2.A.123 The Sweet; PQ-loop; Saliva; MtN3 (Sweet) Family
The eukaryotic proteins of the SWEET family are found in plants, animals, protozoans, bacteria, etc. They have 7 TMSs in a 3+1+3 repeat arrangement. These proteins appear to catalyze facilitated diffusion (entry or export) of sugars across the plant plasma membrane or the endoplasmic reticulum membrane (Takanaga and Frommer, 2010). Plant sweets fall into four subclades (Chen et al., 2010). The tomato genome encodes 29 SWEETs. Feng et al. 2015 analyzed the structures, conserved domains, and
phylogenetic relationships of these proteins, and also analyzed the transcript
levels of SWEET genes in various tissues, organs, and developmental stages in
response to exogenous sugar and adverse environmental stress (e.g., high and low temperatures). They phylogeny of SWEETS has been described (Jia et al. 2017). A database (dbSWEET) of SWEET homologues is freely available to the scientific community at http://bioinfo.iitk.ac.in/bioinfo/dbSWEET/Home (Gupta and Sankararamakrishnan 2018).
On average, angiosperm genomes contain approximately 20 SWEET paralogs, most of which serve distinct
physiological roles. In Arabidopsis, AtSWEET8 and 13 feed the pollen; SWEET 11 and 12 provide sucrose
to MFS-type sucrose transporters for phloem loading; AtSWEET11, 12 and 15 have distinct roles in seed filling; AtSWEET16
and 17 are vacuolar hexose transporters; and SWEET9 is essential for nectar secretion (Eom et al. 2015). The remaining
family members await characterization, and could play roles in the gametophyte and elsewhere in the plant. In rice and cassava, and possibly other systems,
sucrose transporting SWEETs play central roles in pathogen resistance.
Sugar efflux transporters are essential for the maintenance of animal blood glucose levels, plant nectar production, and plant seed and pollen development. Chen et al. (2010) reviewed evidence for a new class of sugar transporters, named SWEETs. At least six out of seventeen Arabidopsis, two out of over twenty rice and two out of seven homologues in Caenorhabditis elegans, and the single copy human protein, mediate glucose transport. Arabidopsis SWEET8 is essential for pollen viability, and the rice homologues SWEET11 and SWEET14 are specifically exploited by bacterial pathogens for virulence by means of direct binding of a bacterial effector to the SWEET promoter. Bacterial symbionts and fungal and bacterial pathogens induce the expression of different SWEET genes, indicating that the sugar efflux function of SWEET transporters is targeted by pathogens and symbionts for nutritional gain. The metazoan homologues may be involved in sugar efflux from intestinal, liver, epididymis and mammary cells.
Plants transport fixed carbon predominantly as sucrose, which is produced in mesophyll cells and imported into phloem cells for translocation throughout the plant. It had not been known how sucrose migrates from sites of synthesis in the mesophyll to the phloem, or which cells mediate efflux into the apoplasm as a prerequisite for phloem loading by the SUT sucrose-H+ (proton) cotransporters. Using optical sucrose sensors, Chen et al. (2012) identified a subfamily of SWEET sucrose efflux transporters. AtSWEET11 and 12 localize to the plasma membrane of the phloem. Mutant plants carrying insertions in AtSWEET11 and 12 are defective in phloem loading, thus revealing a two-step mechanism of SWEET-mediated export from parenchyma cells feeding H+-coupled import into the sieve element-companion cell complex. Restriction of intercellular transport to the interface of adjacent phloem cells may be an effective mechanism to limit the availability of photosynthetic carbon in the leaf apoplasm in order to prevent pathogen infections.
Many bacterial homologues (semisweets) have only 3 TMSs and are half sized, but they nevertheless are members of the MtN3 family with a single 3 TMS repeat unit per polypeptide chain. Other bacterial homologues have 7 TMSs as do most eukaryotic proteins in this family. The SWEET family is large and diverse. These semisweet proteins probably all function as dimeric carriers. The prokaryotic members of this family have been studied and reviewed (Jia et al. 2018).
Arabidopsis SWEETs homo- and heterooligomerize. Xuan et al., (2013) examined mutant SWEET variants for negative dominance to test if oligomerization is necessary for function. Mutation of the conserved Y57 or G58 residues 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, is a 3-TM unit and that a functional transporter contains at least four such domains.
Plant SWEETs play crucial roles in cellular sugar efflux processes: phloem loading, pollen nutrition and nectar secretion. Bacterial SemiSWEETs, consist of a triple-helix bundle, form semi-symmetrical, parallel dimers, thereby generating the translocation pathway. Two SemiSWEET isoforms have been 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 (Xu et al., 2014). In SemiSWEETs and SWEETs, two triple-helix bundles are arranged in a parallel configuration to produce the 6- and (3 + 1 + 3) -transmembrane-helix pores, respectively. Given the similarity of SemiSWEETs and SWEETs to PQ-loop amino acid transporters and to mitochondrial pyruvate carriers (MPCs), the structures characterized by Xu et al., 2014 may also be relevant to other transporters in the SWEET clan.
Latorraca et al. 2017; captured the translocationprocess by crystallography and unguided molecular dynamics simulations, providing an atomic-level description of alternating access transport. Simulations of a SWEET-family transporter initiated from an outward-open, glucose-bound structure spontaneously adopts occluded and inward-open conformations matching crystal structures. Mutagenesis experiments validated simulation predictions suggesting that state transitions are driven by favorable interactions formed upon closure of extracellular and intracellular 'gates' and by an unfavorable transmembrane helix configuration when both gates are closed. This mechanism leads to tight allosteric coupling between gates, preventing them from opening simultaneously. The substrate appears to take a 'free ride' across the membrane without causing major structural rearrangements in the transporter.
Plant SWEET sugar transporters play roles in phloem transport, nectar secretion, pollen nutrition, stress tolerance, and plant-pathogen interactions (Gao et al. 2017). Fify nine family members have been identified in wheat. Phylogenetic relationships, numbers of TMSs, gene structures, and motifs showed that TaSWEETs have 3-7 TMSs fall into four clades with 10 different types of motifs. Examination of the expression patterns of 18 SWEET genes revealed that a few are tissue-specific while most are ubiquitously expressed. Using a stem rust-susceptible cultivar, 'Little Club' (LC) the expression of five SWEETs tested induced following inoculation (Gao et al. 2017). Sugar is transported via SWEETS and semi-SWEETS from the extracellular side (via an outward-open state) to the
intracellular side (inward-open state) through an intermediate occluded
state with both extracellular and intracellular gates closed (Bera and Klauda 2018).
The SWEET family is a member of the TOG superfamily, which is believed to have arisen via the pathway:
2 TMSs --> 4 TMSs --> 8 TMSs --> 7 TMSs --> 3 + 3 TMSs (Shlykov et al. 2012; Yee et al. 2013).
The generalized reation catalyzed by known proteins of this family is:
sugars (in) ⇌ sugars (out)