Tubulin mRNAs of Trypanosoma brucei

γ-Tubulin in Leishmania: cell cycle-dependent changes in subcellular localization and heterogeneity of its isoforms

A panel of six anti-peptide antibodies recognizing epitopes in different regions of the gamma-tubulin molecule was used for the characterization and localization of gamma-tubulin during cell cycle in Leishmania promastigotes. Immunofluorescence microscopy revealed the presence of gamma-tubulin in the basal bodies, posterior pole of the cell, and in the flagellum. Furthermore, the antibodies showed punctuate staining in the subpellicular microtubule. This complex localization pattern was observed in both interphase and dividing cells, where staining of posterior poles and the subpellicular corset was more prominent. In posterior poles, gamma-tubulin co-distributed with the 210-kDa microtubule-interacting protein and the 57-kDa protein immunodetected with anti-vimentin antibody. Immunogold electron microscopy on thin sections of isolated flagella showed that gamma-tubulin was associated with the paraflagellar rod (PFR) that runs adjacent to the axonemal microtubules. Under different extraction conditions, gamma-tubulin in Leishmania was found only in insoluble cytoskeletal fractions, in contrast to tubulin dimers that were both in soluble and cytoskeletal pool. Two-dimensional electrophoresis revealed multiple charge variants of gamma-tubulin. Posttranslational modifications of Leishmania gamma-tubulin might therefore have an important role in the regulation of microtubule nucleation and interaction with other proteins. The complex pattern of gamma-tubulin localization and its properties indicate that gamma-tubulin in Leishmania might have other function(s) besides microtubule nucleation.

Tubulin diversity in trophozoites of Giardia lamblia

Giardia lamblia is a diplomonad that parasitizes the small intestine of vertebrates. The trophozoite is multiflagellar and its cytoskeleton presents a complex organization of microtubular structures. One of these, the adhesive disk, consists of a microtubular spiral. The median body, whose function is not yet determined, is also composed by microtubules. The cell has eight flagella and two microtubule sheets, known as the funis. In this study we used several antibodies and immunofluorescence microscopy to help in the characterization of these structures. We observed that Giardia tubulin reacts with antibodies raised against very distinct immunogens. The antibodies used were against: (1) alpha-tubulin from chicken embryo brain, Trypanosoma brucei, sea urchin sperm, Paramecium, acetylated alpha-tubulin from Paramecium, and tyrosinated alpha-tubulin, (2) beta-tubulin from chicken embryo brain and Physarum polycephalum, and (3) an antibody with specificity to beta-tubulin having as immunogen the FtsZ bacterial protein. Each cytoskeletal structure of Giardia presented a distinct pattern of labeling by the several antibodies used. These data indicate that even being considered one of the most ancient of organisms, Giardia shares similarities (at least in relation to tubulin) with other organisms. They also open some questions about the organization and composition of its microtubular structures.

Trypanosoma cruzi: RNA structure and post-transcriptional control of tubulin gene expression

Changes in tubulin expression are among the biochemical and morphological adaptations that occur during the life cycle of Trypanosomatids. To investigate the mechanism responsible for the differential accumulation of tubulin mRNAs in Trypanosoma cruzi, we determine the sequences of alpha- and beta-tubulin transcripts and analyzed their expression during the life cycle of the parasite. Two beta-tubulin mRNAs of 1.9 and 2.3 kb were found to differ mainly by an additional 369 nucleotides at the end of the 3' untranslated region (UTR). Although their transcription rates are similar in epimastigotes and amastigotes, alpha- and beta-tubulin transcripts are 3- to 6-fold more abundant in epimastigotes than in trypomastigotes and amastigotes. Accordingly, the half-lives of alpha- and beta-tubulin mRNAs are significantly higher in epimastigotes than in amastigotes. Transient transfection experiments indicated that positive regulatory elements occur in the 3' UTR plus downstream intergenic region of the alpha-tubulin gene and that both positive and negative elements occur in the equivalent regions of the beta-tubulin gene.

