact cells, by staining of intact protoplasts with fluorescein isothiocyanate (FITC)-linked concanavalin A (Con A). When cell homogenates of M. luteus are incubated with GDP-[3H]mannose (GDP-Man), [3H]mannosyl units are incorporated into Man1-2-DAG, mannosylphosphorylundecaprenol (Man-P-Undec) and the membrane-associated lipomannan. The addition of amphomycin, an inhibitor of Man-P-Undec synthesis, had no effect on the synthesis of Man1-2-DAG, but blocked the incorporation of [3H]mannose into Man-P-Undec and consequently the lipomannan. These results strongly indicate that GDP-Man is the direct mannosyl donor for the synthesis of Man1-2-DAG, and that the majority of the 50 mannosyl units in the lipomannan are derived from Man-P-Undec. Protease-sensitivity studies with intact and lysed protoplasts indicate that the active sites of the mannosyltransferases catalyzing the formation of Man1-2-DAG and Man-P-Undec are exposed on the inner face, and the Man-P-Undec-mediated reactions occur on the outer surface of the cytoplasmic membrane. Based on all of these results, a topological model is proposed for the lipid-mediated assembly of the membrane-bound lipomannan.

Deficiency of UDP-GlcNAc:Dolichol Phosphate N-Acetylglucosamine-1 Phosphate Transferase (DPAGT1) causes a novel congenital disorder of Glycosylation Type Ij

Defects in the assembly of dolichol-linked oligosaccharide or its transfer to proteins result in severe, multi-system human diseases called Type I congenital disorders of glycosylation. We have identified a novel CDG type, CDG-Ij, resulting from deficiency in UDP-GlcNAc: dolichol phosphate N-acetyl-glucosamine-1 phosphate transferase (GPT) activity encoded by DPAGT1. The patient presents with severe hypotonia, medically intractable seizures, mental retardation, microcephaly, and exotropia. Metabolic labeling of cultured dermal fibroblasts from the patient with [2-(3)H]-mannose revealed lowered incorporation of radiolabel into full-length dolichol-linked oligosaccharides and glycoproteins. In vitro enzymatic analysis of microsomal fractions from the cultured cells indicated that oligosaccharyltransferase activity is normal, but the GPT activity is reduced to approximately 10% of normal levels while parents have heterozygous levels. The patient's paternal DPAGT1 allele contains a point mutation (660A>G) that replaces a highly conserved tyrosine with a cysteine (Y170C). The paternal allele cDNA produces a full-length protein with almost no activity when over-expressed in CHO cells. The maternal allele makes only about 12% normal mature mRNA, while the remainder shows a complex exon skipping pattern that shifts the reading frame encoding a truncated non-functional GPT protein. Thus, we conclude that the DPAGT1 gene defects are responsible for the CDG symptoms in this patient. Hum Mutat 22:144-150, 2003.

Evidence that the wzxE gene of Escherichia coli K-12 encodes a protein involved in the transbilayer movement of a trisaccharide-lipid intermediate in the assembly of enterobacterial common antigen

The assembly of many bacterial cell surface polysaccharides requires the transbilayer movement of polyisoprenoid-linked saccharide intermediates across the cytoplasmic membrane. It is generally believed that transverse diffusion of glycolipid intermediates is mediated by integral membrane proteins called translocases or "flippases." The bacterial genes proposed to encode these translocases have been collectively designated wzx genes. The wzxE gene of Escherichia coli K-12 has been implicated in the transbilayer movement of Fuc4NAc-ManNAcA-GlcNAc-P-P-undecaprenol (lipid III), the donor of the trisaccharide repeat unit in the biosynthesis of enterobacterial common antigen (ECA). Previous studies (Feldman, M. F., Marolda, C. L., Monteiro, M. A., Perry, M. B., Parodi, A. J., and Valvano, M. (1999) J. Biol. Chem. 274, 35129-35138) provided indirect evidence that the wzx(016) gene product of E. coli K-12 encoded a translocase capable of mediating the transbilayer movement of N-acetylglucosaminylpyrophosphorylundecaprenol (GlcNAc-P-P-Und), an early intermediate in the synthesis of ECA and many lipopolysaccharide O antigens. Therefore, genetic and biochemical studies were conducted to determine if the putative Wzx(O16) translocase was capable of mediating the transport of N-acetylglucosaminylpyrophosphorylnerol (GlcNAc-P-P-Ner), a water-soluble analogue of GlcNAc-P-P-Und. [(3)H]GlcNAc-P-P-Ner was transported into sealed, everted cytoplasmic membrane vesicles of E. coli K-12 as well as a deletion mutant lacking both the wzx(016) and wzxC genes. In contrast, [(3)H]GlcNAc-P-P-Ner was not transported into membrane vesicles prepared from a wzxE-null mutant, and metabolic radiolabeling experiments revealed the accumulation of lipid III in this mutant. The WzxE transport system exhibited substrate specificity by recognizing both a pyrophosphoryl-linked saccharide and an unsaturated alpha-isoprene unit in the carrier lipid. These results support the conclusion that the wzxE gene encodes a membrane protein involved in the transbilayer movement of lipid III in E. coli.

Identification and characterization of a cDNA encoding a dolichyl pyrophosphate phosphatase located in the endoplasmic reticulum of mammalian cells

The CWH8 gene in Saccharomyces cerevisiae has been shown recently (Fernandez, F., Rush, J. S., Toke, D. A., Han, G., Quinn, J. E., Carman, G. M., Choi, J.-Y., Voelker, D. R., Aebi, M., and Waechter, C. J. (2001) J. Biol. Chem. 276, 41455-41464) to encode a dolichyl pyrophosphate (Dol-P-P) phosphatase associated with crude microsomal fractions. Mutations in CWH8 result in the accumulation of Dol-P-P, deficiency in lipid intermediate synthesis, defective protein N-glycosylation, and a reduced growth rate. A cDNA (DOLPP1, GenBank accession number AB030189) from mouse brain encoding a homologue of the yeast CWH8 gene is now shown to complement the defects in growth and protein N-glycosylation, and to correct the accumulation of Dol-P-P in the cwh8Delta yeast mutant. Northern blot analyses demonstrate a wide distribution of the DOLPP1 mRNA in mouse tissues. Overexpression of Dolpp1p in yeast, COS, and Sf9 cells produces substantial increases in Dol-P-P phosphatase activity but not in dolichyl monophosphate or phosphatidic acid phosphatase activities in microsomal fractions. Subcellular fractionation and immunofluorescence studies localize the enzyme encoded by DOLPP1 to the endoplasmic reticulum of COS cells. The results of protease sensitivity studies with microsomal vesicles from the lpp1Delta/dpp1Delta yeast mutant expressing DOLPP1 are consistent with Dolpp1p having a luminally oriented active site. The sequence of the DOLPP1 cDNA predicts a polypeptide with 238 amino acids, and a new polypeptide corresponding to 27 kDa is observed when DOLPP1 is expressed in yeast, COS, and Sf9 cells. This study is the first identification and characterization of a cDNA clone encoding an essential component of a mammalian lipid pyrophosphate phosphatase that is highly specific for Dol-P-P. The specificity, subcellular location, and topological orientation of the active site described in the current study strongly support a role for Dolpp1p in the recycling of Dol-P-P discharged during protein N-glycosylation reactions on the luminal leaflet of the endoplasmic reticulum in mammalian cells.

