Structural isoforms of a membrane transport protein from Leishmania enriettii

Kinetoplastid glucose transporters

Protozoa of the order kinetoplastida have colonized many habitats, and several species are important parasites of humans. Adaptation to different environments requires an associated adaptation at a cell's interface with its environment, i.e. the plasma membrane. Sugar transport by the kinetoplastida as a phylogenetically related group of organisms offers an exceptional model in which to study the ways by which the carrier proteins involved in this process may evolve to meet differing environmental challenges. Seven genes encoding proteins involved in glucose transport have been cloned from several kinetoplastid species. The transporters all belong to the glucose transporter superfamily exemplified by the mammalian erythrocyte transporter GLUT1. Some species, such as the African trypanosome Trypanosoma brucei, which undergo a life cycle where the parasites are exposed to very different glucose concentrations in the mammalian bloodstream and tsetse-fly midgut, have evolved two different transporters to deal with this fluctuation. Other species, such as the South American trypanosome Trypanosoma cruzi, multiply predominantly in conditions of relative glucose deprivation (intracellularly in the mammalian host, or within the reduviid bug midgut) and have a single, relatively high-affinity type, transporter. All of the kinetoplastid transporters can also transport d-fructose, and are relatively insensitive to the classical inhibitors of GLUT1 transport cytochalasin B and phloretin.

Hexose uptake in Trypanosoma cruzi: structure-activity relationship between substrate and transporter

The gene encoding a hexose transporter, TcrHt1, from Trypanosoma cruzi has been functionally expressed in mammalian Chinese hamster ovary cells. Kinetic parameters of the heterologously expressed protein are very similar to those of the transporter identified in T. cruzi epimastigotes, confirming that TcrHT1 is the major transporter functioning in these parasites. A detailed analysis of substrate recognition using analogues of D-glucose substituted at each carbon position has been performed. The glucose transporter of T. cruzi does not recognize C-3 or C-6 analogues of D-glucose, whereas these analogues were recognized by the glucose transporter of bloodstream-form T. brucei. As for other kinetoplastid transporters, but in stark contrast to the mammalian GLUT family, TcrHT1 can also transport D-fructose, with relatively high affinity (Km = 0.682 +/- 0.003 mM). Amino acid side-chain-modifying reagents were also used to identify residues of the transporter present at the substrate-binding site. While specific modifiers of cysteine, histidine and arginine all inhibited catalytic activity, protection using substrate was only observed using the arginine-specific reagent, phenylglyoxal. Reagents which modify lysine residues had no effect on transport.

Differential targeting of two glucose transporters from Leishmania enriettii is mediated by an NH2-terminal domain

Leishmania are parasitic protozoa with two major stages in their life cycle: flagellated promastigotes that live in the gut of the insect vector and nonflagellated amastigotes that live inside the lysosomes of the vertebrate host macrophages. The Pro-1 glucose transporter of L. enriettii exists as two isoforms, iso-1 and iso-2, which are both expressed primarily in the promastigote stage of the life cycle. These two isoforms constitute modular structures: they differ exclusively and extensively in their NH2-terminal hydrophilic domains, but the remainder of each isoform sequence is identical to that of the other. We have localized these glucose transporters within promastigotes by two approaches. In the first method, we have raised a polyclonal antibody against the COOH-terminal hydrophilic domain shared by both iso-1 and iso-2, and we have used this antibody to detect the transporters by confocal immunofluorescence microscopy and immunoelectron microscopy. The staining observed with this antibody occurs primarily on the plasma membrane and the membrane of the flagellar pocket, but there is also light staining on the flagellum. We have also localized each isoform separately by introducing an epitope tag into each protein sequence. These experiments demonstrate that iso-1, the minor isoform, resides primarily on the flagellar membrane, while iso-2, the major isoform, is located on the plasma membrane and the flagellar pocket. Hence, each isoform is differentially sorted, and the structural information for targeting each transporter isoform to its correct membrane address resides within the NH2-terminal hydrophilic domain.

Membrane-related processes and overall energy metabolism in Trypanosoma brucei and other kinetoplastid species

An electrochemical proton gradient exists across the plasma membrane and the mitochondrial membrane of the bloodstream form of Trypanosoma brucei. The membrane potential across the plasma membrane and the regulation of the internal pH depend on the temperature. Leishmania donovani regulates its internal pH and maintains a constant electrochemical proton gradient across its plasma membrane under all conditions examined. The mitochondrion of the T. brucei bloodstream form is energized, even though the reactions taking place in it do not result in net ATP synthesis and the Kreb's cycle and the respiratory chain are absent. Glucose is transported across the plasma membrane of T. brucei by a facilitated diffusion carrier, that can transport a wider range of substrates than its mammalian counterparts. Pyruvate exits the cell via a facilitated diffusion transporter as well. Conflicting evidence exists for the mechanism of glucose transport in L. donovani; biochemical evidence suggests proton/glucose symport, while facilitated diffusion is indicated by physiological data.