Sucrose uptake transporters (SUTs) are membrane proteins in the plasma membrane and vacuole membrane. There are three types, type I, II and III that differ in transport properties and in sequence. All SUTs are proton coupled, they transport one sugar molecule and one proton at the same time in the same direction. They are distantly related to the major facilitator superfamily (MFS) of transporters. We are interested in the transport activity of SUTs; figure 1 is a summary of the substrate specificity of some of the SUTs we have studied in the lab.
Figure 1. Substrate specificities of type I, II and III SUTs. Transport activity was assayed in Xenopus oocytes using two-electrode voltage clamping. Results are from Chandran et al. (2003), Sivitz et al. (2005, 2007), Reinders et al. (2006, 2008). This figure is taken from Reinders and Ward (2012).
We found that only type I SUTs transport esculin (a plant coumarin glucoside). Since this sugar is highly fluorescent, we use this substrate to observe SUT activity in yeast. Yeast does not normally take up esculin, but yeast expressing a type I SUT take up esculin and become fluorescent as shown in Fig. 2.
Figure 2. Esculin uptake by yeast expressing sucrose transporter cDNAs. This figure is taken from Gora et al. (2012).
Reinders A, Sivitz AB, Ward JM (2012) Evolution of plant sucrose uptake transporters (SUTs). Front. Plant Sci. 3: 00022
Gora PJ, Reinders A, Ward JM (2012) A novel fluorescent assay for sucrose transporters. Plant Methods 8:13
Sivitz AB, Reinders A, Ward JM (2008) Arabidopsis sucrose transporter AtSUC1 is important for pollen germination and sucrose-induced anthocyanin accumulation. Plant Physiol. 147: 92-100
Reinders A, Sivitz AB, Starker CG, Gantt JS, Ward JM (2008) Functional analysis of LjSUT4, a vacuolar sucrose transporter from Lotus japonicus. Plant Mol. Biol. 68: 289-299
Sivitz AB, Reinders A, Johnson M, Krentz AD, Grof CPL, Perroux JM, Ward JM (2007) Arabidopsis sucrose transporter AtSUC9: high affinity transport activity, intragenic control of expression and early-flowering phenotype. Plant Physiol. 143: 188-198
Sivitz AB, Reinders A, Ward JM (2005) Analysis of the transport activity of barley sucrose transporter HvSUT1. Plant Cell Physiol. 46: 1666-1673
Chandran D, Reinders A, Ward JM (2003) Substrate specificity of the Arabidopsis thaliana sucrose transporter AtSUC2. J. Biol. Chem. 278: 44320-5