Non-fusion proteins expressed in E. coli: detection by alpha-complementation (blue/white selection).
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Base Sequence↗
Biomedical subjects
P T Englund
Publications and source records attributed to P T Englund.
Glycosyl phosphatidylinositol-specific phospholipase C of Trypanosoma brucei: expression in Escherichia coli.
Glycosyl phosphatidylinositol-specific phospholipase C (GPI-PLC) from Trypanosoma brucei cleaves the glycosyl phosphatidylinositol (GPI) anchor of the trypanosome variant surface glycoprotein (VSG) and other GPI structures. We have expressed this enzyme in Escherichia coli, using a protocol designed to produce the native enzyme rather than a fusion protein. We have purified large amounts of GPI-PLC from E. coli membranes, using a single step immunoaffinity technique. The expressed enzyme is identical to its trypanosome counterpart in enzymatic specificity, mobility on SDS-PAGE, and isoelectric point. Recombinant GPI-PLC is a membrane enzyme; it associates with E. coli membranes and, like the T. brucei GPI-PLC, partitions into the detergent phase in Triton X-114 phase separation experiments. The Michaelis constants for the two enzymes are similar (400 nM, with VSG as substrate). The turnover number (kcat, 72 min-1) of the recombinant enzyme (expressed from a. T. brucei rhodesiense WRATat 1.1 cDNA) is about one-tenth that of GPI-PLC from T. brucei brucei (ILTat 1.3).
Animals↗
An analog of myristic acid with selective toxicity for African trypanosomes.
Trypanosoma brucei, the protozoan parasite responsible for African sleeping sickness, evades the host immune response through the process of antigenic variation. The variant antigen, known as the variant surface glycoprotein (VSG), is anchored to the cell surface by a glycosyl phosphatidylinositol (GPI) structure that contains myristate (n-tetradecanoate) as its only fatty acid component. The utilization of heteroatom-containing analogs of myristate was studied both in a cell-free system and in vivo. Results indicated that the specificity of fatty acid incorporation depends on chain length rather than on hydrophobicity. One analog, 10-(propoxy)decanoic acid, was highly toxic to trypanosomes in culture although it is nontoxic to mammalian cells.
Acyl Coenzyme A↗
Fatty acid remodeling: a novel reaction sequence in the biosynthesis of trypanosome glycosyl phosphatidylinositol membrane anchors.
The trypanosome variant surface glycoprotein (VSG) is anchored to the plasma membrane via a glycosyl phosphatidylinositol (GPI). The GPI is synthesized as a precursor, glycolipid A, that is subsequently linked to the VSG polypeptide. The VSG anchor is unusual, compared with anchors in other cell types, in that its fatty acid moieties are exclusively myristic acid. To investigate the mechanism for myristate specificity we used a cell-free system for GPI biosynthesis. One product of this system, glycolipid A', is indistinguishable from glycolipid A except that its fatty acids are more hydrophobic than myristate. Glycolipid A' is converted to glycolipid A through highly specific fatty acid remodeling reactions involving deacylation and subsequent reacylation with myristate. Therefore, myristoylation occurs in the final phase of trypanosome GPI biosynthesis.
Animals↗
Biosynthesis of glycosyl phosphatidylinositol membrane anchors.
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Cell Membrane↗
Selective cleavage of kinetoplast DNA minicircles promoted by antitrypanosomal drugs.
Pentamidine, diminazene aceturate (Berenil), isometamidium chloride (Samorin), and ethidium bromide, which are important antitrypanosomal drugs, promote linearization of Trypanosoma equiperdum minicircle DNA (the principal component of kinetoplast DNA, the mitochondrial DNA in these parasites). This effect occurs at therapeutically relevant concentrations. The linearized minicircles are protease sensitive and are not digested by lambda exonuclease (a 5' to 3' exonuclease), indicating that the break is double stranded and that protein is bound to both 5' ends of the molecule. The cleavage sites map to discrete positions in the minicircle sequence, and the cleavage pattern varies with different drugs. These findings are characteristic for type II topoisomerase inhibitors, and they mimic the effects of the antitumor drug etoposide (VP16-213, a semisynthetic podophyllotoxin analog) on T. equiperdum minicircles. However, the antitrypanosomal drugs differ dramatically from etoposide in that they do not promote detectable formation of nuclear DNA-protein complexes or of strand breaks in nuclear DNA. Selective inhibition of a mitochondrial type II topoisomerase may explain why these antitrypanosomal drugs preferentially disrupt mitochondrial DNA structure and generate dyskinetoplastic trypanosomes (which lack mitochondrial DNA).
