Lipid dependence and basic kinetics of the purified 1,2-diacylglycerol 3-glucosyltransferase from membranes of Acholeplasma laidlawii

Characterization of a Glycolipid Synthase Producing α-Galactosylceramide in Bacteroides fragilis

Glycolipids are complex molecules involved in important cellular processes. Among them, the glycosphingolipid α-galactosylceramide has proven to be of interest in biomedicine for its immunostimulatory capabilities. Given its structural requirements, the use of ceramide glycosyltransferase enzymes capable of synthesizing this molecule under in vivo or in vitro conditions is a potential production strategy. Several GT4 enzymes from Bacteroides fragilis were considered as potential candidates in addition to the known BF9343_3149, but only this one showed glycolipid synthase activity. The enzyme was expressed as a SUMO fusion protein to produce soluble protein. It is a non-processive glycosyltransferase that prefers UDP-Gal over UDP-Glc as a donor substrate, and maximum activity was found at pH 7.3 and around 30-35 °C. It does not require metal cations for activity as other GT4 enzymes, but Zn²⁺ inactivates the enzyme. The reaction occurs when the ceramide lipid acceptor is solubilized with BSA (100% conversion) but not when it is presented in mixed micelles, and anionic lipids do not increase activity, as in other membrane-associated glycolipid synthases. Further protein engineering to increase stability and activity can make feasible the enzymatic synthesis of α-GalCer for biomedical applications.

Two bacterial glycosphingolipid synthases responsible for the synthesis of glucuronosylceramide and α-galactosylceramide

Bacterial glycosphingolipids such as glucuronosylceramide and galactosylceramide have been identified as ligands for invariant natural killer T cells and play important roles in host defense. However, the glycosphingolipid synthases required for production of these ceramides have not been well-characterized. Here, we report the identification and characterization of glucuronosylceramide synthase (ceramide UDP-glucuronosyltransferase [Cer-GlcAT]) in Zymomonas mobilis, a Gram-negative bacterium whose cellular membranes contain glucuronosylceramide. On comparing the gene sequences that encode the diacylglycerol GlcAT in bacteria and plants, we found a homologous gene that is widely distributed in the order Sphingomonadales in the Z. mobilis genome. We first cloned the gene and expressed it in Escherichia coli, followed by protein purification using nickel-Sepharose affinity and gel filtration chromatography. Using the highly enriched enzyme, we observed that it has high glycosyltransferase activity with UDP-glucuronic acid and ceramide as sugar donor and acceptor substrate, respectively. Cer-GlcAT deletion resulted in a loss of glucuronosylceramide and increased the levels of ceramide phosphoglycerol, which was expressed in WT cells only at very low levels. Furthermore, we found sequences homologous to Cer-GlcAT in Sphingobium yanoikuyae and Bacteroides fragilis, which have been reported to produce glucuronosylceramide and α-galactosylceramide, respectively. We expressed the two homologs of the cer-glcat gene in E. coli and found that each gene encodes Cer-GlcAT and Cer-galactosyltransferase, respectively. These results contribute to the understanding of the roles of bacterial glycosphingolipids in host-bacteria interactions and the function of bacterial glycosphingolipids in bacterial physiology.

Dissection of membrane-binding and -remodeling regions in two classes of bacterial phospholipid N-methyltransferases

Bacterial phospholipid N-methyltransferases (Pmts) catalyze the formation of phosphatidylcholine (PC) via successive N-methylation of phosphatidylethanolamine (PE). They are classified into Sinorhizobium-type and Rhodobacter-type enzymes. The Sinorhizobium-type PmtA protein from the plant pathogen Agrobacterium tumefaciens is recruited to anionic lipids in the cytoplasmic membrane via two amphipathic helices called αA and αF. Besides its enzymatic activity, PmtA is able to remodel membranes mediated by the αA domain. According to the Heliquest program, αA- and αF-like amphipathic helices are also present in other Sinorhizobium- and Rhodobacter-type Pmt enzymes suggesting a conserved architecture of α-helical membrane-binding regions in these methyltransferases. As representatives of the two Pmt families, we investigated the membrane binding and remodeling capacity of Bradyrhizobium japonicum PmtA (Sinorhizobium-type) and PmtX1 (Rhodobacter-type), which act cooperatively to produce PC in consecutive methylation steps. We found that the αA regions in both enzymes bind anionic lipids similar to αA of A. tumefaciens PmtA. Membrane binding of PmtX1 αA is enhanced by its substrate monomethyl-PE indicating a substrate-controlled membrane association. The αA regions of all investigated enzymes remodel spherical liposomes into tubular filaments suggesting a conserved membrane-remodeling capacity of bacterial Pmts. Based on these results we propose that the molecular details of membrane-binding and remodeling are conserved among bacterial Pmts.

