Function And Structure Of A Bacterial Na+/Sugar Symporter THE
SYMPORTER
Since long it has been studied the widespread mechanism of
sodium-dependency in the intake of certain solutes against an
electrochemical gradient by cell membranes1. Crane was the first
one to propose a hypothesis for the molecular basis of the process
that included the coupling of the solute with sodium to enter
through the cell membrane and the return of the ion to the
extracellular environment2. The hypothesis was strongly supported
and it is now established that the electrochemical gradient and the
negative charge of the cell membrane drive the accumulation of
certain solutes inside the cell. Back in the 90s, Sarker and
colleagues described the first bacterial example of a
sodium-dependent galactose/glucose transport system for Vibrio
parahaemolyticus3,4. Much later than the its well-described
orthologous, the mammalian Na+-dependent glucose transport system
(SGLT1)2,5. The V. parahaemolyticus Na+/sugar symport unit (vSGLT)
had then its activity characterized and its primary structure
sequenced by the same group (although the nucleotide sequence would
be later on corrected by Turk and colleagues)3,4,68. Although
topological and structural properties of proteins can be reasonably
assumed from primary structure, recent structural studies permit
better understanding of the mechanism and function of the vSGLT.
Moreover, the data sheds light on the general process of
sodium-driven solute transport. PROTEIN ACTIVITY
With regard to the substrate, in vivo and in vitro inhibition
assays demonstrated vSGLT has high affinity for both D-galactose
and D-glucose, and to a lesser extent for D-fucose3,4,8. For sugar
intake, coupled-ion can be Na+ or Li+, although efficiency for the
former is much higher. Na+/substrate stoichiometry for vSGLT was
demonstrated to be of 1:18. According to the kinetic models for
sodium/solute symport, transportation through membrane is
accomplished by means of the alternating-access mechanism9. First
proposed in 1966, it consists of the protein having a binding site
for the molecules to be transported and that it is alternatively
exposed to the intracellular and extracellular environments by
allosteric changes so that molecules can bind in one side and be
released to the other10. PROTEIN STRUCTURE The structure of the
vSGLT consists of 14 transmembrane (TM) helices with both termini
exposed to periplasm11, as predicted by analysis of hydrophobicity,
propensity for reverse-turns and freeze-fracture electron
microscope7,8,12,13. Although the protein packed as a tight dimer
when crystalized11, freeze-fracture electron microscopic studies
indicate that the protein functions as a monomer 8,13. The vSGLT
core structure is composed of 10 TM helices, in which TM2 to TM6
and TM7 to TM11 are structurally similar and related by a symmetry
axis at the center of the cell membrane11,14. This same
five-helices inverted motif has been lately described in other
membrane proteins (BetP, LeuT, Mhp1) that do not share any identity
at the
References
1. Schultz, S. G. & Curran, P. F. Coupled transport of
sodium and organic solutes. Physiol. Rev. 50, 637718 (1970).
2. Crane, R. K., Miller, D. & Bihler, I. in Membr. Transp.
Metab. (Kleinzeller, A. & Kotyk, A.) 43949 (Academic Press,
1960).
3. Sarker, R. I., Ogawa, W., Tsuda, M., Tanaka, S. &
Tsuchiya, T. Characterization of a glucose transport system in
Vibrio parahaemolyticus. J. Bacteriol. 176, 737882 (1994).
4. Sarker, R., Ogawa, W., Tsuda, M., Tanaka, S. & Tsuchiya,
T. Properties of a Na+/galactose (glucose) symport system in Vibrio
parahaemolyticus. Biochim. Biophys. Acta - Biomembr. 1279, 14956
(1996).
5. Wright, E. M., Loo, D. D. F. & Hirayama, B. A. Biology of
human sodium glucose transporters. Physiol. Rev. 91, 73394
(2011).
6. Sarker, R., Okabe, Y., Tsuda, M. & Tsuchiya, T. Sequence
of a Na+/glucose symporter gene and its flanking regions of Vibrio
parahaemolyticus. Biochim. Biophys. Acta - Biomembr. 1281, 14
(1996).
7. Sarker, R., Ogawa, W., Shimamoto, T. & Tsuchiya, T.
Primary Structure and Properties of the Na+/Glucose Symporter
(SglS) of Vibrio parahaemolyticus. J. Bacteriol. 179, 18058
(1997).
8. Turk, E. et al. Molecular characterization of Vibrio
parahaemolyticus vSGLT: a model for sodium-coupled sugar
cotransporters. J. Biol. Chem. 275, 257116 (2000).
9. Loo, D. D. F., Hirayama, B. a, Karakossian, M. H., Meinild,
A.-K. & Wright, E. M. Conformational dynamics of hSGLT1 during
Na+/glucose cotransport. J. Gen. Physiol. 128, 70120 (2006).
10. Jardetzky, O. Simple allosteric model for membrane pumps.
Nature 211, 96970 (1966).
11. Faham, S. et al. The crystal structure of a sodium galactose
transporter reveals mechanistic insights into Na+/sugar symport.
Science 321, 8104 (2008).
12. Turk, E. & Wright, E. M. Membrane Topology Motifs in the
SGLT Cotransporter Family. J. Membr. Biol. 159, 120 (1997).
13. Eskandari, S., Wright, E. M., Kreman, M., Starace, D. M.
& Zampighi, G. a. Structural analysis of cloned plasma membrane
proteins by freeze-fracture electron microscopy. Proc. Natl. Acad.
Sci. U. S. A. 95, 1123540 (1998).