Cell-cycle and developmental regulation of TbRAB31 localisation, a GTP-locked Rab protein from Trypanosoma brucei

Rab proteins are small GTPases that control the direction and timing of vesicle fusion during intracellular trafficking between membraneous compartments. Genome sequencing and EST analysis of Trypanosoma brucei indicates that the trypanosome Rab (TbRAB) gene family, and hence complexity of intracellular transport pathways, is intermediate between Saccharomyces cerevisiae and mammals. TbRAB31 is a constitutively expressed T. brucei Rab protein (formerly Trab7p) and is the product of one of two closely linked TbRAB genes, the other being TbRAB2 (TbRab2p, in: Field H, Ali BRS, Sherwin T, Gull K, Croft SL, Field MC. TbRab2p, a marker for the endoplasmic reticulum of Trypanosoma brucei, localises to the ERGIC in mammalian cells. J Cell Sci 1999; 112:147-156), involved in ER to Golgi transport. TbRAB31 has high homology to members of the Sec4/Ypt1 subfamily of Rab proteins from S. cerevisiae and to Rab13 and Rab11 from higher eukaryotes. Recombinant TbRAB31 binds GTP but, unusually for a Rab protein, has undetectable GTPase activity resulting in a constitutively GTP-bound protein. Antibodies against TbRAB31 recognise a discrete structure located between the kinetoplast and nucleus in interphase procyclic cells; by contrast the structure is morphologically more complex in bloodstream form (BSF) parasites, consisting of at least two foci. TbRAB31 behaviour was also studied during the cell cycle; TbRAB31 always localised to a discrete structure that duplicated very early in mitosis and relocated to daughter cells in a coordinate manner with the basal body and kinetoplast, suggesting the involvement of microtubules. Additional evidence suggests that TbRAB31 localises to the trypanosome Golgi complex. Firstly, the interphase position of TbRAB31 is consistent with a Golgi location. Secondly, the TbRAB31 structure is also recognised by cross-reacting antibodies to mammalian beta-coatomer protein (beta-COP), which localises to the Golgi in mammalian cells. Thirdly, the fluorescent ceramide analogue, BODIPY-TR-ceramide, a reliable marker of the mammalian Golgi apparatus, exhibited overlapping distribution with TbRAB31. The location of BODIPY-TR-ceramide was confirmed at the trypanosome Golgi by histochemistry with diaminobenzidine and electron microscopy.

Disruption of a gene encoding a novel mitochondrial DEAD-box protein in Trypanosoma brucei affects edited mRNAs

The majority of mitochondrial pre-mRNAs in kinetoplastid protozoa such as Trypanosoma, Leishmania, and Crithidia are substrates of a posttranscriptional processing reaction referred to as RNA editing. The process results in the insertion and, to a lesser extent, deletion of uridylates, thereby completing the informational content of the mRNAs. The specificity of the RNA editing reaction is provided by guide RNAs (gRNAs), which serve as templates for the editing apparatus. In addition, the process relies on mitochondrial proteins, presumably acting within a high-molecular-mass ribonucleoprotein complex. Although several enzymatic activities have been implicated in the editing process, no protein has been identified to date. Here we report the identification of a novel mitochondrial DEAD-box protein, which we termed mHel61p. Disruption of the mHEL61 alleles in insect-stage Trypanosoma brucei cells resulted in a reduced growth rate phenotype. On a molecular level, the null mutant showed significantly reduced amounts of edited mRNAs, whereas never-edited and nuclear mRNAs were unaffected. Reexpression of mHel61p in the knockout cell line restored the ability to efficiently synthesize edited mRNAs. The results suggest an involvement of mHel61p in the control of the abundance of edited mRNAs and thus reveal a novel function for DEAD-box proteins.

A common pyrimidine-rich motif governs trans-splicing and polyadenylation of tubulin polycistronic pre-mRNA in trypanosomes

In trypanosomes, the generation of monocistronic mRNAs from polycistronic precursors is achieved via RNA processing, namely trans-splicing of the spliced leader sequence at the 5' end and cleavage/polyadenylation at the 3' end of the mRNA coding region. Recent evidence raised the intriguing possibility that these two reactions are coupled. To begin a dissection of the signals required for mRNA 5'-end and 3'-end formation and to uncover potential interactions between trans-splicing and polyadenylation, we mutagenized the intergenic region between the beta- and alpha-tubulin genes of Trypanosoma brucei. Block substitutions identified the pyrimidine-rich sequences at the alpha-tubulin 3'-splice-acceptor site as a major determinant for accurate trans-splicing downstream and 3'-end formation upstream. In addition to the utilization of cryptic 3'-splice sites, obliteration of the polypyrimidine tracts led to aberrant poly(A)+ site choice, even in the presence of the wild-type poly(A)+ site and neighboring sequences. Taken together, these results indicate that the polypyrimidine-rich sequences act as a bifunctional element that affects RNA processing both upstream and downstream from itself. This is consistent with the possibility that the polypyrimidine tract is recognized by both the trans-splicing and polyadenylation machineries, either sequentially or simultaneously.

