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.evolution | transporter structure

A Bacterial Iron Exporter for Maintenance of Iron Homeostasis

“…The bacterial K ϩ channel contains 8 ␣-helices resulting from tetramerization of identical subunits with 2 helices each (31). The bacterial SemiSWEET sugar transport protein family contains 3 helical domains and oligomerizes to form a channel (32).…”

Glucose-Induced Trophic Shift in an Endosymbiont Dinoflagellate with Physiological and Molecular Consequences

“…Three of these transporters (s6_28295, s6_26300, and s6_19007) are of the SWEET type (Fig. 6B) and contain the highly conserved amino acids known to be critical for transporter function, including the conserved Asn that is associated with Glc binding ) and a Tyr located in the putative intrafacial gate (Xuan et al, 2013;Tao et al, 2015; Fig. 6B, stars).…”

Crosstalk between Two bZIP Signaling Pathways Orchestrates Salt-Induced Metabolic Reprogramming in Arabidopsis Roots

“…In agreement with this finding, MALATE ENZYME, which is associated with the glyoxylate cycle, was also downregulated (Supplemental Data Set 1). Finally, SWEET4 was found as one of the strongest misregulated genes, belonging to a gene family encoding sugar transporters (Chen et al, 2012;Xuan et al, 2013) (Table 1). Saltinduced expression profiles of selected genes (PPDK and FBP) were confirmed by RT-qPCR ( Figures 3D and 3E).…”

“…Moreover, sink-driven transport processes to the root may occur (Ludewig and Flügge, 2013). SWEET4 and SWEET2 are examples of genes misregulated in bzip1 bzip53 that encode putative sugar transporters (Chen et al, 2012;Xuan et al, 2013). In a second phase, the transient increase in Glc, Fru, and Suc returns to initial levels around 24 h after onset of stress.…”