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.
The Escherichia coli uracil:proton symporter UraA is a prototypical member of the nucleobase/ascorbate transporter (NAT) or nucleobase/cation symporter 2 (NCS2) family, which corresponds to the human solute carrier family SLC23. UraA consists of 14 transmembrane segments (TMs) that are organized into two distinct domains, the core domain and the gate domain, a structural fold that is also shared by the SLC4 and SLC26 transporters. Here we present the crystal structure of UraA bound to uracil in an occluded state at 2.5 Å resolution. Structural comparison with the previously reported inward-open UraA reveals pronounced relative motions between the core domain and the gate domain as well as intra-domain rearrangement of the gate domain. The occluded UraA forms a dimer in the structure wherein the gate domains are sandwiched by two core domains. In vitro and in vivo biochemical characterizations show that UraA is at equilibrium between dimer and monomer in all tested detergent micelles, while dimer formation is necessary for the transport activity. Structural comparison between the dimeric UraA and the recently reported inward-facing dimeric UapA provides important insight into the transport mechanism of SLC23 transporters.
Cobalt hydroxide/cadmium sulfide composite was prepared using an easy coprecipitation strategy. The field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) confirmed that Co(OH)2 nanometer particles were modified on CdS. Even without noble-metal cocatalyst, the photocatalytic H2 evolution over CdS after Co(OH)2 loaded was evidently increased. The most excellent Co(OH)2 of 6.8 mol %, resulted in a H2 generation rate of 61 μmol h(-1) g(-1), which exceeded that of pure CdS by a factor of 41 times. Surface photovoltage (SPV) and surface photocurrent (SPC) investigations revealed that the photogenerated electrons could be captured by the loaded Co(OH)2 nanoparticles. The interface formed between Co(OH)2 and CdS is vital to the enhancement of photocatalytic H2 generation. Electrochemical measurement results indicated that another reason for the enhanced photocatalytic activity of Co(OH)2/CdS catalyst is that Co(OH)2 has outstanding H2 generation activity.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.