Expression and characterization of a human cDNA that complements the temperature-sensitive defect in dolichol kinase activity in the yeast sec59-1 mutant: the enzymatic phosphorylation of dolichol and diacylglycerol are catalyzed by separate CTP-mediated kinase activities in Saccharomyces cerevisiae

Dolichol kinase (DK) catalyzes the CTP-mediated phosphorylation of dolichol in eukaryotic cells, the terminal step in dolichyl monophosphate (Dol-P) biosynthesis de novo. In S. cerevisiae, the SEC59 gene encodes a protein essential for the expression of DK, an enzyme activity that is required for cell viability and normal rates of lipid intermediate synthesis and protein N-glycosylation. This study identifies a cDNA clone from human brain that encodes the mammalian homolog of DK (hDK1p). hDK1 is capable of complementing the growth defect, elevating DK activity, and consequently increasing Dol-P levels in vivo and restoring normal N-glycosylation of carboxypeptidase Y at the restrictive temperature in the temperature-sensitive mutant sec59-1. The CTP-mediated phosphorylation of diacylglycerol (DAG) is unaffected by either the temperature-sensitive mutation in the sec59-1 strain, overexpression of the SEC59 gene, or the mammalian homolog hDK1 under conditions that produced a loss or elevation in the level of DK activity. Additionally, overexpression of hDK1p in Sf-9 cells resulted in a 15-fold increase in DK activity but not DAG kinase activity in crude microsomal fractions. The cloned cDNA contains an open reading frame that would encode a protein with 538 amino acids and a molecular weight of 59,268 kDa. Consistent with this prediction, new polypeptides were detected with an apparent molecular weight of 59-60 kDa when His(6)-tagged constructs of hDK1 or the SEC59 gene were expressed in Sf-9 cells or the temperature-sensitive sec59-1 mutant cells, respectively. These results identify the first cDNA clone encoding a protein required for the expression of DK activity, possibly the catalytic subunit, in a mammalian cell, and establish that the phosphorylation of dolichol and DAG are catalyzed by separate kinase activities in yeast.

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.

Requirement of the Lec35 gene for all known classes of monosaccharide-P-dolichol-dependent glycosyltransferase reactions in mammals

The Lec35 gene product (Lec35p) is required for utilization of the mannose donor mannose-P-dolichol (MPD) in synthesis of both lipid-linked oligosaccharides (LLOs) and glycosylphosphatidylinositols, which are important for functions such as protein folding and membrane anchoring, respectively. The hamster Lec35 gene is shown to encode the previously identified cDNA SL15, which corrects the Lec35 mutant phenotype and predicts a novel endoplasmic reticulum membrane protein. The mutant hamster alleles Lec35.1 and Lec35.2 are characterized, and the human Lec35 gene (mannose-P-dolichol utilization defect 1) was mapped to 17p12-13. To determine whether Lec35p was required only for MPD-dependent mannosylation of LLO and glycosylphosphatidylinositol intermediates, two additional lipid-mediated reactions were investigated: MPD-dependent C-mannosylation of tryptophanyl residues, and glucose-P-dolichol (GPD)-dependent glucosylation of LLO. Both were found to require Lec35p. In addition, the SL15-encoded protein was selective for MPD compared with GPD, suggesting that an additional GPD-selective Lec35 gene product remains to be identified. The predicted amino acid sequence of Lec35p does not suggest an obvious function or mechanism. By testing the water-soluble MPD analog mannose-beta-1-P-citronellol in an in vitro system in which the MPD utilization defect was preserved by permeabilization with streptolysin-O, it was determined that Lec35p is not directly required for the enzymatic transfer of mannose from the donor to the acceptor substrate. These results show that Lec35p has an essential role for all known classes of monosaccharide-P-dolichol-dependent reactions in mammals. The in vitro data suggest that Lec35p controls an aspect of MPD orientation in the endoplasmic reticulum membrane that is crucial for its activity as a donor substrate.

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.

Mannose supplementation corrects GDP-mannose deficiency in cultured fibroblasts from some patients with Congenital Disorders of Glycosylation (CDG)

Congenital Disorders of Glycosylation (CDG) are human deficiencies in glycoprotein biosynthesis. Previous studies showed that 1 mM mannose corrects defective protein N-glycosylation in cultured fibroblasts from some CDG patients. We hypothesized that these CDG cells have limited GDP-mannose (GDP-Man) and that exogenous mannose increases the GDP-Man levels. Using a well established method to measure GDP-Man, we found that normal fibroblasts had an average of 23.5 pmol GDP-Man/10(6) cells, whereas phosphomannomutase (PMM)-deficient fibroblasts had only 2.3-2.7 pmol/10(6) cells. Adding 1 mM mannose to the culture medium increased the GDP-Man level in PMM-deficient cells to approximately 15.5 pmol/10(6) cells, but had no significant effect on GDP-Man levels in normal fibroblasts. Similarly, mannose supplementation increased GDP-Man from 4.6 pmol/10(6) cells to 24.6 pmol/10(6) cells in phosphomannose isomerase (PMI)-deficient fibroblasts. Based on the specific activity of the GDP-[(3)H]Man pool present in [2-(3)H]mannose labeled cells, mannose supplementation also partially corrected the impaired synthesis of mannosylphosphoryldolichol (Man-P-Dol) and Glc(0)(-)(3)Man(9)GlcNAc(2)-P-P-Dol. These results confirm directly that deficiencies in PMM and PMI result in lowered cellular GDP-Man levels that are corrected by the addition of mannose. In contrast to these results, GDP-Man levels in fibroblasts from a CDG-Ie patient, who is deficient in Man-P-Dol synthase, were normal and unaffected by mannose supplementation even though mannose addition was found to correct abnormal lipid intermediate synthesis in another study (Kim et al. [2000] J. Clin. Invest., 105, 191-198). The mechanism by which mannose supplementation corrects abnormal protein N-glycosylation in Man-P-Dol synthase deficient cells is unknown, but this observation suggests that the regulation of Man-P-Dol synthesis and utilization may be more complex than is currently understood.