Membrane remodeling capacity of a vesicle-inducing glycosyltransferase

Intracellular vesicles are abundant in eukaryotic cells but absent in the Gram-negative bacterium Escherichia coli. However, strong overexpression of a monotopic glycolipid-synthesizing enzyme, monoglucosyldiacylglycerol synthase from Acholeplasma laidlawii (alMGS), leads to massive formation of vesicles in the cytoplasm of E. coli. More importantly, alMGS provides a model system for the regulation of membrane properties by membrane-bound enzymes, which is critical for maintaining cellular integrity. Both phenomena depend on how alMGS binds to cell membranes, which is not well understood. Here, we carry out a comprehensive investigation of the membrane binding of alMGS by combining bioinformatics methods with extensive biochemical studies, structural modeling and molecular dynamics simulations. We find that alMGS binds to the membrane in a fairly upright manner, mainly by residues in the N-terminal domain, and in a way that induces local enrichment of anionic lipids and a local curvature deformation. Furthermore, several alMGS variants resulting from substitution of residues in the membrane anchoring segment are still able to generate vesicles, regardless of enzymatic activity. These results clarify earlier theories about the driving forces for vesicle formation, and shed new light on the membrane binding properties and enzymatic mechanism of alMGS and related monotopic GT-B fold glycosyltransferases.

Structure-function relationships of membrane-associated GT-B glycosyltransferases

Membrane-associated GT-B glycosyltransferases (GTs) comprise a large family of enzymes that catalyze the transfer of a sugar moiety from nucleotide-sugar donors to a wide range of membrane-associated acceptor substrates, mostly in the form of lipids and proteins. As a consequence, they generate a significant and diverse amount of glycoconjugates in biological membranes, which are particularly important in cell-cell, cell-matrix and host-pathogen recognition events. Membrane-associated GT-B enzymes display two "Rossmann-fold" domains separated by a deep cleft that includes the catalytic center. They associate permanently or temporarily to the phospholipid bilayer by a combination of hydrophobic and electrostatic interactions. They have the remarkable property to access both hydrophobic and hydrophilic substrates that reside within chemically distinct environments catalyzing their enzymatic transformations in an efficient manner. Here, we discuss the considerable progress that has been made in recent years in understanding the molecular mechanism that governs substrate and membrane recognition, and the impact of the conformational transitions undergone by these GTs during the catalytic cycle.

Expression and characterization of a Mycoplasma genitalium glycosyltransferase in membrane glycolipid biosynthesis: potential target against mycoplasma infections

Mycoplasmas contain glycoglycerolipids in their plasma membrane as key structural components involved in bilayer properties and stability. A membrane-associated glycosyltransferase (GT), GT MG517, has been identified in Mycoplasma genitalium, which sequentially produces monoglycosyl- and diglycosyldiacylglycerols. When recombinantly expressed in Escherichia coli, the enzyme was functional in vivo and yielded membrane glycolipids from which Glcβ1,6GlcβDAG was identified as the main product. A chaperone co-expression system and extraction with CHAPS detergent afforded soluble protein that was purified by affinity chromatography. GT MG517 transfers glucosyl and galactosyl residues from UDP-Glc and UDP-Gal to dioleoylglycerol (DOG) acceptor to form the corresponding β-glycosyl-DOG, which then acts as acceptor to give β-diglycosyl-DOG products. The enzyme (GT2 family) follows Michaelis-Menten kinetics. k(cat) is about 5-fold higher for UDP-Gal with either DOG or monoglucosyldioleoylglycerol acceptors, but it shows better binding for UDP-Glc than UDP-Gal, as reflected by the lower K(m), which results in similar k(cat)/K(m) values for both donors. Although sequentially adding glycosyl residues with β-1,6 connectivity, the first glycosyltransferase activity (to DOG) is about 1 order of magnitude higher than the second (to monoglucosyldioleoylglycerol). Because the ratio between the non-bilayer-forming monoglycosyldiacylglycerols and the bilayer-prone diglycosyldiacylglycerols contributes to regulate the properties of the plasma membrane, both synthase activities are probably regulated. Dioleoylphosphatidylglycerol (anionic phospholipid) activates the enzyme, k(cat) linearly increasing with dioleoylphosphatidylglycerol concentration. GT MG517 is shown to be encoded by an essential gene, and the addition of GT inhibitors results in cell growth inhibition. It is proposed that glycolipid synthases are potential targets for drug discovery against infections by mycoplasmas.

Tryptophan residues promote membrane association for a plant lipid glycosyltransferase involved in phosphate stress

Chloroplast membranes contain a substantial excess of the nonbilayer-prone monogalactosyldiacylglycerol (GalDAG) over the biosynthetically consecutive, bilayer-forming digalactosyldiacylglycerol (GalGalDAG), yielding a high membrane curvature stress. During phosphate shortage, plants replace phospholipids with GalGalDAG to rescue phosphate while maintaining membrane homeostasis. Here we investigate how the activity of the corresponding glycosyltransferase (GT) in Arabidopsis thaliana (atDGD2) depends on local bilayer properties by analyzing structural and activity features of recombinant protein. Fold recognition and sequence analyses revealed a two-domain GT-B monotopic structure, present in other plant and bacterial glycolipid GTs, such as the major chloroplast GalGalDAG GT atDGD1. Modeling led to the identification of catalytically important residues in the active site of atDGD2 by site-directed mutagenesis. The DGD synthases share unique bilayer interface segments containing conserved tryptophan residues that are crucial for activity and for membrane association. More detailed localization studies and liposome binding analyses indicate differentiated anchor and substrate-binding functions for these separated enzyme interface regions. Anionic phospholipids, but not curvature-increasing nonbilayer lipids, strongly stimulate enzyme activity. From our studies, we propose a model for bilayer "control" of enzyme activity, where two tryptophan segments act as interface anchor points to keep the substrate region close to the membrane surface. Binding of the acceptor substrate is achieved by interaction of positive charges in a surface cluster of lysines, arginines, and histidines with the surrounding anionic phospholipids. The diminishing phospholipid fraction during phosphate shortage stress will then set the new GalGalDAG/phospholipid balance by decreasing stimulation of atDGD2.

