An analysis of cosmid clones of nuclear DNA from Trypanosoma brucei shows that the genes for variant surface glycoproteins are clustered in the genome

Trypanosoma brucei contains more than a hundred genes coding for the different variant surface glycoproteins (VSGs). Activation of some of these genes involves the duplication of the gene (the basic copy or BC) and transposition of the duplicate to an expression site (yielding the expression-linked copy or ELC). We have cloned large fragments of genomic DNA in cosmid vectors in Escherichia coli. Cosmids containing the BCs of genes 117, 118 and 121 were readily obtained, but DNA containing the ELCs was strongly selected against in the cosmid and plasmid cloning systems used. We have analysed the distribution of VSG genes in the genome using probes for the sequences at the edges of the transposed segment which are partially homologous among these genes. In genomic cosmid clone banks, about 9% of all colonies hybridize with probes from the 5'- and 3'-edges of the transposed segment, showing that these sequences are linked in the genome. Moreover, the 117 and 118 BC cosmids contain several additional putative VSG genes in tandem, as deduced from hybridization and sequence analyses. We conclude that the VSG genes are highly clustered and share common sequences at the borders of the transposed segment.     

Trypanosoma brucei brucei: variability in the association of some variant surface glycoproteins

In our isolation procedure, the surface antigens of the variants AnTat 1.1 and 1.10 (Trypanosoma brucei brucei) are essentially obtained as a disulfide-linked dimer while the AnTat 1.8 surface antigen is found as a mixture of monomer and disulfide-linked dimer. This observation may be related to the localization of the cysteine residues in the protein sequences. In the purification procedure using concanavalin-A Sepharose chromatography, besides the VSG elution by methyl-alpha-D-mannopyranoside, a quantitative elution of still bound VSG may be obtained by the addition of beta-mercaptoethanol to methyl-alpha-D-mannopyrannoside in the elution buffer. The surface antigen of the variant AnTat 1.1 was examined for molecular form at several different times during the release procedure. The disulfide-linked dimer could be observed within 30 min of the surface coat release, indicating its presence within the parasite.     

Selective cleavage of variant surface glycoproteins from Trypanosoma brucei.

Two conformationally distinct regions were revealed by tryptic cleavage of six undenatured variant surface glycoproteins purified from clones of Trypanosoma brucei. Within 5 min, the native glycoproteins (65,000 mol.wt.) were cleaved, yielding a large N-terminal fragment (48,000-55,000 mol.wt. depending on the variant) together with one or more C-terminal fragments. After 30-60 min incubation, further breakdown of the large fragment occurred in some variants. The ultimate large product (40,000-52,000 mol.wt.) was very resistant to further degradation by trypsin (in the absence of denaturation). The distinction between N-terminal and C-terminal domains may be significant in relation to the organization and function of these glycoproteins on the trypanosome surface.     

Identification of peptide mimotopes of Trypanosoma brucei gambiense variant surface glycoproteins

Background

The current antibody detection tests for the diagnosis of gambiense human African trypanosomiasis (HAT) are based on native variant surface glycoproteins (VSGs) of Trypanosoma brucei (T.b.) gambiense. These native VSGs are difficult to produce, and contain non-specific epitopes that may cause cross-reactions. We aimed to identify mimotopic peptides for epitopes of T.b. gambiense VSGs that, when produced synthetically, can replace the native proteins in antibody detection tests.

Methodology/Principal Findings

PhD.-12 and PhD.-C7C phage display peptide libraries were screened with mouse monoclonal antibodies against the predominant VSGs LiTat 1.3 and LiTat 1.5 of T.b. gambiense. Thirty seven different peptide sequences corresponding to a linear LiTat 1.5 VSG epitope and 17 sequences corresponding to a discontinuous LiTat 1.3 VSG epitope were identified. Seventeen of 22 synthetic peptides inhibited the binding of their homologous monoclonal to VSG LiTat 1.5 or LiTat 1.3. Binding of these monoclonal antibodies to respectively six and three synthetic mimotopic peptides of LiTat 1.5 and LiTat 1.3 was significantly inhibited by HAT sera (p<0.05).

Conclusions/Significance

We successfully identified peptides that mimic epitopes on the native trypanosomal VSGs LiTat 1.5 and LiTat 1.3. These mimotopes might have potential for the diagnosis of human African trypanosomiasis but require further evaluation and testing with a large panel of HAT positive and negative sera.     

Characterisation of the asparagine-linked oligosaccharides from Trypanosoma brucei type-I variant surface glycoproteins

The complete primary structures of the Asn-linked oligosaccharides from the conserved glycosylation site of the type-I variant surface glycoproteins of Trypanosoma brucei MITat 1.4 and MITat 1.6 were determined using a combination of exoglycosidase digestions, permethylation analysis, acetolysis and 1H NMR. Both variants contained almost exclusively oligomannose-type oligosaccharides, identical in structure to those of mammalian glycoproteins. The oligosaccharides ranged in size from (Man)9(GlcNAc)2 to (Man)5(GlcNAc)2. The relative abundance of each component was similar in both variants. The major components were (Man)8(GlcNAc)2 and (Man)7(GlcNAc)2 with slightly less (Man)9(GlcNAc)2 and (Man)6(GlcNAc)2 and much less (Man)5(GlcNAc)2. Both variants also contained the same structural isomers. The close similarity of the oligomannose series indicates identical processing at the conserved site in both variants.     

