SWEET transporters

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]

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]

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.

Plant SWEETs fall into four subclades.[10] The tomato genome encodes 29 SWEETs.[12]

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.

• Solute carrier family
• TOG Superfamily
• Transporter Classification Database
• Glucose transporter
• Transport protein

As of 2 February 2016, this article is derived in whole or in part from Transporter Classification Database. The copyright holder has licensed the content in a manner that permits reuse under CC BY-SA 3.0 and GFDL. All relevant terms must be followed. The original text was at "2.A.123 The Sweet; PQ-loop; Saliva; MtN3 (Sweet) Family"