High-yield expression and purification of a monotopic membrane glycosyltransferase

Membrane proteins are essential to many cellular processes. However, the systematic study of membrane protein structure has been hindered by the difficulty in obtaining large quantities of these proteins. Protein overexpression using Escherichia coli is commonly used to produce large quantities of protein, but usually yields very little membrane protein. Furthermore, optimization of the expressing conditions, as well as the choice of detergent and other buffer components, is thought to be crucial for increasing the yield of stable and homogeneous protein. Herein we report high-yield expression and purification of a membrane-associated monotopic protein, the glycosyltransferase monoglucosyldiacylglycerol synthase (alMGS), in E. coli. Systematic optimization of protein expression was achieved through controlling a few basic expression parameters, including temperature and growth media, and the purifications were monitored using a fast and efficient size-exclusion chromatography (SEC) screening method. The latter method was shown to be a powerful tool for fast screening and for finding the optimal protein-stabilizing conditions. For alMGS it was found that the concentration of detergent was just as important as the type of detergent, and a low concentration of n-dodecyl-beta-D-maltoside (DDM) (approximately 1x critical micelle concentration) was the best for keeping the protein stable and homogeneous. By using these simply methods to optimize the conditions for alMGS expression and purification, the final expression level increase by two orders of magnitude, reaching 170 mg of pure protein per litre culture.

Biophysical regulation of lipid biosynthesis in the plasma membrane

We present a cellular model of lipid biosynthesis in the plasma membrane that couples biochemical and biophysical features of the enzymatic network of the cell-wall-less Mycoplasma Acholeplasma laidlawii. In particular, we formulate how the stored elastic energy of the lipid bilayer can modify the activity of curvature-sensitive enzymes through the binding of amphipathic alpha-helices. As the binding depends on lipid composition, this results in a biophysical feedback mechanism for the regulation of the stored elastic energy. The model shows that the presence of feedback increases the robustness of the steady state of the system, in the sense that biologically inviable nonbilayer states are less likely. We also show that the biophysical and biochemical features of the network have implications as to which enzymes are most efficient at implementing the regulation. The network imposes restrictions on the steady-state balance between bilayer and nonbilayer lipids and on the concentrations of particular lipids. Finally, we consider the influence of the length of the amphipathic alpha-helix on the efficacy of the feedback and propose experimental measurements and extensions of the modeling framework.

In vitro biosynthesis of ether-type glycolipids in the methanoarchaeon Methanothermobacter thermautotrophicus

The biosynthesis of archaeal ether-type glycolipids was investigated in vitro using Methanothermobacter thermautotrophicus cell-free homogenates. The sole sugar moiety of glycolipids and phosphoglycolipids of the organism is the beta-D-glucosyl-(1-->6)-D-glucosyl (gentiobiosyl) unit. The enzyme activities of archaeol:UDP-glucose beta-glucosyltransferase (monoglucosylarchaeol [MGA] synthase) and MGA:UDP-glucose beta-1,6-glucosyltransferase (diglucosylarchaeol [DGA] synthase) were found in the methanoarchaeon. The synthesis of DGA is probably a two-step glucosylation: (i) archaeol + UDP-glucose --> MGA + UDP, and (ii) MGA + UDP-glucose --> DGA + UDP. Both enzymes required the addition of K(+) ions and archaetidylinositol for their activities. DGA synthase was stimulated by 10 mM MgCl(2), in contrast to MGA synthase, which did not require Mg(2+). It was likely that the activities of MGA synthesis and DGA synthesis were carried out by different proteins because of the Mg(2+) requirement and their cellular localization. MGA synthase and DGA synthase can be distinguished in cell extracts greatly enriched for each activity by demonstrating the differing Mg(2+) requirements of each enzyme. MGA synthase preferred a lipid substrate with the sn-2,3 stereostructure of the glycerol backbone on which two saturated isoprenoid chains are bound at the sn-2 and sn-3 positions. A lipid substrate with unsaturated isoprenoid chains or sn-1,2-dialkylglycerol configuration exhibited low activity. Tetraether-type caldarchaetidylinositol was also actively glucosylated by the homogenates to form monoglucosyl caldarchaetidylinositol and a small amount of diglucosyl caldarchaetidylinositol. The addition of Mg(2+) increased the formation of diglucosyl caldarchaetidylinositol. This suggested that the same enzyme set synthesized the sole sugar moiety of diether-type glycolipids and tetraether-type phosphoglycolipids.

