No 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.

SemiSWEETs and SWEETs are mono- and disaccharide transporters present from Archaea to higher plants and humans1-3. SWEETs play crucial roles in cellular sugar efflux processes, i.e. phloem loading4, pollen nutrition5 and nectar secretion6. Their bacterial homologs, SemiSWEETs, are among the smallest known transporters1,3. Here we show SemiSWEET, consisting of a triple-helix-bundle (THB), forms a symmetric parallel dimer to create the translocation pathway. Two SemiSWEET isoforms were crystallized in apparent open and occluded states, indicating that SemiSWEETs/SWEETs are transporters that undergo rocking-type movements during the transport cycle. The topology of THB is similar to the basic building block in MFS transporters (GLUTs, SUTs), indicating that they may have evolved from an ancestral THB into a parallel configuration to produce 6/6+1 transmembrane-helix pores for SemiSWEETs/SWEETs, and an antiparallel configuration of 2×2 THBs to generate 12 transmembrane-helix pores for MFS transporters. Given the similarity of SemiSWEETs/SWEETs to PQ-loop amino acid transporters and mitochondrial MPC organic acid transporters, the structures characterized here may also be relevant for other MtN3 clan transporters7-9.

Netrins are secreted proteins that regulate axon guidance and neuronal migration. DCC is a well-established Netrin-1 receptor mediating attractive responses. We provide evidence that its close relative neogenin is also a functional Netrin-1 receptor that acts with DCC to mediate guidance in vivo. We determined the structures of a functional Netrin-1 region, alone and in complexes with neogenin or DCC. Netrin-1 has a rigid elongated structure containing two receptor-binding sites at opposite ends through which it brings together receptor molecules. The ligand/receptor complexes reveal two distinct architectures: a 2:2 heterotetramer and a continuous ligand/receptor assembly. The differences result from different lengths of the linker connecting receptor domains FN4 and FN5, which differs among DCC and neogenin splice variants, providing a basis for diverse signaling outcomes.

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Mitochondrial calcium uptake is critical for regulating ATP production, intracellular calcium signalling, and cell death. This uptake is mediated by a highly selective calcium channel called the mitochondrial calcium uniporter (MCU). Here, we determined the structures of the pore-forming MCU proteins from two fungi by X-ray crystallography and single-particle cryo-electron microscopy. The stoichiometry, overall architecture, and individual subunit structure differed markedly from those described in the recent nuclear magnetic resonance structure of Caenorhabditis elegans MCU. We observed a dimer-of-dimer architecture across species and chemical environments, which was corroborated by biochemical experiments. Structural analyses and functional characterization uncovered the roles of key residues in the pore. These results reveal a new ion channel architecture, provide insights into calcium coordination, selectivity and conduction, and establish a structural framework for understanding the mechanism of mitochondrial calcium uniporter function.

The light aroma type liquor is widely welcomed by consumers due to its pleasant fruity and floral aroma, particularly in northern China. To answer the puzzling question of which key aroma compounds are responsible for the typical aroma, three typical liquors were studied in this paper. A total of 66 aroma compounds were identified in three liquors by means of gas chromatography-olfactometry (GC-O) coupled with mass spectrometry (MS), and 27 odorants were further screened out as the important odorants according to quantitative study and odor activity values (OAVs). For OAV calculation, odor thresholds of the odorants were determined in a hydroalcoholic solution at 46% ethanol by volume. The typical light type aroma dominated by fruity and floral notes was successfully simulated by dissolving these important odorants in the 46% vol hydroalcoholic solution in their natural concentrations. Omission experiments further confirmed β-damascenone and ethyl acetate as the key odorants and revealed the significance of the entire group of esters, particularly ethyl lactate, geosmin, acetic acid, and 2-methylpropanoic acid, for the overall aroma of the light aroma type Chinese liquor.

The volatile and semi-volatile compounds of Chinese rice wines were extracted by headspace solid phase microextraction (HS-SPME) and analyzed by gas chromatography-mass spectrum (GC-MS). The original Chinese rice wine samples were diluted with deionized water to a final concentration of 6% (v/v) ethanol and then extracted by HS-SPME. The samples were pre-equilibrated at 50°C for 15 min and extracted with stirring at the same temperature for 45 min prior to injection into a GC-MS. A total of 97 volatile and semi-volatile compounds were identified from ten Chinese typical rice wine samples, including 13 alcohols, 8 acids, 28 esters, 4 aldehydes and ketones, 17 aromatic compounds, 3 lactones, 6 phenols, 3 sulphides, 9 furans and 6 nitrogen-containing compounds, of which, 39 compounds were firstly reported in Chinese rice wine. By stepwise linear discrimination analysis, the ten Chinese rice wine samples could be classified into three groups according to the production area, and in particular, the production technologies.