The Structural Pathway for Water Permeation through Sodium-Glucose Cotransporters

Abstract

Although water permeation across cell membranes occurs through several types of membrane proteins, the only permeation mechanism resolved at atomic scale is that through aquaporins. Crystallization of the Vibrio parahaemolyticus sodium-galactose transporter (vSGLT) allows investigation of putative water permeation pathways through both vSGLT and the homologous human Na-glucose cotransporter (hSGLT1) using computational methods. Grand canonical Monte Carlo and molecular dynamics simulations were used to stably insert water molecules in both proteins, showing the presence of a water-filled pathway composed of ∼100 water molecules. This provides a structural basis for passive water permeation that is difficult to reconcile with the water cotransport hypothesis. Potential-of-mean-force calculations of water going through the crystal structure of vSGLT shows a single barrier of 7.7 kCal mol(-1), in agreement with previously published experimental data for cotransporters of the SGLT family. Electrophysiological and volumetric experiments performed on hSGLT1-expressing Xenopus oocytes showed that the passive permeation pathway exists in different conformational states. In particular, experimental conditions that aim to mimic the conformation of the crystal structure displayed passive water permeability. These results provide groundwork for understanding the structural basis of cotransporter water permeability.

Stochastic steps in secondary active sugar transport

“…This, or closely related conformational states, are poised to release both Na + and sugar through that passage, and consistent with a strict alternating-access mechanism, the X-ray structure exhibits a thick outer gate that occludes the substrate site from the extracellular space. Simulations of vSGLT have revealed that this outer gate is permeable to water, which crosses the transporter through the sugar binding site, and that water flow is modulated by the transient formation of an opening on the extracellular face of the transporter (18)(19)(20)(21). In 2 of our 21 simulations, the galactose escapes to the extracellular bulk through the same opening.…”

The Sodium Glucose Cotransporter SGLT1 Is an Extremely Efficient Facilitator of Passive Water Transport

“…Thus, the introduced mutation removes a barrier that obstructs the Na ϩ leak (either sterical or electrostatic) but does not eliminate a significant barrier for water movement. Previous oocyte experiments report contradictory results: removal of external Na ϩ either eliminated the water conducting ability of SGLT1 (27) or augmented it (28).…”

“…The agreement is surprising, because the SGLT1 conformations in cells and vesicles are not necessarily the same: The membrane potential is maintained at Ϫ30 mV in living MDCK cells, whereas it is either positive inside shrunken vesicles (because of the up-concentration of Na ϩ ) or clamped to 0 by the protonophore CCCP and the K ϩ -ionophore valinomycin. Previous oocyte experiments also revealed a weak potential dependence: the water permeability of SGLT1 overexpressing oocytes changed by only ϳ20% when the membrane potential was decreased from Ϫ20 to Ϫ100 mV (28).…”

Addition of Whey Peptides to a Carbohydrate-electrolyte Drink Enhances its Effect on the Early Treatment of Dehydration in Rats

“…The hypotheses regarding the mechanism of the water absorption-promoting effect of SGLT1 are roughly divided into two types: (1) passive water transport driven by the local osmotic gradient formed by the transport of the substrates (Duquette et al, 2001;Sasseville et al, 2011), and (2) active water transport coupled with the transport of the substrates at the molecular level (Loo et al, 1996(Loo et al, , 2002. Peptide transport by PepT1 may also promote water absorption, as described by these mechanisms.…”

“…Na + can therefore be absorbed efficiently when it is ingested with glucose. It was reported that water absorption is promoted by SGLT1 along with the absorption of Na + and glucose (Duquette et al, 2001;Loo et al, 1996Loo et al, , 2002Sasseville et al, 2011).…”