Monoglucosyldiacylglycerol, a Foreign Lipid, Can Substitute for Phosphatidylethanolamine in Essential Membrane-associated Functions in Escherichia coli

The mechanisms by which lipid bilayer properties govern or influence membrane protein functions are little understood, but a liquid-crystalline state and the presence of anionic and nonbilayer (NB)-prone lipids seem important. An Escherichia coli mutant lacking the major membrane lipid phosphatidylethanolamine (NB-prone) requires divalent cations for viability and cell integrity and is impaired in several membrane functions that are corrected by introduction of the "foreign" NB-prone neutral glycolipid alpha-monoglucosyldiacylglycerol (MGlcDAG) synthesized by the MGlcDAG synthase from Acholeplasma laidlawii. Dependence on Mg(2+) was reduced, and cellular yields and division malfunction were greatly improved. The increased passive membrane permeability of the mutant was not abolished, but protein-mediated osmotic stress adaptation to salts and sucrose was recovered by the presence of MGlcDAG. MGlcDAG also restored tryptophan prototrophy and active transport function of lactose permease, both critically dependent on phosphatidylethanolamine. Three mechanisms can explain the observed effects: NB-prone MGlcDAG improves the quenched lateral pressure profile across the bilayer; neutral MGlcDAG dilutes the high anionic lipid surface charge; MGlcDAG provides a neutral lipid that can hydrogen bond and/or partially ionize. The reduced dependence on Mg(2+) and lack of correction by high monovalent salts strongly support the essential nature of the NB properties of MGlcDAG.

Lateral organization in Acholeplasma laidlawii lipid bilayer models containing endogenous pyrene probes

In membranes of the small prokaryote Acholeplasma laidlawii bilayer- and nonbilayer-prone glycolipids are major species, similar to chloroplast membranes. Enzymes of the glucolipid pathway keep certain important packing properties of the bilayer in vivo, visualized especially as a monolayer curvature stress ('spontaneous curvature'). Two key enzymes depend in a cooperative fashion on substantial amounts of the endogenous anionic lipid phosphatidylglycerol (PG) for activity. The lateral organization of five unsaturated A. laidlawii lipids was analyzed in liposome model bilayers with the use of endogenously produced pyrene-lipid probes, and extensive experimental designs. Of all lipids analyzed, PG especially promoted interactions with the precursor diacylglycerol (DAG), as revealed from pyrene excimer ratio (Ie/Im) responses. Significant interactions were also recorded within the major nonbilayer-prone monoglucosylDAG (MGlcDAG) lipids. The anionic precursor phosphatidic acid (PA) was without effects. Hence, a heterogeneous lateral lipid organization was present in these liquid-crystalline bilayers. The MGlcDAG synthase when binding at the PG bilayer interface, decreased acyl chain ordering (increase of membrane free volume) according to a bis-pyrene-lipid probe, but the enzyme did not influence the bulk lateral lipid organization as recorded from DAG or PG probes. It is concluded that the concentration of the substrate DAG by PG is beneficial for the MGlcDAG synthase, but that binding in a proper orientation/conformation seems most important for activity.

Structural features of glycosyltransferases synthesizing major bilayer and nonbilayer-prone membrane lipids in Acholeplasma laidlawii and Streptococcus pneumoniae

In membranes of Acholeplasma laidlawii two consecutively acting glucosyltransferases, the (i) alpha-monoglucosyldiacylglycerol (MGlcDAG) synthase (alMGS) (EC ) and the (ii) alpha-diglucosyl-DAG (DGlcDAG) synthase (alDGS) (EC ), are involved in maintaining (i) a certain anionic lipid surface charge density and (ii) constant nonbilayer/bilayer conditions (curvature packing stress), respectively. Cloning of the alDGS gene revealed related uncharacterized sequence analogs especially in several Gram-positive pathogens, thermophiles and archaea, where the encoded enzyme function of a potential Streptococcus pneumoniae DGS gene (cpoA) was verified. A strong stimulation of alDGS by phosphatidylglycerol (PG), cardiolipin, or nonbilayer-prone 1,3-DAG was observed, while only PG stimulated CpoA. Several secondary structure prediction and fold recognition methods were used together with SWISS-MODEL to build three-dimensional model structures for three MGS and two DGS lipid glycosyltransferases. Two Escherichia coli proteins with known structures were identified as the best templates, the membrane surface-associated two-domain glycosyltransferase MurG and the soluble GlcNAc epimerase. Differences in electrostatic surface potential between the different models and their individual domains suggest that electrostatic interactions play a role for the association to membranes. Further support for this was obtained when hybrids of the N- and C-domain, and full size alMGS with green fluorescent protein were localized to different regions of the E. coli inner membrane and cytoplasm in vivo. In conclusion, it is proposed that the varying abilities to bind, and sense lipid charge and curvature stress, are governed by typical differences in charge (pI values), amphiphilicity, and hydrophobicity for the N- and (catalytic) C-domains of these structurally similar membrane-associated enzymes.

