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)



This family belongs to the Transporter-Opsin-G protein-coupled receptor (TOG) Superfamily.

 

References:

Bera, I. and J.B. Klauda. (2018). Structural Events in a Bacterial Uniporter Leading to Translocation of Glucose to the Cytosol. J. Mol. Biol. 430: 3337-3352.

Chen, L.Q., B.H. Hou, S. Lalonde, H. Takanaga, M.L. Hartung, X.Q. Qu, W.J. Guo, J.G. Kim, W. Underwood, B. Chaudhuri, D. Chermak, G. Antony, F.F. White, S.C. Somerville, M.B. Mudgett, and W.B. Frommer. (2010). Sugar transporters for intercellular exchange and nutrition of pathogens. Nature 468: 527-532.

Chen, L.Q., X.Q. Qu, B.H. Hou, D. Sosso, S. Osorio, A.R. Fernie, and W.B. Frommer. (2012). Sucrose efflux mediated by SWEET proteins as a key step for phloem transport. Science 335: 207-211.

Chu, Z., B. Fu, H. Yang, C. Xu, Z. Li, A. Sanchez, Y.J. Park, J.L. Bennetzen, Q. Zhang, and S. Wang. (2006). Targeting xa13, a recessive gene for bacterial blight resistance in rice. Theor Appl Genet 112: 455-461.

Eom, J.S., L.Q. Chen, D. Sosso, B.T. Julius, I.W. Lin, X.Q. Qu, D.M. Braun, and W.B. Frommer. (2015). SWEETs, transporters for intracellular and intercellular sugar translocation. Curr. Opin. Plant Biol. 25: 53-62.

Feng CY., Han JX., Han XX. and Jiang J. (2015). Genome-wide identification, phylogeny, and expression analysis of the SWEET gene family in tomato. Gene. 573(2):261-72.

Gao, Y., Z.Y. Wang, V. Kumar, X.F. Xu, P. Yuan, X.F. Zhu, T.Y. Li, B.L. Jia, and Y.H. Xuan. (2017). Genome-wide identification of the SWEET gene family in wheat. Gene. [Epub: Ahead of Print]

Ge, Y.X., G.C. Angenent, P.E. Wittich, J. Peters, J. Franken, M. Busscher, L.M. Zhang, E. Dahlhaus, M.M. Kater, G.J. Wullems, and T. Creemers-Molenaar. (2000). NEC1, a novel gene, highly expressed in nectary tissue of Petunia hybrida. Plant J. 24: 725-734.

Guan, Y.F., X.Y. Huang, J. Zhu, J.F. Gao, H.X. Zhang, and Z.N. Yang. (2008). RUPTURED POLLEN GRAIN1, a member of the MtN3/saliva gene family, is crucial for exine pattern formation and cell integrity of microspores in Arabidopsis. Plant Physiol. 147: 852-863.

Gupta, A. and R. Sankararamakrishnan. (2018). dbSWEET: An Integrated Resource for SWEET Superfamily to Understand, Analyze and Predict the Function of Sugar Transporters in Prokaryotes and Eukaryotes. J. Mol. Biol. [Epub: Ahead of Print]

Hamada, M., S. Wada, K. Kobayashi, and N. Satoh. (2005). 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.

Hamada, M., S. Wada, K. Kobayashi, and N. Satoh. (2007). Novel genes involved in Ciona intestinalis embryogenesis: characterization of gene knockdown embryos. Dev Dyn 236: 1820-1831.

Jia, B., L. Hao, Y.H. Xuan, and C.O. Jeon. (2018). New Insight Into the Diversity of SemiSWEET Sugar Transporters and the Homologs in Prokaryotes. Front Genet 9: 180.

Jia, B., X.F. Zhu, Z.J. Pu, Y.X. Duan, L.J. Hao, J. Zhang, L.Q. Chen, C.O. Jeon, and Y.H. Xuan. (2017). Integrative View of the Diversity and Evolution of SWEET and SemiSWEET Sugar Transporters. Front Plant Sci 8: 2178.

