Structures of bacterial homologues of SWEET transporters in two distinct conformations

The X-ray crystal structures of two bacterial homologues of the SWEET sugar transporters are solved in two conformational states, and comparison of these states suggests that transport occurs via a ‘rocker-switch’ mechanism. There are many similarities between the sugar transporter proteins used by prokaryotes, plants and animals. This paper presents the X-ray crystal structures of two bacterial homologues of the SWEET sugar transporters that are involved in various sugar efflux processes (for example, phloem loading, pollen nutrition, and nectar secretion) in plants in and glucose transport in animals. The overall three-dimensional structures of both proteins — SemiSWEET from Vibrio sp. in an outward-facing, open state and Leptospira biflexa SemiSWEET in the occluded state — differ markedly from those of the MFS and SGLT sugar transporter superfamilies but all three transporter types feature a triple-helix bundle . Comparisons of the conformations of the open and occluded states of SemiSWEET suggest that transport occurs via a 'rocker-switch' mechanism. 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.