A Glycosomal Protein (p60) Which Is Predominantly Expressed in Procyclic Trypanosoma brucei

Large-scale identification of tubulin-binding proteins provides insight on subcellular trafficking, metabolic channeling, and signaling in plant cells

Microtubules play an essential role in the growth and development of plants and are known to be involved in regulating many cellular processes ranging from translation to signaling. In this article, we describe the proteomic characterization of Arabidopsis tubulin-binding proteins that were purified using tubulin affinity chromatography. Microtubule co-sedimentation assays indicated that most, if not all, of the proteins in the tubulin-binding protein fraction possessed microtubule-binding activity. Two-dimensional gel electrophoresis of the tubulin-binding protein fraction was performed, and 86 protein spots were excised and analyzed for protein identification. A total of 122 proteins were identified with high confidence using LC-MS/MS. These proteins were grouped into six categories based on their predicted functions: microtubule-associated proteins, translation factors, RNA-binding proteins, signaling proteins, metabolic enzymes, and proteins with other functions. Almost one-half of the proteins identified in this fraction were related to proteins that have previously been reported to interact with microtubules. This study represents the first large-scale proteomic identification of eukaryotic cytoskeleton-binding proteins, and provides insight on subcellular trafficking, metabolic channeling, and signaling in plant cells.

Identification of a rice RNA- and microtubule-binding protein as the multifunctional protein, a peroxisomal enzyme involved in the beta -oxidation of fatty acids

The control of subcellular mRNA localization and translation is often mediated by protein factors that are directly or indirectly associated with the cytoskeleton. We report the identification and characterization of a rice seed protein that possesses both RNA and microtubule binding activities. In vitro UV cross-linking assays indicated that this protein binds to all mRNA sequences tested, although there was evidence for preferential binding to RNAs that contained A-C nucleotide sequence motifs. The protein was purified to homogeneity using a two-step procedure, and amino acid sequencing identified it as the multifunctional protein (MFP), a peroxisomal enzyme known to possess a number of activities involved in the beta-oxidation of fatty acids. The recombinant version of this rice MFP binds to RNA in UV cross-linking and gel mobility shift experiments, co-sediments specifically with microtubules, and possesses at least two enzymatic activities involved in peroxisomal fatty acid beta-oxidation. Taken together these data suggest that MFP has an important role in mRNA physiology in the cytoplasm, perhaps in regulating the localization or translation of mRNAs through an interaction with microtubules, in addition to its peroxisomal function.

Expression of the Trypanosoma brucei phosphoenolpyruvate carboxykinase gene in Saccharomyces cerevisiae

Plasmid pTbp60B (Kueng et al., J. Biol. Chem. 264 (1989) 5203-5209) was employed to obtain, through the polymerase chain reaction, the Trypanosoma brucei gene coding for phosphoenolpyruvate (PEP) carboxykinase, and then cloned into the yeast expression plasmid pYES2. The cloned gene was completely sequenced and the expression plasmid transformed into Saccharomyces cerevisiae PUK-3B (MATalpha pck1 ura3 ade1) competent cells. Gene expression took place upon induction with 2% galactose, and the recombinant T. brucei PEP carboxykinase was purified to near homogeneity. The basic molecular and catalytic characteristics of the recombinant enzyme were determined, and they showed to be essentially similar to those reported for wild type T. brucei PEP carboxykinase (Hunt and Köhler, Biochim. Biophys. Acta 1249 (1995) 15-22). The expression system here described is a reliable non-pathogenic source of T. brucei PEP carboxykinase.

The role of proteolysis during differentiation of Trypanosoma brucei from the bloodstream to the procyclic form