A repetitive protein from Trypanosoma brucei which caps the microtubules at the posterior end of the cytoskeleton

The major structural component of the cyto-architecture of Trypanosoma brucei is a microtubular array which envelopes the entire cell body and which is in close contact with the overlying cell membrane. A cytoskeletal protein has been identified which is associated with the microtubules of this array at their posterior ends exclusively. This protein, Gb4, is coded for by a very large gene which consists of numerous, tandemly linked repeat units of 0.6 kb length. Despite the large size of the gene, and also of the corresponding mRNA, the mature Gb4 protein has a size of only 28 kDa. Gb4 is well conserved between different species of African trypanosomes.

Genomic and transcriptional linkage of the genes for calmodulin, EF-hand 5 protein, and ubiquitin extension protein 52 in Trypanosoma brucei

We report genomic linkage of a pair of tandem, identical ubiquitin-extension protein 52 (EP52) genes, a novel EF-hand superfamily member gene (EFH5), and the calmodulin gene cluster in Trypanosoma brucei. The intergenic regions of these four genes are short: about 108 bp between the calmodulin gene C and the EFH5 gene, about 111 bp between the EFH5 gene and the ubiquitin-EP52/1 gene, and about 116 bp between the ubiquitin-EP52/1 and -EP52/2 genes. RNA molecules that span these three intergenic regions have been detected by polymerase chain reaction, which suggests that the genes are transcribed in a polycistronic manner. Transcription of the calmodulin, EFH5, and ubiquitin-EP52 genes in isolated nuclei is rapidly inactivated by UV irradiation, which further strengthens the hypothesis that this cluster of three different genes is transcribed in a polycistronic manner and suggests that they are under the control of a single distant upstream promoter. These results suggest that polycistronic transcription is common in trypanosomes and will probably be found for most, if not all, protein-encoding genes. The presence of at least three housekeeping genes with different known or potential regulatory functions within a polycistronic unit suggests that regulation of transcription initiation plays an important role in the coordinated expression of housekeeping genes in trypanosomes.

Temporal order of RNA-processing reactions in trypanosomes: rapid trans splicing precedes polyadenylation of newly synthesized tubulin transcripts

Many trypanosome genes are expressed as part of large polycistronic transcription units. This finding suggests that regulation of mRNA biogenesis may emphasize RNA-processing reactions more so than in other organisms. This study was undertaken to understand the temporal order of two RNA-processing reactions, trans splicing and polyadenylation, in the maturation of trypanosome mRNAs in vivo. Kinetic studies revealed rapid trans splicing of alpha-tubulin, beta-tubulin, and actin pre-mRNAs within 1 to 2 min after synthesis of the 3' splice site. Furthermore, following blockage of pre-mRNA synthesis, newly synthesized spliced leader RNA cannot be used for trans splicing, suggesting that trypanosomes do not accumulate substantial amounts of pre-mRNA which can provide splice acceptor sites. Thus, trans splicing is cotranscriptional. In addition, we show that trans splicing precedes polyadenylation in the processing of trypanosome tubulin pre-mRNAs.

Genomic and transcriptional linkage of the genes for calmodulin, EF-hand 5 protein, and ubiquitin extension protein 52 in Trypanosoma brucei

We report genomic linkage of a pair of tandem, identical ubiquitin-extension protein 52 (EP52) genes, a novel EF-hand superfamily member gene (EFH5), and the calmodulin gene cluster in Trypanosoma brucei. The intergenic regions of these four genes are short: about 108 bp between the calmodulin gene C and the EFH5 gene, about 111 bp between the EFH5 gene and the ubiquitin-EP52/1 gene, and about 116 bp between the ubiquitin-EP52/1 and -EP52/2 genes. RNA molecules that span these three intergenic regions have been detected by polymerase chain reaction, which suggests that the genes are transcribed in a polycistronic manner. Transcription of the calmodulin, EFH5, and ubiquitin-EP52 genes in isolated nuclei is rapidly inactivated by UV irradiation, which further strengthens the hypothesis that this cluster of three different genes is transcribed in a polycistronic manner and suggests that they are under the control of a single distant upstream promoter. These results suggest that polycistronic transcription is common in trypanosomes and will probably be found for most, if not all, protein-encoding genes. The presence of at least three housekeeping genes with different known or potential regulatory functions within a polycistronic unit suggests that regulation of transcription initiation plays an important role in the coordinated expression of housekeeping genes in trypanosomes.

