Identification of membrane-associated proteins in Trypanosoma brucei encoding an internal, EARLRAEE amino acid repeat

Role of protein translocation pathways across the endoplasmic reticulum in Trypanosoma brucei

The translocation of secretory and membrane proteins across the endoplasmic reticulum (ER) membrane is mediated by co-translational (via the signal recognition particle (SRP)) and post-translational mechanisms. In this study, we investigated the relative contributions of these two pathways in trypanosomes. A homologue of SEC71, which functions in the post-translocation chaperone pathway in yeast, was identified and silenced by RNA interference. This factor is essential for parasite viability. In SEC71-silenced cells, signal peptide (SP)-containing proteins traversed the ER, but several were mislocalized, whereas polytopic membrane protein biogenesis was unaffected. Surprisingly trypanosomes can interchangeably utilize two of the pathways to translocate SP-containing proteins except for glycosylphosphatidylinositol-anchored proteins, whose level was reduced in SEC71-silenced cells but not in cells depleted for SRP68, an SRP-binding protein. Entry of SP-containing proteins to the ER was significantly blocked only in cells co-silenced for the two translocation pathways (SEC71 and SRP68). SEC63, a factor essential for both translocation pathways in yeast, was identified and silenced by RNA interference. SEC63 silencing affected entry to the ER of both SP-containing proteins and polytopic membrane proteins, suggesting that, as in yeast, this factor is essential for both translocation pathways in vivo. This study suggests that, unlike bacteria or other eukaryotes, trypanosomes are generally promiscuous in their choice of mechanism for translocating SP-containing proteins to the ER, although the SRP-independent pathway is favored for glycosylphosphatidylinositol-anchored proteins, which are the most abundant surface proteins in these parasites.

Down-regulation of the trypanosomatid signal recognition particle affects the biogenesis of polytopic membrane proteins but not of signal peptide-containing proteins

Protein translocation across the endoplasmic reticulum is mediated by the signal recognition particle (SRP). In this study, the SRP pathway in trypanosomatids was down-regulated by two approaches: RNA interference (RNAi) silencing of genes encoding SRP proteins in Trypanosoma brucei and overexpression of dominant-negative mutants of 7SL RNA in Leptomonas collosoma. The biogenesis of both signal peptide-containing proteins and polytopic membrane proteins was examined using endogenous and green fluorescent protein-fused proteins. RNAi silencing of SRP54 or SRP68 in T. brucei resulted in reduced levels of polytopic membrane proteins, but no effect on the level of signal peptide-containing proteins was observed. When SRP deficiency was achieved in L. collosoma by overexpression of dominant-negative mutated 7SL RNA, a major effect was observed on polytopic membrane proteins but not on signal peptide-containing proteins. This study included two trypanosomatid species, tested various protein substrates, and induced depletion of the SRP pathway by affecting either the levels of SRP binding proteins or that of SRP RNA. Our results demonstrate that, as in bacteria but in contrast to mammalian cells, the trypanosome SRP is mostly essential for the biogenesis of membrane proteins.

Population sub-structuring among Trypanosoma evansi stocks

To investigate the population genetic structure of Trypanosoma evansi from domesticated animals, we have analysed 112 stocks from camels, buffaloes, cattle and horses using the tandemly repeated coding sequence (MORF2) and minisatellite markers 292 and cysteine-rich acidic integral membrane protein (CRAM). We recorded a total of six alleles at the MORF2 locus, seven at 292 and 12 at the CRAM loci. Nei's genetic distance showed reduced allelic diversity between buffaloes and cattle stocks (1.2) as compared to the diversity between camels and buffaloes (3.75) and camels and cattle stock (1.69). The mean index of association (IA=0.92) significantly deviated from zero, and the average number of multilocus genotypes (G/N ratio) was 0.21. Twenty-four multilocus genotypes were defined from the combination of alleles at the three loci. The Kenyan sub-populations showed Fst=0.28 and analysis of molecular variance showed significant divergence (22.7%) between the Laikipia, Kulal and Galana regions. The regional and host distribution of multi-locus genotypes significant population differentiation and high Nei's genetic distances suggest existence of genetic sub-structuring within T. evansi stocks while the few multi-locus genotypes and deviation of association index from zero indicate the lack of recombination. In conclusion, this study reveals that some genetic sub-structuring does occur within T. evansi, which has a clonal population structure.

