Differential regulation of multiple glucose transporter genes in Leishmania mexicana

We have studied the structure and expression of glucose transporter genes in the parasitic protozoan Leishmania mexicana. Three distinct glucose transporter isoforms, LmGT1, LmGT2, and LmGT3, are encoded by single copy genes that are clustered together at a single locus. Quantitation of Northern blots reveals thatLmGT2 mRNA is present at ∼15-fold higher level in promastigotes, the insect stage of the parasite life cycle, compared with amastigotes, the intracellular stage of the life cycle that lives within the mammalian host. In contrast, LmGT1 andLmGT3 mRNAs are expressed at similar levels in both life cycle stages. Transcription of the LmGT genes in promastigotes and axenically cultured amastigotes occurs at similar levels, as measured by nuclear run-on transcription. Consequently, the ∼15-fold up-regulation of LmGT2 mRNA levels in promastigotes compared with amastigotes must be controlled at the post-transcriptional level. Measurement of LmGT2 RNA decay in promastigotes and axenic amastigotes treated with actinomycin D suggests that differential mRNA stability may play a role in regulating glucose transporter mRNA levels in the two life cycle stages

Biochemistry and molecular genetics of Leishmania glucose transporters

Glucose is utilized as a significant source of metabolic energy by Leishmania parasites. This sugar is accumulated by the parasite via a specific carrier-mediated transport system located in the parasite membrane. Parasites may also contain another transporter that shuttles glucose between the cytoplasm and the glycosome, a membrane-bound organelle where the early steps of glycolysis occur. The transport systems of both the insect stage promastigotes and the intracellular amastigotes have been characterized and shown to have kinetic properties that are consistent with the different physio-logical environments of the insect gut and the macrophage phagolysosome. Several genes have been cloned from Leishmania species which encode proteins with substantial sequence similarity to glucose transporters from mammals and lower eukaryotes. Two of these genes are expressed preferentially in the promastigote stage of the life cycle, where glucose is more readily available and more rapidly transported and metabolized than in the intracellular amastigotes. One of these two developmentally-regulated genes has been functionally expressed in Xenopus oocytes and shown to encode a glucose transporter. A third gene encodes a protein that is also a member of the glucose transporter family on the basis of sequence similarity and proposed secondary structure. However, the significant differences between this protein and the other two suggest that it is likely to transport a different substrate. Functional expression will be required to define the specific biochemical role of each gene within the parasite