Animals↗
Genomic organization, chromosomal localization, and developmentally regulated expression of the glycosyl-phosphatidylinositol-specific phospholipase C of Trypanosoma brucei.
The surface of the bloodstream form of the African trypanosome, Trypansoma brucei, is covered with about 10(7) molecules of the variant surface glycoprotein (VSG), a protein tethered to the plasma membrane by a glycosyl-phosphatidylinositol (GPI) membrane anchor. This anchor is cleavable by an endogenous GPI-specific phospholipase C (GPI-PLC). GPI-PLC activity is down regulated when trypanosomes differentiate from the bloodstream form to the procyclic form found in the tsetse fly vector. We have mapped the GPI-PLC locus in the trypanosome genome and have examined the mechanism for this developmental regulation in T. brucei. Southern blot analysis indicates a single-copy gene for GPI-PLC, with two allelic variants distinguishable by two NcoI restriction fragment length polymorphisms. The gene was localized solely to a chromosome in the two-megabase compression region by contour-clamped homogeneous electric field gel electrophoresis. No rearrangement of the GPI-PLC gene occurs during differentiation to procyclic forms, which could potentially silence GPI-PLC gene expression. Enzymological studies give no indication of a diffusible inhibitor of GPI-PLC activity in procyclic forms, and Western immunoblot analysis reveals no detectable GPI-PLC polypeptide in these forms. Therefore, it is highly unlikely that the absence of GPI-PLC activity in procyclic forms is due to posttranslational control. Northern (RNA) blot analysis reveals barely detectable levels of GPI-PLC mRNA in procyclic forms; therefore, regulation of GPI-PLC activity in these forms correlates with the steady-state mRNA level.
Animals↗
Assessing heterogeneity of the high-mannose glycopeptide gp432 on the variant surface glycoprotein of trypanosomes: a comparison of plasma desorption mass spectrometry and radiolabeling techniques.
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Amino Acid Sequence↗
Biosynthesis of the glycosyl phosphatidylinositol membrane anchor of the trypanosome variant surface glycoprotein. Origin of the non-acetylated glucosamine.
Non-acetylated glucosamine is an unusual structural feature shared by all glycosyl phosphatidylinositol (GPI) lipids, including a variety of membrane anchors, the leishmanial lipophosphoglycan, and a mediator of insulin action. We proposed previously a pathway for biosynthesis of glycolipid A, the precursor of the GPI membrane anchor of the trypanosome variant surface glycoprotein (Masterson, W. J., Doering, T. L., Hart, G. W., and Englund, P. T. (1989) Cell 56, 793-800). In this paper we characterize in more detail the initial steps of GPI assembly. The first and committed step in the pathway is the transfer of GlcNAc, from UDP-GlcNAc, to endogenous phosphatidylinositol to form N-acetylglucosaminyl phosphatidylinositol (GlcNAc-PI). The GlcNAc-PI is then efficiently deacetylated to form glucosaminyl phosphatidylinositol (GlcN-PI), the substrate for subsequent reactions en route to glycolipid A.
Acetylation↗
A novel pathway for glycan assembly: biosynthesis of the glycosyl-phosphatidylinositol anchor of the trypanosome variant surface glycoprotein.
