Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Structures of bacterial homologues of SWEET transporters in two distinct conformations


SWEETs and their prokaryotic homologues are monosaccharide and disaccharide transporters that are present from Archaea to plants and humans1,2,3. SWEETs play crucial roles in cellular sugar efflux processes: that is, in phloem loading4, pollen nutrition5 and nectar secretion6. Their bacterial homologues, which are called SemiSWEETs, are among the smallest known transporters1,3. Here we show that SemiSWEET molecules, which consist of a triple-helix bundle, form symmetrical, parallel dimers, thereby generating the translocation pathway. Two SemiSWEET isoforms were crystallized, one in an apparently open state and one in an occluded state, indicating that SemiSWEETs and SWEETs are transporters that undergo rocking-type movements during the transport cycle. The topology of the triple-helix bundle is similar yet distinct to that of the basic building block of animal and plant major facilitator superfamily (MFS) transporters (for example, GLUTs and SUTs). This finding indicates two possibilities: that SWEETs and MFS transporters evolved from an ancestral triple-helix bundle or that the triple-helix bundle represents convergent evolution. In SemiSWEETs and SWEETs, two triple-helix bundles are arranged in a parallel configuration to produce the 6- and 6 + 1-transmembrane-helix pores, respectively. In the 12-transmembrane-helix MFS transporters, four triple-helix bundles are arranged into an alternating antiparallel configuration, resulting in a much larger 2 × 2 triple-helix bundle forming the pore. Given the similarity of SemiSWEETs and SWEETs to PQ-loop amino acid transporters and to mitochondrial pyruvate carriers (MPCs), the structures characterized here may also be relevant to other transporters in the MtN3 clan7,8,9. The insight gained from the structures of these transporters and from the analysis of mutations of conserved residues will improve the understanding of the transport mechanism, as well as allow comparative studies of the different superfamilies involved in sugar transport and the evolution of transporters in general.

This is a preview of subscription content, access via your institution

Relevant articles

Open Access articles citing this article.

Access options

Rent or buy this article

Prices vary by article type



Prices may be subject to local taxes which are calculated during checkout

Figure 1: Structure of Vibrio sp. SemiSWEET.
Figure 2: Structure of L. biflexa SemiSWEET.
Figure 3: Glucose transport by L. biflexa SemiSWEET and A. thaliana SWEET1.
Figure 4: Alternating access by the rocking-type movement of two protomers.

Accession codes

Primary accessions

Protein Data Bank

Data deposits

Crystallographic coordinates and structure factors of L. biflexa SemiSWEET and Vibrio sp. SemiSWEET have been deposited in the Protein Data Bank database under accession numbers 4QNC and 4QND, respectively.


  1. Sosso, D., Chen, L. Q. & Frommer, W. B. in Encyclopedia of Biophysics Vol. 5 (ed. Roberts, G. ) 2556–2558 (Springer, 2013)

    Google Scholar 

  2. Chen, L. Q. et al. Sugar transporters for intercellular exchange and nutrition of pathogens. Nature 468, 527–532 (2010)

    Article  ADS  CAS  Google Scholar 

  3. Xuan, Y. H. et al. Functional role of oligomerization for bacterial and plant SWEET sugar transporter family. Proc. Natl Acad. Sci. USA 110, E3685–E3694 (2013)

    Article  CAS  Google Scholar 

  4. Chen, L. Q. et al. Sucrose efflux mediated by SWEET proteins as a key step for phloem transport. Science 335, 207–211 (2012)

    Article  ADS  CAS  Google Scholar 

  5. Sun, M. X., Huang, X. Y., Yang, J., Guan, Y. F. & Yang, Z. N. Arabidopsis RPG1 is important for primexine deposition and functions redundantly with RPG2 for plant fertility at the late reproductive stage. Plant Reprod. 26, 83–91 (2013)

    Article  CAS  Google Scholar 

  6. Lin, I. W. et al. Nectar secretion requires sucrose phosphate synthases and the sugar transporter SWEET9. Nature 508, 546–549 (2014)

    Article  ADS  CAS  Google Scholar 

  7. Jezegou, A. et al. Heptahelical protein PQLC2 is a lysosomal cationic amino acid exporter underlying the action of cysteamine in cystinosis therapy. Proc. Natl Acad. Sci. USA 109, E3434–E3443 (2012)