Sorting signals required for trafficking of the cysteine-rich acidic repetitive transmembrane protein in Trypanosoma brucei

In trypanosomatids, endocytosis and exocytosis are restricted to the flagellar pocket (FP). The cysteine-rich acidic repetitive transmembrane (CRAM) protein is located at the FP of Trypanosoma brucei and potentially functions as a receptor or an essential component for lipoprotein uptake. We characterized sorting determinants involved in efficient trafficking of CRAM to and from the FP of T. brucei. Previous studies indicated the presence of signals in the CRAM C terminus, specific for its localization to the FP and for efficient endocytosis (H. Yang, D. G. Russell, B. Zeng, M. Eiki, and M.G.-S. Lee, Mol. Cell. Biol. 20:5149-5163, 2000.) To delineate functional domains of putative sorting signals, we performed a mutagenesis series of the CRAM C terminus. Subcellular localization of CRAM mutants demonstrated that the amino acid sequence between -5 and -14 (referred to as a transport signal) is essential for exporting CRAM from the endoplasmic reticulum to the FP, and mutations of amino acids at -12 (V), -10 (V), or -5 (D) led to retention of CRAM in the endoplasmic reticulum. Comparison of the endocytosis efficiency of CRAM mutants demonstrated that the sequence from amino acid -5 to -23 (referred to as a putative endocytosis signal) is required for efficient endocytosis and overlaps with the transport signal. Apparently the CRAM-derived sorting signal can efficiently interact with the T. brucei micro1 adaptin, and mutations at amino acids essential for the function of the transport signal abolished the interaction of the signal with T. brucei micro1, strengthening the hypothesis of the involvement of the clathrin- and adaptor-dependent pathway in trafficking of CRAM via the FP.

ProtRepeatsDB: a database of amino acid repeats in genomes

Background

Genome wide and cross species comparisons of amino acid repeats is an intriguing problem in biology mainly due to the highly polymorphic nature and diverse functions of amino acid repeats. Innate protein repeats constitute vital functional and structural regions in proteins. Repeats are of great consequence in evolution of proteins, as evident from analysis of repeats in different organisms. In the post genomic era, availability of protein sequences encoded in different genomes provides a unique opportunity to perform large scale comparative studies of amino acid repeats. ProtRepeatsDB is a relational database of perfect and mismatch repeats, access to which is designed as a resource and collection of tools for detection and cross species comparisons of different types of amino acid repeats.

Description

ProtRepeatsDB (v1.2) consists of perfect as well as mismatch amino acid repeats in the protein sequences of 141 organisms, the genomes of which are now available. The web interface of ProtRepeatsDB consists of different tools to perform repeat s; based on protein IDs, organism name, repeat sequences, and keywords as in FASTA headers, size, frequency, gene ontology (GO) annotation IDs and regular expressions (REGEXP) describing repeats. These tools also allow formulation of a variety of simple, complex and logical queries to facilitate mining and large-scale cross-species comparisons of amino acid repeats. In addition to this, the database also contains sequence analysis tools to determine repeats in user input sequences.

Conclusion

ProtRepeatsDB is a multi-organism database of different types of amino acid repeats present in proteins. It integrates useful tools to perform genome wide queries for rapid screening and identification of amino acid repeats and facilitates comparative and evolutionary studies of the repeats. The database is useful for identification of species or organism specific repeat markers, interspecies variations and polymorphism.

CZH proteins: a new family of Rho-GEFs

The Rho family of small GTPases are important regulators of multiple cellular activities and, most notably, reorganization of the actin cytoskeleton. Dbl-homology (DH)-domain-containing proteins are the classical guanine nucleotide exchange factors (GEFs) responsible for activation of Rho GTPases. However, members of a newly discovered family can also act as Rho-GEFs. These CZH proteins include: CDM (Ced-5, Dock180 and Myoblast city) proteins, which activate Rac; and zizimin proteins, which activate Cdc42. The family contains 11 mammalian proteins and has members in many other eukaryotes. The GEF activity is carried out by a novel, DH-unrelated domain named the DOCKER, CZH2 or DHR2 domain. CZH proteins have been implicated in cell migration, phagocytosis of apoptotic cells, T-cell activation and neurite outgrowth, and probably arose relatively early in eukaryotic evolution.