Kanno, Y., T. Oikawa, Y. Chiba, Y. Ishimaru, T. Shimizu, N. Sano, T. Koshiba, Y. Kamiya, M. Ueda, and M. Seo. (2016). AtSWEET13 and AtSWEET14 regulate gibberellin-mediated physiological processes. Nat Commun 7: 13245.

Latorraca, N.R., N.M. Fastman, A.J. Venkatakrishnan, W.B. Frommer, R.O. Dror, and L. Feng. (2017). Mechanism of Substrate Translocation in an Alternating Access Transporter. Cell 169: 96-107.e12.

Shlykov, M.A., W.H. Zheng, J.S. Chen, and M.H. Saier, Jr. (2012). Bioinformatic characterization of the 4-Toluene Sulfonate Uptake Permease (TSUP) family of transmembrane proteins. Biochim. Biophys. Acta. 1818: 703-717.

Takanaga, H. and W.B. Frommer. (2010). Facilitative plasma membrane transporters function during ER transit. FASEB J. 24: 2849-2858.

Tao, Y., L.S. Cheung, S. Li, J.S. Eom, L.Q. Chen, Y. Xu, K. Perry, W.B. Frommer, and L. Feng. (2015). Structure of a eukaryotic SWEET transporter in a homotrimeric complex. Nature 527: 259-263.

Wang, L., L. Yao, X. Hao, N. Li, W. Qian, C. Yue, C. Ding, J. Zeng, Y. Yang, and X. Wang. (2018). Tea plant SWEET transporters: expression profiling, sugar transport, and the involvement of CsSWEET16 in modifying cold tolerance in Arabidopsis. Plant Mol. Biol. 96: 577-592.

Wu, Z., K.M. Soliman, J.J. Bolton, S. Saha, and J.N. Jenkins. (2008). Identification of differentially expressed genes associated with cotton fiber development in a chromosomal substitution line (CS-B22sh). Funct Integr Genomics 8: 165-174.

Xu Y., Tao Y., Cheung LS., Fan C., Chen LQ., Xu S., Perry K., Frommer WB. and Feng L. (2014). Structures of bacterial homologues of SWEET transporters in two distinct conformations. Nature. 515(7527):448-52.

Xuan, Y.H., Y.B. Hu, L.Q. Chen, D. Sosso, D.C. Ducat, B.H. Hou, and W.B. Frommer. (2013). Functional role of oligomerization for bacterial and plant SWEET sugar transporter family. Proc. Natl. Acad. Sci. USA 110: E3685-3694.

Yang, B., A. Sugio, and F.F. White. (2006). Os8N3 is a host disease-susceptibility gene for bacterial blight of rice. Proc. Natl. Acad. Sci. USA 103: 10503-10508.

Yee, D.C., M.A. Shlykov, A. Västermark, V.S. Reddy, S. Arora, E.I. Sun, and M.H. Saier, Jr. (2013). The transporter-opsin-G protein-coupled receptor (TOG) superfamily. FEBS J. 280: 5780-5800.

Zhang, G., S.S. Liu, X.J. Yang, Y. Chen, L.L. Liu, and S.X. Guo. (2016). [Molecular cloning and characterization of a novel DoSWEET1 gene from Dendrobium officinale]. Yao Xue Xue Bao 51: 991-997.

Zhu, L.Q., Z.K. Bao, W.W. Hu, J. Lin, Q. Yang, and Q.H. Yu. (2015). Cloning and functional analysis of goat SWEET1. Genet Mol Res 14: 17124-17133.

Examples:

TC#NameOrganismal TypeExample
2.A.123.1.1

Alfalfa Nodulin MtN3

Plants

MtN3 of Medicago truncatula (P93332)

 
2.A.123.1.10

Golgi/E.R. Sweet1 glucose/galactose facilitator (Km ≥ 50mM) (Chen et al. 2010)

Animals

Sweet1 of Caenorhabditis elegans (O45102)

 
2.A.123.1.11

The sea squirt sugar transporter, Rga or Sweet1; required for normal development (Hamada et al., 2007; Chen et al., 2010).