Farnesol is utilized for isoprenoid biosynthesis in plant cells via farnesyl pyrophosphate formed by successive monophosphorylation reactions

The ability of Nicotiana tabacum cell cultures to utilize farnesol (F-OH) for sterol and sesquiterpene biosynthesis was investigated. [(3)H]F-OH was readily incorporated into sterols by rapidly growing cell cultures. However, the incorporation rate into sterols was reduced by greater than 70% in elicitor-treated cell cultures whereas a substantial proportion of the radioactivity was redirected into capsidiol, an extracellular sesquiterpene phytoalexin. The incorporation of [(3)H]F-OH into sterols was inhibited by squalestatin 1, suggesting that [(3)H]F-OH was incorporated via farnesyl pyrophosphate (F-P-P). Consistent with this possibility, N. tabacum proteins were metabolically labeled with [(3)H]F-OH or [(3)H]geranylgeraniol ([(3)H]GG-OH). Kinase activities converting F-OH to farnesyl monophosphate (F-P) and, subsequently, F-P-P were demonstrated directly by in vitro enzymatic studies. [(3)H]F-P and [(3)H]F-P-P were synthesized when exogenous [(3)H]F-OH was incubated with microsomal fractions and CTP. The kinetics of formation suggested a precursor-product relationship between [(3)H]F-P and [(3)H]F-P-P. In agreement with this kinetic pattern of labeling, [(32)P]F-P and [(32)P]F-P-P were synthesized when microsomal fractions were incubated with F-OH and F-P, respectively, with [gamma-(32)P]CTP serving as the phosphoryl donor. Under similar conditions, the microsomal fractions catalyzed the enzymatic conversion of [(3)H]GG-OH to [(3)H]geranylgeranyl monophosphate and [(3)H]geranylgeranyl pyrophosphate ([(3)H]GG-P-P) in CTP-dependent reactions. A novel biosynthetic mechanism involving two successive monophosphorylation reactions was supported by the observation that [(3)H]CTP was formed when microsomes were incubated with [(3)H]CDP and either F-P-P or GG-P-P, but not F-P. These results document the presence of at least two CTP-mediated kinases that provide a mechanism for the utilization of F-OH and GG-OH for the biosynthesis of isoprenoid lipids and protein isoprenylation.

The LPP1 and DPP1 gene products account for most of the isoprenoid phosphate phosphatase activities in Saccharomyces cerevisiae

Two genes in Saccharomyces cerevisiae, LPP1 and DPP1, with homology to a mammalian phosphatidic acid (PA) phosphatase were identified and disrupted. Neither single nor combined deletions resulted in growth or secretion phenotypes. As observed previously (Toke, D. A., Bennett, W. L., Dillon, D. A., Wu, W.-I., Chen, X., Ostrander, D. B., Oshiro, J., Cremesti, A., Voelker, D. R., Fischl, A. S., and Carman, G. M. (1998) J. Biol. Chem. 273, 3278-3284; Toke, D. A., Bennett, W. L., Oshiro, J., Wu, W.-I., Voelker, D. R., and Carman, G. M. (1998) J. Biol. Chem. 273, 14331-14338), the disruption of DPP1 and LPP1 produced profound losses of Mg2+-independent PA phosphatase activity. The coincident attenuation of hydrolytic activity against diacylglycerol pyrophosphate prompted an examination of the effects of these disruptions on hydrolysis of isoprenoid pyrophosphates. Disruption of either LPP1 or DPP1 caused respective decreases of about 25 and 75% in Mg2+-independent hydrolysis of several isoprenoid phosphates by particulate fractions isolated from these cells. The particulate and cytosolic fractions from the double disruption (lpp1Delta dpp1Delta) showed essentially complete loss of Mg2+-independent hydrolytic activity toward dolichyl phosphate (dolichyl-P), dolichyl pyrophosphate (dolichyl-P-P), farnesyl pyrophosphate (farnesyl-P-P), and geranylgeranyl pyrophosphate (geranylgeranyl-P-P). However, a modest Mg2+-stimulated activity toward PA and dolichyl-P was retained in cytosol from lpp1Delta dpp1Delta cells. The action of Dpp1p on isoprenyl pyrophosphates was confirmed by characterization of the hydrolysis of geranylgeranyl-P-P by the purified protein. These results indicate that LPP1 and DPP1 account for most of the hydrolytic activities toward dolichyl-P-P, dolichyl-P, farnesyl-P-P, and geranylgeranyl-P-P but also suggest that yeast contain other enzymes capable of dephosphorylating these essential isoprenoid intermediates.

Topological studies on the enzymes catalyzing the biosynthesis of Glc-P-dolichol and the triglucosyl cap of Glc3Man9GlcNAc2-P-P-dolichol in microsomal vesicles from pig brain: use of the processing glucosidases I/II as latency markers

In the current model for Glc3Man9GlcNAc2-P-P-Dol assembly, Man5GlcNAc2-P-P-Dol, Man-P-Dol, and Glc-P-Dol are synthesized on the cytoplasmic face of the ER and diffuse transversely to the lumenal leaflet where the synthesis of the lipid-bound precursor oligosaccharide is completed. To establish the topological sites of Glc-P-Dol synthesis and the lipid-mediated glucosyltransfer reactions involved in Glc3Man9GlcNAc2-P-P-Dol synthesis in ER vesicles from pig brain, the trypsin-sensitivity of Glc-P-Dol synthase activity and the Glc-P-Dol:Glc0-2Man9GlcNAc2-P-P-Dol glucosyltransferases (GlcTases) was examined in sealed microsomal vesicles. Since ER vesicles from brain do not contain glucose 6-phosphate (Glc 6-P) phosphatase activity, the latency of the lumenally oriented, processing glucosidase I/II activities was used to assess the intactness of the vesicle preparations. Comparative enzymatic studies with sealed ER vesicles from brain and kidney, a tissue that contains Glc 6-P phosphatase, demonstrate the reliability of using the processing glucosidase activities as latency markers for topological studies with microsomal vesicles from non-gluconeogenic tissues lacking Glc 6-P phosphatase. The results obtained from the trypsin-sensitivity assays with sealed microsomal vesicles from brain are consistent with a topological model in which Glc-P-Dol is synthesized on the cytoplasmic face of the ER, and subsequently utilized by the three Glc-P-Dol-mediated GlcTases after "flip-flopping" to the lumenal monolayer.

