Structure of a eukaryotic SWEET transporter in a homotrimeric complex

Abstract

Eukaryotes rely on efficient distribution of energy and carbon skeletons between organs in the form of sugars. Glucose in animals and sucrose in plants serve as dominant distribution forms. Cellular sugar uptake and release require vesicular and/or plasma membrane transport proteins. Humans and plants use related proteins from three superfamilies for sugar translocation: the major facilitator superfamily (MFS), the sodium solute symporter Family (SSF; only animal kingdom), and SWEETs1-5. SWEETs carry mono- and disaccharides6 across vacuolar or plasma membranes. Plant SWEETs play key roles in sugar translocation between compartments, cells, and organs, notably in nectar secretion7, phloem loading for long distance translocation8, pollen nutrition9, and seed filling10. Plant SWEETs cause pathogen susceptibility by sugar leakage from infected cells3,11,12. The vacuolar AtSWEET2 sequesters sugars in root vacuoles; loss-of-function increases susceptibility to Pythium infection13. Here we show that its orthologue, the vacuolar glucose transporter OsSWEET2b from rice, consists of an asymmetrical pair of triple-helix-bundles (THBs), connected by an inversion linker helix (TM4) to create the translocation pathway. Structural and biochemical analyses show OsSWEET2b in an apparent inward (cytosolic) open state forming homomeric trimers. TM4 tightly interacts with the first THB within a protomer and mediates key contacts among protomers. Structure-guided mutagenesis of the close paralogue SWEET1 from Arabidopsis identified key residues in substrate translocation and protomer crosstalk. Insights into the structure-function relationship of SWEETs is valuable for understanding the transport mechanism of eukaryotic SWEETs and may be useful for engineering sugar flux.

Glucose-Induced Trophic Shift in an Endosymbiont Dinoflagellate with Physiological and Molecular Consequences

“…Three of these transporters (s6_28295, s6_26300, and s6_19007) are of the SWEET type (Fig. 6B) and contain the highly conserved amino acids known to be critical for transporter function, including the conserved Asn that is associated with Glc binding ) and a Tyr located in the putative intrafacial gate (Xuan et al, 2013;Tao et al, 2015; Fig. 6B, stars).…”

Glucose Homeostasis

“…The SWEET class of transporters is predicted to have seven transmembrane helices. Eukaryotic SWEETs have a predicted topology comprising a repeat of three membrane-spanning domains that are connected by an inversion linker helix with extracellular N-terminus and intracellular C-terminus [15][16][17]. Cytosolic C-terminus of the SWEETs is very long and may serve as a docking platform for protein interactions [18].…”

A genome-wide analysis of SWEET gene family in cotton and their expressions under different stresses

“…The OsSWEET2b protein is the only eukaryotic SWEET protein with resolved three-dimensional structure so far (Tao et al 2015). SWEET proteins generally contain seven transmembrane domains and two MtN3_slv domains (Chen et al 2010;Chen et al 2012;Talbot 2010;Baker et al 2012), the N-terminus and C-terminus are outside and inside the cytoplasm of the cell, respectively.…”