The in vitro differentiation of Trypanosoma brucei from bloodstream to procyclic (insect) forms is accompanied by diminishing variant surface glycoprotein (VSG) and increasing levels of procyclin and phosphoenolpyruvate carboxykinase (PEPCK). In this study, we examined the fate of several glycolytic enzymes of T. brucei during differentiation. We observed a down-regulation of glycosomal phosphoglycerate kinase (gPGK) during differentiation. In contrast, intracellular levels of glycosomal glyceraldehyde-3-phosphate dehydrogenase (gGAPDH), aldolase (ALD), and phosphoglucoisomerase (PGI) remained unchanged during differentiation and apparently continued to be synthesized in the procyclic form. To determine the potential role of proteasomes and other proteases during the differentiation process, we tested the effect of lactacystin, a specific inhibitor of proteasome activity, and morpholinourea-Phe-homoPhe-benz-alpha-pyrone (P27), a selective inhibitor of cysteine proteases, on the in vitro differentiation of T. brucei. Cells differentiated normally in the presence of 1 microM lactacystin, which confirmed our previous observation that this differentiation does not require crossing any phase boundaries in the cell cycle (Mutomba and Wang, Mol Biochem Parasitol 1996;80:89-102). But the cells thus differentiated did not increase in number and retained gPGK. Cells differentiated under 2 microM P27 also proceeded at a normal rate but failed to multiply and retained gPGK. However, most of the differentiated cells under 2 microM P27 also retained VSG on the cell membrane surface and expressed higher levels of procyclin suggesting that a cysteine protease(s) may be involved in releasing VSG and partially reducing procyclin during differentiation. This cysteine protease(s) has been tentatively identified in the procyclic cells as a 48 kDa protein through labeling of cysteine protease(s) with a biotinylated P27 homolog K02 (morpholinourea-Phe-homoPhe-vinylsulfone).

Effects of aphidicolin and hydroxyurea on the cell cycle and differentiation of Trypanosoma brucei bloodstream forms

The effects of aphidicolin (APH) and hydroxyurea (HU) on the cell cycle and differentiation of Trypanosoma brucei bloodstream forms were studied. APH (0.1 microgram ml-1) inhibited cell division, but did not inhibit DNA synthesis. Most of the cells were arrested in the G2 phase of the cell cycle, with each cell containing two kinetoplasts, but only one nucleus. Recovery of the arrested cells showed a 24-h lag period compared to controls. Higher concentrations of APH (1 and 10 micrograms ml-1) were required to inhibit DNA synthesis, but the cells failed to resume growth after removal of the drug. Incubation of cells with HU (7.5 micrograms ml-1) did not inhibit DNA synthesis, but arrested cells after duplicating both the kinetoplast and the nucleus. Recovery from drug arrest also showed a 24-h lag period. We therefore conclude that neither APH nor HU arrests T. brucei at the G1/S phase boundary as anticipated. The mechanisms of cell cycle arrest by APH and HU are not through inhibition of DNA synthesis, but rather through unidentified pathways, leading to growth arrest prior to nuclear division and cytokinesis respectively. Since the arrested cells do not resume normal development immediately following drug removal, APH and HU should be regarded as unsuitable agents for synchronizing T. brucei bloodstream forms. T. brucei bloodstream forms arrested with either APH or HU differentiated normally into procyclic forms in vitro, indicating that a cycle of cell division is not required for initiation of differentiation, and that the process can be initiated and completed when cells are arrested at the G2/M and M/G1 phase boundaries.

Phosphoenolpyruvate carboxykinase from Trypanosoma cruzi. Purification and physicochemical and kinetic properties

Phospho enolpyruvate carboxykinase (PEPCK) has been purified to homogeneity from epimastigotes of the Tul 0 strain of Trypanosoma cruzi. The physicochemical parameters determined allowed the calculation of an average molecular mass of 120 kDa; the subunit molecular mass, about 61 kDa, is in good agreement with the value of 58.6 kDa recently determined from the sequence by Sommer et al. (FEBS Lett. 359 (1994) 125-129). The PEPCK from T. cruzi presented, in addition to its molecular mass, typical properties of other ATP-linked PEPCKs, namely strict specificity for ADP in the carboxylation reaction and lower specificity in the decarboxylation and exchange reactions, and synergistic activation by CdCl2 or MgCl2 when added in addition to MnCl2. The enzyme presented hysteretic behaviour, shown by a lag period in the carboxylation reaction, which was affected by dilution and preincubation. The decarboxylation reaction catalyzed by the T. cruzi PEPCK was not inhibited by excess of ATP-Mn. The apparent Km values for the carboxylation reaction, including the low value for PEP (0.035 mM) are compatible with an important role of PEPCK, as suggested by previous NMR experiments, on the CO2 fixation in vivo which leads to succinate excretion during aerobic fermentation of glucose.