Temporal order of RNA-processing reactions in trypanosomes: rapid trans splicing precedes polyadenylation of newly synthesized tubulin transcripts

Many trypanosome genes are expressed as part of large polycistronic transcription units. This finding suggests that regulation of mRNA biogenesis may emphasize RNA-processing reactions more so than in other organisms. This study was undertaken to understand the temporal order of two RNA-processing reactions, trans splicing and polyadenylation, in the maturation of trypanosome mRNAs in vivo. Kinetic studies revealed rapid trans splicing of alpha-tubulin, beta-tubulin, and actin pre-mRNAs within 1 to 2 min after synthesis of the 3' splice site. Furthermore, following blockage of pre-mRNA synthesis, newly synthesized spliced leader RNA cannot be used for trans splicing, suggesting that trypanosomes do not accumulate substantial amounts of pre-mRNA which can provide splice acceptor sites. Thus, trans splicing is cotranscriptional. In addition, we show that trans splicing precedes polyadenylation in the processing of trypanosome tubulin pre-mRNAs.

Evolution of the retrotransposons TRS/ingi and of the tubulin genes in trypanosomes

The African trypanosomes have genomes of high plasticity, as demonstrated for instance by their ability to shuffle their genes around, coding for variant-specific surface glycoproteins (VSGs). Another indication of their genome plasticity is the presence of multiple retro-elements. The retrotransposon-like element TRS/ingi is present in many copies in the genome of trypanosomes. One particular derivative of TRS/ingi, called TUBIS, had previously been found to interrupt a tubulin gene in a particular strain of T. brucei. Here both TRS/ingi and TUBIS were studied by hybridizing genomic DNA of various strains and species of trypanosomes with suitable probes in order to elucidate the evolution of this family of retro-elements. The TSR/ingi elements are highly repeated and have very long open reading frames, while TUBIS clearly is a truncated, inactivated form of this element, found in only one particular chromosomal location. Both elements were shown to be present in several strains and species of the subgenus Trypanozoon, in particular in T. brucei brucei, T. gambiense, T. rhodesiense, T. equiperdum and T. evansi. They could not be detected in species of other subgenera, in particular in T. congolense and T. cruzi. These findings suggest that the retrotransposon TRS/ingi was acquired by trypanosomes only after divergence of present day subgenera. The TUBIS element was found in exactly the same chromosomal location (at the 3' end of the tubulin gene cluster) in many different strains and species of the subgenus Trypanozoon. This shows that the element was transposed to this location before speciation of the subgenus. Although, TRS/ingi is unlikely to be involved directly in VSG switching, it may have contributed to the genome plasticity of trypanosomes.

A ribosomal RNA gene promoter at the telomere of a mini-chromosome in Trypanosoma brucei

The parasitic protozoan Trypanosoma brucei has some hundred mini-chromosomes of 50-150 kb, which mainly consist of telomeric repeats, sub-telomeric repeats and internal 177-bp repeats. Their primary function seems to be to expand the repertoire of non-transcribed sub-telomeric variant surface glycoprotein (VSG) genes. Here we report that two of the smaller mini-chromosomes (55 and 60 kb) contain sequences homologous to the ribosomal RNA gene promoter region. We have targeted by homologous recombination the neomycin phosphotransferase (neo(r)) gene behind the promoter on the 55 kb chromosome and show that this promoter mediates the efficient synthesis of properly trans-spliced and polyadenylated neo mRNA. The resulting high resistance to G418 (a neo analogue) is stable in the absence of drug showing that mitotic segregation of this mini-chromosome is precise. Downstream of the transcription start the wild-type version of the ribosomal promoter is flanked by telomeric repeats. The absence of the sub-telomeric repeats found in other T.brucei chromosome ends suggests that the rDNA-telomeric junction has been formed by de novo addition of telomeric repeats to a broken chromosome end (healing). Our results provide a plausible explanation for the alpha-amanitin-resistant transcription of telomeric repeats in T.brucei reported by Rudenko and Van der Ploeg and they show that trypanosomes can efficiently use RNA polymerase I for the expression of sub-telomeric genes, supporting the notion that the alpha-amanitin-resistant transcription of sub-telomeric VSG genes may also be catalyzed by this enzyme.