Characterization of Two Protein Disulfide Isomerases from the Endocytic Pathway of Bloodstream Forms of Trypanosoma brucei

Proteins from the endocytic pathway in bloodstream forms of Trypanosome brucei are modified by the addition of linear poly-N-acetyllactosamine side chains, which permits their isolation by tomato lectin affinity chromatography. Antibodies against this tomato lectin binding fraction were employed to screen a cDNA expression library from bloodstream forms of T. brucei. Two cDNAs were prominent among those selected. These cDNAs coded for two putative protein disulfide isomerases (PDIs) that respectively contained one and two double-cysteine redox-active sites and corresponded to a single domain PDI and a class 1 PDI. Assays of the purified recombinant proteins demonstrated that both proteins possess isomerase activity, but only the single domain PDI had a reducing activity. These PDIs possess a number of unusual features that distinguish them from previously characterized PDIs. The expression of both is developmentally regulated, they both co-localize with markers of the endocytic pathway, and both are modified by N-glycosylation. The larger PDI possesses N-glycans containing poly-N-acetyllactosamine, a modification that is indicative of processing in the Golgi and suggests the presence of a novel trafficking pathway for PDIs in trypanosomes. Although generally PDIs are considered essential, neither activity appeared to be essential for the growth of trypanosomes, at least in vitro.

Trypanosoma brucei glycoproteins contain novel giant poly-N-acetyllactosamine carbohydrate chains

The flagellar pocket of the bloodstream form of the African sleeping sickness parasite Trypanosoma brucei contains material that binds the beta-d-galactose-specific lectin ricin (Brickman, M. J., and Balber, A. E. (1990) J. Protozool. 37, 219-224). Glycoproteins were solubilized from bloodstream form T. brucei cells in 8 M urea and 3% SDS and purified by ricin affinity chromatography. Essentially all binding of ricin to these glycoproteins was abrogated by treatment with peptide N-glycosidase, showing that the ricin ligands are attached to glycoproteins via N-glycosidic linkages to asparagine residues. Glycans released by peptide N-glycosidase were resolved by Bio-Gel P-4 gel filtration into two fractions: a low molecular mass mannose-rich fraction and a high molecular mass galactose and N-acetylglucosamine-rich fraction. The latter fraction was further separated by high pH anion exchange chromatography and analyzed by gas chromatography mass spectrometry, one- and two-dimensional NMR, electrospray mass spectrometry, and methylation linkage analysis. The high molecular mass ricin-binding N-glycans are based on a conventional Manalpha1-3(Manalpha1-6)Manbeta1-4-GlcNAcbeta1-4GlcNAc core structure and contain poly-N-acetyllactosamine chains. A significant proportion of these structures are extremely large and of unusual structure. They contain an average of 54 N-acetyllactosamine (Galbeta1-4GlcNAc) repeats per glycan, linked mostly by -4GlcNAcbeta1-6Galbeta1-interrepeat linkages, with an average of one -4GlcNAcbeta1-3(-4GlcNAcbeta1-6)Galbeta1- branch point in every six repeats. These structures, which also bind tomato lectin, are twice the size reported for the largest mammalian poly-N-acetyllactosamine N-linked glycans and also differ in their preponderance of -4GlcNAcbeta1-6Galbeta1- over -4GlcNacbeta1-3Galbeta1- interrepeat linkages. Molecular modeling suggests that -4GlcNAcbeta1-6Galbeta1- interrepeat linkages produce relatively compact structures that may give these giant N-linked glycans unique physicochemical properties. Fluorescence microscopy using fluorescein isothiocyanatericin indicates that ricin ligands are located mainly in the flagellar pocket and in the endosomal/lysosomal system of the trypanosome.

