SWEET transporters

Available protein structures:
Pfam	structures / ECOD
PDB	RCSB PDB; PDBe; PDBj
PDBsum	structure summary

SemiSWEET PQ-loop
Identifiers
Symbol	PQ-loop
Pfam	PF03083
Pfam clan	PQ-loop
InterPro	IPR006603
SMART	SM00679
TCDB	2.A.123
OPM superfamily	415
OPM protein	5ctg
Available protein structures:
Pfam	structures / ECOD
PDB	RCSB PDB; PDBe; PDBj
PDBsum	structure summary

The SWEET family, 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:[1]

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.[10]

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.[10] 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

3.1 A resolution structure of a eukaryotic SWEET transporter in a homotrimer (5CTG, OsSWEET2). Each different color represents one subunit.

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.[15] 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.

References

Saier, MH Jr. "2.A.123 The Sweet; PQ-loop; Saliva; MtN3 (Sweet) Family". Transporter Classification Database. Saier Lab Bioinformatics Group / SDSC.
Takanaga H, Frommer WB (August 2010). "Facilitative plasma membrane transporters function during ER transit". FASEB Journal. 24 (8): 2849–58. doi:10.1096/fj.09-146472. PMC 3230527. PMID 20354141.
Kanno Y, Oikawa T, Chiba Y, Ishimaru Y, Shimizu T, Sano N, Koshiba T, Kamiya Y, Ueda M, Seo M (October 2016). "AtSWEET13 and AtSWEET14 regulate gibberellin-mediated physiological processes". Nat Commun. 7 (13245): 13245. Bibcode:2016NatCo...713245K. doi:10.1038/ncomms13245. PMC 5095183. PMID 27782132.
"Nanosensors | Department of Plant Biology". dpb.carnegiescience.edu. Retrieved 1 March 2016.
Bermejo C, Ewald JC, Lanquar V, Jones AM, Frommer WB (August 2011). "In vivo biochemistry: quantifying ion and metabolite levels in individual cells or cultures of yeast". The Biochemical Journal. 438 (1): 1–10. doi:10.1042/BJ20110428. PMID 21793803. S2CID 26944897.
Jones AM, Grossmann G, Danielson JÅ, Sosso D, Chen LQ, Ho CH, Frommer WB (June 2013). "In vivo biochemistry: applications for small molecule biosensors in plant biology". Current Opinion in Plant Biology. 16 (3): 389–95. doi:10.1016/j.pbi.2013.02.010. PMC 3679211. PMID 23587939.
Jones AM, Ehrhardt DW, Frommer WB (May 2012). "A never ending race for new and improved fluorescent proteins". BMC Biology. 10: 39. doi:10.1186/1741-7007-10-39. PMC 3342923. PMID 22554191.
Okumoto S, Jones A, Frommer WB (1 January 2012). "Quantitative imaging with fluorescent biosensors". Annual Review of Plant Biology. 63: 663–706. doi:10.1146/annurev-arplant-042110-103745. PMID 22404462.
Hou BH, Takanaga H, Grossmann G, Chen LQ, Qu XQ, Jones AM, Lalonde S, Schweissgut O, Wiechert W, Frommer WB (October 2011). "Optical sensors for monitoring dynamic changes of intracellular metabolite levels in mammalian cells". Nature Protocols. 6 (11): 1818–33. doi:10.1038/nprot.2011.392. PMID 22036884. S2CID 21852318.
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 (November 2010). "Sugar transporters for intercellular exchange and nutrition of pathogens". Nature. 468 (7323): 527–32. Bibcode:2010Natur.468..527C. doi:10.1038/nature09606. PMC 3000469. PMID 21107422.
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 (December 2015). "Seed filling in domesticated maize and rice depends on SWEET-mediated hexose transport". Nature Genetics. 47 (12): 1489–93. doi:10.1038/ng.3422. PMID 26523777. S2CID 6985808.
Feng CY, Han JX, Han XX, Jiang J (December 2015). "Genome-wide identification, phylogeny, and expression analysis of the SWEET gene family in tomato". Gene. 573 (2): 261–72. doi:10.1016/j.gene.2015.07.055. PMID 26190159.
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 (April 2014). "Nectar secretion requires sucrose phosphate synthases and the sugar transporter SWEET9". Nature. 508 (7497): 546–9. Bibcode:2014Natur.508..546L. doi:10.1038/nature13082. PMID 24670640. S2CID 4384123.
Chen LQ, Lin IW, Qu XQ, Sosso D, McFarlane HE, Londoño A, Samuels AL, Frommer WB (March 2015). "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. doi:10.1105/tpc.114.134585. PMC 4558658. PMID 25794936.
Xu Y, Tao Y, Cheung LS, Fan C, Chen LQ, Xu S, Perry K, Frommer WB, Feng L (November 2014). "Structures of bacterial homologues of SWEET transporters in two distinct conformations". Nature. 515 (7527): 448–452. Bibcode:2014Natur.515..448X. doi:10.1038/nature13670. PMC 4300204. PMID 25186729.
Yee DC, Shlykov MA, Västermark A, Reddy VS, Arora S, Sun EI, Saier MH (November 2013). "The transporter-opsin-G protein-coupled receptor (TOG) superfamily". The FEBS Journal. 280 (22): 5780–800. doi:10.1111/febs.12499. PMC 3832197. PMID 23981446.