SWEET transporters explained

The SWEET family (Sugars Will Eventually Be Exported Transporter), also known as the PQ-loop, Saliva or MtN3 family (TC# 2.A.123), is a family of sugar transporters and a member of the TOG superfamily. The proteins of the SWEET family have been found in plants, animals, protozoans, and bacteria. Eukaryotic family members have 7 transmembrane segments (TMSs) in a 3+1+3 repeat arrangement.[1]

Function

Proteins of the SWEET family appear to catalyze facilitated diffusion (entry or export) of sugars across the plant plasma membrane or the endoplasmic reticulum membrane.[2]

They also seem to transport other metabolites, like gibberellins. [3]

Transport Reaction

The generalized reaction catalyzed by known proteins of this family is:

sugars (in) ⇌ sugars (out)

Discovery

SWEETs were originally identified in Arabidopsis thaliana, in a screen for novel facilitators of transmembrane glucose transport. In this experiment, several previously uncharacterized membrane proteins were selected to be screened. These uncharacterized membrane proteins were assayed for glucose transport ability by expression in HEK293T (human embryonic kidney) cells, which have negligible glucose transport ability in the normal state. These membrane proteins were co-expressed with a fluorescent FRET (Förster resonance energy transfer) glucose sensor localized to the endoplasmic reticulum (ER).[4] [5] [6] [7] [8] [9] Glucose movement from the cytoplasm to the ER of the HEK293T cells was monitored by quantifying changes in FRET ratio. By using this assay, the first member of the SWEET family, AtSWEET1, was identified. Other potential family members were identified by sequence homology.

Homologues

Chen et al. (2010) reviewed evidence for a new class of sugar transporters, named SWEETs.[10] Those that mediate glucose transport include at least six out of seventeen sugar homologues in Arabidopsis (i.e., TC#s 2.A.123.1.3, 2.A.123.1.5, 2.A.123.1.9, 2.A.123.1.13), two out of over twenty porters in rice (TC#s 2.A.123.1.6 and 2.A.123.1.18), two out of seven homologues in Caenorhabditis elegans (i.e., TC# 2.A.123.1.10) and the single copy human protein (SLC50A1 of Homo sapiens, TC# 2.A.123.1.4). Without Arabidopsis SWEET8 (TC# 2.A.123.1.5), pollen is not viable. The corn homolog ZmSWEET4c was shown to be involved in seed filling.[11]

Currently classified members of the SWEET transporter family can be found in the Transporter Classification Database.

SWEETs in plants

Plant SWEETs fall into four subclades. The tomato genome encodes 29 SWEETs.[12]

SWEET9 in Nectar Secretion

Lin et al., 2014, examined the role of SWEET9 in nectaries. SWEET9 is a member of clade 3. A homologue in petunias had been shown to have an inverse correlation between expression and starch content in nectaries. Mutation and overexpression of SWEET9 in Arabidopsis led to corresponding loss of and increase in nectar secretion, respectively. After showing that SWEET9 is involved in nectar secretion, the next step was to determine at which phase of the process SWEET9 has its function. The 3 options were: phloem unloading, or uptake or efflux from nectary parenchyma. A combination of localization studies and starch accumulation assays showed that SWEET9 is involved in sucrose efflux from the nectary parenchyma.[13]

SWEETs 11, 12, and 15 in Embryo Nutrition

Chen et al., 2015, asked what SWEETs are involved in providing nutrition to an embryo. The team noticed that mRNA and protein for SWEETs 11, 12, and 15 are each expressed at high levels during some stage of embryo development. Each gene was subsequently mutated to generate a sweet11;12;15 triple mutant which lacked activity in each of the three genes. This triple mutant was shown to have delayed embryo development; that is, the seeds of the triple mutant were significantly smaller than that of the wild type at the same time during development. The starch content of the seed coat was higher than the wild-type, and the starch content of the embryo was lower than the wild-type. Additionally, protein levels were shown to be maternally controlled: in a sweet11;12;15 mutant crossed with a wild-type plant, the mutant phenotype was only seen when sweet11;12;15 was used as the maternal plant.[14]

Structure

Many bacterial homologues have only 3 TMSs and are half sized, but they nevertheless are members of the SWEET family with a single 3 TMS repeat unit. Other bacterial homologues have 7 TMSs as do most eukaryotic proteins in this family. The SWEET family is large and diverse. Based on 3-D structural analyses, it is likely that these paired 3 TMS SWEET family members function as carriers.

Bacterial SemiSWEETs, consist of a triple-helix bundle in a 1-3-2 conformation, with TM3 sandwiched between TM1 and TM2.[15] The structures also show tryptophan and asparagine residues interacting with the sugar; point mutations of these residues to alanine destroys the hexose transport function of SemiSWEET. 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.[16]

Several crystal structures are available on RCSB for members of the SWEET/SemiSWEET/PQ-loop/Saliva/MtN3 family.

See also

Further reading

Notes and References

  1. Web site: 2.A.123 The Sweet; PQ-loop; Saliva; MtN3 (Sweet) Family. Transporter Classification Database. Saier Lab Bioinformatics Group / SDSC. Saier. MH Jr..
  2. Takanaga H, Frommer WB . Facilitative plasma membrane transporters function during ER transit . FASEB Journal . 24 . 8 . 2849–58 . August 2010 . 20354141 . 3230527 . 10.1096/fj.09-146472 . free .
  3. Kanno Y, Oikawa T, Chiba Y, Ishimaru Y, Shimizu T, Sano N, Koshiba T, Kamiya Y, Ueda M, Seo M . AtSWEET13 and AtSWEET14 regulate gibberellin-mediated physiological processes . Nat Commun . 7 . 13245 . October 2016 . 13245 . 27782132 . 5095183 . 10.1038/ncomms13245 . 2016NatCo...713245K .
  4. Web site: Nanosensors Department of Plant Biology. dpb.carnegiescience.edu. 2016-03-01.
  5. Bermejo C, Ewald JC, Lanquar V, Jones AM, Frommer WB . In vivo biochemistry: quantifying ion and metabolite levels in individual cells or cultures of yeast . The Biochemical Journal . 438 . 1 . 1–10 . August 2011 . 21793803 . 10.1042/BJ20110428 . 26944897 .
  6. Jones AM, Grossmann G, Danielson JÅ, Sosso D, Chen LQ, Ho CH, Frommer WB . In vivo biochemistry: applications for small molecule biosensors in plant biology . Current Opinion in Plant Biology . 16 . 3 . 389–95 . June 2013 . 23587939 . 3679211 . 10.1016/j.pbi.2013.02.010 . 2013COPB...16..389J .
  7. Jones AM, Ehrhardt DW, Frommer WB . A never ending race for new and improved fluorescent proteins . BMC Biology . 10 . 39 . May 2012 . 22554191 . 3342923 . 10.1186/1741-7007-10-39 . free .
  8. Okumoto S, Jones A, Frommer WB . Quantitative imaging with fluorescent biosensors . Annual Review of Plant Biology . 63 . 663–706 . 2012-01-01 . 22404462 . 10.1146/annurev-arplant-042110-103745 .
  9. Hou BH, Takanaga H, Grossmann G, Chen LQ, Qu XQ, Jones AM, Lalonde S, Schweissgut O, Wiechert W, Frommer WB . Optical sensors for monitoring dynamic changes of intracellular metabolite levels in mammalian cells . Nature Protocols . 6 . 11 . 1818–33 . October 2011 . 22036884 . 10.1038/nprot.2011.392 . 21852318 .
  10. Chen LQ, Hou BH, Lalonde S, Takanaga H, Hartung ML, Qu XQ, Guo WJ, Kim JG, Underwood W, Chaudhuri B, Chermak D, Antony G, White FF, Somerville SC, Mudgett MB, Frommer WB . Sugar transporters for intercellular exchange and nutrition of pathogens . Nature . 468 . 7323 . 527–32 . November 2010 . 21107422 . 3000469 . 10.1038/nature09606 . 2010Natur.468..527C .
  11. Sosso D, Luo D, Li QB, Sasse J, Yang J, Gendrot G, Suzuki M, Koch KE, McCarty DR, Chourey PS, Rogowsky PM, Ross-Ibarra J, Yang B, Frommer WB . Seed filling in domesticated maize and rice depends on SWEET-mediated hexose transport . Nature Genetics . 47 . 12 . 1489–93 . December 2015 . 26523777 . 10.1038/ng.3422 . 6985808 .
  12. Feng CY, Han JX, Han XX, Jiang J . Genome-wide identification, phylogeny, and expression analysis of the SWEET gene family in tomato . Gene . 573 . 2 . 261–72 . December 2015 . 26190159 . 10.1016/j.gene.2015.07.055 .
  13. Lin IW, Sosso D, Chen LQ, Gase K, Kim SG, Kessler D, Klinkenberg PM, Gorder MK, Hou BH, Qu XQ, Carter CJ, Baldwin IT, Frommer WB . Nectar secretion requires sucrose phosphate synthases and the sugar transporter SWEET9 . Nature . 508 . 7497 . 546–9 . April 2014 . 24670640 . 10.1038/nature13082 . 2014Natur.508..546L . 4384123 .
  14. Chen LQ, Lin IW, Qu XQ, Sosso D, McFarlane HE, Londoño A, Samuels AL, Frommer WB . A cascade of sequentially expressed sucrose transporters in the seed coat and endosperm provides nutrition for the Arabidopsis embryo . The Plant Cell . 27 . 3 . 607–19 . March 2015 . 25794936 . 4558658 . 10.1105/tpc.114.134585 .
  15. Xu Y, Tao Y, Cheung LS, Fan C, Chen LQ, Xu S, Perry K, Frommer WB, Feng L . Structures of bacterial homologues of SWEET transporters in two distinct conformations . Nature . 515 . 7527 . 448–452 . November 2014 . 25186729 . 4300204 . 10.1038/nature13670 . 2014Natur.515..448X .
  16. Yee DC, Shlykov MA, Västermark A, Reddy VS, Arora S, Sun EI, Saier MH . The transporter-opsin-G protein-coupled receptor (TOG) superfamily . The FEBS Journal . 280 . 22 . 5780–800 . November 2013 . 23981446 . 3832197 . 10.1111/febs.12499 .

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