RNA polymerase I transcribes procyclin genes and variant surface glycoprotein gene expression sites in Trypanosoma brucei

In eukaryotes, RNA polymerase (pol) I exclusively transcribes the large rRNA gene unit (rDNA) and mRNA is synthesized by RNA pol II. The African trypanosome, Trypanosoma brucei, represents an exception to this rule. In this organism, transcription of genes encoding the variant surface glycoprotein (VSG) and the procyclins is resistant to alpha-amanitin, indicating that it is mediated by RNA pol I, while other protein-coding genes are transcribed by RNA pol II. To obtain firm proof for this concept, we generated a T. brucei cell line which exclusively expresses protein C epitope-tagged RNA pol I. Using an anti-protein C immunoaffinity matrix, we specifically depleted RNA pol I from transcriptionally active cell extracts. The depletion of RNA pol I impaired in vitro transcription initiated at the rDNA promoter, the GPEET procyclin gene promoter, and a VSG gene expression site promoter but did not affect transcription from the spliced leader (SL) RNA gene promoter. Fittingly, induction of RNA interference against the RNA pol I largest subunit in insect-form trypanosomes significantly reduced the relative transcriptional efficiency of rDNA, procyclin genes, and VSG expression sites in vivo whereas that of SL RNA, alphabeta-tubulin, and heat shock protein 70 genes was not affected. Our studies unequivocally show that T. brucei harbors a multifunctional RNA pol I which, in addition to transcribing rDNA, transcribes procyclin genes and VSG gene expression sites.

Galactose metabolism is essential for the African sleeping sickness parasite Trypanosoma brucei

The tsetse fly-transmitted protozoan parasite Trypanosoma brucei is the causative agent of human African sleeping sickness and the cattle disease Nagana. The bloodstream form of the parasite uses a dense cell-surface coat of variant surface glycoprotein to escape the innate and adaptive immune responses of the mammalian host and a highly glycosylated transferrin receptor to take up host transferrin, an essential growth factor. These glycoproteins, as well as other flagellar pocket, endosomal, and lysosomal glycoproteins, are known to contain galactose. The parasite is unable to take up galactose, suggesting that it may depend on the action of UDP-glucose 4'-epimerase for the conversion of UDP-Glc to UDP-Gal and subsequent incorporation of galactose into glycoconjugates via UDP-Gal-dependent galactosyltransferases. In this paper, we describe the cloning of T. brucei galE, encoding T. brucei UDP-Glc-4'-epimerase, and functional characterization by complementation of a galE-deficient Escherichia coli mutant and enzymatic assay of recombinant protein. A tetracycline-inducible conditional galE null mutant of T. brucei was created using a transgenic parasite expressing the TETR tetracycline repressor protein gene. Withdrawal of tetracycline led to a cessation of cell division and substantial cell death, demonstrating that galactose metabolism in T. brucei proceeds via UDP-Glc-4'-epimerase and is essential for parasite growth. After several days without tetracycline, cultures spontaneously recovered. These cells were shown to have undergone a genetic rearrangement that deleted the TETR gene. The results show that enzymes and transporters involved in galactose metabolism may be considered as potential therapeutic targets against African trypanosomiasis.

Sequence requirements for trafficking of the CRAM transmembrane protein to the flagellar pocket of African trypanosomes

CRAM is a cysteine-rich acidic transmembrane protein, highly expressed in the procyclic form of Trypanosoma brucei. Cell surface expression of CRAM is restricted to the flagellar pocket of trypanosomes, the only place where receptor mediated endocytosis takes place in the parasite. CRAM can function as a receptor and was hypothesized to be a lipoprotein receptor of trypanosomes. We study mechanisms involved in the presentation and routing of CRAM to the flagellar pocket of insect- and bloodstream-form trypanosomes. By deletional mutagenesis, we found that deleting up to four amino acids from the C terminus of CRAM did not affect the localization of CRAM at the flagellar pocket. Shortening the CRAM protein by 8 and 19 amino acids from the C terminus resulted in the distribution of the CRAM protein in the endoplasmic reticulum (ER) (the CRAM protein is no longer uniquely sequestered at the flagellar pocket). This result indicates that the truncation of the CRAM C terminus affected the transport efficiency of CRAM from the ER to the flagellar pocket. However, when CRAM was truncated between 29 and 40 amino acids from the C terminus, CRAM was not only distributed in the ER but also located to the flagellar pocket and spread to the cell surface and the flagellum. Replacing the CRAM transmembrane domain with the invariant surface glycoprotein 65-derived transmembrane region did not affect the flagellar pocket location of CRAM. These results indicate that the CRAM cytoplasmic extension may exhibit two functional domains: one domain near the C terminus is important for efficient export of CRAM from the ER, while the second domain is of importance for confining CRAM to the flagellar pocket membrane.