    Article  CAS  Google Scholar 

  8. Herzig, S. et al. Identification and functional expression of the mitochondrial pyruvate carrier. Science 337, 93–96 (2012)

    Article  ADS  CAS  Google Scholar 

  9. Bricker, D. K. et al. A mitochondrial pyruvate carrier required for pyruvate uptake in yeast, Drosophila, and humans. Science 337, 96–100 (2012)

    Article  ADS  CAS  Google Scholar 

  10. Wright, E. M. Glucose transport families SLC5 and SLC50. Mol. Aspects Med. 34, 183–196 (2013)

    Article  CAS  Google Scholar 

  11. Cura, A. J. & Carruthers, A. Role of monosaccharide transport proteins in carbohydrate assimilation, distribution, metabolism, and homeostasis. Compr. Physiol. 2, 863–914 (2012)

    PubMed  PubMed Central  Google Scholar 

  12. Lalonde, S., Wipf, D. & Frommer, W. B. Transport mechanisms for organic forms of carbon and nitrogen between source and sink. Annu. Rev. Plant Biol. 55, 341–372 (2004)

    Article  CAS  Google Scholar 

  13. Kumar, H. et al. Structure of sugar-bound LacY. Proc. Natl Acad. Sci. USA 111, 1784–1788 (2014)

    Article  ADS  CAS  Google Scholar 

  14. Deng, D. et al. Crystal structure of the human glucose transporter GLUT1. Nature 510, 121–125 (2014)

    Article  ADS  CAS  Google Scholar 

  15. Abramson, J. & Wright, E. M. Structure and function of Na+-symporters with inverted repeats. Curr. Opin. Struct. Biol. 19, 425–432 (2009)

    Article  CAS  Google Scholar 

  16. Antony, G. et al. Rice xa13 recessive resistance to bacterial blight is defeated by induction of the disease susceptibility gene Os-11N3 . Plant Cell 22, 3864–3876 (2010)

    Article  CAS  Google Scholar 

  17. Hamada, M., Wada, S., Kobayashi, K. & Satoh, N. Ci-Rga, a gene encoding an MtN3/saliva family transmembrane protein, is essential for tissue differentiation during embryogenesis of the ascidian Ciona intestinalis . Differentiation 73, 364–376 (2005)

    Article  CAS  Google Scholar 

  18. Yan, N. Structural advances for the major facilitator superfamily (MFS) transporters. Trends Biochem. Sci. 38, 151–159 (2013)

    Article  CAS  Google Scholar 

  19. von Heijne, G. & Gavel, Y. Topogenic signals in integral membrane proteins. Eur. J. Biochem. 174, 671–678 (1988)

    Article  CAS  Google Scholar 

  20. Niittylä, T., Fuglsang, A. T., Palmgren, M. G., Frommer, W. B. & Schulze, W. X. Temporal analysis of sucrose-induced phosphorylation changes in plasma membrane proteins of Arabidopsis . Mol. Cell. Proteomics 6, 1711–1726 (2007)

    Article  Google Scholar 

  21. Ponting, C. P., Mott, R., Bork, P. & Copley, R. R. Novel protein domains and repeats in Drosophila melanogaster: insights into structure, function, and evolution. Genome Res. 11, 1996–2008 (2001)

    Article  CAS  Google Scholar 

  22. Zhai, Y., Heijne, W. H., Smith, D. W. & Saier, M. H., Jr Homologues of archaeal rhodopsins in plants, animals and fungi: structural and functional predications for a putative fungal chaperone protein. Biochim. Biophys. Acta 1511, 206–223 (2001)

    Article  CAS  Google Scholar 

  23. Henderson, P. J., Giddens, R. A. & Jones-Mortimer, M. C. Transport of galactose, glucose and their molecular analogues by Escherichia coli K12. Biochem. J. 162, 309–320 (1977)

    Article  CAS  Google Scholar 

  24. Forrest, L. R., Kramer, R. & Ziegler, C. The structural basis of secondary active transport mechanisms. Biochim. Biophys. Acta 1807, 167–188 (2011)

    Article  CAS  Google Scholar 

  25. Kaback, H. R., Smirnova, I., Kasho, V., Nie, Y. & Zhou, Y. The alternating access transport mechanism in LacY. J. Membr. Biol. 239, 85–93 (2011)

    Article  CAS  Google Scholar 

  26. Forrest, L. R. & Rudnick, G. The rocking bundle: a mechanism for ion-coupled solute flux by symmetrical transporters. Physiology 24, 377–386 (2009)

