AtSTP3, a green leaf-specific, low affinity monosaccharide-H+ symporter of Arabidopsis thaliana

This paper describes the expression analyses of the AtSTP3 gene of Arabidopsis thaliana, the functional characterization of the encoded protein as a new monosaccharide transporter, and introduces the AtSTP gene family. The kinetic properties of the AtSTP3 protein (for sugar transport protein 3) were studied in a hexose transport deficient mutant of Schizosaccharomyces pombe. AtSTP3 represents a new monosaccharide transporter that is composed of 514 amino acids and has a calculated molecular mass of 55.9 kDa. Kinetic analyses in yeast showed that AtSTP3 is a low affinity, energy-dependent H+ symporter with a K-m for D-glucose of 2 mM. RNase protection analyses revealed that AtSTP3 is expressed in leaves and floral tissue of Arabidopsis. This expression pattern of the AtSTP3 gene was confirmed in AtSTP3 promoter-beta-glucuronidase (GUS) plants showing AtSTP3-driven GUS activity in green leaves, such as cotelydons, rosette and stalk leaves and sepals. Wounding caused an induction of GUS activity in the transgenic plants and an increase of AtSTP3 mRNA levels in Arabidopsis wild-type plants. Polymerase chain reaction analyses with degenerate primers identified additional new AtSTP genes and revealed that AtSTP3 is the member of a large family of at least 14 homologous genes coding for putative monosaccharide-H+ symporters (AtSTPs).