Transbilayer movement of Glc-P-dolichol and its function as a glucosyl donor: protein-mediated transport of a water-soluble analog into sealed ER vesicles from pig brain

The results described in the accompanying article support the model in which glucosylphosphoryldolichol (Glc-P-Dol) is synthesized on the cytoplasmic face of the ER, and functions as a glucosyl donor for three Glc-P-Dol:Glc0-2Man9-GlcNAc2-P-P-Dol glucosyltransferases (GlcTases) in the lumenal compartment. In this study, the enzymatic synthesis and structural characterization by NMR and electrospray-ionization tandem mass spectrometry of a series of water-soluble beta-Glc-P-Dol analogs containing 2-4 isoprene units with either the cis - or trans -stereoconfiguration in the beta-position are described. The water-soluble analogs were (1) used to examine the stereospecificity of the Glc-P-Dol:Glc0-2Man9GlcNAc2-P-P-Dol glucosyltransferases (GlcTases) and (2) tested as potential substrates for a membrane protein(s) mediating the transbilayer movement of Glc-P-Dol in sealed ER vesicles from rat liver and pig brain. The Glc-P-Dol-mediated GlcTases in pig brain microsomes utilized [3H]Glc-labeled Glc-P-Dol10, Glc-P-(omega, c )Dol15, Glc-P(omega, t,t )Dol20, and Glc-P-(omega, t,c )Dol20as glucosyl donors with [3H]Glc3Man9GlcNAc2-P-P-Dol the major product labeled in vitro. A preference was exhibited for C15-20 substrates containing an internal cis -isoprene unit in the beta-position. In addition, the water-soluble analog, Glc-P-Dol10, was shown to enter the lumenal compartment of sealed microsomal vesicles from rat liver and pig brain via a protein-mediated transport system enriched in the ER. The properties of the ER transport system have been characterized. Glc-P-Dol10was not transported into or adsorbed by synthetic PC-liposomes or bovine erythrocytes. The results of these studies indicate that (1) the internal cis -isoprene units are important for the utilization of Glc-P-Dol as a glucosyl donor and (2) the transport of the water-soluble analog may provide an experimental approach to assay the hypothetical "flippase" proposed to mediate the transbilayer movement of Glc-P-Dol from the cytoplasmic face of the ER to the lumenal monolayer.

Geranylgeraniol overcomes the block of cell proliferation by lovastatin in C6 glioma cells

It is well documented that 3-hydroxy-3-methylglutaryl-CoA reductase inhibitors prevent cultured mammalian cells from progressing through the cell cycle, suggesting a critical role for a mevalonate-derived product. Recently, it has been shown that free geranylgeraniol (GG-OH) and farnesol (F-OH) can be utilized by C6 glioma cells for protein isoprenylation. The ability of GG-OH and F-OH to restore protein geranylgeranylation or farnesylation selectively has enabled us to examine the possibility that mevalonate is essential for cell proliferation because it is a precursor of farnesyl pyrophosphate or geranylgeranyl pyrophosphate, the isoprenyl donors involved in the posttranslational modification of key regulatory proteins. In this study we report that GG-OH, as well as mevalonate, overcomes the arrest of cell proliferation of C6 glioma cells treated with lovastatin, as assessed by increased cell numbers and a stimulation in [3H]thymidine incorporation. The increase in cell number and [3H]thymidine incorporation were significantly lower when F-OH was added. Under these conditions [3H]mevalonate and [3H]GG-OH are actively incorporated into a set of isoprenylated proteins in the size range of small, GTP-binding proteins (19-27 kDa) and a polypeptide with the molecular size (46 kDa) of the smaller isoform of 2 ',3'-cyclic nucleotide 3'-phosphodiesterase. Analysis of the proteins metabolically labeled by [3H]mevalonate and [3H]GG-OH reveals the presence of labeled proteins containing geranylgeranylated cysteinyl residues. Consistent with geranylgeranylated proteins playing a critical role in the entry of C6 cells into the cell cycle, a (phosphonoacetamido)oxy derivative of GG-OH, a drug previously shown to interfere with protein geranylgeranylation, prevented the increase in cell number when mevalonate or GG-OH was added to lovastatin-treated cells. These results strongly suggest that geranylgeranylated proteins are essential for progression of C6 cells into the S phase of the cell cycle and provide the first evidence that the "salvage" pathway for the utilization of the free isoprenols is physiologically significant in the CNS.

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.

An electrospray-ionization tandem mass spectrometry method for determination of the anomeric configuration of glycosyl 1-phosphate derivatives

A rapid, simple, and sensitive method is described for the determination of the anomeric configuration of sugar 1-phosphates, sugar nucleotides, and polyisoprenyl-phospho-sugars. Negative-ion electrospray ionization of picomole amounts of glycosyl 1-phosphate derivatives produces an intense signal of the [M-H]-deprotonated molecule which, by collision-induced dissociation, decomposes in a characteristic manner depending on cis/trans configuration of the 2-hydroxyl and phosphate groups of the glycosyl residue. A distinct feature of the product ion spectra of glycosyl 1-P and polyisoprenyl-P-sugars with cis configuration is the presence of abundant ions that correspond to the [M-H2O-H]- dehydration product and the [R-PO4-(C2H3O]- fragment arising from a cleavage across the sugar ring, where R is -H or -polyprenyl/dolichyl for glycosyl 1-P and polyisoprenyl-P-sugar, respectively. These two fragments, [M-H2O-H]- and [R-PO4-(C2H3O)]- are absent from the product ion spectra of sugar 1-P and polyisoprenyl-P-sugars with trans configuration. For sugar nucleotides, compounds with cis configuration produce, in tandem mass spectrometry, only one abundant fragment of nucleoside monophosphate, whereas those with trans configuration give nucleoside diphosphate as a major fragment ion. Accordingly, the anomeric configuration of a glycosyl 1-phosphate derivative can be easily determined by using electrospray-ionization tandem mass spectrometry provided that the glycosyl residue of known absolute configuration has a free 2-hydroxyl group and no other charge location.

Biosynthesis of mycobacterial lipoarabinomannan

The mycobacterial lipoglycans, lipomannan (LM) and lipoarabinomannan (LAM), are potent immunomodulators in tuberculosis and leprosy. Little is known of their biosynthesis, other than being based on phosphatidylinositol (PI), and they probably originate in the phosphatidylinositol mannosides (PIMs; PIMans). A novel form of cell-free incubation involving in vitro and in situ labeling with GDP-[14C]Man of the polyprenyl-P-mannoses (C35/C50-P-Man) and the simpler PIMs of mycobacterial membranes, reisolation of the [14C]Man-labeled membranes, and in situ chase demonstrated the synthesis of a novel alpha(1-->6)-linked linear form of LM at the expense of the C35/C50-P-Man. There was little or no synthesis under these conditions of PIMan5 with its terminal alpha(1-->2)Man unit or the mature LM or LAM with copious alpha(1-->2)Man branching. Synthesis of the linear LM, but not of the simpler PIMan2, was susceptible to amphomycin, a lipopeptide antibiotic that specifically inhibits polyprenyl-P-requiring translocases. A mixture of P[3H]I and P[3H]IMan2 was incorporated into the linear LM, supporting other evidence that, like the PIMs, LM and LAM, it is a lipid-linked mannooligosaccharide and a new member of the mycobacterial glycosylphosphatidylinositol lipoglycan/glycolipid class. Hence, the simpler PIMs originate in PI and GDP-Man, but further growth of the linear backbone emanates from C35-/C50-P-Man and is amphomycin-sensitive. The origin of the alpha(1-->2)Man branches of mature PIMan5, LM, and LAM is not known at this time but is probably GDP-Man.