    Article  CAS  Google Scholar 

  27. Khare, D., Oldham, M. L., Orelle, C., Davidson, A. L. & Chen, J. Alternating access in maltose transporter mediated by rigid-body rotations. Mol. Cell 33, 528–536 (2009)

    Article  CAS  Google Scholar 

  28. De Zutter, J. K., Levine, K. B., Deng, D. & Carruthers, A. Sequence determinants of GLUT1 oligomerization: analysis by homology-scanning mutagenesis. J. Biol. Chem. 288, 20734–20744 (2013)

    Article  CAS  Google Scholar 

  29. Loqué, D., Lalonde, S., Looger, L. L., von Wirén, N. & Frommer, W. B. A cytosolic trans-activation domain essential for ammonium uptake. Nature 446, 195–198 (2007)

    Article  ADS  Google Scholar 

  30. Sun, J. et al. Crystal structure of the plant dual-affinity nitrate transporter NRT1.1. Nature 507, 73–77 (2014)

    Article  ADS  CAS  Google Scholar 

  31. Kabsch, W. XDS. Acta Crystallogr. D 66, 125–132 (2010)

    CAS  Google Scholar 

  32. Diederichs, K. & Karplus, P. A. Improved R-factors for diffraction data analysis in macromolecular crystallography. Nature Struct. Biol. 4, 269–275 (1997)

    Article  CAS  Google Scholar 

  33. Otwinowski, Z. & Minor, W. Processing of X-ray diffraction data collected in oscillation mode. Methods Enzymol. 276, 307–326 (1997)

    Article  CAS  Google Scholar 

  34. Schneider, T. R. & Sheldrick, G. M. Substructure solution with SHELXD. Acta Crystallogr. D 58, 1772–1779 (2002)

    Article  Google Scholar 

  35. Adams, P. D. et al. PHENIX: a comprehensive Python-based system for macromolecular structure solution. Acta Crystallogr. D 66, 213–221 (2010)

    CAS  Google Scholar 

  36. McCoy, A. J. Solving structures of protein complexes by molecular replacement with Phaser. Acta Crystallogr. D 63, 32–41 (2007)

    Article  CAS  Google Scholar 

  37. Cowtan, K. ‘dm': An automated procedure for phase improvement by density modification. Joint CCP4 and ESF-EACBM Newsletter on Protein Crystallography 31, 34–38 (1994)

    Google Scholar 

  38. Emsley, P. & Cowtan, K. Coot: model-building tools for molecular graphics. Acta Crystallogr. D 60, 2126–2132 (2004)

    Article  Google Scholar 

  39. Chen, V. B. et al. MolProbity: all-atom structure validation for macromolecular crystallography. Acta Crystallogr. D 66, 12–21 (2010)

    Article  CAS  Google Scholar 

  40. DeLano, W. L. The PyMOL molecular graphics system. (DeLano Scientific, Palo Alto, California, 2008)

  41. Sun, L. et al. Crystal structure of a bacterial homologue of glucose transporters GLUT1–4. Nature 490, 361–366 (2012)

    Article  ADS  CAS  Google Scholar 

  42. Wieczorke, R. et al. Concurrent knock-out of at least 20 transporter genes is required to block uptake of hexoses in Saccharomyces cerevisiae . FEBS Lett. 464, 123–128 (1999)

    Article  CAS  Google Scholar 

  43. Gietz, R. D. & Schiestl, R. H. Large-scale high-efficiency yeast transformation using the LiAc/SS carrier DNA/PEG method. Nature Protocols 2, 38–41 (2007)

    Article  CAS  Google Scholar 

Download references


We thank the staff at beamlines 23ID-B and 23ID-D (APS, Argonne National Laboratory) and S. Russi and the staff at beamlines 11-1 and 12-2 (SSRL, SLAC National Laboratory) for assistance at the synchrotrons. We thank the Kobilka laboratory for help and advice on the LCP. This work was made possible by support from Stanford University and the Harold and Leila Y. Mathers Charitable Foundation to L.F. and from the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences at the US Department of Energy (DOE) under grant number DE-FG02-04ER15542 to W.B.F. Part of this work is based upon research conducted at the APS on the Northeastern Collaborative Access Team beamlines, which are supported by a grant from the National Institute of General Medical Sciences (P41 GM103403) from the National Institutes of Health. Use of the APS, an Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory, was supported by the DOE under contract number DE-AC02-06CH11357.