Efficient production of functional mRNA mediated by RNA polymerase I in Trypanosoma brucei

The unicellular eukaryote Trypanosoma brucei evades the immune defence of its mammalian host by antigenic variation. The genes for variant-specific surface glycoproteins (VSGs) are expressed within large multicistronic transcription units. Mature messenger RNAs are produced by trans-splicing and polyadenylation. A remarkable feature of the transcription of VSG genes is its insensitivity to the RNA polymerase II inhibitor alpha-amanitin. This has led to the speculation that RNA polymerase I, normally only involved in the transcription of ribosomal RNA genes, also mediates expression of these surface antigen genes. In higher eukaryotes, however, transcripts produced by RNA polymerase I were found to be poor substrates for processing into mature mRNAs. In contrast, we show here that the RNA polymerase I of T. brucei can mediate the efficient production of functional mRNA for neomycin phosphotransferase. This exceptional ability may be related to the unusual way in which pre-mRNAs are capped in trypanosomes. In most eukaryotes, mRNAs are modified at their 5' end by a capping activity associated with RNA polymerase II; in trypanosomes, mRNAs acquire their 5'-cap from capped mini-exon donor RNA by trans-splicing, a process that could be independent of the RNA polymerase producing the pre-mRNA.

Permeable trypanosome cells as a model system for transcription and trans-splicing

We have established conditions for Trypanosoma brucei permeable cells to study transcription and trans-splicing. We found that the concentration of monovalent and, to a lesser extent, divalent ions plays a critical role for the expression of a number of different genes. Most remarkably, the synthesis of the spliced leader (SL) RNA was optimal at 20 mM KCl, whereas higher potassium concentrations were inhibitory. In addition, MgCl2 concentrations above 3 mM led to the accumulation of a 3' end shortened SL RNA species, which has been previously reported not to participate in trans-splicing. Using conditions optimal for the synthesis of the SL RNA, we observed accurate trans-splicing of newly-synthesized alpha-tubulin RNA. Moreover, we detected the SL intron both joined to high molecular weight RNAs in the form of branched Y-structures and as a free linear molecule, which rapidly turned over. Furthermore, ionic concentrations that inhibit the synthesis of the SL RNA produced exclusively unspliced alpha-tubulin RNA, thus demonstrating that transcription and trans-splicing can be uncoupled.

The major component of the paraflagellar rod of Trypanosoma brucei is a helical protein that is encoded by two identical, tandemly linked genes

The flagellum of the parasitic hemoflagellate Trypanosoma brucei contains two major structures: (a) the microtubule axoneme, and (b) a highly ordered, filamentous array, the paraflagellar rod (PFR). This is a complex, three-dimensional structure, of yet unknown function, that extends along most of the axoneme and is closely linked to it. Its major structural component is a single protein of 600 amino acids. This PFR protein can assume two different conformations, resulting in two distinct bands of apparent molecular masses of 73 and 69 kD in SDS-gel electrophoresis. Secondary structure predictions indicate a very high helix content. Despite its biochemical similarity to the intermediate filament proteins (solubility properties, amino acid composition, and high degree of helicity), the PFR protein does not belong in this class of cytoskeletal proteins. The PFR protein is coded for by two tandemly linked genes of identical nucleotide sequence. Both genes are transcribed into stable mRNAs of very similar length that carry the mini-exon sequence at their 5' termini.

The tubulin gene cluster of Trypanosoma brucei starts with an intact beta-gene and ends with a truncated beta-gene interrupted by a retrotransposon-like sequence

In Trypanosoma brucei the genes coding for alpha- and beta-tubulin are arranged in a single cluster comprising about ten alternating alpha and beta genes. The cluster starts at its 5' end with an intact beta gene. In this area, a 1.1-kb segment upstream of the mini-exon addition site has been sequenced. Several potential transcription signals were seen. The 3' end of the cluster terminates in the middle of a beta gene, making it an inactive truncated pseudogene. The adjoining 1.7-kb sequence shows a high degree of similarity with a retrotransposon-like (RTnL) element previously characterized elsewhere in the trypanosome genome. This is the first report of an RTnL element interrupting a protein-coding gene of T. brucei.

Polycistronic transcripts in trypanosomes and their accumulation during heat shock: evidence for a precursor role in mRNA synthesis

Maturation of mRNA precursors in trypanosomes involves an apparent trans splicing event in which a 39-nucleotide miniexon sequence, common to all trypanosome mRNAs, is joined to the 5' end of a protein-coding exon. We demonstrate that the processing machinery responsible for the maturation of tubulin mRNA precursors in Trypanosoma brucei can be disrupted by heat shock. This results in an accumulation of polycistronic RNA species and a decrease in the abundance of branched splicing intermediates. At normal temperatures, tubulin polycistronic transcripts were also detected and were shown in pulse-chase experiments to be abundantly synthesized and very rapidly turned over. These results, combined with results of the heat shock experiments, suggest that these polycistronic transcripts are the precursors of the (monocistronic) tubulin mRNAs of trypanosomes.

