Characterization of an in vitro assay for import of 3-phosphoglycerate kinase into the glycosomes of Trypanosoma brucei

Purine metabolite and energy charge analysis of Trypanosoma brucei cells in different growth phases using an optimized ion-pair RP-HPLC/UV for the quantification of adenine and guanine pools

Human African Trypanosomiasis (HAT) is caused by the protozoan parasite Trypanosoma brucei. Although trypanosomes are well-studied model organisms, only little is known about their adenine and guanine nucleotide pools. Besides being building blocks of RNA and DNA, these nucleotides are also important modulators of diverse biochemical cellular processes. Adenine nucleotides also play an important role in the regulation of metabolic energy. The energetic state of cells is evaluated by the energy charge which gives information about how much energy is available in form of high energy phosphate bonds of adenine nucleotides. A sensitive and reproducible ion-pair RP-HPLC/UV method was developed and optimized, allowing the quantification of guanine and adenine nucleosides/nucleotides in T. brucei. With this method, the purine levels and their respective ratios were investigated in trypanosomes during logarithmic, stationary and senescent growth phases. Results of this study showed that all adenine and guanine purines under investigation were in the low mM range. The energy charge was found to decrease from logarithmic to static and to senescent phase whereas AMP/ATP, ADP/ATP and GDP/GTP ratios increased in the same order. In addition, the AMP/ATP ratio varied as the square of the ADP/ATP ratio, indicating AMP to be the key energy sensor molecule in trypanosomes.

Characterization of glycosomal RING finger proteins of trypanosomatids

The glycosomes of trypanosomatids are essential organelles that are evolutionarily related to peroxisomes of other eukaryotes. The peroxisomal RING proteins-PEX2, PEX10 and PEX12-comprise a network of integral membrane proteins that function in the matrix protein import cycle. Here, we describe PEX10 and PEX12 in Trypanosoma brucei, Leishmania major, and Trypanosoma cruzi. We expressed GFP fusions of each T. brucei coding region in procyclic form T. brucei, where they localized to glycosomes and behaved as integral membrane proteins. Despite the weak transmembrane predictions for TbPEX12, protease protection assays demonstrated that both the N and C termini are cytosolic, similar to mammalian PEX12. GFP fusions of T. cruzi PEX10 and L. major PEX12 also localized to glycosomes in T. brucei indicating that glycosomal membrane protein targeting is conserved across trypanosomatids.

Characterisation of phosphoglycerate kinase genes in Leishmania major and evidence for the absence of a third closely related gene or isoenzyme

We have characterised the phosphoglycerate kinases (PGKs) in L. major and studied their mRNA and protein expression. Interestingly we have found evidence for only two tandemly linked PGK genes which correspond to the PGK gene B and C homologue in Trypanosoma and Crithidia. The primary structure of the leishmanial PGK genes B and C are virtually identical and differed only by the presence of a 62 amino acid extension at the carboxyl terminal of the PGK gene C homologue which is therefore likely to contain the translocation signal for glycosomal topogenesis. Indeed, the PGK gene C protein was found to be glycosomal (gPGK) while the PGK gene B protein was found to be cytosolic (cPGK). Both PGK genes are expressed in L. major promastigotes with the cPGK transcript expressed at a much higher level (4-5-fold) than the gPGK transcript. Similarly the relative cPGK isoenzyme activity was found to be approximately 4-fold higher than that of the gPGK isoenzyme. Surprisingly in L. major we have found no evidence for the PGK gene A present in all other trypanosomatids studied to date (Trypanosoma brucei, Trypanosoma congolense and Crithidia fasciculata). We therefore consider the possible evolutionary and functional significance of a trypanosomatid with only two PGK isoenzymes.

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.

A phosphoglycerate kinase-like molecule localized to glycosomal microbodies: evidence that the topogenic signal is not at the C-terminus

The phosphoglycerate kinase (PGK) gene complex of Trypanosoma brucei contains three tandemly linked related genes. One gene encodes a cytoplasmic PGK, while another encodes a PGK isozyme localized to glycosomal microbodies. In this communication, we report that the third gene in this complex encodes a 56-kDa molecule which is also localized to the glycosomal core. DNA sequence analysis indicates that this gene contains multiple substitutions and a large insertion in the amino domain, but that it is very similar to the other PGK isozymes in the carboxy domain. The C-terminal tripeptide is identical to that of the cytoplasmic isozyme, and only one conservative change occurs in the last 25 amino acids. The encoded protein, p56, thus contrasts with the many peroxisomal microbody proteins in which the C-terminal tripeptide contains sufficient information for targeting to peroxisomes. Multiple mechanisms may exist for targeting proteins to the protein cores of microbody organelles. Comparisons of the DNA sequences of several alleles suggest that homologous recombination plays a role in the generation of allelic diversity.

Recognition of a peroxisomal tripeptide entry signal by the glycosomes of Trypanosoma brucei

Trypanosomes compartmentalise the first 9 enzymes of glycolysis and glycerol metabolism in peroxisome-like microbodies known as glycosomes. The identity of the sequences that direct proteins into the glycosome has until now been uncertain. We show here that the peroxisomal tripeptide entry signal is sufficient to cause association of a bacterial enzyme with the glycosomes of Trypanosoma brucei. However, it works less efficiently than the C-terminal 21 amino acids of trypanosome glycosomal phosphoglycerate kinase.