Author information

Authors and Affiliations



Y.X., W.B.F. and L.F. conceived and designed experiments. Y.X. and Y.T. performed expression, purification, crystallization, data collection and crystallography. C.F. performed functional experiments. S.X. performed biochemical characterization. L.S.C. and L.-Q.C. performed alignments and functional experiments. K.P. performed data collection and assisted crystallography. L.F. contributed to crystallization, data collection and crystallography. Y.X., Y.T., L.S.C., C.F., L.-Q.C., W.B.F. and L.F. analysed the data. L.F. and W.B.F. wrote the manuscript.

Corresponding author

Correspondence to Liang Feng.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Sequence alignment.

The secondary structure is shown above the alignment. Protein sequences were aligned using ClustalW and manually adjusted. The N-terminal part (first THB) and C-terminal part (second THB) of A. thaliana SWEET1 were separately used for alignment and are labelled A. thaliana SWEET1-N and A. thaliana SWEET1-C, respectively.

Extended Data Figure 2 Vibrio sp. SemiSWEET dimer in the crystal.

Crystal packing in the lipid bilayer environment shows the formation of the dimer. One protomer is shown in green, and the other is shown in purple.

Extended Data Figure 3 Subunit stoichiometry of SemiSWEET.

a, Purified Vibrio sp. SemiSWEET (predicted molecular weight, 11.1 kDa) remains as a dimer after SDS–PAGE. b, Crosslinking of purified B. japonicum SemiSWEET (theoretical molecular weight, 10.1 kDa) in detergent solution. Increasing amounts of disuccinimidyl suberate (DSS; 0.02 mM, 0.2 mM and 2 mM) were used to crosslink B. japonicum SemiSWEET. The protein was analysed by SDS–PAGE and stained with Coomassie blue.

Extended Data Figure 4 Crystal packing of L. biflexa SemiSWEET.

The crystal lattice structure of L. biflexa SemiSWEET shows dimer formation in the membrane. One protomer is shown in red, and the other is shown in blue.

Extended Data Figure 5 Conserved tryptophan and asparagine residues in A. thaliana SWEET1 are required for transport activity.

a, Functional analysis of A. thaliana SWEET1–GFP (green fluorescent protein) fusion transport activity in the EBY4000 yeast strain. The point mutations W56A, N73A, W176A and N192A failed to complement the growth defect of EBY4000 in synthetic medium supplemented with 2% glucose as the only carbon source. Growth was unaffected in control medium containing 2% maltose. An empty vector and A. thaliana SWEET1–GFP were used as the negative and positive controls, respectively. bg, Membrane localization of the wild-type and mutant A. thaliana SWEET1 was confirmed by imaging of GFP. The empty vector negative control (b) shows the cytoplasmic localization of GFP, while the wild-type and W56A, N73A, W176A and N192A mutants (cg) show membrane localization, suggesting that the phenotype of the mutated transporters is not due to mislocalization.

Extended Data Figure 6 Transmembrane-helix arrangements in SWEET, MFS and G-protein-coupled receptor (GPCR) proteins.

Ribbon representations of a three-transmembrane-helix unit in a SWEET (left), a GPCR (centre) and an MFS (right) are shown (top), with their respective topologies (bottom). The same colour scheme is used for all three proteins.

Extended Data Figure 7 Proposed transport model and pathway for building SWEET and MFS transporters.

a, Alternating access model for SemiSWEET transport. One protomer is shown in purple, and the other is shown in green. The putative substrate is shown as an orange hexagon. b, Proposed pathway for constructing SemiSWEET, SWEET or MFS transporters from the primitive THB. The same colour scheme is used for all THBs. The position of TM3 relative to TM1 and TM2 of the THB in SWEET or MFS is illustrated by coloured circles, viewed from the N terminus. Note that the THB in SWEET and the THB in MFS are roughly mirror images of each other.

Extended Data Table 1 Data collection and refinement statistics

Supplementary information

Supplementary Information

This file contains a Supplementary Discussion and additional references. (PDF 79 kb)

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Xu, Y., Tao, Y., Cheung, L. et al. Structures of bacterial homologues of SWEET transporters in two distinct conformations. Nature 515, 448–452 (2014).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

This article is cited by


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing