Hexose transporters of tomato: molecular cloning, expression analysis and functional characterization

A full-length (LeHT2) and two partial (LeHT1 and LeHT3) cDNA clones, encoding hexose transporters, were isolated from tomato (Lycopersicon esculentum) fruit and flower cDNA libraries. Southern blot analysis confirmed the presence of a gene family of hexose transporters in tomato consisting of at least three members. The full-length cDNA (LeHT2) encodes a protein of 523 amino acids, with a calculated molecular mass of 57.6 kDa. The predicted protein has 12 putative membrane-spanning domains and belongs to the Major Facilitator Superfamily of membrane carriers. The three clones encode polypeptides that are homologous to other plant monosaccharide transporters and contain conserved amino acid motifs characteristic of this superfamily. Expression of the three genes in different organs of tomato was investigated by quantitative PCR. LeHT1 and LeHT3 are expressed predominantly in sink tissues, with both genes showing highest expression in young fruit and root tips. LeHT2 is expressed at relatively high levels in source leaves and certain sink tissues such as flowers. LeHT2 was functionally expressed in a hexose transport-deficient mutant (RE700A) of Saccharomyces cerevisiae. LeHT2-dependent transport of glucose in RE700A exhibited properties consistent with the operation of an energy-coupled transporter and probably a H+/hexose symporter. The K-m of the symporter for glucose is 45 muM.