Differentiation of a culture-adapted mutant bloodstream form of Trypanosoma brucei into the procyclic form results in growth arrest of the cells

The bloodstream forms of Trypanosoma brucei monomorphic strain 427 serially passaged in rats can differentiate in vitro equally well in HMI-9, HMI-10, SDM-79 or Cunningham's medium into the insect (procyclic) forms by a simple temperature shift from 37 to 26 degrees C in the presence of citrate and cis-aconitate. The procyclic forms thus generated can also continue to multiply at 26 degrees C without replacing the culture medium. The same strain of T. brucei pre-adapted to grow as bloodstream forms in HMI-10 medium at 37 degrees C is also capable of differentiating showing a similar rate of variant surface glycoprotein (VSG) disappearance and appearance of phosphoenolpyruvate carboxykinase (PEPCK) under the same experimental conditions. However, appearance of both procyclin mRNA and procyclin protein is much delayed and the resulting procyclic forms cannot multiply. The culture-adapted bloodstream forms are capable of infecting rats, and the cells thus harvested from the rats can differentiate but cannot multiply in the same manner as the original culture-adapted bloodstream forms. Apparently, a certain variant has been selected during the adaptation of T. brucei bloodstream forms from rat blood to the culture medium. This variant could be a useful tool for identifying the genes involved in differentiation of T. brucei and multiplication of the procyclic forms. Comparison of the protein profiles between the wild-type and the variant during differentiation showed that a major protein band of about 70 kDa remained in the non-dividing variant procyclic forms but vanished in the rapidly dividing wild type procyclic forms. N-terminal determinations indicated that the 70-kDa protein band consists of bovine serum albumin and fetuin. Presumably these two serum proteins are actively taken up by the bloodstream forms via endocytosis. Since the procyclic forms are incapable of endocytosis, the serum proteins may be rapidly diluted in the growing wild type procyclic cells but remain unchanged in the non-dividing procyclic cells of the variant. Further studies are underway in trying to identify the key distinctions between these two lines of cells at the molecular level.

Purification and characterization of phospho enol pyruvate carboxykinase from Trypanosoma brucei

ATP-dependent phospho enol pyruvate carboxykinase (EC 4.1.1.49; PEPCK, ATP) was purified from glycosomes of cultured procyclic Trypanosoma brucei to electrophoretic homogeneity. The purified enzyme exhibited a mean specific activity of 83 units mg-1, as measured in the carboxylation direction at 30 degrees C. A similar activity was obtained for the decarboxylation reaction. The enzyme was shown to be a homodimer in solution with a subunit molecular mass of 59 kDa. Amino acid sequence analysis suggested that the PEPCK (ATP) is identical to the trypanosomal protein p60, the sequence of which was previously predicted from the corresponding nucleotide sequence by other investigators. The basic nature of the enzyme was indicated by a high isoelectric point (pH 8.9). The enzyme was found to be strictly dependent on adenosine nucleotides for activity, as well as on the presence of Mn2+. Mg2+ was found to be ineffective as activator of the trypanosomal enzyme, but a combination of subsaturating (< or = 300 microM) concentrations of Mn2+ and high concentrations of Mg2+ caused a synergistic effect on the carboxylation activity, indicating a dual cation requirement. Mn2+ is necessary to activate the enzyme and Mn2+ or Mg2+ most likely forms the cation-nucleotide complex as the active form of the substrate. Relatively high (5 mM) levels of ATP were required to produce a significant inhibition of the carboxylation reaction. Quinolinic acid, a structural analogue of oxaloacetate, completely inhibited the decarboxylation reaction at a 1 mM concentration. The apparent Michaelis constants of the enzyme were 490 microM for PEP, 37 microM for oxaloacetate, 40 microM for ADP, 10.3 microM for ATP, 970 microM for Mn2+ and 26 mM for HCO3-. Endogenous substrate concentrations were found to be 327 nmol PEP, 1486 nmol ADP, 4200 nmol ATP and 11.5 nmol Mn2+ (ml cell volume)-1. Our kinetic data suggest that under physiological conditions PEPCK (ATP) in T. brucei is bidirectional and that its activity is regulated primarily by mass action. The physiological relevance of the enzyme in procyclic T. brucei is discussed.

Phosphoenolpyruvate carboxykinase of Trypanosoma brucei is targeted to the glycosomes by a C-terminal sequence

Import of proteins into the glycosomes of T. brucei resembles the peroxisomal protein import in that C-terminal SKL-like tripeptide sequences can function as targeting signals. Many of the glycosomal proteins do not, however, possess such C-terminal tripeptide signals. Among these, phosphoenolpyruvate carboxykinase (PEPCK (ATP)) was thought to be targeted to the glycosomes by an N-terminal or an internal targeting signal. A limited similarity to the N-terminal targeting signal of rat peroxisomal thiolase exists at the N-terminus of T. brucei PEPCK. However, we found that this peroxisomal targeting signal does not function for glycosomal protein import in T. brucei. Further studies of the PEPCK gene revealed that the C-terminus of the predicted protein does not correspond to the previously deduced protein sequence of 472 amino acids due to a -1 frame shift error in the original DNA sequence. Readjusting the reading frame of the sequence results in a predicted protein of 525 amino acids in length ending in a tripeptide serine-arginine-leucine (SRL), which is a potential targeting signal for import into the glycosomes. A fusion protein of firefly luciferase, without its own C-terminal SKL targeting signal, and T. brucei PEPCK is efficiently imported into the glycosomes when expressed in procyclic trypanosomes. Deletion of the C-terminal SRL tripeptide or the last 29 amino acids of PEPCK reduced the import only by about 50%, while a deletion of the last 47 amino acids completely abolished the import. These results suggest that T. brucei PEPCK may contain a second, internal glycosomal targeting signal upstream of the C-terminal SRL sequence.

