A glucose transporter can mediate ribose uptake: definition of residues that confer substrate specificity in a sugar transporter.
Sugars, the major energy source for many organisms, must be transported across biological membranes. Glucose is the most abundant sugar in human plasma and in many other biological systems and has been the primary focus of sugar transporter studies in eukaryotes. We have previously cloned and characterized a family of glucose transporter genes from the protozoan parasite Leishmania. These transporters, called LmGT1, LmGT2, and LmGT3, are homologous to the well characterized glucose transporter (GLUT) family of mammalian glucose transporters. We have demonstrated that LmGT proteins are important for parasite viability. Here we show that one of these transporters, LmGT2, is a more effective carrier of the pentose sugar d-ribose than LmGT3, which has a 6-fold lower relative specificity (V(max)/K(m)) for ribose. A pair of threonine residues, located in the putative extracellular loops joining transmembrane helices 3 to 4 and 7 to 8, define a filter that limits ribose approaching the exofacial substrate binding pocket in LmGT3. When these threonines are substituted by alanine residues, as found in LmGT2, the LmGT3 permease acquires ribose permease activity that is similar to that of LmGT2. The location of these residues in hydrophilic loops supports recent suggestions that substrate recognition is separated from substrate binding and translocation in this important group of transporters.
Naula
Christina M
CM
Faculty of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, United Kingdom.
Logan
Flora J
FJ
Logan
Flora M
FM
Wong
Pui Ee
PE
Barrett
Michael P
MP
Burchmore
Richard J
RJ
D019943:Amino Acid Substitution; D000818:Animals; D001665:Binding Sites; D051246:Glucose Transport Proteins, Facilitative / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D007894:Leishmania mexicana / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D005810:Multigene Family / Q000502:physiology; D017433:Protein Structure, Secondary; D015800:Protozoan Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity
D051246:Glucose Transport Proteins, Facilitative; D015800:Protozoan Proteins