Animals

Rga of Ciona intestinalis (F6U696)

 
2.A.123.1.12Sugar transporter SWEET1 (Protein saliva)Animals

Slv of Drosophila melanogaster

 
2.A.123.1.13

Bidirectional sugar (sucrose) transporter SWEET11 (AtSWEET11).  Oligomerization, probably to the dimeric form, has been demonstrated (Xuan et al. 2013). Important for phloem loading.

Plants

SWEET11 of Arabidopsis thaliana

 
2.A.123.1.14Sugar transporter SWEET1Amoebaslc50a1 of Dictyostelium discoideum
 
2.A.123.1.15

SWEET homologue of 375 aas and 7 TMSs in a 3 + 4 arrangement.

Stramenopiles (Marine diatom)

SWEET homologue of Phaeodactylum tricornutum

 
2.A.123.1.16

Uncharacterized protein of 262 aas and 7 TMSs

Rhodophyta (Algae)

UP of Galdieria sulphuraria (Red alga)

 
2.A.123.1.17

MtN3-like protein of 686 aas and 7-8 TMSs

Alveolata

MtN3-like protein of Plasmodium falciparum

 
2.A.123.1.18

SWEET2b sugar transporter. Sequesters sugars in root vacuoles.  The 3-d structure is known. The subunit consists of two asymetic triple helix bundles (TMSs 1-3 and 5-7) connected by TMS4. SWEET2b is in an apparent inward (cytosolic) open state forming homomeric trimers. TMS4 tightly interacts with the first triple-helix bundle within a protomer and mediates key contacts among protomers (Tao et al. 2015).

SWEET2b of Oryza sativa

 
2.A.123.1.19

Sweet1 (SLC50A1).  99.6% identical to the goat (Capra hircus) mammary gland epithelial homologue which has been characterized (Zhu et al. 2015).

Sweet1 of Ovis aries (sheep)

 
2.A.123.1.2

Sweet family member of 305 aas and 7 TMSs.  Mediates both low-affinity uptake and efflux of sugars across the membrane. (Wu et al., 2008)

Plants

Sweet of Citrus clementina

 
2.A.123.1.20

Uncharacterized protein of 197 aas and 7 TMSs

UP of Acidimicrobium sp. BACL27

 
2.A.123.1.21

Uncharacterized duplicated protein of 709 aas and 15 TMSs in a 7 + 7 + 1 arrangement. The protein contains two 7 TMS Sweet domains followed by an Atrophin-1 domain. There is no close homologue in the NCBI database, suggesting that it could be a result of a sequencing artifact.

UP of Ananas comosus

 
2.A.123.1.22

Uncharacterized protein of 1089 aas and 28 TMSs in a 7 + 7 + 7 + 7 arrangement.

UP of Phytophthora ramorum (Sudden oak death agent)

 
2.A.123.1.23

Vacuolar texose transporter of 230 aas and 7 TMSs, Sweet16 (Eom et al. 2015). It plays an iimportant role in cold tolerance (Wang et al. 2018).

Sweet16 of Arabidopsis thaliana (Mouse-ear cress)

 
2.A.123.1.24

Sweet9 of 258 aas and 7 TMSs.  Important for nectar secretion (Eom et al. 2015).

Sweet9 of Arabidopsis thaliana (Mouse-ear cress)

 
2.A.123.1.25

Sweet family member of 89 aas and 3 TMSs.  Associated with a trehalose phosphatase, possibly suggesting al role in trehalose transport.

Semi-sweet of Methanobacterium lacus

 
2.A.123.1.26

Sweet13 of 294 aas and 7 TMSs.  Transports the plant hormone, gibberellin (GA).  Sweet14 also transports gibberellin.  A double mutant has a defect in anther dehiscence. This mutant also exhibits altered long distant transport of exogenously applied GA and altered responses to GA during germination and seedling stages (Kanno et al. 2016)

SWEET13 of Arabidopsis thaliana (Mouse-ear cress)

 
2.A.123.1.27

SWEET transporter 1 of 262 aas and 7 TMSs. Plays a role in the D. officinale symbiotic germination process (Zhang et al. 2016).