TUBIS, a fossilized retroposon in the tubulin gene cluster of Trypanosoma brucei

The genome of Trypanosoma brucei contains many copies of TRS/ingi, a retroposon like element of about 5 kb length. One vestigial member of this family, TUBIS, is found in the tubulin gene cluster. The element has sequence homologies to reverse transcriptase. The similarity to TRS/ingi is lost after 4 kb, resulting in a 3' truncated element. The sequence following downstream shows similarities to 5' flanking regions of VSG genes.

Cloning and characterization of the gene encoding ATP-dependent phospho-enol-pyruvate carboxykinase in Trypanosoma cruzi: comparison of primary and predicted secondary structure with host GTP-dependent enzyme

The complete nucleotide (nt) sequence of the PEPCK gene encoding Trypanosoma cruzi phospho-enol-pyruvate carboxykinase (PEPCK; ATP dependent, EC 4.1.1.49) has been determined. The predicted primary sequence has 473 amino acids (aa) with a calculated molecular mass of 52.5 kDa. The ubiquitous spliced leader is present at nt position -60 from the AUG start codon in PEPCK mRNA; the coding region is followed by a long 3'-non-coding region of 777 nt. Northern and Southern blot analysis showed that the PEPCK mRNA is 2.7 kb long and that the PEPCK gene is polymorphic in T. cruzi, with more than one copy in the genome of the epimastigote form. Comparison of the available aa sequences of ATP(protozoa, yeast and bacteria)- and GTP(vertebrates, insects, helminths and fungi)-dependent PEPCKs showed that the former lack two characteristic, highly conserved regions present in the GTP-dependent enzymes: one is associated with the binding of PEP while the second is frequently labeled as 'catalytic' and contains a conserved Cys residue of unusual reactivity. On the other hand, two consensus sequences with conserved predicted secondary structure were identified in all PEPCKs, independent of their nt specificity; one of them is a divalent metal-binding site previously identified in pyruvate kinase by X-ray crystallographic studies.

Transient inhibition of protein synthesis accompanies differentiation of Trypanosoma brucei from bloodstream to procyclic forms

It has been widely believed that bloodstream forms of Trypanosoma brucei must be first transformed into intermediary and/or short-stumpy forms in the bloodstream of the mammalian host before differentiation to the procyclic culture form can occur. In our recent studies, the pleomorphic T. brucei strain TREU667 was found to differentiate directly from the long-slender bloodstream form to the procyclic form in Cunningham's medium at 26 degrees C [7]. In the present investigation, the same was found true for another pleomorphic strain of T. brucei, STIB366D. Four independent monomorphic strains of T. brucei were tested; two, #427 and EATRO164, were found capable of differentiating in vitro directly into procyclic forms, whereas the other two, TREU667/RP-56 and EATRO110, could not. There is thus no correlation between the capability of differentiating in vitro and the ability of being converted from long-slender to intermediary and short-stumpy bloodstream forms. Two additional markers for following differentiation, other than observing morphological changes, were tested. Assays for the emerging phosphoenolpyruvate carboxykinase (PEPCK) by immunoblottings worked well, with results agreeing closely with the morphological change. But immunoblottings of glycosomal phosphoglycerate kinase (gPGK) failed to demonstrate a significant decrease in the protein level upon completion of differentiation. Apparently, gPGK has a rather long half-life and is unsuitable as a marker of differentiation. When temperature was dropped from 37 degrees C to 26 degrees C at the starting point of in vitro differentiation, protein synthetic activity in the pleomorphic T. brucei TREU667 bloodstream form was decreased by 4-fold. When the activity was gradually brought back to and beyond the original level after a day's incubation, the profile of newly synthesized proteins was that of the procyclic form. A monomorphic variant of TREU667, RP-56, which is incapable of differentiating in vitro, has a much higher protein synthetic activity than its pleomorphic parent in the bloodstream form. This high activity and the bloodstream profile of proteins thus synthesized were unaffected by the decreased temperature in Cunningham's medium until cell death. We thus conclude that a general inhibition of protein synthesis in bloodstream forms caused by temperature drop may be among the early events triggering differentiation into the procyclic form.