Analysis of ploidy (in megabase chromosomes) in Trypanosoma brucei after genetic exchange

The megabase chromosomes of Trypanosoma brucei are normally diploid, but the extent to which this ploidy is maintained when parasites undergo genetic exchange is not known. To investigate this issue, a panel of 30 recombinant clones resulting from the co-transmission through tsetse flies of three different parental T. brucei lines in all pair-wise combinations (STIB 247, STIB 386 and TREU 927/4) were examined. These clones are products of 28 different mating events; four of them result from self-fertilisation and the others are F1 hybrids. DNA contents of the three parental lines were determined by flow cytometry and shown to differ only slightly with DNA content increasing in the order 927/4 < 247 < 386. Flow cytometry of the recombinant clones indicated DNA contents were similar to the parents in 28 clones and raised approximately 1.5 times the parental values in only two. The two F1 hybrid progeny with raised DNA contents were shown by marker analysis to be trisomic for seven independent loci indicating that they were probably triploid whereas progeny with DNA contents similar to parental values inherited a single allele from each parent for four independent loci indicating that they were diploid.

Characterization of tGLP-1, a Golgi and lysosome-associated, transmembrane glycoprotein of African trypanosomes

Purification of endosomal/lysosomal vesicles of Trypanosoma brucei brucei bloodstream forms and generation of monoclonal antibodies led to the isolation of antibodies directed against an 85 kDa, Golgi and endocytic traffic-associated protein termed tGLP-1, Trypanosoma Golgi/lysosome protein-1. Preliminary immunoelectron microscopical analysis revealed that the protein is present in, but not restricted to, the limiting membrane of multivesicular lysosomes and is more abundant in bloodstream forms compared to the procyclic stage. The corresponding gene was cloned and is present as a single copy. Blast searches did not reveal any homologies to other proteins and genes published. The nucleotide sequence of the gene (1848 base pairs) predicted a type 1 membrane topology with an N-terminal signal sequence (20 aa), a luminal domain with 2 N-glycosylation sites (524 aa), a transmembrane domain (23 aa), and a long cytosolic tail domain (49 aa). Polyclonal antibodies raised against the cytosolic tail confirmed the localization of the gene product to multivesicular lysosomes but revealed that the majority of the protein was in the Golgi apparatus. Colabelling with an antibody against p67, a lysosomal glycoprotein of trypanosomes, revealed extensive overlap between the proteins with opposing relative abundance. Expression of the tGLP-1 open reading frame in Leishmania resulted in Golgi localization, and in Toxoplasma, in localization to both the Golgi and endoplasmic reticulum. These data indicate conservation in the functionality of the Golgi-targeting sequence of tGLP-1.

A high level of mixed Trypanosoma brucei infections in tsetse flies detected by three hypervariable minisatellites

The issue of whether genetic exchange occurs at a significant frequency in natural populations of Trypanosoma brucei is controversial and one of the arguments against a high frequency has been the apparent lack of host infections with mixtures of trypanosome genotypes. Three minisatellite markers (MS42, CRAM, 292) within the coding regions of three genes have been identified and PCR based methods developed for detecting variation at these loci using crude lysates of infected blood as templates. Initial PCR analysis, using primers flanking the repeats, of DNA from two cloned stocks of the parasite has shown that two DNA fragments of different size were amplified from each stock. Analysis of the inheritance of these fragments into the F1 progeny of crosses demonstrated that the different size fragments were alleles that segregated in a Mendelian manner. The alleles at each of the three loci segregated independently consistent with their localisation on three different chromosomes. Analysis of a series of cloned isolates from tsetse flies showed that these loci were highly variable giving heterozygosities of 94% and the identification of 12 distinct alleles in a sample of 17 cloned isolates. In order to determine whether isolates are heterogeneous in terms of trypanosome genotype, the allelic variation at these three loci was examined in uncloned samples from tsetse flies isolated in Kiboko, Kenya and Lugala, Uganda. A significant proportion of the isolates (36% in Lugala and 47% in Kiboko) contained more than two alleles at one or more of the loci thus demonstrating that a high proportion of tsetse flies were infected with more than one genotype of trypanosomes. This was established, unequivocally, for two isolates by generating a series of cloned trypanosome lines from each and determining the genotype of each clone; one isolate (927) contained seven different genotypes with a high proportion of the possible combinations of alleles at each locus. These results indicate the possibility of frequent genetic exchange in the field, they imply that a significant proportion of mammalian hosts must contain mixtures of different trypanosome genotypes and they demonstrate the advantages of using minisatellite markers for the analysis of the population structure of T. brucei.