The trypanosome variant surface glycoprotein (VSG), like many other eukaryotic cell surface proteins, is anchored to the plasma membrane by a glycosyl-phosphatidylinositol (GPI) moiety. This glycolipid is assembled first as a precursor (glycolipid A) that is then covalently attached to the newly synthesized polypeptide. We have developed a trypanosome cell-free system capable of performing all of the steps in the biosynthesis of the glycan portion of glycolipid A. Using [3H]sugar nucleotides as substrates, several biosynthetic intermediates have been identified. From structural analyses of these intermediates, we propose a pathway for GPI biosynthesis. Based on comparisons between the VSG GPI anchor and similar structures in other cells, we believe that this same pathway will apply to the GPI anchors, and the related insulin-mediator compound, of higher eukaryotes.
Animals↗
Drug-promoted cleavage of kinetoplast DNA minicircles. Evidence for type II topoisomerase activity in trypanosome mitochondria.
Minicircle DNA, the major component of the mitochondrial DNA of trypanosomes (kinetoplast DNA), is linearized when living Trypanosoma equiperdum cells are treated with inhibitors of mammalian type II topoisomerases and then lysed with sodium dodecyl sulfate. A variety of intercalating and nonintercalating compounds (the epipodophyllotoxins, 4'-(9-acridinylamino)-methanesulfon-m-anisidine, 2-methyl-9-hydroxyellipticine, and acriflavine) are active, but novobiocin and specific gyrase inhibitors (the quinolones) are not. The linearized minicircles are in a DNA-protein complex, as their electrophoretic mobility is increased by Proteinase K treatment. They are digested by exonuclease III but not by lambda exonuclease, indicating that the protein must be linked to both 5' ends. Drug-induced cleavage sites vary with different compounds and are found throughout the minicircle sequence. These results indicate that trypanosome mitochondria contain a type II topoisomerase with some properties similar to those of type II topoisomerases in the nucleus of higher eukaryotes. A maximum of 12% of all minicircles is cleaved in the presence of VP16-213, indicating there are at least 600 molecules of mitochondrial type II topoisomerase/cell or about one enzyme/8 kilobases of minicircle DNA.
Animals↗
Replication of kinetoplast DNA in Trypanosoma equiperdum. Minicircle H strand fragments which map at specific locations.
The mitochondrial DNA of trypanosomes, kinetoplast DNA, is a network containing thousands of topologically interlocked minicircles. Minicircles are replicated as free molecules after being detached from the network. The minicircle L strand appears to be synthesized continuously and the H strand discontinuously. This paper describes properties of Trypanosoma equiperdum minicircle H strand fragments which could be Okazaki fragments. These fragments constitute a family of molecules of discrete sizes (ranging from about 70 to 1000 nucleotides) which map to specific locations. Three of the most prominent fragments, a 73-mer, 83-mer, and 138-mer, map at contiguous or overlapping sites. Based on their position relative to the initiation site for L strand synthesis, the 73-mer may be the first Okazaki fragment to be synthesized and either the 83-mer or the 138-mer may be the second. The 5' end of the 73-mer lies within a sequence, GGGCGT, found at a similar location in minicircles of all trypanosomatid species. During the maturation of free minicircles and after their reattachment to the networks there appears to be continued extension and ligation of the H strand fragments. However, the ligation of the 73-mer, 83-mer, and 138-mer to the rest of the H strand is delayed; their eventual ligation results in covalent closure of the minicircles.
Animals↗
A glycolipid from Trypanosoma brucei related to the variant surface glycoprotein membrane anchor.
The variant surface glycoprotein (VSG) of Trypanosoma brucei is covalently linked to a phosphatidylinositol-containing glycolipid which serves as a membrane anchor. We previously identified a molecule, glycolipid A, which appears to be a biosynthetic precursor to the anchor [9]. In this paper we describe a related molecule, glycolipid C, which is similar to glycolipid A but which is more hydrophobic. Chromatographic analyses indicate that the polar head groups in glycolipids A and C are similar or identical. Both glycolipids contain phosphatidylinositol, but the inositol in glycolipid C is modified by a hydrophobic moiety. Since treatment of glycolipid C with mild alkali results in partial conversion to a molecule chromatographically identical to glycolipid A, it is likely that glycolipid C has an alkali-sensitive hydrophobic group, such as a fatty acid, linked to its inositol moiety.