The cytosolic and glycosomal isoenzymes of glyceraldehyde-3-phosphate dehydrogenase in Trypanosoma brucei have a distant evolutionary relationship

Trypanosoma brucei contains two isoenzymes for glyceraldehyde-3-phosphate dehydrogenase: one enzyme resides in a microbody-like organelle, the glycosome; the other is found in the cytosol. Previously we have reported the characterization of the gene for the glycosomal enzyme [Michels, P. A. M., Poliszczak, A., Osinga, K. A., Misset, O., Van Beeumen, J., Wierenga, R. K., Borst, P. & Opperdoes, F. R. (1986) EMBO J. 5, 1049-1056]. Here we describe the cloning and analysis of the gene that codes for the cytosolic isoenzyme. The gene encodes a polypeptide of 330 amino acids, with a calculated molecular mass of 35440 Da. The two isoenzymes are only 55% identical. The cytosolic glyceraldehyde-3-phosphate dehydrogenase differs from the glycosomal enzyme in the following respects: (a) its subunit molecular mass is 3.4 kDa smaller due to the absence of insertions and a small C-terminal extension which are unique to the glycosomal protein; (b) the cytosolic enzyme has a lower pI (7.9, as compared to 9.3 for the glycosomal isoenzyme), which is due to a reduction in the excess of positively charged amino acids (the calculated net charges of the polypeptides are +2 and +11, respectively). We have compared the amino acid sequences of the two T. brucei glyceraldehyde-3-phosphate dehydrogenases, with 24 available sequences of the corresponding enzyme of other organisms from various phylogenetic groups. On the basis of this comparison an evolutionary tree was constructed. This analysis strongly supports the theory that T. brucei diverged early in evolution from the main eukaryotic branch of the phylogenetic tree. Further, two separate branches for the lineages leading to Trypanosoma are inferred from the amino acid sequences, suggesting that the genes for the two glyceraldehyde-3-phosphate dehydrogenases of the trypanosome are distantly related and must have been acquired independently by the trypanosomal ancestor. The branching determined with the glycosomal enzyme precedes that found with the cytosolic enzyme. The available data do not allow us to decide which of the two genes originally belonged to the trypanosome lineage and which entered the cell later by horizontal gene transfer.

The association of glycosomal enzymes and microtubules: a physiological phenomenon or an experimental artifact?

Subpellicular microtubules isolated from Trypanosoma brucei parasites were fractionated on a phosphocellulose column, and the trypanosomal p52 microtubule-associated protein was eluted along with two other proteins of 41 and 36 kDa. These proteins were found to be the glycosomal enzymes aldolase (41 kDa) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH, 36 kDa) by enzyme activity, antibody cross-reaction, and N-terminal sequencing. These enzymes were coprecipitated with tubulin in the presence of taxol, and aldolase had the capacity to polymerize tubulin and crosslink microtubules. Immunolocalization of anti-aldolase and anti-GAPDH antibodies did not show an interaction between these enzymes and the subpellicular microtubules. The question whether the copurification of aldolase and the subpellicular microtubules could reflect a physiological phenomenon or may be an experimental artifact is discussed.

Trypanosoma brucei: two-dimensional gel analysis of the major glycosomal proteins during the life cycle

Kinetoplastid organisms possess a unique organelle, the glycosome, which compartmentalizes the Embden-Meyerhof segment of glycolysis and several other metabolic pathways. In Trypanosoma brucei many of the enzyme activities localized to the glycosome are stage regulated. Two-dimensional gel analysis was used to examine the characteristics, expression, and biosynthesis of the major glycosomal proteins. Two-dimensional gel maps of glycosomes from slender bloodforms and late intermediate-stumpy bloodforms (the precursors of procyclic forms) were indistinguishable, while those of procyclic form glycosomes showed extensive differences. Glycosomal phosphoenolpyruvate carboxykinase and malate dehydrogenase were identified to have subunit molecular weights of 60 and 34 kDa, respectively. We detected two hitherto undescribed glycosomal proteins, one of which is found only in bloodforms. All of the major proteins, except glucose phosphate isomerase, were highly basic. Stage regulation of glycosomal enzyme activities correlated with stage regulation of specific protein biosynthesis.