Candidate glycophospholipid precursor for the glycosylphosphatidylinositol membrane anchor of Trypanosoma brucei variant surface glycoproteins

Trypanosoma brucei variant surface glycoproteins are apparently synthesized with a hydrophobic carboxyl-terminal peptide that is cleaved and replaced by a complex glycosylphosphatidylinositol membrane anchor within 1 min of the completion of polypeptide synthesis. The rapidity of this carboxyl-terminal modification suggests the existence of a prefabricated core glycolipid that would be transferred en bloc to the variant surface glycoprotein polypeptide. We report the purification and chemical characterization of a glycolipid from T. brucei that has properties consistent with a role as a variant surface glycoprotein glycolipid donor. This candidate glycolipid precursor has been defined by thin-layer chromatography of extracts of trypanosomes metabolically labeled with radioactive myristic acid, ethanolamine, glucosamine, mannose, and phosphate and by enzymatic, chemical, and gas chromatographic-mass spectrometric analysis. Mild alkali released 100% of the myristic acid, and reaction with phospholipase A2 released 50%. Nitrous acid deamination generated dimyristylphosphatidylinositol, and periodate oxidation released phosphatidic acid. Treatment of purified glycolipid with phosphatidylinositol-specific phospholipase C released dimyristylglycerol and a water-soluble glycan that was sized on Bio-Gel P-4 columns. The candidate precursor contained mannose, myristic acid, phosphate, and ethanolamine with an unsubstituted amino group, but not galactose.     

Physical studies of Trypanosoma brucei variant surface glycoproteins and their antigenic determinants

Secondary structure determinations have been carried out on two antigenically related variant surface glycoproteins (VSG's) from Trypanosoma brucei, WaTat 1.1 and WaTat 1.12. The two molecules, which bear highly homologous amino-terminal sequences, showed subtle differences in their circular dichroism (CD). Computer analysis revealed that the contribution of alpha helix to the secondary structure of the VSG's was 49% for WaTat 1.1 and 52% for WaTat 1.12. Unfolding studies using guanidine hydrochloride suggested that the WaTat 1.12 VSG was slightly more resistant than WaTat 1.1 VSG to the effect of this reagent. The membrane form of WaTat 1.1 VSG purified by reverse-phase high-performance liquid chromatography gave CD and fluorescence spectra indicative of a partially unfolded or denatured molecule. It was also shown that the antigenic differences between the VSG's were due to surface-oriented topographically assembled epitopes which were highly sensitive to structural perturbations. Monoclonal antibodies specific for these epitopes bound to four discreet determinants on WaTat 1.1, one of which was absent from WaTat 1.12.     

Crystallization of amino-terminal domains and domain fragments of variant surface glycoproteins from Trypanosoma brucei brucei

Crystals were produced from variant surface glycoproteins (VSG) of Trypanosoma brucei brucei antigenic variants MITat 1.2, 1.6, and ILTat 1.22, 1.23, 1.24, 1.25, and 1.26. Purified VSGs had molecular weights from 60,000 to 68,000 on sodium dodecyl sulfate-polyacrylamide gels, whereas the crystals obtained were composed of polypeptides of approximate Mr 40,000-50,000. Amino-terminal amino acid sequences determined from the crystallized VSGs were identical to sequences obtained from the respective intact proteins, indicating that the crystals contained VSG amino-terminal fragments. Crystallization conditions and lattice dimensions of the crystals are given.     

Characterization of Trypanosoma brucei gambiense variant surface glycoprotein LiTat 1.5

At present, all available diagnostic antibody detection tests for Trypanosoma brucei gambiense human African trypanosomiasis are based on predominant variant surface glycoproteins (VSGs), such as VSG LiTat 1.5. During investigations aiming at replacement of the native VSGs by recombinant proteins or synthetic peptides, the sequence of VSG LiTat 1.5 was derived from cDNA and direct N-terminal amino acid sequencing. Characterization of the VSG based on cysteine distribution in the amino acid sequence revealed an unusual cysteine pattern identical to that of VSG Kinu 1 of T. b. brucei. Even though both VSGs lack the third of four conserved cysteines typical for type A N-terminal domains, they can be classified as type A.     

