Figure 1. Characterization of SWEET transporters.

a, Identification of glucose transport activity for AtSWEET1 by co-expression with cytosolic FRET glucose sensor FLIPglu600μΔ 13V in HEK293T cells¹⁵. Individual cells were analysed by quantitative ratio imaging of CFP and Venus emission (acquisition interval 5 s; Fc/D corresponds to normalized emission intensity ratio¹⁵). HEK293T/FLIPglu600μΔ 13V cells were perfused with medium, followed by square pulses of increasing glucose (Gluc.) concentrations. Orange line indicates cells expressing sensor alone; blue line indicates cells co-expressing sensor and AtSWEET1; accumulation of glucose is shown by a negative FRET ratio change (blue line; mean – s.d.; n > 10). b, FRET imaging of glucose efflux from cytosol into ER. FLIPglu600μΔ13V^ER was targeted to the ER lumen¹⁵ (compare with panel a; acquisition interval 10 s; mean – s.d.; n > 10). c, Cartoon for SWEET influx across plasma membrane and efflux from cytosol to ER. Cytosolic FLIPglu600μΔ 13V identifies glucose import from the extracellular face (extracellular N terminus). FLIPglu600μΔ13V^ER measures transport from the intracellular side (cytosolic C terminus). d, Complementation of yeast EBY4000 (ref. ¹⁷) lacking 18 hexose transporter genes with AtSWEET1, AtSWEET8, or yeast HXT5; control, empty vector. e, Glucose accumulation in EBY4000 co-expressing AtSWEET1 and FLII¹²Pglu700μδ6 before and after glucose addition (two cycles before glucose addition; mean ± s.d.; n = 3). f, Kinetics of [¹⁴C]glucose accumulation by AtSWEET1 in EBY4000 (mean ± s.d., n = 3). g, AtSWEET1-mediated uptake of 1 mM [¹⁴C]glucose, [¹⁴C]galactose or [¹⁴C]sucrose into oocytes (mean ± s.d.; n = 8 cells). h, [¹⁴C]glucose efflux from oocytes expressing AtSWEET1 (mean ± s.e.; n ≥ 10 cells; P < 0.0005). i, Confocal imaging of AtSWEET1–YFP in leaves of stably transformed Arabidopsis (panel width = 176 μm). j, Structural model of SWEETs based on hydrophobicity plots (duplication of three transmembrane helices; red/blue triangles).