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Structure of a eukaryotic SWEET transporter in a homotrimeric complex
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Structure of a eukaryotic SWEET transporter in a homotrimeric complex
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Journal Article
Structure of a eukaryotic SWEET transporter in a homotrimeric complex
2015
Overview
The X-ray crystal structure is presented of a seven-transmembrane eukaryotic SWEET glucose transporter, revealing the link between seven-transmembrane eukaryotic SWEETs and their three-transmembrane bacterial homologues and providing insight into eukaryotic sugar transport mechanisms.
Structure of a SWEET sugar transporter
SWEET sugar transporters are involved in various processes in plants and in glucose transport in animals. The authors report the first X-ray crystal structure of a eukaryotic SWEET glucose transporter, a vacuolar glucose transporter from rice. The structure (of the inward-open state) shows that this transporter forms homomeric trimers. It contains seven transmembrane helices — in contrast to the three helices reported for bacterial homologues — suggesting a molecular basis for understanding functional cross-talk and coupling of SWEET transporters.
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 the dominant distribution forms. Cellular sugar uptake and release require vesicular and/or plasma membrane transport proteins. Humans and plants use proteins from three superfamilies for sugar translocation: the major facilitator superfamily (MFS), the sodium solute symporter family (SSF; only in the animal kingdom), and SWEETs
1
,
2
,
3
,
4
,
5
. SWEETs carry mono- and disaccharides
6
across vacuolar or plasma membranes. Plant SWEETs play key roles in sugar translocation between compartments, cells, and organs, notably in nectar secretion
7
, phloem loading for long distance translocation
8
, pollen nutrition
9
, and seed filling
10
. Plant SWEETs cause pathogen susceptibility possibly by sugar leakage from infected cells
3
,
11
,
12
. The vacuolar
Arabidopsis thaliana
AtSWEET2 sequesters sugars in root vacuoles; loss-of-function mutants show increased susceptibility to
Pythium
infection
13
. Here we show that its orthologue, the vacuolar glucose transporter OsSWEET2b from rice (
Oryza sativa
), consists of an asymmetrical pair of triple-helix bundles, connected by an inversion linker transmembrane 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 triple-helix bundle 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 are valuable for understanding the transport mechanism of eukaryotic SWEETs and may be useful for engineering sugar flux.
Publisher
Nature Publishing Group UK,Nature Publishing Group
Subject
/ 82/80
/ 82/83
/ Arabidopsis Proteins - chemistry
/ Arabidopsis Proteins - genetics
/ Arabidopsis Proteins - metabolism
/ Dextrose
/ Glucose
/ Glucose Transport Proteins, Facilitative - chemistry
/ Glucose Transport Proteins, Facilitative - genetics
/ Glucose Transport Proteins, Facilitative - metabolism
/ Humanities and Social Sciences
/ Humans
/ letter
/ Monosaccharide Transport Proteins - chemistry
/ Monosaccharide Transport Proteins - genetics
/ Monosaccharide Transport Proteins - metabolism
/ Mutation
/ Phloem
/ Plasma
/ Pollen
/ Protein Structure, Quaternary
/ Protein Structure, Secondary
/ Proteins
/ Science
/ Structure-Activity Relationship
/ Sugar