Animals↗
Synthesis and processing of kinetoplast DNA minicircles in Trypanosoma equiperdum.
Kinetoplast DNA, the mitochondrial DNA in trypanosomes, is a giant network containing topologically interlocked minicircles. Replication occurs on free minicircles that have been detached from the network. In this paper, we report studies on the synthesis and processing of the minicircle L and H strands. Analysis of free minicircles from Trypanosoma equiperdum by two-dimensional agarose gel electrophoresis indicated that elongating L strands are present on theta structures. Hybridization studies indicated that L-strand elongation is continuous and unidirectional, starting near nucleotide 805 and proceeding around the entire minicircle. The theta structures segregate into monomeric progeny minicircles, and those with a newly synthesized L strand have a 8-nucleotide gap between nucleotides 805 and 814 (J. M. Ntambi, T. A. Shapiro, K. A. Ryan, and P. T. Englund, J. Biol. Chem. 261:11890-11895, 1986). These molecules are reattached to the network, where repair of the gap takes place. Of the molecules labeled during a 10-min pulse with [3H]thymidine, gap filling occurred on half within about 15 min and on virtually all by 60 min; however, there was no detectable covalent closure of the newly synthesized L strand by 60 min.
Animals↗
Biosynthesis of a variant surface glycoprotein of Trypanosoma brucei. Processing of the glycolipid membrane anchor and N-linked oligosaccharides.
The variant surface glycoprotein (VSG) of the ILTat 1.3 variant of Trypanosoma brucei has two asparagine-linked glycan moieties, as well as a phosphatidylinositol glycan membrane anchor. We have investigated the structure and processing of each of these oligosaccharides through analysis of the intact protein and of glycopeptides. Processing has been examined by comparing glycan structures purified from an immature intracellular form (58 kDa) of VSG with those of the mature form (59 kDa) found on the parasite surface. We find exclusively high mannose oligosaccharides (Man4-7-GlcNAc2) at Asn-432 in both the immature 58-kDa and mature 59-kDa forms. In contrast, the "core" oligosaccharide of Asn-419 (Man3-GlcNAc2) appears to be nearly quantitatively processed to a complex biantennary structure [Gal-GlcNAc-Man)2-Man-GlcNAc2) during VSG maturation. The asparagine-linked structures at Asn-419, but not those at Asn-432, are resistant to endo-beta-N-acetylglucosaminidase H within 30 s of biosynthesis. This suggests possible novel and selective mechanisms for glycosylation in African trypanosomes. Finally, we show that the carboxyl-terminal glycolipid is galactosylated (3-4 residues) relatively late in VSG biosynthesis. Phosphatidylinositol glycans have been identified on a growing number of eukaryotic membrane proteins. This report provides a direct demonstration of the processing of such a glycolipid anchor following its attachment to protein.
Amino Acid Sequence↗
A knotted free minicircle in kinetoplast DNA.
Kinetoplast DNA, the mitochondrial DNA of trypanosomes, is a network containing thousands of minicircles that are topologically interlocked. The minicircle replication intermediates are free molecules that have been released from the network. We report here that one form of free minicircles is a trefoil knot. Identification of this knotted structure is based on its electrophoretic and sedimentation properties, its response to treatments with restriction enzymes or topoisomerase II, and its appearance by electron microscopy. Except for its topology, the knotted minicircle closely resembles a previously described replication intermediate with a unique gap in the newly synthesized L strand.
Animals↗
cDNA encoding the glycosyl-phosphatidylinositol-specific phospholipase C of Trypanosoma brucei.
VSG lipase of Trypanosoma brucei specifically cleaves the glycosyl-phosphatidylinositol membrane anchor of the trypanosome variant surface glycoprotein (VSG), releasing this protein from the plasma membrane. It also cleaves similar membrane anchors on some mammalian proteins. VSG lipase may play a role in processes such as parasite differentiation or antigenic variation. We describe here the cloning and sequencing of a cDNA encoding VSG lipase from T. brucei.
Amino Acid Sequence↗
Replication of kinetoplast DNA in trypanosomes.
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Animals↗