Molecular cloning of p67, a lysosomal membrane glycoprotein from Trypanosoma brucei

We have previously characterized a highly glycosylated membrane protein (p67) in Trypanosoma brucei spp that is apparently targeted to lysosomes in a developmentally regulated manner. Antibody to native p67 identified a partial cDNA clone from a T. b. rhodesiense expression library and RT-PCR was used to complete the sequence of the cDNA. Equal levels of p67 transcript are detected in both procyclic and bloodstream stages of the life cycle. The 2771 nt cDNA contains a 1980 nt orf encoding a 659 amino acid polypeptide (72,567 Da). Hydropathy analysis predicts a Type I membrane topology (N to C): an N-terminal signal sequence, a large hydrophilic lumenal domain with 14 N-glycosylation sites, a trans-membrane domain (19 aa), and a short (24 aa) cytoplasmic domain. Peptide microsequencing of purified p67 identified nine residues identical to the deduced amino acid sequence, confirming the identity of the cDNA and defining the signal sequence cleavage site. Antibody to p67 protein produced in E. coli recognizes the same spectrum of native p67 glycoforms as the antibody used to clone the cDNA. All features of the deduced amino acid sequence are consistent with the known properties of the native protein and suggest a structure similar to mammalian LAMPS. The cytoplasmic domain contains two putative di-leucine targeting motifs similar to those involved in lysosomal targeting in vertebrate cells. Our results suggest that a single p67 polypeptide, or a group of highly related polypeptides, is synthesized in both bloodstream and procyclic trypanosomes and that subsequent post-translational processing and lysosomal targeting is subject to stage-specific regulation.

Conserved organization of genes in trypanosomatids

Trypanosomatids are unicellular protozoan parasites which constitute some of the most primitive eukaryotes. Leishmania spp, Trypanosoma cruzi and members of the Trypanosoma brucei group, which cause human diseases, are the most studied representatives of this large family. Here we report a comparative analysis of a large genomic region containing glucose transporter genes in three Salivarian trypanosomes (T. brucei, T. congolense and T. vivax), T. cruzi and Leishmania donovani. In T. brucei, the 8 kb (upstream) and 14 kb (downstream) regions flanking the glucose transporter genes cluster contain two and six new genes, respectively, six of them encoding proteins homologous to known eukaryotic proteins (phosphatidylinositol 3 kinase, ribosomal protein S12, DNAJ and three small G-proteins--Rab1, YPT6 and ARL3). This gene organization is identical in T. brucei, T. congolense and T. vivax suggesting that Salivarian trypanosomes have a high level of conservation in gene organization. In T. cruzi and Leishmania, the overall organization of this cluster is conserved, with insertion of additional genes when compared with T. brucei. Phylogenetic reconstitution based on glucose transporters is in accord with the monophyly of the genus Trypanosoma and the early separation of T. vivax within Salivarian trypanosomes. On the basis of gene organization, biochemical characteristics of isoforms and phylogeny, we discuss the genesis of the glucose transporter multigene family in Salivarian trypanosomes.

Surface coats and secretory trafficking in African trypanosomes

Recent advances in transfection technology have been exploited to address fundamental questions relating to secretory trafficking in African trypanosomes. Targeted gene disruptions and ectopic expression of the major stage-specific surface proteins have provided unexpected insights into both the function and assembly of the essential parasite surface coats. A growing list of novel secretory cargo molecules, as well as advances in the characterization of trypanosomal secretory machinery, provide a unique model system for the study of eukaryotic secretory processes.

Analysis of trypanosomal endocytic organelles using preparative free-flow electrophoresis