Geranylgeraniol promotes entry of UT-2 cells into the cell cycle in the absence of mevalonate

Although UT-2 cells, a mutant clone of Chinese hamster ovary cells, have been shown to require mevalonate for growth due to a deficiency in 3-hydroxy-3-methylglutaryl-CoA reductase, the precise mevalonate-derived product(s) essential for proliferation has not been identified. These studies show that UT-2 cells proliferate in the presence of free geranylgeraniol (GG-OH), as well as mevalonate. Cell growth was optimal when the culture medium was supplemented with 5-10 microM GG-OH. Under these growth conditions [3H]GG-OH is actively incorporated into UT-2 proteins. Prominent [3H]geranylgeranylated polypeptides in the size range (19-27 kDa) of the small GTP-binding proteins are observed by SDS-PAGE. Analysis of the butanol-soluble products released from the metabolically labeled proteins by digestion with Pronase E reveals that the proteins contain [3H]geranylgeranylated cysteine residues. Even though [3H]farnesol is also incorporated into cysteinyl residues of a different set of UT-2 proteins, farnesol added at 10 microM did not satisfy the mevalonate requirement for cell growth. These results show that UT-2 cells divide in the presence of exogenously supplied GG-OH, providing evidence that one or more geranylgeranylated proteins are essential for entry of UT-2 cells, and probably other mammalian cells, into the cell cycle.

Polyisoprenyl phosphate specificity of UDP-GlcNAc:undecaprenyl phosphate N-acetylglucosaminyl 1-P transferase from E.coli

N-Acetyl-D-glucosaminylpyrophosphorylundecaprenol (GlcNAc-P-P-Und), an intermediate in the biosynthesis of the enterobacterial common antigen in E.coli and some O-antigen chains in gram-negative bacteria, is formed by the transfer of GlcNAc 1-P from UDP-GlcNAc to Und-P, analogous to the reaction forming GlcNAc-P-P-dolichol (GlcNAc-P-P-Dol) in mammalian cells. Since the microsomal enzyme from animal cells exhibits a strong preference for Dol-P, which contains a saturated alpha-isoprene unit, the polyisoprenyl phosphate specificity of the homologous bacterial enzyme was characterized. The enzyme remained bound to the membrane fraction when spheroplasts, formed by lysozyme-EDTA treatment, were lysed in hypotonic buffer. GlcNAc-P-P-Und synthase (GPT) activity was elevated in a strain of E.coli bearing the rfe gene, which encodes GPT on a multicopy plasmid, and virtually absent from rfe null mutants. GPT actively utilized fully unsaturated polyprenyl phosphate (Poly-P) substrates with maximal activity seen with (C55) Und-P, but was unable to utilize (C55)Dol-P. This substrate specificity contrasts with the microsomal GPT from pig brain, which actively utilized (C55)Dol-P, but not Und-P, as substrate. GPT activity bound to particulate fractions from three strains of bacilli also exhibited a strict preference for fully unsaturated Poly-P substrates. Unexpectedly, E.coli GPT activity cofractionated with the cytosolic marker enzyme, beta-galactosidase, and not the membrane-bound enzyme, D-lactate dehydrogenase, in cells disrupted in a French pressure cell. The properties and polyisoprenyl phosphate specificity of the soluble form of GPT were identical to the activity associated with the membrane preparations obtained from spheroplasts. The evolutionary and functional significance of the use of polyisoprenyl glycosyl carrier lipids with saturated alpha-isoprene units in eukaryotes remains uncertain.

Determination of the anomeric configuration of glycosyl esters of nucleoside pyrophosphates and polyisoprenyl phosphates by fast-atom bombardment tandem mass spectrometry

Collision-induced dissociation of the deprotonated molecules of glycosyl esters of nucleoside pyrophosphates and polyisoprenyl (dolichyl and polyprenyl) phosphates results in distinct fragmentation patterns that depend on cis-trans configuration of the phosphodiester and 2″ (or 2', respectively)-hydroxyl groups of the glycosyl residue. At the collision-offset voltage of 0. 5 V, sugar nucleotides with cis configuration produce only one very abundant fragment of nucleoside monophosphate, whereas compounds with trans configuration give weak signals for nucleoside di- and mono-phosphates and their dehydration products. These fragmentation patterns are largely preserved at higher collision energy, with the exception that, for sugar nucleotides with trans configuration, the characteristic signals are much more abundant and a novel diagnostic fragment of [ribosyl(deoxyribosyl)-5'-P2O5 - H](-) is generated. In the case of polyisoprenyl-P-sugars, polyisoprenyl phosphate ion is the only fragment observed for compounds with trans configuration, whereas in compounds with cis configuration, this ion is accompanied by another abundant fragment, which is derived from the cleavage across the sugar ring and corresponds to [polyisoprenyl-PO4-(C2H3O)](-). The relative intensity ratio of the latter ion to the [polyisoprenyl-HPO4](-) ion is close to 1 for compounds with cis configuration, but it is only about 0. 01 for compounds with trans configuration. This ratio may serve, therefore, as a diagnostic value for determination of the anomeric configuration of glycosyl esters of polyisoprenyl phosphates. It is proposed that the observed differences in fragmentation patterns of cis-trans sugar nucleotides and polyisoprenyl-P-sugars could be explained in terms of kinetic stereoelectronic effect, and a speculative mechanism of fragmentation of compounds with trans configuration is presented. For compounds with cis configuration, formation of a hydrogen bond between the C-2″(2') hydroxyl and the phosphate group could play a crucial role in directing the specific fragmentation reactions. Consequently, the described empirical rules would hold only for compounds that have a free 2″(2')-hydroxyl group and no alternative charge location. Owing to its simplicity, sensitivity, and tolerance of impurities, fast-atom bombardment-tandem mass spectrometry represents a suitable method for determination of the anomeric linkage of glycosyl esters of nucleoside pyrophosphates and polyisoprenyl phosphates if the absolute configuration of glycosyl residue is known and the compound fulfills the above-mentioned requirements.

