title: Functional role of oligomerization for bacterial and plant SWEET sugar transporter family.
abstract: Eukaryotic sugar transporters of the MFS and SWEET superfamilies consist of 12 and 7 α-helical transmembrane domains (TMs), respectively. Structural analyses indicate that MFS transporters evolved from a series of tandem duplications of an ancestral 3-TM unit. SWEETs are heptahelical proteins carrying a tandem repeat of 3-TM separated by a single TM. Here, we show that prokaryotes have ancestral SWEET homologs with only 3-TM and that the Bradyrhizobium japonicum SemiSWEET1, like Arabidopsis SWEET11, mediates sucrose transport. Eukaryotic SWEETs most likely evolved by internal duplication of the 3-TM, suggesting that SemiSWEETs form oligomers to create a functional pore. However, it remains elusive whether the 7-TM SWEETs are the functional unit or require oligomerization to form a pore sufficiently large to allow for sucrose passage. Split ubiquitin yeast two-hybrid and split GFP assays indicate that Arabidopsis SWEETs homo- and heterooligomerize. We examined mutant SWEET variants for negative dominance to test if oligomerization is necessary for function. Mutation of the conserved Y57 or G58 in SWEET1 led to loss of activity. Coexpression of the defective mutants with functional A. thaliana SWEET1 inhibited glucose transport, indicating that homooligomerization is necessary for function. Collectively, these data imply that the basic unit of SWEETs, similar to MFS sugar transporters, is a 3-TM unit and that a functional transporter contains at least four such domains. We hypothesize that the functional unit of the SWEET family of transporters possesses a structure resembling the 12-TM MFS structure, however, with a parallel orientation of the 3-TM unit.
authors.affiliation: Department of Plant Biology, Carnegie Institution for Science, Stanford, CA 94305.
authors.forename: Yuan Hu
authors.initials: YH
authors.lastname: Xuan
authors.forename: Yi Bing
authors.initials: YB
authors.lastname: Hu
authors.forename: Li-Qing
authors.initials: LQ
authors.lastname: Chen
authors.forename: Davide
authors.initials: D
authors.lastname: Sosso
authors.forename: Daniel C
authors.initials: DC
authors.lastname: Ducat
authors.forename: Bi-Huei
authors.initials: BH
authors.lastname: Hou
authors.forename: Wolf B
authors.initials: WB
authors.lastname: Frommer
mesh_terms: D000596:Amino Acids / Q000378:metabolism; D017360:Arabidopsis / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism*; D001692:Biological Transport; D020369:Bradyrhizobium / Q000378:metabolism; D050260:Carbohydrate Metabolism; D005816:Genetic Complementation Test; D005947:Glucose / Q000378:metabolism; D049452:Green Fluorescent Proteins / Q000378:metabolism; D026901:Membrane Transport Proteins / Q000737:chemistry / Q000378:metabolism*; D008954:Models, Biological; D005810:Multigene Family; D010802:Phylogeny; D010940:Plant Proteins / Q000737:chemistry / Q000378:metabolism*; D055503:Protein Multimerization; D017433:Protein Structure, Secondary; D012441:Saccharomyces cerevisiae / Q000378:metabolism; D015398:Signal Transduction; D013329:Structure-Activity Relationship; D013395:Sucrose / Q000378:metabolism*
chemical_list: D000596:Amino Acids; D001426:Bacterial Proteins; D026901:Membrane Transport Proteins; D010940:Plant Proteins; D049452:Green Fluorescent Proteins; D013395:Sucrose; D005947:Glucose
keywords: evolution; transporter structure
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