Coexistence of domains with distinct order and polarity in fluid bacterial membranes

In this study we sought the detection and characterization of bacterial membrane domains. Fluorescence generalized polarization (GP) spectra of laurdan-labeled Escherichia coli and temperature dependencies of both laurdan's GP and fluorescence anisotropy of 1,3-diphenyl-1,3,5-hexatriene (DPH) (rDPH) affirmed that at physiological temperatures, the E. coli membrane is in a liquid-crystalline phase. However, the strong excitation wavelength dependence of rlaurdan at 37 degrees C reflects membrane heterogeneity. Time-resolved fluorescence emission spectra, which display distinct biphasic redshift kinetics, verified the coexistence of two subpopulations of laurdan. In the initial phase, <50 ps, the redshift in the spectral mass center is much faster for laurdan excited at the blue edge (350 nm), whereas at longer time intervals, similar kinetics is observed upon excitation at either blue or red edge (400 nm). Excitation in the blue region selects laurdan molecules presumably located in a lipid domain in which fast intramolecular relaxation and low anisotropy characterize laurdan's emission. In the proteo-lipid domain, laurdan motion and conformation are restricted as exhibited by a slower relaxation rate, higher anisotropy and a lower GP value. Triple-Gaussian decomposition of laurdan emission spectra showed a sharp phase transition in the temperature dependence of individual components when excited in the blue but not in the red region. At least two kinds of domains of distinct polarity and order are suggested to coexist in the liquid-crystalline bacterial membrane: a lipid-enriched and a proteolipid domain. In bacteria with chloramphenicol (Cam)-inhibited protein synthesis, laurdan showed reduced polarity and restoration of an isoemissive point in the temperature-dependent spectra. These results suggest a decrease in membrane heterogeneity caused by Cam-induced domain dissipation.

Sequence properties of the 1,2-diacylglycerol 3-glucosyltransferase from Acholeplasma laidlawii membranes. Recognition of a large group of lipid glycosyltransferases in eubacteria and archaea

Synthesis of the nonbilayer-prone alpha-monoglucosyldiacylglycerol (MGlcDAG) is crucial for bilayer packing properties and the lipid surface charge density in the membrane of Acholeplasma laidlawii. The gene for the responsible, membrane-bound glucosyltransferase (alMGS) (EC ) was sequenced and functionally cloned in Escherichia coli, yielding MGlcDAG in the recombinants. Similar amino acid sequences were encoded in the genomes of several Gram-positive bacteria (especially pathogens), thermophiles, archaea, and a few eukaryotes. All of these contained the typical EX(7)E catalytic motif of the CAZy family 4 of alpha-glycosyltransferases. The synthesis of MGlcDAG by a close sequence analog from Streptococcus pneumoniae (spMGS) was verified by polymerase chain reaction cloning, corroborating a connection between sequence and functional similarity for these proteins. However, alMGS and spMGS varied in dependence on anionic phospholipid activators phosphatidylglycerol and cardiolipin, suggesting certain regulatory differences. Fold predictions strongly indicated a similarity for alMGS (and spMGS) with the two-domain structure of the E. coli MurG cell envelope glycosyltransferase and several amphipathic membrane-binding segments in various proteins. On the basis of this structure, the alMGS sequence charge distribution, and anionic phospholipid dependence, a model for the bilayer surface binding and activity is proposed for this regulatory enzyme.

alpha-methylene ordering of acyl chains differs in glucolipids and phosphatidylglycerol from Acholeplasma laidlawii membranes: (2)H-NMR quadrupole splittings from individual lipids in mixed bilayers

A Acholeplasma laidlawii strain A-EF22 was grown in a medium supplemented with alpha-deuterated oleic acid. Phosphatidylglycerol (PG), the glucolipids monoglucosyldiacylglycerol (MGlcDAG), diglucosyldiacylglycerol (DGlcDAG) and monoacyldiglucosyldiacylglycerol, and the phosphoglucolipid glycerophosphoryldiglucosyldiacylglycerol (GPDGlcDAG) were purified, and the phase behaviour and molecular ordering for the individual lipids, as well as for mixtures of the lipids, were studied by (2)H-, (31)P-NMR and X-ray scattering methods. The chemical structure of all the A. laidlawii lipids, except PG, has been determined and verified previously; here also the chemical structure of PG was verified, utilising mass spectrometry and (1)H and (13)C high resolution NMR spectroscopy. For the first time, lipid dimers were found in the mass spectrometry measurements. The major findings in this work are: (1) addition of 50 mol% of PG to the non-lamellar-forming lipid MGlcDAG does not significantly alter the transition temperature between lamellar and non-lamellar phases; (2) the (2)H-NMR quadrupole splitting patterns obtained from the lamellar liquid crystalline phase are markedly different for PG on one hand, and DGlcDAG and GPDGlcDAG on the other hand; and (3) mixtures of PG and DGlcDAG or MGlcDAG give rise to (2)H-NMR spectra consisting of a superposition of splitting patterns of the individual lipids. These remarkable features show that the local ordering of the alpha-carbon of the acyl chains is different for PG than for MGlcDAG and DGlcDAG, and that this difference is preserved when PG is mixed with the glucolipids. The results obtained are interpreted in terms of differences in molecular shape and hydrophilicity of the different polar headgroups.