Polycistronic transcripts in trypanosomes and their accumulation during heat shock: evidence for a precursor role in mRNA synthesis

Maturation of mRNA precursors in trypanosomes involves an apparent trans splicing event in which a 39-nucleotide miniexon sequence, common to all trypanosome mRNAs, is joined to the 5' end of a protein-coding exon. We demonstrate that the processing machinery responsible for the maturation of tubulin mRNA precursors in Trypanosoma brucei can be disrupted by heat shock. This results in an accumulation of polycistronic RNA species and a decrease in the abundance of branched splicing intermediates. At normal temperatures, tubulin polycistronic transcripts were also detected and were shown in pulse-chase experiments to be abundantly synthesized and very rapidly turned over. These results, combined with results of the heat shock experiments, suggest that these polycistronic transcripts are the precursors of the (monocistronic) tubulin mRNAs of trypanosomes.

Large microtubule-associated protein of T. brucei has tandemly repeated, near-identical sequences

The parasitic protozoon Trypanosoma brucei contains a highly organized membrane skeleton, consisting of a dense array of parallel, singlet microtubules that are laterally interconnected and that are also in tight contact with the overlying cell membrane. A high molecular weight, heat-stable protein from this membrane skeleton was isolated that is localized along the microtubules. Protease digestion experiments and sequencing of a cloned gene segment showed that most of the protein is built up by more than 50 nearly identical tandem repeats with a periodicity of 38 amino acids.

Post-transcriptional control of the differential expression of phosphoglycerate kinase genes in Trypanosoma brucei

The genes for the cytosolic and glycosomal phosphoglycerate kinases (PGK) of Trypanosoma brucei are found in a compact tandem array together with a third PGK-related gene, expressed at low level. Expression of the two PGK genes is differentially regulated in the life cycle of T. brucei: the glycosomal PGK and its mRNA are abundant in the mammalian stage of the cycle but not in the insect stage, whereas the reverse is found for the cytosolic PGK and its mRNA. Nevertheless, our experiments indicate that the mRNAs for both isoenzymes are derived from a common precursor. Nuclease protection experiments using fragments cloned into single-stranded DNA vectors show the presence of low abundance RNA species running through one gene into the next. Indeed minor RNA species larger than the mature mRNAs are visible in overexposed RNA blots. Analysis of nascent RNA in a nuclear run-on assay indicates that the entire PGK gene array is transcribed at an equal rate throughout in both life cycle stages. We conclude that the PGK genes are part of one large multicistronic transcription unit, which is processed to yield the individual mRNAs with concomitant addition of the 5' 35-nucleotide mini-exon sequence, characteristic of all trypanosome mRNAs. It follows that the steady-state levels of the PGK mRNAs are controlled post-transcriptionally.

A 60-kDa cytoskeletal protein from Trypanosoma brucei brucei can interact with membranes and with microtubules

The cytoskeleton of eukaryotic cells is a major determinant of cellular architecture and of many cellular functions. In addition to or in place of the transcellular cytoskeleton, many eukaryotic cells also contain membrane-associated cytoskeletal structures (membrane skeletons), which are important for cellular structure and function. The membrane skeleton of the parasitic hemoflagellate Trypanosoma brucei consists of a dense array of singlet microtubules (subpellicular microtubules), which are tightly associated to the overlying cell membrane. This study reports the identification of a microtubule-associated protein from Trypanosoma brucei that constitutes a component of the link between this microtubular array and the cell membrane. The protein can bind in vitro both to microtubules and to membrane vesicles or liposomes. Furthermore, it can crosslink microtubules and membrane vesicles, suggesting that it exerts a similar function in the membrane skeleton.

