Purification and characterization of an alpha-galactosyltransferase from Trypanosoma brucei.
(1/2346)
A membrane-associated galactosyltransferase from Trypanosoma brucei was purified 34000-fold by affinity chromatography on UDP-hexanolamine-Sepharosetrade mark. Using SDS/PAGE under reducing conditions, the isolated enzyme ran as a relatively broad band with apparent molecular masses of 53 kDa and 52 kDa, indicative of glycosylation and the existence of two isoforms. N-Glycosylation of the enzyme was subsequently confirmed using Western blotting and either specific binding of concanavalin A or peptide-N4-(N-acetylglucosaminyl)asparagine amidase digestion. The de-N-glycosylated enzyme ran with apparent molecular masses of 51 kDa and 50 kDa, indicative of a single N-glycosylation site. The galactosyltransferase exhibited a pH optimum at 7.2 and had a pronounced requirement for Mn2+ ions (KM=2.5 mM) for its action. The transferase activity was independent of the concentration of Triton X-100. The enzyme was capable of transferring galactose from UDP-galactose to a variety of galactose-based acceptors in alpha-glycosidic linkages. The apparent KM values for UDP-galactose and for the preferred acceptor substrate N-acetyl-lactosamine are 46 microM and 4.5 mM respectively. From these results we would like to suggest that the galactosyltransferase functions in the processing of terminal N-acetyl-lactosamine structures of trypanosomal glycoproteins. (+info)
S-myristoylation of a glycosylphosphatidylinositol-specific phospholipase C in Trypanosoma brucei.
(2/2346)
Covalent modification with lipid can target cytosolic proteins to biological membranes. With intrinsic membrane proteins, the role of acylation can be elusive. Herein, we describe covalent lipid modification of an integral membrane glycosylphosphatidylinositol-specific phospholipase C (GPI-PLC) from the kinetoplastid Trypanosoma brucei. Myristic acid was detected on cysteine residue(s) (i.e. thiomyristoylation). Thiomyristoylation occurred both co- and post-translationally. Acylated GPI-PLC was active against variant surface glycoprotein (VSG). The half-life of fatty acid on GPI-PLC was 45 min, signifying the dynamic nature of the modification. Deacylation in vitro decreased activity of GPI-PLC 18-30-fold. Thioacylation, from kinetic analysis, activated GPI-PLC by accelerating the conversion of a GPI-PLC.VSG complex to product. Reversible thioacylation is a novel mechanism for regulating the activity of a phospholipase C. (+info)
The involvement of gRNA-binding protein gBP21 in RNA editing-an in vitro and in vivo analysis.
(3/2346)
RNA editing in the parasitic organism Trypanosoma brucei is characterised by the insertion and deletion of uridylate residues into otherwise incomplete primary transcripts. The processing reaction is a required pathway for the expression of most mitochondrial genes and proceeds by a cascade of enzyme-catalysed steps. RNA editing involves one or more macromolecular ribonucleoprotein complexes which are likely to interact with additional components as the reaction proceeds. Here we examined the involvement of the gRNA-binding polypeptide gBP21, a protein which has been demonstrated to be associated with active RNA editing complexes. We show that in vitro RNA editing can be suppressed by the addition of a gBP21-specific antibody or by immunodepletion of the protein. By creating a gBP21 knockout mutant we analysed the requirement for the protein in vivo. gBP21(-) trypanosomes are viable as bloodstream stage cells and contain edited mRNAs. However, the knockout mutant is not capable of differentiating from the bloodstream to the insect life cycle stage in vitro. Moreover, mutant cells are characterised by a low mitochondrial transcript abundance. Together, these data establish that gBP21 contributes a non-essential function to the RNA editing reaction and further suggest that the protein is involved in additional mitochondrial processes which impact a larger pool of mitochondrial transcripts. (+info)
Characterization of a novel trypanosome lytic factor from human serum.
(4/2346)
Natural resistance of humans to the cattle pathogen Trypanosoma brucei brucei has been attributed to the presence in human serum of nonimmune factors that lyse the parasite. Normal human serum contains two trypanosome lytic factors (TLFs). TLF1 is a 500-kDa lipoprotein, which is reported to contain apolipoprotein A-I (apoA-I), haptoglobin-related protein (Hpr), hemoglobin, paraoxonase, and apoA-II, whereas TLF2 is a larger, poorly characterized particle. We report here a new immunoaffinity-based purification procedure for TLF2 and TLF1, as well as further characterization of the components of each purified TLF. Immunoaffinity-purified TLF1 has a specific activity 10-fold higher than that of TLF1 purified by previously described methods. Moreover, we find that TLF1 is a lipoprotein particle that contains mainly apoA-I and Hpr, trace amounts of paraoxonase, apoA-II, and haptoglobin, but no detectable hemoglobin. Characterization of TLF2 reveals that it is a 1,000-kDa protein complex containing mainly immunoglobulin M, apoA-I, and Hpr but less than 1% detectable lipid. (+info)
Differences between the trypanosomal and human GlcNAc-PI de-N-acetylases of glycosylphosphatidylinositol membrane anchor biosynthesis.