Novel processive and nonprocessive glycosyltransferases from Staphylococcus aureus and Arabidopsis thaliana synthesize glycoglycerolipids, glycophospholipids, glycosphingolipids and glycosylsterols

A processive diacylglycerol glucosyltransferase has recently been identified from Bacillus subtilis [Jorasch, P., Wolter, F.P., Zähringer, U., and Heinz, E. (1998) Mol. Microbiol. 29, 419-430]. Now we report the cloning and characterization of two other genes coding for diacylglycerol glycosyltransferases from Staphylococcus aureus and Arabidopsis thaliana; only the S. aureus enzyme shows processivity similar to the B. subtilis enzyme. Both glycosyltransferases characterized in this work show unexpected acceptor specificities. We describe the isolation of the ugt106B1 gene (GenBank accession number Y14370) from the genomic DNA of S. aureus and the ugt81A1 cDNA (GenBank accession number AL031004) from A. thaliana by PCR. After cloning and expression of S. aureus Ugt106B1 in Escherichia coli, SDS/PAGE of total cell extracts showed strong expression of a protein having the predicted size of 44 kDa. Thin-layer chromatographic analysis of the lipids extracted from the transformed E. coli cells revealed several new glycolipids and phosphoglycolipids not present in the controls. These lipids were purified from lipid extracts of E. coli cells expressing the S. aureus gene and identified by NMR and mass spectrometry as 1, 2-diacyl-3-[O-beta-D-glucopyranosyl]-sn-glycerol, 1, 2-diacyl-3-[O-beta-D-glucopyranosyl-(1-->6)-O-beta-D-glucopyrano-+ ++syl] -sn-glycerol, 1, 2-diacyl-3-[O-beta-D-glucopyranosyl-(1-->6)-O-beta-D-glucopyranosyl-( 1-->6)-O-beta-D-glucopyranosyl]-sn-glycerol, sn-3'-[O-beta-D-glucopyranosyl]-phosphatidylglycerol and sn-3'-[O-(6"'-O-acyl)-beta-D-glucopyranosyl-(1"'-->6")-O-beta-D-gluco pyranosyl]-sn-2'-acyl-phospha-tidylglycerol. A 1, 2-diacyl-3-[O-beta-D-galactopyranosyl]-sn-glycerol was isolated from extracts of E. coli cells expressing the ugt81A1 cDNA from A. thaliana. The enzymatic activities expected to catalyze the synthesis of these compounds were confirmed by in vitro assays with radioactive substrates. Experiments with several of the above described glycolipids as 14C-labeled sugar acceptors and unlabeled UDP-glucose as glucose donor, suggest that the ugt106B1 gene codes for a processive UDP-glucose:1, 2-diacylglycerol-3-beta-D-glucosyltransferase, whereas ugt81A1 codes for a nonprocessive diacylglycerol galactosyltransferase. As shown in additional assays with different lipophilic acceptors, both enzymes use diacylglycerol and ceramide, but Ugt106B1 also accepts glucosyl ceramide as well as cholesterol and cholesterol glucoside as sugar acceptors.

The nonbilayer/bilayer lipid balance in membranes. Regulatory enzyme in Acholeplasma laidlawii is stimulated by metabolic phosphates, activator phospholipids, and double-stranded DNA

In membranes of Acholeplasma laidlawii a single glucosyltransferase step between the major, nonbilayer-prone monoglucosyl-diacylglycerol (MGlcDAG) and the bilayer-forming diglucosyl-diacylglycerol (DGlcDAG) is important for maintenance of lipid phase equilibria and curvature packing stress. This DGlcDAG synthase is activated in a cooperative fashion by phosphatidylglycerol (PG), but in vivo PG amounts are not enough for efficient DGlcDAG synthesis. In vitro, phospholipids with an sn-glycero-3-phosphate backbone, and no positive head group charge, functioned as activators. Different metabolic, soluble phosphates could supplement PG for activation, depending on type, amount, and valency. Especially efficient were the glycolytic intermediates fructose 1,6-bisphosphate and ATP, active at cellular concentrations on the DGlcDAG but not on the preceding MGlcDAG synthase. Potencies of different phosphatidylinositol (foreign lipid) derivatives differed with numbers and positions of their phosphate moieties. A selective stimulation of the DGlcDAG, but not the MGlcDAG synthase, by minor amounts of double-stranded DNA was additive to the best phospholipid activators. These results support two types of activator sites on the enzyme: (i) lipid-phosphate ones close to the membrane interphase, and (ii) soluble (or particulate)-phosphate ones further out from the surface. Thereby, the nonbilayer (MGlcDAG) to bilayer (DGlcDAG) lipid balance may be integrated with the metabolic status of the cell and potentially also to membrane and cell division.

Transcription- and translation-dependent changes in membrane dynamics in bacteria: testing the transertion model for domain formation

Cell cycle events have been proposed to be triggered by the formation of membrane domains in the process of coupled transcription, translation and insertion ('transertion') of nascent membrane and exported proteins. Disruption of domain structure should lead to changes in membrane dynamics. Membrane viscosity of Escherichia coli and Bacillus subtilis decreased after inhibition of protein synthesis by chloramphenicol or puromycin, or of RNA initiation by rifampicin, but not after inhibition of RNA elongation by streptolydigin or amino acid starvation of a stringent strain. The decrease caused by inhibitors of protein synthesis was prevented by streptolydigin if added simultaneously, but was not reversed if added later. The drug-induced decrease in membrane viscosity is energy dependent: it did not happen in KCN-treated cells. All treatments decreasing membrane viscosity also induced nucleoid compaction and fusion. Inhibition of macromolecular synthesis without membrane perturbation caused nucleoids to expand. Changes in membrane dynamics were also displayed during a nutritional shift-down transition that causes imbalance in macromolecular syntheses. The results are consistent with the transertion model, predicting dissipation of membrane domains by termination of protein synthesis or detachment of polysomes from DNA; domain structure is conserved if the transertion process is 'frozen'.