Subcellular sequestration of an antigenically unique beta-tubulin

Tubulin from Trypanosoma brucei was characterized by Western blotting using well defined monoclonal antibodies reacting with alpha- or beta-tubulin and a new monoclonal antibody, 1B41, raised against a microtubule-enriched fraction of T. brucei, which specifically reacts with the beta-subunit of tubulin from either T. brucei or rat brain. This antibody has been used to examine the subcellular distribution of the corresponding antigen in T. brucei by indirect immunofluorescence. The epitope recognized by 1B41 is restricted to a thin line extending from the basal body region to the anterior end of the cell body. To determine the relationship between the immunoreactive zone and the flagellum, double-label immunofluorescence was performed in both interphase and mitotic cells with 1B41 and a flagellar marker, the monoclonal antibody 5E9, specific for the paraflagellar rod polypeptides of trypanosomes. These experiments revealed that the immunoreactive tubulin was contained in a part of the subpellicular cytoskeleton that remained in a constant spatial correspondence with the flagellum throughout the cell division cycle. The beta-tubulin recognized by 1B41 may be segregated into the microtubular structures associated with a cisterna of the endoplasmic reticulum forming the subflagellar microtubule quartet (SFMQ). These results suggest that the presence of an antigenically unique beta-tubulin defines a subpopulation of microtubules possessing specific dynamic properties that may be involved in the morphogenesis of daughter cells during the division of T. brucei.

Transcription of the intergenic regions of the tubulin gene cluster of Trypanosoma brucei: evidence for a polycistronic transcription unit in a eukaryote

The tubulin genes of T. brucei are clustered in a tightly packed array of alternating alpha- and beta-genes. The steady state mRNA contains one abundant mRNA species each for alpha- and beta-tubulin, both carrying the identical 35 nt mini-exon sequence at their 5'-ends. We have used in vitro run-on transcription assays to investigate the mechanism of tubulin gene transcription in T. brucei. Our results show that the regions between the individual tubulin genes are transcribed at the same rate as are the genes themselves. On the other hand, transcripts containing the intergenic regions could not be detected by Northern analysis or in vivo labelling experiments. We conclude that putative transcripts from the intergenic regions have a half-life of less than one minute. These results suggest that the tubulin gene cluster is transcribed as a single contiguous transcription unit yielding a primary transcript which is rapidly processed into individual mRNAs by the polyadenylation and mini-exon trans splicing machineries.

RNA turnover in Trypanosoma brucei

Regulation of variant surface glycoprotein (VSG) mRNA turnover in Trypanosoma brucei was studied in bloodstream forms, in procyclic cells, and during in vitro transformation of bloodstream forms to procyclic cells by approach-to-equilibrium labeling and pulse-chase experiments. Upon initiation of transformation at 27 degrees C in the presence of citrate-cis-aconitate, the half-life of VSG mRNA was reduced from 4.5 h in bloodstream forms to 1.2 h in transforming cells. Concomitantly, an approximately 25-fold decrease in the rate of transcription was observed, resulting in a 100-fold reduction in the steady-state level of de novo-synthesized VSG mRNA. This low level of expression was maintained for at least 7 h, finally decreasing to an undetectable level after 24 h. Transcription of the VSG gene in established procyclic cells was undetectable. For comparison, the turnover of polyadenylated and nonpolyadenylated RNA, beta-tubulin mRNA, and mini-exon-derived RNA (medRNA) was studied. For medRNA, no significant changes in the rate of transcription or stability were observed during differentiation. In contrast, while the rate of transcription of beta-tubulin mRNA in in vitro-cultured bloodstream forms, transforming cells, and established procyclic cells was similar, the half life was four to five times longer in procyclic cells (t1/2, 7 h) than in cultured bloodstream forms (t1/2, 1.4 h) or transforming cells (t1/2, 1.7 h). Inhibition of protein synthesis in bloodstream forms at 37 degrees Celsius caused a dramatic 20-fold decrease in the rate of VSG mRNA synthesis and a 6-fold decrease in half-life to 45 min, while beta-tubulin mRNA was stabilized 2- to 3-fold and mRNA stability remained unaffected. It is postulated that triggering transformation or inhibiting protein synthesis induces changes in the abundance of the same regulatory molecules which effect the shutoff of VSG gene transcription in addition to shortening the half-life of VSG mRNA.

