Abstract
Plants are dependent on controlled sugar uptake for correct organ development and sugar storage, and apoplastic sugar depletion is a defense strategy against microbial infections like rust and mildew. Uptake of glucose and other monosaccharides is mediated by Sugar Transport Proteins, proton-coupled symporters from the Monosaccharide Transporter (MST) superfamily. We present the 2.4 Å structure of Arabidopsis thaliana high affinity sugar transport protein, STP10, with glucose bound. The structure explains high affinity sugar recognition and suggests a proton donor/acceptor pair that links sugar transport to proton translocation. It contains a Lid domain, conserved in all STPs, that locks the mobile transmembrane domains through a disulfide bridge, and creates a protected environment which allows efficient coupling of the proton gradient to drive sugar uptake. The STP10 structure illuminates fundamental principles of sugar transport in the MST superfamily with implications for both plant antimicrobial defense, organ development and sugar storage.
Highlights
Plants are dependent on controlled sugar uptake for correct organ development and sugar storage, and apoplastic sugar depletion is a defense strategy against microbial infections like rust and mildew
Fifty three members of the Monosaccharide Transporter Superfamily have been identified in Arabidopsis thaliana alone, of which 14 constitute the Sugar Transport Protein family[1,14] (Supplementary Fig. 1 and Supplementary Table 1)
The overall structure adopts a Major Facilitator fold with 12 transmembrane helices (M1-M12) divided in two domains (N and C domain) with a quasi-twofold symmetry perpendicular to the membrane plane (Fig. 1a). These are joined by an intracellular helical bundle (ICH) domain
Summary
The μM affinity of STP10 for glucose can be explained by the substrate’s interaction to residues in the N domain (Phe[39] (M1b), Ile[184] (M5) and in particular Leu[43] (M1b)) that creates a hydrophobic interaction surface for the substrate (Fig. 1c) This tight and hydrophobic interaction surface is not found in human sugar facilitators or bacterial sugar/H+ symporters, where the interaction distance is longer and the corresponding residue is polar[17,18,19] (Supplementary Fig. 7). By breaking the disulfide bridge and increasing flexibility, the proton/donor acceptor pair becomes much more sensitive to the extracellular pH, either directly through a change in Asp[42] pKa value or through a requirement for a stronger proton gradient to drive transport, and this indirectly affect substrate turnover at higher pH, without affecting Kd (Supplementary Fig. 9)
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