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Import of triosephosphate isomerase into the glycosomes of Trypanosoma brucei.
S. DE WALQUE1 , F. R. OPPERDOES2 AND P. A. M. MICHELS1
1Laboratoire de Biochimie, Université catholique de Louvain and 2International Institute of Cellular and Molecular Pathology, Research Unit for Tropical Diseases, Brussels, Belgium.
In trypanosomes, glycolysis is compartmentalised (OPPERDOES, 1987). The first seven enzymes of the glycolytic pathway are contained in peroxisome-like organelles called glycosomes.
Glycosomes are organelles devoid of DNA into which proteins are post-translationally imported. The import of proteins into glycosomes resembles that of peroxisomes. Most striking is the conservation of the targeting signals between proteins imported into both organelles. The targeting of proteins into the glycosome can be mediated by either a C-terminal (SOMMER et al., 1992, BLATTNER et al., 1992) or a N-terminal signal (BLATTNER et al., 1995). There remain, however, glycosomal proteins which do not contain either of these signals, for instance triosephosphate isomerase (TIM). Here we present the results of our studies on the identification of the signal directing TIM from T. brucei into the glycosomes.
Procyclic trypanosomes were transiently transfected with constructs coding for fusion proteins of different segments of TIM with the reporter protein CAT (chloramphenicol acetyl transferase). The intracellular localization of the fusion proteins was determined after digitonin fractionation of the transiently transfected trypanosomes. Two fractions were taken: a cytosolic and an organellar one. The presence of CAT in the two fractions was measured using a CAT-ELISA system (Boehringer-Mannheim).
As a positive control, we used the reporter protein CAT extended with the C-terminal tripeptide of glyceraldehydephosphate dehydrogenase of T. brucei (-AKL) which as already been described as an efficient glycosomal targeting signal (SOMMER et al., 1992). This protein is imported efficiently into glycosomes. The CAT protein without targeting signal remains cytosolic.
A fusion protein consisting of the entire sequence of TIM and CAT is also targeted to glycosomes. A protein containing the first 131 amino acids of TIM fused to CAT is not imported. The fusion protein is efficiently imported, however, when the first 172 amino acids of TIM are used. When a fragment of only 42 amino acids, delimited by residues 131 to 172, is fused onto CAT the protein is still detected almost completely in the organellar fraction.
An alignment of the three available TIM sequences of Trypanosomatidae with the sequence of cytoplasmic TIM from other organisms shows several characteristic features in the 42 amino-acid fragment that seems to contain the import signal. A two amino-acid insertion specific of the Trypanosomatidae sequences and several conserved residues are present in this segment. They might constitute a targeting signal.
Acknowledgements (optional, Times 10)
This work was financially supported by a grant of the FRSM (contract n° 3.4534.95). S. de Walque is recipient of a FRIA fellowship.
References (optional, Authors in Times 12 small caps, journal names in italics, volume numbers in bold)
OPPERDOES, F. R., (1987) Annu. Rev. Microbiol. 41:127-151.
SOMMER, J. M., CHENG, Q.-L., KELLER, G.-A., and WANG, C. C. (1992) Mol. Biol. Cell 3:749-759.
BLATTNER, J., SWINKELS, B., DÖRSAM, H., PROPERO, T., SUBRAMANI, S., and CLAYTON, C. (1992) J. Cell Biol. 119:1126-1136.
BLATTNER, J., DÖRSAM, H. and CLAYTON, C. E. (1995) FEBS Lett. 360:310-314.
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