Comparative analysis of STP6 and STP10 unravels molecular selectivity in Sugar Transport Proteins

ABSTRACT The distribution of sugars is crucial for plant energy, signaling, and defense mechanisms. Sugar Transport Proteins (STPs) are Sugar Porters that mediate proton-driven cellular uptake of glucose. Some STPs also transport fructose, while others remain highly selective for only glucose. What determines this selectivity, allowing STPs to distinguish between compounds with highly similar chemical composition, remains unknown. Here, we present the structure of Arabidopsis thaliana STP6 in an inward occluded conformational state with glucose bound and demonstrate its role as both a glucose and fructose transporter. We perform a comparative analysis of STP6 with the glucose-selective STP10 using in-vivo and in-vitro systems, demonstrating how different experimental setups strongly influence kinetic transport properties. We analyze the properties of the monosaccharide binding site and show that the position of a single methyl group in the binding site is sufficient to shuffle glucose and fructose specificity, providing detailed insights into the fine-tuned dynamics of affinity-induced specificity for sugar uptake. Altogether these findings enhance our understanding of sugar selectivity in STPs and more broadly Sugar Porter proteins. SIGNIFICANCE STATEMENT Understanding the mechanisms of sugar transport in plants is essential for advancing agricultural practices and enhancing plant resilience. This study reveals the structural basis of sugar selectivity in Sugar Transport Proteins of Arabidopsis thaliana. By comparing the dual-specific STP6, transporting both glucose and fructose with the glucose-selective STP10 across multiple experimental setups, we show that difference as subtle as the position of a single methyl group in the binding site can control sugar specificity. These findings enhance our understanding of sugar selectivity by Sugar Transport Proteins and more broadly Sugar Porter proteins and lay the groundwork for engineering crops with improved energy efficiency and pathogen resistance. Competing Interest Statement The authors have declared no competing interest. Footnotes revisions of oocyte data with more datapoints. Addition of two new co-authors.