Convergence of three steroid receptor pathways in the mediation of nongenotoxic hepatocarcinogenesis

The mechanisms by which peroxisome proliferators are able to regulate metabolic processes such as fat metabolism, while at the same time creating an environment for the development of hepatocellular carcinomas, is a central issue in the non-genotoxic carcinogenesis field. The convergence of two members of the steroid receptor family (peroxisome proliferator-activated receptor, PPAR; and retinoid X receptor, RXR) has provided strong support for an oxidative stress component in this carcinogenesis process, but has yet to define clearly a pathway for the classical tumor promotion events associated with peroxisome proliferation. The findings presented here integrate a third member of the steroid receptor family into this process and suggest a novel autocrine loop and mechanism for creating both oxidative stress and tumor promotion. A central regulatory component in this pathway is farnesol which has recently been shown to induce transcription mediated by the steroid receptor family member, farnesoid X receptor (FXR). In this report, it is clearly demonstrated that farnesol can also upregulate the transcriptional events of PPAR, but most likely through a different farnesoid metabolite, resulting in the regulation of an entirely different set of genetic components. Deregulation of the activities of these receptors offers a provocative mechanism for explaining the hepatocarcinogenic effects of peroxisome proliferators in chronically treated rodents.

Selective inhibition of cholesterol biosynthesis in brain cells by squalestatin 1

The effect of squalestatin 1 (SQ) on squalene synthase and other enzymes utilizing farnesyl pyrophosphate (F-P-P) as substrate was evaluated by in vitro enzymological and in vivo metabolic labeling experiments to determine if the drug selectively inhibited cholesterol biosynthesis in brain cells. Direct in vitro enzyme studies with membrane fractions from primary cultures of embryonic rat brain (IC50 = 37 nM), pig brain (IC50 = 21 nM), and C6 glioma cells (IC50 = 35 nM) demonstrated that SQ potently inhibited squalene synthase activity but had no effect on the long-chain cis-isoprenyltransferase catalyzing the conversion of F-P-P to polyprenyl pyrophosphate (Poly-P-P), the precursor of dolichyl phosphate (Dol-P). SQ also had no effect on F-P-P synthase; the conversion of [3H]F-P-P to geranylgeranyl pyrophosphate (GG-P-P) catalyzed by partially purified GG-P-P synthase from bovine brain; the enzymatic farnesylation of recombinant H-p21ras by rat brain farnesyltransferase; or the enzymatic geranylgeranylation of recombinant Rab 1A, catalyzed by rat brain geranylgeranyltransferase. Consistent with SQ selectively blocking the synthesis of squalene, when C6 glial cells were metabolically labeled with [3H]mevalonolactone, the drug inhibited the incorporation of the labeled precursor into squalene and cholesterol (IC50 = 3-5 microM) but either had no effect or slightly stimulated the labeling of Dol-P, ubiquinone (CoQ), and isoprenylated proteins. These results indicate that SQ blocks cholesterol biosynthesis in brain cells by selectively inhibiting squalene synthase.(ABSTRACT TRUNCATED AT 250 WORDS)

Transmembrane movement of a water-soluble analogue of mannosylphosphoryldolichol is mediated by an endoplasmic reticulum protein

Based on topological studies mannosylphosphoryldolichol (Man-P-Dol) is synthesized on the cytoplasmic face of the RER, but functions as a mannosyl donor in Glc3Man9GlcNAc2-P-P-dolichol biosynthesis after the mannosyl-phosphoryl headgroup diffuses transversely to the luminal compartment. The transport of mannosylphosphorylcitronellol (Man-P-Cit), a water-soluble analogue of Man-P-Dol, by microsomal vesicles from mouse liver, has been investigated as a potential experimental approach to determine if a membrane protein(s) mediates the transbilayer movement of Man-P-Dol. For these studies beta-[3H]Man-P-Cit was synthesized enzymatically with a partially purified preparation of Man-P-undecaprenol synthase from Micrococcus luteus. The uptake of the radiolabeled water-soluble analogue was found to be (a) time dependent; (b) stereoselective; (c) dependent on an intact permeability barrier; (d) saturable; (e) protease-sensitive; and (f) highest in ER-enriched vesicles relative to Golgi complex-enriched vesicles and intact mitochondria. Consistent with the involvement of a membrane protein, the analogue did not enter synthetic phosphatidylcholine-liposomes. [3H]Man-P-Cit also was not transported by human erythrocytes. These results indicate that the transport of Man-P-Cit by sealed microsomal vesicles from mouse liver is mediated by a membrane protein transport system. It is possible that the same membrane protein(s) participates in the transbilayer movement of Man-P-Dol in the ER.

Farnesol is utilized for protein isoprenylation and the biosynthesis of cholesterol in mammalian cells

Evidence has been obtained indicating that free farnesol (F-OH) can be utilized for isoprenoid biosynthesis in mammalian cells. When rat C6 glial cells and an African green monkey kidney cell line (CV-1) were incubated with [3H]F-OH, radioactivity was incorporated into cholesterol, ubiquinone (CoQ) and isoprenylated proteins. The incorporation of label from [3H]F-OH into cholesterol in C6 and CV-1 cells was blocked by squalestatin 1 (SQ) which specifically inhibits the conversion of farnesyl pyrophosphate (F-P-P) to squalene. This result strongly suggests that cholesterol, and probably CoQ and protein, is metabolically labeled via F-P-P. SDS-PAGE analysis of the delipidated protein fractions from C6 and CV-1 cells revealed several labeled polypeptides. Consistent with these proteins being modified by isoprenylation of cysteine residues. Pronase E digestion released a major labeled product with the chromatographic mobility of [3H]farnesyl-cysteine (F-Cys). A different set of polypeptides was labeled when C6 and CV-1 cells were incubated with [3H]geranylgeraniol (GG-OH). Both sets of proteins appear to be metabolically labeled by [3H]mevalonolactone, and [3H]-labeled F-Cys and geranylgeranyl-cysteine (GG-Cys) were liberated from these proteins by Pronase E treatment. These cellular and biochemical studies indicate that F-OH can be used for isoprenoid biosynthesis and protein isoprenylation in mammalian cells after being converted to F-P-P by phosphorylation reactions that remain to be elucidated.