Sweet transporter of Dendrobium officinale

 
2.A.123.1.3

Senescence-associated protein 29, SAG29 (SWEET15)

Plants

SAG29 of Arabidopsis thaliana (Q9FY94)

 
2.A.123.1.4

Stromal cell protein (SCP) homologue, HsSWEET1 or RAG1AP1. Transports glucose and galactose bidirectionally. Present in the ER, Golgi and plasma membrane (Chen et al., 2010).

Animals

SLC50A1 of Homo sapiens

 
2.A.123.1.5

Ruptured pollen Grain-1, Sweet8 or Nodulin MtN3 family protein (essential for pollen viability). (Guan et al., 2008; Chen et al. 2010).

Plants

RPGI of Arabidoposis thaliana (Q8LFH5)

 
2.A.123.1.6

Host disease susceptible protein, Xa13 or Os8N3, for bacterial blight (Yang et al., 2006; Chu et al., 2006). Bidirectional sugar transporter, Sweet 11 (Chen et al., 2010)

Plants

Oryza sativa (Q6YZF3)

 
2.A.123.1.7

Nec1; predominantly expressed in nectaries; involved in sugar metabolism and nectar secretion (Ge et al., 2000)

Plants

Nec1 of Petunia hybrida (Q9FPN0)

 
2.A.123.1.8

Rga (Recombination-activating gene 1) (Hamada et al., 2005)

Animals

Rga of Mus musculus (Q9CXK4)

 
2.A.123.1.9

Sweet1: bidirectional low affinity glucose uniporter, Km = ~9 mM (Does not transport mannose, fructose or galactose) (Chen et al. 2010). The structure is know, and three regions, each containing several well conserved essential residues, comprise the substrate-binding pocket, the extrafacial gate, and the intrafacial gate (Xuan et al. 2013; Tao et al. 2015).  The orthologous SWEET1 in Camellia sinensis (tea) transports glucose, glucose analogues, and other hexoses (Wang et al. 2018).

 

 

 

Plants

Sweet1 of Arabidopsis thalinana (Q8L9J7)

 
Examples:

TC#NameOrganismal TypeExample
2.A.123.2.1

The 7 TMS (242aa) bacterial MtN3 homologue

Bacteria

MtN3 homologue of Mycoplasma arthritidis (B3PMT4)

 
2.A.123.2.10

SWEET homologue of 125 aas and 3 TMSs; resembles 2.A.123.2.3 with all 3 TMSs overlapping.

SWEET homologue of Phytophthora sojae (Soybean stem and root rot agent) (Phytophthora megasperma f. sp. glycines)

 
2.A.123.2.11

SWEET homologue of 125 aas and 3 TMSs.  Closely related to 2.A.123.2.10.

SWEET homologue of Phytophthora parasitica

 
2.A.123.2.12

SWEET homologue of 84 aas and 3 TMSs

SWEET homologue of Methanocella conradii

 
2.A.123.2.13

Uncharacterized protein of 728 aas and 5 putative TMSs with 4 N-terminal TMSs, where the first 3 are homologous to semisweets of 3 TMSs. The long sequence with one large centrally located peak of hydrophobicity includes several recognized protein domains following the SWEET domain in the following order:  Cache-3 - Cache-1 - dimerization interface domain - HAMP domain - followed by two PAS domains.  Another protein (UniProt acc #I3IJJ5 of 762 aas), has residues 3 - 635 aas) showing 83% sequence identity with residues 96 - 728 in 2.A.123.2.13.

UP of Candidatus Jettenia caeni

 
2.A.123.2.14

SemiSWEET of 86 aas and 3 TMSs specific for sucrose. The basic unit of SWEETs may be a 3-TMS unit, and it has been suggested that a functional transporter contains at least four such domains, although this suggestion has not been substantiated (Xuan et al. 2013).