Mapping of segmental antigenic determinants on structurally related variant surface glycoproteins of Trypanosoma brucei

To study common and variant specific antigenic determinants on variant surface glycoproteins from Trypanosoma brucei, we have selected four serologically cross-reacting variant populations. Monoclonal antibodies were raised against the purified variant surface glycoproteins from each variant trypanosome population. Six monoclonal antibodies bind to segmental epitopes and one binds to a topographically assembled epitope. Amino acid compositions of these variant surface glycoproteins reveal striking conservation of certain residues including cysteine and charged amino acids. We also find that all seven monoclonal antibodies used in this study bind to protein determinants not exposed on the surface of the living trypanosome. Only one monoclonal antibody exhibits homologous specificity, while the remainder display cross-reactivity for three or all four variant surface glycoproteins. In addition, polyacrylamide gel electrophoresis peptide mapping and Western blots probed with each monoclonal antibody reveal significant peptide homologies. Furthermore, two pairs of monoclonal antibodies recognize two epitopes that are possibly immunodominant. The significance of these findings is discussed in terms of the structural similarities and differences among variant surface glycoproteins.     

Coordinate transcription of variant surface glycoprotein genes and an expression site associated gene family in Trypanosoma brucei

Trypanosoma brucei variant surface glycoprotein (VSG) genes are activated either by duplicative (DA) transposition of the gene to a pre-activated expression site or by nonduplicative (NDA) activation of a previously silent telomeric gene. We have obtained a recombinant clone spanning the 5' barren region of the expression linked copy of the duplicated VSG gene 117a. By DNA sequence and hybridization analyses we have identified a pleomorphic family of 14-25 non-VSG genes that lie upstream of both DA and NDA VSG expression sites. These expression site associated genes (ESAGs) encode 1.2 kb poly(A)+ mRNAs that are specifically transcribed from the active VSG expression telomere in mammalian bloodstream stages of T. brucei but, in common with VSG genes, are not transcribed in procyclic culture forms. cDNA and genomic sequences predict open reading frames that are conserved in the two ESAGs examined.     

Trypanosoma brucei: radioimmunoassay of variant surface glycoproteins from organisms grown in vitro and in vivo

Bloodstream forms of Trypanosoma brucei that were infective for mammals, when grown in vitro at 37 C for 29 days or 25 months had amounts of variant surface glycoprotein similar to the amounts from bloodstream forms isolated from infected rat blood. The amounts were measured by competition radioimmunoassays for both unique and cross-reacting determinants and the results were the same, providing evidence that a single type of variant surface glycoprotein was measured. Neither radioimmunoassay detected determinants of variant surface glycoproteins in trypanosomes transformed by culturing at 27 C to insect forms not infective for mammals.     

Molecular characterization of two invariant surface glycoproteins specific for the bloodstream stage of Trypanosoma brucei.

In the accompanying paper (Ziegelbauer, K., and Overath, P. (1992) J. Biol. Chem. 267, 10791-10796), two invariant surface glycoproteins, ISG65 and ISG75, were identified in the mammalian stage of the parasitic protozoan, Trypanosoma brucei. In this study, the genes coding for these proteins have been isolated. Their nucleotide sequence suggests no relationship to other known genes and predicts polypeptides with NH2-terminal signal sequences, hydrophilic extracellular domains, single trans-membrane alpha-helices, and short cytoplasmic domains. ISG65 and ISG75 are expressed in bloodstream forms (70,000 and 50,000 molecules/cell, respectively) but not in the insect midgut stage. They can be detected in all T. brucei brucei variant clones investigated. Both polypeptides are distributed over the entire surface of the parasite.     

Identification of mimotopes with diagnostic potential for Trypanosoma brucei gambiense variant surface glycoproteins using human antibody fractions

Background

At present, screening of the population at risk for gambiense human African trypanosomiasis (HAT) is based on detection of antibodies against native variant surface glycoproteins (VSGs) of Trypanosoma brucei (T.b.) gambiense. Drawbacks of these native VSGs include culture of infective T.b. gambiense trypanosomes in laboratory rodents, necessary for production, and the exposure of non-specific epitopes that may cause cross-reactions. We therefore aimed at identifying peptides that mimic epitopes, hence called “mimotopes,” specific to T.b. gambiense VSGs and that may replace the native proteins in antibody detection tests.

Methodology/Principal Findings

A Ph.D.-12 peptide phage display library was screened with polyclonal antibodies from patient sera, previously affinity purified on VSG LiTat 1.3 or LiTat 1.5. The peptide sequences were derived from the DNA sequence of the selected phages and synthesised as biotinylated peptides. Respectively, eighteen and twenty different mimotopes were identified for VSG LiTat 1.3 and LiTat 1.5, of which six and five were retained for assessment of their diagnostic performance. Based on alignment of the peptide sequences on the original protein sequence of VSG LiTat 1.3 and 1.5, three additional peptides were synthesised. We evaluated the diagnostic performance of the synthetic peptides in indirect ELISA with 102 sera from HAT patients and 102 endemic negative controls. All mimotopes had areas under the curve (AUCs) of ≥0.85, indicating their diagnostic potential. One peptide corresponding to the VSG LiTat 1.3 protein sequence also had an AUC of ≥0.85, while the peptide based on the sequence of VSG LiTat 1.5 had an AUC of only 0.79.

Conclusions/Significance

We delivered the proof of principle that mimotopes for T.b. gambiense VSGs, with diagnostic potential, can be selected by phage display using polyclonal human antibodies.