In this paper we demonstrate the power of preparative free-flow electrophoresis (FFE) for the study of endocytosis by African trypanosomes. Endocytosis of extracellular macromolecules by these parasites occurs through a specialized region of the parasite called the flagella pocket. The uptake of fluid phase markers such as horseradish peroxidase (HRP) into the various compartments of the endocytic pathway of bloodstream forms of Trypanosoma brucei brucei was manipulated by regulating the external environment (e.g., by altering the temperature of incubation). The various subcellular compartments were then separated by free-flow electrophoresis (FFE) or isopycnic density gradient centrifugation and analyzed for marker uptake. At low temperatures, HRP was found predominantly in the flagellar pocket. Increasing the temperature resulted in a time-dependent uptake of HRP into more positively charged endosomal fractions. However, little HRP activity was detected in lysosomal compartments, suggesting that either HRP had not yet entered the lysosome or was degraded immediately upon entry. Through the use of FFE we were able to identify and analyze compartments of the endosomal pathway that were not possible to identify by density gradient centrifugation alone. Although the differences in FFE separation of the endocytic compartments as seen in HRP uptake were striking, the minor changes seen within the lysosomal system were more subtle, as depicted in the protease profiles. In conclusion, we show that preparative FFE is a powerful technique for the analysis and separation of flagellar pocket-derived membranes from other endosomal and lysosomal compartments of African trypanosomes.

Characterization of a novel, stage-specific, invariant surface protein in Trypanosoma brucei containing an internal, serine-rich, repetitive motif

A new surface membrane protein, invariant surface glycoprotein termed ISG100, was identified in Trypanosoma brucei, using catalyzed surface, radioiodination of intact cells. This integral membrane glycoprotein was purified by a combination of detergent extraction, lectin-affinity, and ion-exchange chromatography followed by preparative SDS-polyacrylamide gel electrophoresis. The protein was expressed only in bloodstream forms of the parasite, was heavily N-glycosylated, and was present in different clonal variants of the same serodeme as well as in different serodemes. The gene for this protein was isolated by screening a cDNA expression library with antibodies against the purified protein followed by screening of a genomic library. The nucleotide sequence of the gene (4050 base pairs) predicted a highly reiterative polypeptide containing three distinct domains, a unique N-terminal domain of about 10 kDa containing three potential N-glycosylation sites, which was followed by a large internal domain consisting entirely of 72 consecutive copies of a serine-rich, 17-amino acid motif (approximately 113 kDa) and terminated with an apparent transmembrane spanning region of about 3.3 kDa. The internal repeat region of this gene (3672 base pairs) represents the largest reiterative coding sequence to be fully characterized in any species of trypanosome. There was no significant homology with other known proteins, and overall the predicted protein was extremely hydrophobic. Unlike the genes for other surface proteins, the gene encoding ISG100 was present as a single copy. Although present in the flagellar pocket, ISG100 was predominantly associated with components of the pathways for endo/exocytosis, such as intracellular vesicles located in the proximity of the pocket as well a large, electron-lucent perinuclear digestive vacuole.

Endocytosis and secretion in trypanosomatid parasites — Tumultuous traffic in a pocket

Trypanosomatids are flagellated protozoan parasites of invertebrates, vertebrates and plants. Some species, found in the subtropics and tropics, cause chronic diseases in humans and domestic animals. The surface of the trypanosomatid provides a shield against environmental challenges, ligands for interaction with host cells, as well as receptors and transporters for the uptake of nutrients. Communication between the parasite and its environment is confined to the flagellar pocket, an invagination of the plasma membrane around the base of the flagellum. In this review, the authors discuss endocytosis, secretion and membrane trafficking in Trypanosoma and Leishmania.

Gene cloning and cellular localization of a membrane-bound acid phosphatase of Leishmania mexicana

In a previous publication, we described the purification of a membrane-bound acid phosphatase of Leishmania mexicana as a heterogeneously N-glycosylated protein of an apparent molecular mass of 70000-72000 expressed in both the promastigote and the amastigote stage of the parasite [19]. Screening of a genomic DNA library of L. mexicana with degenerate oligonucleotides designed according to the NH2-terminus of the protein led to the cloning of the lmmbap gene, which is present in one copy per haploid genome. The open reading frame predicts a protein of 516 amino acids composed of a signal sequence, a large hydrophilic region, a trans-membrane alpha-helix and a short cytoplasmic tail. The sequence of the hydrophilic region is homologous to acid phosphatases from other organisms. While in wild-type promastigotes, the acid phosphatase is located in the endosomal/lysosomal compartment between the flagellar pocket and the nucleus, overexpression leads to its abundant exposure on the cell surface. In cells transfected with a construct lacking the region corresponding to the trans-membrane and the cytoplasmic parts, the resulting altered acid phosphatase is efficiently secreted into the culture medium. The potential of this system for studies on membrane trafficking in kinetoplastid organisms is discussed.