Mannosylphosphoryldolichol-mediated O-mannosylation of yeast glycoproteins: stereospecificity and recognition of the alpha-isoprene unit by a purified mannosyltransferase

Mannosylphosphoryldolichol (Man-P-Dol):protein O-mannosyltransferase (PMT1) was solubilized by extracting a crude microsomal fraction from Saccharomyces cerevisiae with 1.2% Chaps-0.5% desoxycholate and purified 120-fold by standard chromatographic procedures. These very stable, partially purified preparations of PMT1 catalyzed the transfer of mannosyl units from exogenous Man-P-Dol to serine/threonine residues in the synthetic peptide acceptor, Tyr-Asn-Pro-Thr-Ser-Val-NH2, forming O-mannosidic linkages of the alpha-configuration. The specificity of yeast PMT1 was defined with respect to the recognition of the saturated alpha-isoprene unit, the chain length of the dolichyl moiety, and the anomeric configuration of the mannosyl-phosphoryl linkage of the lipophilic mannosyl donor. When Man-P-Dol95 and mannosylphosphorylpolyprenol (Man-P-Poly95), which contains a fully unsaturated polyprenyl chain, were compared as substrates, the initial rate for peptide mannosylation was dramatically higher with Man-P-Dol95 relative to Man-P-Poly95. The chain length of the dolichyl moiety also influenced the mannolipid-enzyme interaction as the partially purified PMT1 had a higher affinity for Man-P-Dol95 than for Man-P-Dol55. When beta-Man-P-Dol95 was compared with chemically synthesized alpha-Man-P-Dol95 as a mannosyl donor, a strict stereo-specificity was observed for the presence of a beta-mannosyl-phosphoryl linkage. In summary, a procedure for isolating a stable, partially purified preparation of PMT1 from S. cerevisiae is described. Enzymological studies with these preparations of PMT1 provide the first evidence that the recognition of the lipophilic mannosyl donor is stereospecific. These results also demonstrate that maximal O-mannosylation of serine/threonine residues in yeast glycoproteins catalyzed by the partially purified preparation of PMT1 requires the presence of a saturated alpha-isoprene unit in the dolichyl moiety of Man-P-Dol.

Method for the determination of cellular levels of guanosine-5'-diphosphate-mannose based on a weak interaction with concanavalin A at low pH

A rapid and simple two-step procedure for the quantitative analysis of GDP-mannose (GDP-Man) recovered in ethanol extracts of cultured mammalian cells is described. GDP-Man is initially separated from water-soluble metabolites and other nucleotide sugars, including UDP-glucose (UDP-Glc) and GDP-fucose (GDP-Fuc), due to a weak, alpha-mannoside-specific interaction with concanavalin A (Con A)-Sepharose at pH 3.5. The specificity and pH dependence of the GDP-Man-Con A interaction have been characterized. The partially purified fraction from Con A-Sepharose can be further purified by high-performance anion-exchange chromatography on a Partisil-10 SAX silica gel column, and the concentration of GDP-Man was determined by monitoring the HPLC column eluate for absorbance at 254 nm. This procedure provides a simple means of calculating the specific activity of cellular GDP-[3H]Man pools, metabolically labeled with [2-3H]mannose. Using this new procedure, the relative rates of Glc3Man9Glc-NAc2-P-P-dolichol (Oligo-P-P-Dol) biosynthesis and protein N-glycosylation were assayed in C 6 rat glial tumor cells, COS P6 cells, Chinese hamster ovary (CHO) cells, and mouse L929 cells by metabolic labeling with [2-3H]mannose. A comparison of the relative rates of incorporation of [2-3H]mannose into Oligo-P-P-Dol and N-linked oligosaccharides in four different cultured cell lines demonstrates that misleading results can be obtained if the calculation of the biosynthetic rates is not based on the specific activity of the nucleotide sugar pools.

Mannolipid donor specificity of glycosylphosphatidylinositol mannosyltransferase-I (GPIMT-I) determined with an assay system utilizing mutant CHO-K1 cells

The microsomal enzyme glycosylphosphatidylinositol mannosyltransferase I (GPIMT-I) catalyses the transfer of a mannosyl residue from beta-mannosylphosphoryldolichol (beta-Man-P-Dol) to glucosamine-alpha(1,6)(acyl)phosphatidylinositol (GlcN-aPI) to form Man alpha(1,4)GlcN-aPI (ManGlcN-aPI), an intermediate in glycosylphosphatidylinositol (GPI) synthesis. While the transfer of [3H]mannosyl units to endogenous GlcN-aPI was not seen when membrane fractions from normal Chinese hamster ovary (CHO) K1 cells were incubated with exogenous [3H]Man-P-Dol, GPIMT-I activity could be characterized with an in vitro enzyme assay system employing membrane fractions from Lec15 or Lec35 cells. These CHO cell mutants apparently contain elevated levels of endogenous GlcN-aPI due to the inability to synthesize (Lec15) or utilize (Lec35) beta-Man-P-Dol in vivo. The presence of a saturated alpha-isoprene unit in the dolichyl moiety is required for optimal GPIMT-I activity since beta-mannosylphosphorylpolyprenol (beta-Man-P-Poly), which contains a fully unsaturated polyisoprenyl chain, was only 50% as effective as beta-[3H]Man-P-Dol as a mannosyl donor. When beta-[3H]-Man-P-Dol and alpha-[3H]Man-P-Dol were compared as substrates, GPIMT-I exhibited a strict stereospecificity for the mannolipid containing the beta-mannosyl-phosphoryl linkage. beta-[3H]Man-P-dolichols containing 11 or 19 isoprenyl units were equally effective substrates for GPIMT-I. Membrane fractions from Lec 9, a CHO mutant that apparently lacks polyprenol reductase activity and synthesizes very little beta-Man-P-Dol, but accumulates beta-Man-P-Poly, synthesized no detectable Man-GlcN-aPI when incubated with beta-[3H]Man-P-Dol in vitro. This indirect assay suggests that GlcN-aPI does not accumulate in Lec 9 cells, possibly because it is mannosylated via beta-Man-P-Poly, or perhaps the small amount of Man-P-Dol formed by the mutant in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

Utilization of geranylgeraniol for protein isoprenylation in C6 glial cells

When rat C6 glial cells were incubated with [3H]geranylgeraniol (GGol), radioactivity was incorporated into a delipidated protein fraction. SDS-PAGE analysis of the protein fraction labeled by [3H]GGol revealed a 46 kDa polypeptide and a group of labeled polypeptides (19-27 kDa) in the same size range as the small GTP-binding proteins. A similar pattern of labeled polypeptides was seen when C6 cells were metabolically labeled with [14C]mevalonolactone. An isotopically labeled product, chromatographically identical to geranylgeranylcysteine (GG-Cys), was released by Pronase E digestion of the [3H]GGol-labeled protein. When the protein fraction from cells metabolically labeled with [3H]mevalonolactone was digested with Pronase E, two radiolabeled products were released with the chromatographic mobilities of farnesyl-cysteine (F-Cys) and GG-Cys. These studies suggest that C6 glial cells are capable of converting GGol to geranylgeranyl pyrophosphate (GG-P-P), or perhaps a novel "activated" form of the allylic isoprenol, that can be utilized for protein isoprenylation reactions.