Molecular biology and pathogenicity of mycoplasmas

The recent sequencing of the entire genomes of Mycoplasma genitalium and M. pneumoniae has attracted considerable attention to the molecular biology of mycoplasmas, the smallest self-replicating organisms. It appears that we are now much closer to the goal of defining, in molecular terms, the entire machinery of a self-replicating cell. Comparative genomics based on comparison of the genomic makeup of mycoplasmal genomes with those of other bacteria, has opened new ways of looking at the evolutionary history of the mycoplasmas. There is now solid genetic support for the hypothesis that mycoplasmas have evolved as a branch of gram-positive bacteria by a process of reductive evolution. During this process, the mycoplasmas lost considerable portions of their ancestors' chromosomes but retained the genes essential for life. Thus, the mycoplasmal genomes carry a high percentage of conserved genes, greatly facilitating gene annotation. The significant genome compaction that occurred in mycoplasmas was made possible by adopting a parasitic mode of life. The supply of nutrients from their hosts apparently enabled mycoplasmas to lose, during evolution, the genes for many assimilative processes. During their evolution and adaptation to a parasitic mode of life, the mycoplasmas have developed various genetic systems providing a highly plastic set of variable surface proteins to evade the host immune system. The uniqueness of the mycoplasmal systems is manifested by the presence of highly mutable modules combined with an ability to expand the antigenic repertoire by generating structural alternatives, all compressed into limited genomic sequences. In the absence of a cell wall and a periplasmic space, the majority of surface variable antigens in mycoplasmas are lipoproteins. Apart from providing specific antimycoplasmal defense, the host immune system is also involved in the development of pathogenic lesions and exacerbation of mycoplasma induced diseases. Mycoplasmas are able to stimulate as well as suppress lymphocytes in a nonspecific, polyclonal manner, both in vitro and in vivo. As well as to affecting various subsets of lymphocytes, mycoplasmas and mycoplasma-derived cell components modulate the activities of monocytes/macrophages and NK cells and trigger the production of a wide variety of up-regulating and down-regulating cytokines and chemokines. Mycoplasma-mediated secretion of proinflammatory cytokines, such as tumor necrosis factor alpha, interleukin-1 (IL-1), and IL-6, by macrophages and of up-regulating cytokines by mitogenically stimulated lymphocytes plays a major role in mycoplasma-induced immune system modulation and inflammatory responses.

A UDP glucosyltransferase from Bacillus subtilis successively transfers up to four glucose residues to 1,2-diacylglycerol: expression of ypfP in Escherichia coli and structural analysis of its reaction products

We have isolated the ypfP gene (accession number P54166) from genomic DNA of Bacillus subtilis Marburg strain 60015 (Freese and Fortnagel, 1967) using PCR. After cloning and expression in E. coli, SDS-PAGE showed strong expression of a protein that had the predicted size of 43.6 kDa. Chromatographic analysis of the lipids extracted from the transformed E. coli revealed several new glycolipids. These glycolipids were isolated and their structures determined by nuclear magnetic resonance (NMR) and mass spectrometry. They were identified as 3-[O-beta-D-glucopyranosyl-(1-->6)-O-beta-D-glucopyranosyl]-1,2-diacylgl ycerol, 3-[O-beta-D-glucopyranosyl-(1-->6)-O-beta-D-glucopyranosyl-(1-->6)-O-bet a-D-glucopyranosyl]-1,2-diacylglycerol and 3-[O-beta-D-glucopyranosyl-(1-->6)-O-beta-D-glucopyranosyl-(1-->6)-O-bet a-D-glucopyranosyl-(1-->6)-O-beta-D-glucopyranosyl]-1,2-diacylglycerol. The enzymatic activity expected to catalyse the synthesis of these compounds was confirmed by in vitro assays with radioactive substrates. In these assays, one additional glycolipid was formed and tentatively identified as 3-[O-beta-D-glucopyranosyl]-1,2-diacylglycerol, which was not detected in the lipid extract of transformed cells. Experiments with some of the above-described glycolipids as 14C-labelled sugar acceptors and unlabelled UDP-glucose as glucose donor suggest that the ypfP gene codes for a new processive UDP-glucose: 1,2-diacylglycerol-3-beta-D-glucosyl transferase. This glucosyltransferase can use diacylglycerol, monoglucosyl-diacylglycerol, diglucosyl diacylglycerol or triglucosyl diacylglycerol as sugar acceptor, which, apart from the first member, are formed by repetitive addition of a glucopyranosyl residue in beta (1-->6) linkage to the product of the preceding reaction.

The comparative metabolism of the mollicutes (Mycoplasmas): the utility for taxonomic classification and the relationship of putative gene annotation and phylogeny to enzymatic function in the smallest free-living cells