SemiSWEET of Bradyrhizobium diazoefficiens

 
2.A.123.2.15

Putative uncharacterized protein of 99 aas and 3 TMSs

Hypothetical protein UW38 of Candidatus Saccharibacteria bacterium

 
2.A.123.2.2

3 TMS MtN3 homologue (85aas)

Bacteria

MtN3 of MtN3 of Prochlorococcus marinus (A2BS89)

 
2.A.123.2.3

Half sized (3 TMS) bacterial MtN3 protein homologue (85aas)

Bacteria

MtN3 homologue of Fusobacterium mortiferum (C3WG44)

 
2.A.123.2.4

3 TMS bacterial MtN3 homologue (96aas)

Bacteria

MtN3 homologue of Leptospira interrogans (Q8F4F7)

 
2.A.123.2.5

3 TMS Sweet homologue, MJ_0110 (93aas)

Archaea

MJ_0110 of Methanocaldococcus jannashii (Q57574)

 
2.A.123.2.6

SemiSWEET half glucose transporter of 93 aas and 3 TMSs with an N-terminal amphipathic α-helix.  The protein occurs as a tight homodimer with the translocation channel between the two monomers.  The 3-d structure is known at 2.4 Å resolution revealing the outward open conformation (Xu et al. 2014). The occluded state of the Vibrio sp. N418 SemiSWEET (9.A.58.3.1) has been solved at 1.7 Å resolution (Xu et al. 2014).  The presence of these two states argues in favor of a carrier (rocker switch) mechanism rather than a channel-type mechanism (Xu et al. 2014).

Spirochaetes

SemiSWEET of Leptospira biflexa

 
2.A.123.2.7

SemiSWEET homologue of 89 aas and 3 TMSs

Proteobacteria

SemiSWEET of Rickettsia bellii

 
2.A.123.2.8

SWEET homologue of 141 aas and 4 putative TMSs.

SWEET homologue of Anabaena variabilis

 
2.A.123.2.9

SWEET homologue of 231 aas and 7 TMSs

SWEET homologue of Mycoplasma hyopneumoniae

 
Examples:

TC#NameOrganismal TypeExample
2.A.123.3.1

SemiSWEET half putative sugar transporter of 97 aas and 3 TMSs with an N-terminal amphipathic α-helix.  The protein occurs as a tight homodimer with the translocation channel between the two monomers.  The 3-d structure is known at 1.7 Å resolution revealing the occluded conformation (Xu et al. 2014). The outward open state of the Leptospira biflexa SemiSWEET (2.a.123.2.6) has been solved at 2.4 Å resolution (Xu et al. 2014).  The presence of these two states argues in favor of a carrier (rocker switch) mechanism rather than a channel-type mechanism (Xu et al. 2014).

Proteobacteria

SemiSWEET of Vibrio sp. N418

 
2.A.123.3.2

Uncharacterized protein of 107 aas.

Proteobacteria

UP of Rhodobacteraceae bacterium KLH11

 
2.A.123.3.3

Uncharacterized conserved protein of 273 aas and 7 TMSs containing PQ loop repeats.

UP of Geodermatophilus obscurus

 
2.A.123.3.4

Uncharacterized protein of 97 aas and 3 TMSs.

UP of Photobacterium leiognathi

 
Examples:

TC#NameOrganismal TypeExample
2.A.123.4.1

Membrane protein of 209 aas and 7 TMSs with a putative N-terminal lipid A disaccharide synthase domain.  This protein is a member of the "Lipid A Biosynthesis, N-terminal (LAB_N) domain in Pfam/CDD, related to the SWEET family.

Bacteroidetes

Membrane protein of Gramella forsetii

 
2.A.123.4.2

Uncharacterized protein of 115 aas and 3 TMSs.

Proteobacteria

UP of Frateuria aurantia (Acetobacter aurantius)

 
2.A.123.4.3

Uncharacterized protein (putative lipid A synthesis protein domain) of 115 aas and 3 TMSs.

Proteobacteria

UP of Stenotrophomonas maltophilia (Pseudomonas maltophilia) (Xanthomonas maltophilia)