PRIMARY STRUCTURE, GENOMIC ORGANIZATION AND HETEROLOGOUS EXPRESSION OF A GLUCOSE TRANSPORTER FROM ARABIDOPSIS-THALIANA

Both genomic and full length cDNA clones of an Arabidopsis thaliana sugar carrier, STP1, have been obtained using a cDNA clone of the H+/hexose-cotransporter from the green alga Chlorella kessleri as hybridization probe. The peptide predicted from these sequences is 522 amino acids long and has a molecular weight of 57 518 kd. This higher plant sugar carrier contains 12 putative transmembrane segments and is highly homologous to the H+/hexose cotransporter from Chlorella, with an overall identity in the amino acid sequence of 47.1%. It is also homologous to the human HepG2 glucose transporter (28.4%), and other sugar carriers from man, rat, yeast and Escherichia coli. The definite proof for the function of the STP1 protein as a hexose transporter and data on its substrate specificity were obtained by heterologous expression in the fission yeast Schizosaccharomyces pombe. Transformed yeast cells transport D-glucose with a 100-fold lower K(M) value than control cells. Moreover only the transformed cells were able to accumulate the non-metabolizable D-glucose analogue 3-O-methyl-D-glucose, indicating that the Arabidopsis carrier catalyses an energy dependent, active uptake of hexoses. Expression of STP1 mRNA is low in heterotrophic tissues like roots or flowers. High levels of expression are found in leaves.