Before the discovery of SWEETs, the MSTs and SUTs that had been identified in plants were members of the major facilitator superfamily (MFS). The N-terminal and C-terminal ends of these proteins are both located on the intracellular side and typically contain 12 α-helical transmembrane domains (TMs). There is a large cytoplasmic loop located on the intracellular side in the middle of the MFS protein, which divides the protein into two domains
[1][2][3]. The N-terminal and C-terminal domains each contain six TMs. The topological structures of these two domains are very similar and exist in a pseudo-quadratic axisymmetric manner. The six TMs that form each domain can be split into two groups of three TMs that symmetrically repeat units in an anti-parallel manner
[1][2][3]. This unique folding method is designated MFS fold
[4].
The plant SWEETs are members of the MtN3/saliva family (PF03083). Its N-terminus and C-terminus are located on the outside and inside of the cytoplasm, respectively. Plant SWEETs generally contain seven TMs (
Figure 1a,b). The fourth TM is less conservative and primarily acts as a link. It divides the protein into two MtN3/saliva domains that each contain three TMs that form a “3-1-3” structure
[5]. The three TMs of each MtN3/saliva domain are arranged in the form of “TM1-TM3-TM2” to form a triple-helix bundle (THB) (
Figure 1a,b). It is apparent that the topological structure of SWEETs differs significantly from those of MSTs and SUTs. This difference could be an important reason why SWEET can transport sugar intracellularly to extracellularly. In addition, the SWEETs of prokaryotes only contain one MtN3/saliva domain composed of three TMs
[6]. Therefore, they have been designated SemiSWEETs. It can be hypothesized that one MtN3/saliva domain in prokaryotes underwent replication or horizontal gene transfer, which is defined as the transmission of DNA between different genomes, during the process of evolution, which led to the production of SWEET proteins in eukaryotes that contain two MtN3/saliva domains.
The results of truncation and complementation experiments demonstrated that SWEETs must undergo oligomerization to form homologous or heteromultimers to transport sugars
[6]. The most likely scenario is that the eukaryotic SWEETs form dimers, while the prokaryotic SemiSWEETs form tetramers.
A. thaliana SWEET proteins can form at least eight homopolymers and 47 heteropolymers
[6] (
Figure 1c). A high-resolution three-dimensional structural analysis of the bacterial SemiSWEET proteins proved that two SemiSWEET protein monomers form a basic translocation pore unit by forming a symmetrical homodimer
[7][8][9][10]. In addition, the tryptophan residue from TM2 and the asparagine residue from TM3 are the key sites for the SemiSWEET protein to be able to transport sugars
[7][8][9][10]. Rice
OsSWEET2b was the first eukaryotic SWEET protein for which a three-dimensional structure was resolved
[11]. It showed that a single
OsSWEET2b protein monomer can form a basic translocation pore unit and that TM4 and THB1 are closely linked to constitute the N-terminal region. In addition, THB2 constitutes the C-terminal region. This explains why the truncation of
AtSWEET1 protein into THB1 + TM4 and THB2 and their co-expression enables the transportation of glucose
[6]. In contrast, THB1 and TM4 + THB2 that have been truncated and co-expressed cannot transport glucose
[6]. Moreover, owing to the inconsistent tightness of TM4 with THB1 and THB2, THB1 and THB2 are structurally asymmetrical, which obviously differs from the symmetrical arrangement of the two THBs in the SemiSWEET homodimer of prokaryotes
[11]. Cysteine residues from TM2, asparagine residues from TM3 and TM7 and phenylalanine residues from TM6 are the key sites for
OsSWEET2 to transport glucose
[11]. Recently, the crystal structure of
A. thaliana AtSWEET13 with a resolution of 2.8-Å has been obtained. The researchers observed an inward-facing conformation of
AtSWEET13 with the substrate analog 2′-deoxycytidine-5′-monophosphate bound to the central cavity
[12]. There are 10 amino acid residues in the
AtSWEET13 protein that play an important role in the recognition and binding of substrates. They are designated Ser20 from TM1 (Ser20
TM1), Leu23
TM1, Asn54
TM2, Trp58
TM2, Asn76
TM3, Ser142
TM5, Met145
TM5, Asn176
TM6, Trp180
TM6 and Asn196
TM7. Up to now, structural analysis has revealed that SemiSWEET or SWEET proteins have three conformations: outward open conformation, inward open conformation and occluded conformation. These results laid a structural foundation to elucidate the mechanism by which the SWEET protein binds to substrates and transports sugar. Based on this, a rocking-type motion theory was proposed
[7]. In OsWEET2b, the proline residues on TM1, TM2, TM5 and TM6 may be the key factors that promote the transition between different conformations.