2.A.21 The Solute:Sodium Symporter (SSS) Family
Members of the SSS family catalyze solute:Na+ symport. The solutes transported may be sugars, amino acids, organo cations such as choline, nucleosides, inositols, vitamins, urea or anions, depending on the system. Members of the SSS family have been identified in bacteria, archaea and animals, and all functionally well-characterized members normally catalyze solute uptake via Na+ symport. The human placental multivitamin symporter cotransports an anionic vitamin with two Na+. In the rabbit Na+:D-glucose cotransporter, SGLT1, the glucose translocation pathway probably involves TMSs 10-13, and the binding site for the inhibitor, phlorizin, involves loop 13 (residues 604-610). Cation binding in the N-terminal domain may induce transport-related conformational changes. A conserved tyrosine in the first transmembrane segment of solute:sodium symporters is involved in Na+-coupled substrate co-transport (Mazier et al., 2011). Mechanistic aspects of Na+ binding sites in LeuT-like fold symporters has been discussed in detail (Perez and Ziegler 2013).
In the human homologue (hSGLT1), H+ can replace Na+, but the apparent affinity for glucose reduces 20x from 0.3 mM to 6 mM. The apparent affinity for H+ is 6 μM, 1000x higher than for Na+ (6 mM). The transport stoichiometry is 1 glucose:2 Na+ or H+. If Asp204 is replaced by glutamate (D204E), the apparent affinity for H+ increases >20x with no change in apparent Na+ affinity. The D204N or D204C mutation promotes phlorizin-sensitive H+ currents that are 10x greater than Na+ currents, and the glucose:H+ stoichiometry is then as great as 1:145. The mutant system thus behaves as a glucose-gated H+ channel.
Proteins of the SSS vary in size from about 400 residues to about 700 residues and probably possess thirteen to fifteen putative transmembrane helical spanners (TMSs). They generally share a core of 13 TMSs, but different members of the family may have different numbers of TMSs. A 13 TMS topology with a periplasmic N-terminus and a cytoplasmic C-terminus has been experimentally determined for the proline:Na+ symporter, PutP, of E. coli. Residues important for substrate and Na+ binding in PutP are found in TMSs 2, 7 and 9 as well as adjacent loops (Jung, 2002). A 14 TMS topology with periplasmic N- and C-termini has been established for the V. parahaemolyticus SglT carrier. SglT transports sugar:Na with a 1:1 stoichiometry. However, MctP of Rhizobium leguminosarum may take up monocarboxylates via an H+ symport mechanism as a dependency on Na+ could not be demonstrated and uptake was strongly inhibited by 10 μM CCCP.
Faham et al., 2008 reported the crystal structure of a member of the solute sodium symporters (SSS), the Vibrio parahaemolyticus sodium/galactose symporter (vSGLT). The approximately 3.0 angstrom structure contains 14 transmembrane (TM) helices in an inward-facing conformation with a core structure of inverted repeats of 5 TM helices (TM2 to TM6 and TM7 to TM11). Galactose is bound in the center of the core, occluded from the outside solutions by hydrophobic residues. The architecture of the core is similar to that of the leucine transporter (LeuT) (TC#2.A.22.4.2) from the NSS family. Modeling the outward-facing conformation based on the LeuT structure, in conjunction with biophysical data, provided insight into structural rearrangements for active transport (Faham et al., 2008).
Some bacterial sensor kinases (2.A.21.9.1 and 2.A.22.9.2) have N-terminal, 12 TMS, sensor domains that regulate the C-terminal kinase domains. The latter are homologous to the kinase domain of NtrB (Pao and Saier, 1995). The N-terminal sensor domains are homologous, but distantly related to members of the SSS. The closest homologues are PutP of E. coli (2.A.21.2.1) and PanF of E. coli (2.A.21.1.1). Homologous regulatory domains are found in Agrobacterium, Mesorhizobium, Sinorhizobium, Vibrio cholera and Bacillus species. While it is clear that these domains function as sensors, it is not known if they also transport the small molecules they sense.
The generalized transport reaction catalyzed by the members of this family is:
solute (out) + nNa+ (out) → solute (in) + nNa+ (in).