Sequence analysis of a 14.2 kb fragment of Saccharomyces cerevisiae chromosome XIV that includes the ypt53, tRNALeu and gsr m2 genes and four new open reading frames

As part of the EU yeast genome program, a fragment of 14,262 bp from the left arm of Saccharomyces cerevisiae chromosome XIV has been sequenced. This fragment corresponds to cosmid 14-14b and is located roughly 130 kb from the centromere. It contains four new open reading frames which encode potential proteins of more than 99 amino acids, as well as the ypt53, tRNALeu and gsr moffenes. The putative protein N2212 is similar to the ribosomal protein S7 from humans. N2215 contains several predicted transmembrane elements. N2231 contains regions which are rich in acidic, as well as basic, residues which could from alpha-helical structures. Similar regions are found in a variety of proteins including glutamic acid rich protein, trichohyalin, caldesmon, Tb-29 and several cytoskeleton-interacting proteins.

An M(r) 145,000 low-density lipoprotein (LDL)-binding protein is conserved throughout the Kinetoplastida order

In view of the importance of the low-density lipoprotein (LDL)-receptor in Trypanosoma brucei, we have examined whether other bloodstream trypanosomes of medical and veterinary importance (T.b. rhodesiense, T. equiperdum, T. vivax, T. congolense), but also related parasites developing in mammalian (Leishmania donovani) and non-mammalian hosts (Crithidia luciliae and Phytomonas sp. isolated from Euphorbia), would possess an LDL-receptor of their own. (1) All these parasites specifically accumulate human 125I-LDL with a relatively 2.5-fold higher rate for bloodstream trypanosomes. (2) A mixture of monoclonal antibodies raised against T.b. brucei LDL-receptor inhibit binding of LDL to all species but with different efficiency. (3) A single glycoprotein of similar M(r) (gp145) is isolated by LDL-affinity chromatography from all the above species, as well as from both human serum-resistant and sensitive strain of T.b. rhodesiense, and from the bodonid member of the Kinetoplastida Trypanoplasma borelli. (4) Several control experiments including 35S-metabolic labeling of procyclic T.b. brucei and of C. luciliae followed by LDL-affinity chromatography or immunoprecipitation demonstrate that gp145 is indeed synthesised by the parasites and is not a contaminant of the experimental system. (5) In immunoblots and ELISA, these gp145 cross-react with the polyclonal and monoclonal antibodies raised against the LDL-receptor of T.b. brucei, the highest degree of cross-reactivity being found among the members of the Trypanozoon subgroup. (6) Finally, immunisation of mice with the purified LDL-receptor from one strain of T.b. brucei is not sufficient to confer durable protection against another strain of this parasite.

Low temperature reversibly inhibits transport from tubular endosomes to a perinuclear, acidic compartment in African trypanosomes

We have used electron microscopy and flow cytofluorimetry to study endocytosis and intracellular transport of fluid phase bovine serum albumen gold complexes and membrane bound concanavalin A through endosomal compartments of bloodstream forms of Trypanosoma brucei rhodesiense. Both markers were rapidly endocytosed from the flagellar pocket. Within 20 minutes at 37 degrees C the markers reached a large, vesicular, perinuclear compartment that stained heavily with the CB1 monoclonal antibody. Neither marker left the flagellar pocket and entered cells at 4 degrees C. When cells were incubated at 12 degrees C, both markers entered the cell and were transported to collecting tubules, a tubular endosomal compartment that receives endocytosed material from coated endocytic vesicles. However, no material was transported from collecting tubules to the late, perinuclear compartment at 12 degrees C. The morphology of collecting tubule membranes was specifically altered at 12 degrees C; tubules became shorter and were arrayed near the flagellar pocket. The morphological alteration and the block in transport of endocytic markers to the perinuclear compartment seen at 12 degrees C were reversed 10 minutes after cells were returned to 37 degrees C. We also used flow cytofluorimetric measurements of pH dependent fluorescence quenching to measure the pH of the terminal endocytic compartment. Fluoresceinated lectins accumulated in a terminal compartment with a pH of 6.0-6.1, a value considerably higher than that of mammalian lysosomes. Fluorescence from fluoresceinated lectins in this terminal endocytic compartment was dequenched when bloodstream forms were incubated in the presence of chloroquine.