Abstract. Trypanosomes compartmentalize most of

African trypanosomes compartmentalize glycolysis in a microbody, the glycosome. When growing in the mammalian bloodstream, trypanosomes contain only a rudimentary mitochondrion, and the first seven glycolytic enzymes, including phosphoglycerate kinase, are located in the glycosome. Procyclic trypanosomes, growing in the gut of tsetse f lies, possess a fully developed mitochondrion that is active in oxidative phosphorylation. The first six glycolytic enzymes are still glycosomal, but phosphoglycerate kinase is now found in the cytosol. We demonstrate here that bloodstream trypanosomes are killed by expression of cytosolic phosphoglycerate kinase. The toxicity depends on both enzyme activity and cytosolic location. One possible explanation is that cytosolic phosphoglycerate kinase creates an ATPgenerating shunt in the cytosol, thus preventing full ATP regeneration in the glycosome and ultimately inhibiting the first, ATP-consuming, steps of glycolysis.All members of the order Kinetoplastida contain microbodies harboring glycolytic enzymes (1). Some enzymes are exclusive to the glycosome, whereas others are present in both glycosome and cytosol. In addition, the compartmentalization is developmentally regulated to varying degrees depending on the species. Kinetoplastid metabolism is simplest in the African trypanosome Trypanosoma brucei (2). T. brucei multiplies extracellularly as ''bloodstream forms'' in the blood and tissue fluids of mammals, and as ''procyclic forms'' in the gut of the tsetse fly vector. The bloodstream forms possess only a rudimentary mitochondrion and survive exclusively by substratelevel phosphorylation, with glucose-abundantly available in the environment-as the only energy source. In tsetse flies, in which amino acids are the predominant substrate; the mitochondrion is well developed, with citric acid cycle enzymes and a respiratory chain (2).One of the many enzymatic differences between bloodstream and procyclic forms is the location of phosphoglycerate kinase (PGK). T. brucei has three PGK genes (3, 4). One, PGKA, encodes a minor glycosomal variant (PGKA) that is expressed at low levels in both bloodstream and procyclic forms (5, 6). The second gene, PGKB, encodes the major cytosolic enzyme PGKB, which is present only in procyclic forms. The third gene, PGKC, encodes the major glycosomal enzyme PGKC, expressed only in bloodstream forms (3). PGKC is directed to the glycosome by a signal sequence present at the end of a 20-amino acid C-terminal extension (7-9). The developmental regulation of PGKB and PGKC expression is mediated posttranscriptionally by sequences in the 3Ј untranslated regions of the mRNAs (10). Here we describe experiments showing that correct developmentally regulated compartmentalization of PGK is vital for bloodstream trypanosome survival. MATERIALS AND METHODSPlasmid Constructs. Plasmids for inducible expression of PGK genes were constructed by replacing the chloramphenicol acetyltransferase or luciferase cassettes in pHD 615, pHD 616, or pHD 451 (11) with the ...

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The kinetoplastid protozoa infect hosts ranging from invertebrates to plants and mammals, causing diseases of medical and economic importance. They are the earliest-branching organisms in eucaryotic evolution to have either mitochondria or peroxisome-like microbodies. Investigation of their protein trafficking enables us to identify characteristics that have been conserved throughout eucaryotic evolution and also reveals how far variations, or alternative mechanisms, are possible. Protein trafficking in kinetoplastids is in many respects similar to that in higher eucaryotes, including mammals and yeasts. Differences in signal sequence specificities exist, however, for all subcellular locations so far examined in detail--microbodies, mitochondria, and endoplasmic reticulum--with signals being more degenerate, or shorter, than those of their higher eucaryotic counterparts. Some components of the normal array of trafficking mechanisms may be missing in most (if not all) kinetoplastids: examples are clathrin-coated vesicles, recycling receptors, and mannose 6-phosphate-mediated lysosomal targeting. Other aspects and structures are unique to the kinetoplastids or are as yet unexplained. Some of these peculiarities may eventually prove to be weak points that can be used as targets for chemotherapy; others may turn out to be much more widespread than currently suspected.

The glycosomes of trypanosomes are related to eukaryotic peroxisomes. For many glycosomal and peroxisomal proteins, a C-terminal SKL-like tripeptide known as PTS-1 serves as the targeting signal. For peroxisomes, a second N-terminal signal (PTS-2) was demonstrated on rat 3-ketoacyl-CoA thiolase. Several glycosomal proteins do not bear a PTS-1. One such protein, fructose bisphosphate aldolase, has a PTS-2 homology at its N-terminus. To find out whether the PTS-2 pathway exists in trypanosomes, we expressed chloramphenicol acetyltransferase fusion proteins bearing N-terminal segments of either rat thiolase or trypanosome aldolase. The mammalian PTS-2 clearly mediated glycosomal import. The aldolase N-terminus mediated import with variable efficiency depending on the length of the appended sequence. These results provide evidence for the existence of the PTS-2 pathway in trypanosomes.