RNA turnover in Trypanosoma brucei

Regulation of variant surface glycoprotein (VSG) mRNA turnover in Trypanosoma brucei was studied in bloodstream forms, in procyclic cells, and during in vitro transformation of bloodstream forms to procyclic cells by approach-to-equilibrium labeling and pulse-chase experiments. Upon initiation of transformation at 27 degrees C in the presence of citrate-cis-aconitate, the half-life of VSG mRNA was reduced from 4.5 h in bloodstream forms to 1.2 h in transforming cells. Concomitantly, an approximately 25-fold decrease in the rate of transcription was observed, resulting in a 100-fold reduction in the steady-state level of de novo-synthesized VSG mRNA. This low level of expression was maintained for at least 7 h, finally decreasing to an undetectable level after 24 h. Transcription of the VSG gene in established procyclic cells was undetectable. For comparison, the turnover of polyadenylated and nonpolyadenylated RNA, beta-tubulin mRNA, and mini-exon-derived RNA (medRNA) was studied. For medRNA, no significant changes in the rate of transcription or stability were observed during differentiation. In contrast, while the rate of transcription of beta-tubulin mRNA in in vitro-cultured bloodstream forms, transforming cells, and established procyclic cells was similar, the half life was four to five times longer in procyclic cells (t1/2, 7 h) than in cultured bloodstream forms (t1/2, 1.4 h) or transforming cells (t1/2, 1.7 h). Inhibition of protein synthesis in bloodstream forms at 37 degrees Celsius caused a dramatic 20-fold decrease in the rate of VSG mRNA synthesis and a 6-fold decrease in half-life to 45 min, while beta-tubulin mRNA was stabilized 2- to 3-fold and mRNA stability remained unaffected. It is postulated that triggering transformation or inhibiting protein synthesis induces changes in the abundance of the same regulatory molecules which effect the shutoff of VSG gene transcription in addition to shortening the half-life of VSG mRNA.

Subpellicular and flagellar microtubules of Trypanosoma brucei brucei contain the same alpha-tubulin isoforms

The cytoskeleton of the parasitic hemoflagellate Trypanosoma brucei brucei essentially consists of two microtubule-based structures: a subpellicular layer of singlet microtubules, which are in close contact with the cell membrane, and the flagellar axoneme. In addition, the cells contain a small pool of soluble tubulin. Two-dimensional gel electrophoretic analysis of the tubulins present in these subcellular compartments revealed two distinct electrophoretic isoforms of alpha-tubulin, termed alpha 1 and alpha 3. alpha 1-Tubulin most likely represents the primary translation product, while alpha 3-tubulin is a posttranslationally acetylated derivative of alpha 1-tubulin. In the pool of soluble cytoplasmic tubulin, alpha 1 is the predominant species, while the very stable flagellar microtubules contain almost exclusively the alpha 3-tubulin isoform. The subpellicular microtubules contain both isoforms. Neither of the two alpha-tubulin isoforms is organelle specific, but the alpha 3 isoform is predominantly located in stable microtubules.

Expression of the alpha and beta tubulin genes of the African trypanosome in Escherichia coli

The African trypanosome, Trypanosoma brucei, contains multiple genes for both alpha- and beta-tubulins, which code for similar if not identical proteins. Studies of the structure and function of trypanosome microtubules have been limited due to the difficulties in obtaining sufficient amounts of purified tubulin. To produce large amounts of purified tubulin for studies of structure and function and to begin developing a system for producing systematic alterations of tubulin structure we have cloned and expressed the alpha- and beta-tubulin genes of T. brucei in Escherichia coli to produce the unfused proteins. Controlled high-level expression of both alpha- and beta-tubulin was achieved using a plasmid vector, pOTS, in which expression is controlled by phage lambda promoter/operator and a temperature-sensitive lambda repressor. The tubulins produced are insoluble, as has been found for many other proteins expressed to high levels in E. coli; they are readily purified to near homogeneity by chromatography on DEAE-cellulose in 7 M urea. N-terminal analysis of the purified proteins indicates that they are initiated correctly and that the N-formyl group is removed from the initiating methionine. This factor will probably prove important in the reconstitution of biologically active tubulin.

The common 5' terminal sequence on trypanosome mRNAs: a target for anti-messenger oligodeoxynucleotides

Several mature mRNAs of Trypanosoma brucei were previously shown to have a common 5' terminal sequence of 35 nucleotides (nt) encoded by a separate mini-exon. To verify whether all trypanosome mRNAs contain this mini-exon sequence at their 5' end, we have tested oligodeoxynucleotides complementary to different parts of the 35 nt leader sequence for their ability to inhibit translation of total trypanosome mRNA. All oligomers tested inhibited translation of trypanosome mRNAs in a wheat germ extract. They had no effect on translation of Brome mosaic virus mRNA and of a trypanosome mRNA for phosphoglycerate kinase modified to remove the mini-exon sequence. Three different 12mers inhibited translation 35-60%; both the 22- and 34mer inhibited translation 95-100%. Incorporation of amino acids decreased proportionally in all protein bands detected in high resolution polyacrylamide gels. Our results show that all trypanosome mRNAs that yield a product detectable in gel contain a mini-exon sequence. We infer that most, if not all, trypanosome mRNAs contain a 5' terminal mini-exon sequence acquired by discontinuous synthesis.