Substrate Specificity of the Arabidopsis thaliana Sucrose Transporter AtSUC2

Abstract

The Arabidopsis sucrose transporter AtSUC2 is expressed in the companion cells of the phloem (specialized vascular tissue) and is essential for the long distance transport of carbohydrates within the plant. A variety of glucosides are known to inhibit sucrose uptake into yeast expressing AtSUC2; however, it remains unknown whether glucosides other than sucrose could serve as transported substrates. By expression of AtSUC2 in Xenopus oocytes and two-electrode voltage clamping, we have tested the ability of AtSUC2 to transport a range of physiological and synthetic glucosides. Sucrose induced inward currents with a K 0.5 of 1.44 mM at pH 5 and a membrane potential of ؊137 mV. Of the 24 additional sugars tested, 8 glucosides induced large inward currents allowing kinetic analysis. These glucosides were maltose, arbutin (hydroquinone-␤-D-glucoside), salicin (2-(hydroxymethyl)phenyl-␤-D-glucoside), ␣-phenylglucoside, ␤-phenylglucoside, ␣-paranitrophenylglucoside, ␤-paranitrophenylglucoside, and paranitrophenyl-␤-thioglucoside. In addition, turanose and ␣-methylglucoside induced small but significant inward currents indicating that they were transported by At-SUC2. The results indicate that AtSUC2 is not highly selective for ␣-over ␤-glucosides and may function in transporting glucosides besides sucrose into the phloem, and the results provide insight into the structural requirements for transport by AtSUC2.Plant sucrose transporters (SUTs, also named SUCs) 1 are integral membrane proteins within the glycoside-pentosidehexuronide cation symporter family. SUTs are expressed in the plasma membrane of companion cells and sieve elements within the phloem, a specialized tissue within the plant vasculature, and catalyze the H ϩ -coupled uptake of sucrose. Sucrose is the primary photosynthetic product that is transported by the phloem to heterotrophic tissues such as roots, developing leaves, and seeds (1, 2). The Arabidopsis genome encodes nine SUT homologs (3). One of these, AtSUC2 (4), functions as the main phloem-loading transporter. AtSUC2 is highly expressed in the companion cells of source (photosynthetic) leaves (5) and is essential for the long distance transport of sucrose as evidenced by the severe phenotype of insertional mutants (6). As demonstrated by the [ 14 C]sucrose uptake of yeast expressing AtSUC2, the transporter has a high affinity for sucrose (K m ϭ 0.8 mM) (4) compared with Arabidopsis homologs AtSUT4 (7) and AtSUT2 (8).In plant cell protoplasts or plasma membrane vesicles, sucrose uptake is inhibited by a variety of glucosides (9, 10) providing insight into the structural requirements for binding to sucrose transporters. Results indicated that none of the fructosyl hydroxyls of sucrose interact specifically with the transporter (9). The ability of phenylthioglucoside to inhibit sucrose uptake and to serve as a transported substrate further indicated that the fructosyl moiety of sucrose presents a hydrophobic surface required for binding (9, 10). Substitutions of the glucosyl hydroxyls 3, 4, and 6 of ...

Expression of Sucrose Transporter cDNAs Specifically in Companion Cells Enhances Phloem Loading and Long-Distance Transport of Sucrose but Leads to an Inhibition of Growth and the Perception of a Phosphate Limitation

“…AtSUC2 lines). Collectively, these results suggest partial rescue based on relative expression and imply that the capacities of AtSUC2, AtSUC1, and ZmSUT1 for moving Suc across membranes in planta are roughly equal, consistent with kinetic parameters measured in heterologous systems (Chandran et al, 2003;Carpaneto et al, 2005;Sivitz et al, 2008).…”

Proton-Dependent Coniferin Transport, a Common Major Transport Event in Differentiating Xylem Tissue of Woody Plants

“…S6, A and B). The possibility that a Suc transporter would be involved in coniferin transport (Chandran et al, 2003;Sivitz et al, 2007) was tested, but coniferin transport was barely influenced in the presence of Suc (Supplemental Fig. S6C).…”

Impaired Function of the Tonoplast-Localized Sucrose Transporter in Rice, OsSUT2, Limits the Transport of Vacuolar Reserve Sucrose and Affects Plant Growth

“…Compared with other tonoplast SUTs, including LjSUT4, this represents a relatively high affinity for Suc Weschke et al, 2000;Reinders et al, 2008). Analysis of [ 14 C]Suc uptake into yeast is thought to underestimate K m because SUT activity depletes the proton motive force and membrane potential is not controlled (Chandran et al, 2003). This tends to decrease the V max measured and therefore decreases the apparent K m .…”