Activation of GlcNAc-P-P-dolichol synthesis by mannosylphosphoryldolichol is stereospecific and requires a saturated alpha-isoprene unit

Exogenous mannosylphosphoryldolichol (Man-P-Dol) has previously been shown to stimulate UDP-GlcNAc:dolichyl phosphate N-acetylglucosamine 1-phosphate transferase (GPT1), the enzyme catalyzing the biosynthesis of N-acetylglucosaminylpyrophosphoryldolichol (GlcNAc-P-P-Dol). To define the structural specificity of the mannolipid-mediated activation of GPT1, the ability of a variety of mannosylphosphorylisoprenols to stimulate GlcNAc-lipid biosynthesis in microsomal preparations from retinas of the embryonic chick has been tested. For these comparisons several Man-P-isoprenols were synthesized enzymatically and chemically. The catalytic efficiency of activation expressed as the Vmax/Ka ratio was substantially higher for Man-P-Dol95 than for mannosylphosphorylpolyprenol95 (Man-P-Poly95), demonstrating that the saturated alpha-isoprene unit of the dolichyl moiety influences the mannolipid-enzyme interaction. The degree of activation increased with chain length and hydrophobicity of the dolichyl moiety when Man-P-dolichols containing 2, 11, and 19 isoprene units were evaluated. A strict stereospecificity was exhibited as beta-Man-P-Dol95 provided a 100-fold greater stimulation than the corresponding alpha-stereoisomer. The recognition of the saturated alpha-isoprene unit, the influence of chain length, and the strict stereospecificity of the interaction between beta-Man-P-Dol and GPT1 extend the description of the mannolipid-enzyme interaction and provide strong new evidence that Man-P-Dol levels can influence the rate of GlcNAc-P-P-Dol synthesis.

Induction of dolichyl-saccharide intermediate biosynthesis corresponds to increased long chain cis-isoprenyltransferase activity during the mitogenic response in mouse B cells

There are large increases in the rates of Glc3-Man9GlcNAc2-P-P-Dol (Oligo-P-P-Dol) biosynthesis and protein N-glycosylation during the proliferative response of murine B lymphocytes (B cells) to bacterial lipopolysaccharide (LPS). To learn more about the regulation of dolichyl-saccharide biosynthesis, the possible relationships between developmental changes in specific steps in dolichyl phosphate (Dol-P) and N-acetyl-glucosaminylpyrophosphoryldolichol (GlcNAc-P-P-Dol) biosynthesis and the induction of Oligo-P-P-Dol biosynthesis were investigated. These studies describe an impressive induction of long chain cis-isoprenyltransferase (cis-IPTase) activity, an enzyme system required for the chain elongation stage in de novo Dol-P synthesis, which corresponded to the striking increase in the rate of Oligo-P-P-Dol biosynthesis in LPS-activated B cells. The cellular level and specific activity of cis-IPTase increase 15-fold in LPS-treated cells with relatively unaltered affinity for isopentenyl pyrophosphate. The rates of Dol-P and Oligo-P-P-Dol synthesis increased substantially when cis-IPTase activity was induced, suggesting a regulatory relationship between the level of cis-IPTase activity and lipid intermediate synthesis. Distinctly different developmental patterns were observed for cis-IPTase and HMG-CoA reductase activity, and when sterol biosynthesis was drastically inhibited by lovastatin, the rate of synthesis of Dol-P was slightly higher in the presence of the drug. Modest elevations in the cellular levels of dolichol kinase, Dol-P phosphatase, and UDP-GlcNAc:Dol-P N-acetylglucosaminylphosphoryltransferase (L-G1PT) activities were also observed, but these changes were relatively small compared with the increases in cis-IPTase activity and the rates of Dol-P, Gl-cNAc-P-P-Dol, and Oligo-P-P-Dol synthesis. The expression of the L-G1PT gene is an early event in the developmental program for Oligo-P-P-Dol synthesis, but GlcNAc-P-P-Dol formation is apparently not rate-limiting. In summary, large increases in cis-IPTase activity and the rate of Dol-P biosynthesis appear to play a key regulatory role in the induction of Oligo-P-P-Dol biosynthesis during the proliferative response of B cells to LPS, and the biosynthetic pathways for Dol-P and cholesterol are regulated independently in dividing B cells.

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.

Mannosylphosphoryldolichol-mediated reactions in oligosaccharide-P-P-dolichol biosynthesis. Recognition of the saturated alpha-isoprene unit of the mannosyl donor by pig brain mannosyltransferases

The specificity of Man-P-Dol:Man5-8GlcNAc2-P-P-Dol (Oligo-P-P-Dol) mannosyltransferase activity in pig brain was investigated by comparing a variety of mannosylphosphorylisoprenols as mannosyl donors. For this comparison the beta-Man-P-isoprenols were synthesized using a partially purified preparation of mannosylphosphorylundecaprenol (Man-P-Undec) synthase from Micrococcus luteus. The bacterial mannosyltransferase efficiently catalyzed the transfer of mannose from GDP-[3H]Man to a series of defined isoprenyl monophosphate substrates. Two alpha-Man-P-dolichols were synthesized chemically and also examined as substrates. When exogenous beta-[3H]Man-P-Dol95 was tested as a substrate for Man-P-Dol:Oligo-P-P-Dol mannosyltransferase activity in pig brain microsomes, [3H]mannose was actively transferred to endogenous Oligo-P-P-Dol acceptors. The major enzymatically labeled product was Man9GlcNAc2-P-P-Dol. Under identical conditions beta-[3H]mannosylphosphorylpolyprenol (Man-P-Poly95) was an extremely poor substrate, indicating that the saturated alpha-isoprene unit of the dolichyl moiety is critical for recognition of the lipophilic mannosyl donor by the endoplasmic reticulum-associated mannosyltransferase(s). When Man-P-dolichols containing 2, 11, or 19 isoprene units were compared, the initial rates for the mannosyl transfer reactions and the affinity of the enzyme(s) for the mannophospholipid substrate increased with the length and hydrophobicity of the polyisoprenol chain. The anomeric configuration of the mannosyl moiety is apparently essential because the brain mannosyltransferases exhibited a strong preference for beta-Man-P-dolichols over the corresponding chemically synthesized alpha-stereoisomers. These results: 1) describe a simple two-step procedure for obtaining a partially purified preparation of Man-P-Undec synthase that efficiently synthesizes a variety of beta-Man-P-isoprenols; 2) indicate that pig brain Man-P-Dol:Oligo-P-P-Dol mannosyltransferase activity is relatively specific for lipophilic mannosyl donors containing 19 isoprene units with a beta-Man 1-P group attached to the saturated alpha-isoprene unit of dolichol; and 3) emphasize the importance of the reduction of the alpha-isoprene unit in the biosynthesis and function of Dol-P in mammalian cells.