(5/2346)
De-N-acetylation of N-acetylglucosaminyl-phosphatidylino-sitol (GlcNAc-PI) is the second step of glycosylphosphatidylino-sitol (GPI) membrane anchor biosynthesis in eukaryotes. This step is a prerequisite for the subsequent processing of glucosaminyl-phosphatidylinositol (GlcN-PI) that leads to mature GPI membrane anchor precursors, which are transferred to certain proteins in the endoplasmic reticulum. In this article, we used a direct de-N-acetylase assay, based on the release of [14C]acetate from synthetic GlcN[14C]Ac-PI and analogues thereof, and an indirect assay, based on the mannosylation of GlcNAc-PI analogues, to study the substrate specificities of the GlcNAc-PI de-N-acetylase activities of African trypanosomes and human (HeLa) cells. The HeLa enzyme was found to be more fastidious than the trypanosomal enzyme such that, unlike the trypanosomal enzyme, it was unable to act on a GlcNAc-PI analogue containing 2-O-octyl-d- myo -inositol or on the GlcNAc-PI diastereoisomer containing l- myo -inositol (GlcNAc-P(l)I). These results suggest thatselective inhibition of the trypanosomal de-N-acetylase may be possible and that this enzyme should be considered as a possible therapeutic target. The lack of strict stereospecificity of the trypanosomal de-N-acetylase for the d- myo -inositol component was also seen for the trypanosomal GPI alpha-manno-syltransferases when GlcNAc-P(l)I was added to the trypanosome cell-free system, but not when GlcN-P(l)I was used. In an attempt to rationalize these data, we modeled the structure and dynamics of d-GlcNAcalpha1-6d- myo -inositol-1-HPO4-( sn )-3-glycerol and its diastereoisomer d-GlcNAcalpha1-6l- myo -inositol-1-HPO4-( sn )-3-glycerol. These studies indicate that the latter compound visits two energy minima, one of which resembles the low-energy conformer of former compound. Thus, it is conceivable that the trypanosomal de-N-acetylase acts on GlcNAc-P(l)I when it occupies a GlcNAc-PI-likeconformation and that GlcN-P(l)I emerging from the de-N-acetylase may be channeled to the alpha-mannosyltransferases in this conformation. (+info)
Telomerase in kinetoplastid parasitic protozoa.
(6/2346)
We have identified telomerase activity in extracts of three evolutionarily diverse kinetoplastid species: Trypanosoma brucei, Leishmania major, and Leishmania tarentolae. Telomerase activity was initially detected in extracts from insect form cells of all three kinetoplastid species by using a modification of the one-tube telomere repeat amplification protocol [Kim, N., et al. (1994) Science 266, 2011-2015], although better results were subsequently achieved with the two-tube telomere repeat amplification protocol [Autexier, C., Pruzan, R., Funk, W. & Greider, C. (1996) EMBO J. 15, 5928-5935]. The activity in T. brucei extracts was sufficiently robust to enable its detection in a direct assay of telomerase; enzyme processivity was found to be relatively low. The in vitro properties of telomerase suggest a possible templating domain sequence for the telomerase RNA of T. brucei. Telomerase activity is likely to contribute to telomere maintenance in these parasitic organisms and provides a new target for chemotherapeutic intervention. (+info)
The anti-influenza virus drug rimantadine has trypanocidal activity.
(7/2346)
We report here that bloodstream forms of the African trypanosome, Trypanosoma brucei, are sensitive to the anti-influenza virus drug rimantadine (50% inhibitory concentration of 1.26 micrograms ml-1 at pH 7.4). The activity is pH dependent and is consistent with a mechanism involving inhibition of the ability to regulate internal pH. Rimantadine is also toxic to the trypanosomatid parasites Trypanosoma cruzi and Leishmania major. (+info)
Combinatorial selection of high affinity RNA ligands to live African trypanosomes.
(8/2346)
African trypanosomiasis is a parasitic disease caused by a specific class of protozoan organisms. The best-studied representative of that group is Trypanosoma brucei which is transmitted by tsetse flies and multiplies in the blood of many mammals. Trypanosomes evade the immune system by altering their surface structure which is dominated by a layer of a variant surface glycoprotein (VSG). Although invariant surface proteins exist, they are inaccessible to the humoral immune response. Using a combinatorial selection method in conjunction with live trypanosomes as the binding target, we show that short RNA ligands (aptamers) for constant surface components can be isolated. We describe the selection of three classes of RNA aptamers that crosslink to a single 42 kDa protein located within the flagellar pocket of the parasite. The RNAs associate rapidly and with high affinity. They do not discriminate between two different trypanosome VSG variant strains and, furthermore, are able to bind to other trypanosome strains not used in the selection protocol. Thus, the aptamers have the potential to function as markers on the surface of the extracellular parasite and as such they might be modified to function as novel drugs against African trypanosomiasis. (+info)