Mollicutes or mycoplasmas are a class of wall-less bacteria descended from low G + C% Gram-positive bacteria. Some are exceedingly small, about 0.2 micron in diameter, and are examples of the smallest free-living cells known. Their genomes are equally small; the smallest in Mycoplasma genitalium is sequenced and is 0.58 mb with 475 ORFs, compared with 4.639 mb and 4288 ORFs for Escherichia coli. Because of their size and apparently limited metabolic potential, Mollicutes are models for describing the minimal metabolism necessary to sustain independent life. Mollicutes have no cytochromes or the TCA cycle except for malate dehydrogenase activity. Some uniquely require cholesterol for growth, some require urea and some are anaerobic. They fix CO2 in anaplerotic or replenishing reactions. Some require pyrophosphate not ATP as an energy source for reactions, including the rate-limiting step of glycolysis: 6-phosphofructokinase. They scavenge for nucleic acid precursors and apparently do not synthesize pyrimidines or purines de novo. Some genera uniquely lack dUTPase activity and some species also lack uracil-DNA glycosylase. The absence of the latter two reactions that limit the incorporation of uracil or remove it from DNA may be related to the marked mutability of the Mollicutes and their tachytelic or rapid evolution. Approximately 150 cytoplasmic activities have been identified in these organisms, 225 to 250 are presumed to be present. About 100 of the core reactions are graphically linked in a metabolic map, including glycolysis, pentose phosphate pathway, arginine dihydrolase pathway, transamination, and purine, pyrimidine, and lipid metabolism. Reaction sequences or loci of particular importance are also described: phosphofructokinases, NADH oxidase, thioredoxin complex, deoxyribose-5-phosphate aldolase, and lactate, malate, and glutamate dehydrogenases. Enzymatic activities of the Mollicutes are grouped according to metabolic similarities that are taxonomically discriminating. The arrangements attempt to follow phylogenetic relationships. The relationships of putative gene assignments and enzymatic function in My. genitalium, My. pneumoniae, and My. capricolum subsp. capricolum are specially analyzed. The data are arranged in four tables. One associates gene annotations with congruent reports of the enzymatic activity in these same Mollicutes, and hence confirms the annotations. Another associates putative annotations with reports of the enzyme activity but from different Mollicutes. A third identifies the discrepancies represented by those enzymatic activities found in Mollicutes with sequenced genomes but without any similarly annotated ORF. This suggests that the gene sequence is significantly different from those already deposited in the databanks and putatively annotated with the same function. Another comparison lists those enzymatic activities that are both undetected in Mollicutes and not associated with any ORF. Evidence is presented supporting the theory that there are relatively small gene sequences that code for functional centers of multiple enzymatic activity. This property is seemingly advantageous for an organism with a small genome and perhaps under some coding restraint. The data suggest that a concept of "remnant" or "useless genes" or "useless enzymes" should be considered when examining the relationship of gene annotation and enzymatic function. It also suggests that genes in addition to representing what cells are doing or what they may do, may also identify what they once might have done and may never do again.

Purification of a phosphatase which hydrolyzes phosphatidic acid, a key intermediate in glucolipid synthesis in Acholeplasma laidlawii A membranes

A phosphatidic acid phosphatase (PAP; EC 3.1.3.4.), dephosphorylating phosphatidic acid (PA) to diacylglycerol (DAG), was identified and purified from the plasma membrane of Acholeplasma laidlawii A. After four purification steps, including membrane preparation, Tween 20 solubilization, preparative gel electrophoresis and electro-elution, PAP was purified about 400 times to near homogeneity. The molecular weight of PAP was according to SDS-polyacrylamide gel electrophoresis approximately 25 kDa and the enzyme was a stable and integral membrane protein. It is proposed to catalyze the first enzymatic step in the important glucolipid pathway of A. laidlawii. No essential cofactors or activator lipids were found. However, some divalent cations and phosphate analogues were potent inhibitors. Beside the in vivo substrate (PA), PAP was found to dephosphorylate p-nitrophenylphosphate. This less stringent specificity makes alternative in vivo functions for PAP plausible, the importance which is discussed.

Activating amphiphiles cause a conformational change of the 1,2-diacylglycerol 3-glucosyltransferase from Acholeplasma laidlawii membranes according to proteolytic digestion

1,2-Diacylglycerol 3-glucosyltransferase synthesizes the major nonbilayer-prone lipid monoglucosyldiacylglycerol (MGlcDAG) in the membrane of Acholeplasma laidlawii, which is important for the spontaneous curvature, and is a regulatory site for the lipid surface charge density. A potential connection between activity and a conformational change of this enzyme, governed by essential lipid activators, was studied with purified MGlcDAG synthase in different lipid aggregates. Critical fractions of anionic phospholipids 1, 2-dioleoyl-phosphatidylglycerol (DOPG) and 1,2-dioleoyl-phosphatidylserine (DOPS) were essential for the restoration of enzyme activity, while the zwitterionic 1,2-dioleoyl-phosphatidylcholine (DOPC) and the uncharged diglucosyldiacylglycerol (DGlcDAG) were not. Proteolytic resistance had a very good correlation with the enzyme activity in various lipid-CHAPS mixed micelles. Anionic lipids DOPG and DOPS could protect the exposed MGlcDAG synthase from digestion, whereas DOPC and DGlcDAG could not. Similar features were observed in liposome bilayers. Likewise, the detergent dodecylphosphoglycerol (PGD), with a phosphatidylglycerol-like headgroup, could also stimulate the MGlcDAG synthase activity efficiently with a concomitant protection toward proteolytic digestion. Neither proteolytic resistance nor restored enzyme activity was observed using soluble glycerol 3-phosphate. It is concluded that in addition to critical amounts, both the negatively charged headgroup and hydrophobic chains of the activator amphiphiles, but not a certain aggregate curvature, seem necessary for a proper conformation and the resulting active state of the MGlcDAG synthase.