Structural and functional significance of water permeation through cotransporters

Abstract

Membrane transporters, in addition to their major role as specific carriers for ions and small molecules, can also behave as water channels. However, neither the location of the water pathway in the protein nor their functional importance is known. Here, we map the pathway for water and urea through the intestinal sodium/glucose cotransporter SGLT1. Molecular dynamics simulations using the atomic structure of the bacterial transporter vSGLT suggest that water permeates the same path as Na + and sugar. On a structural model of SGLT1, based on the homology structure of vSGLT, we identified and mutated residues lining the sugar transport pathway to cysteine. The mutants were expressed in Xenopus oocytes, and the unitary water and urea permeabilities were determined before and after modifying the cysteine side chain with reversible methanethiosulfonate reagents. The results demonstrate that water and urea follow the sugar transport pathway through SGLT1. The changes in permeability, increases or decreases, with side-chain modifications depend on the location of the mutation in the region of external or internal gates, or the sugar binding site. These changes in permeability are hypothesized to be due to alterations in steric hindrance to water and urea, and/or changes in protein folding caused by mismatching of side chains in the water pathway. Water permeation through SGLT1 and other transporters bears directly on the structural mechanism for the transport of polar solutes through these proteins. Finally, in vitro experiments on mouse small intestine show that SGLT1 accounts for two-thirds of the passive water flow across the gut.ater is indispensable for life as we know it, and water flow across cell membranes is central to normal physiology from single cells to complex organisms including humans. The pathways for water permeation across membranes include the lipid bilayer and water channels (aquaporins). However, it has become clear that other membrane proteins also transport water. Prominent examples are the sodium-coupled glucose and amino acid cotransporters, SGLT1 and GAT1 (1-4) and the cotransporters from the SLC12 family such as the KCC and NKCC1 (5). In this study, we focus entirely on cotransporters as water channels, i.e., the water transport induced by an osmotic gradient. Nonosmotic water transport has been reviewed (5, 6).One feature that distinguishes cotransporters from conventional water channels, aquaporins, is that the water permeability of transporters depends on the conformational state of the protein, i.e., specific competitive inhibitors block the water pathway. For example, phlorizin blocks water permeation through SGLT1, and SKF89976A blocks water and urea permeation through the sodiumcoupled GABA transporter GAT1 (3, 7). Experimental information about the water pathway through the transport proteins is not available, and the physiological significance of water permeation has not been established.Molecular dynamic (MD) simulations using the atomic structure of the bacterial homolog vSGLT ha...

SUR1‐TRPM4 and AQP4 form a heteromultimeric complex that amplifies ion/water osmotic coupling and drives astrocyte swelling

“…This result is consistent with SUR1 providing four additional AQP4 binding sites in the SUR1-TRPM4 channel versus the TRPM4 channel ( Figure 9). Notably, the close apposition of SUR1-TRPM4 and AQP4 is similar to the arrangement of ion and water channels observed in water co-transporters (MacAulay et al, 2004;Zeuthen et al, 2016). Structural studies are currently underway to examine the proposed model in Figure 9.…”

“…In these "water co-transporters", osmolyte influx is immediately translated to water influx via the generation of small localized osmotic pressures (Duquette, Bissonnette, & Lapointe, 2001;Lapointe, Gagnon, Poirier, & Bissonnette, 2002). Water cotransporters are highly efficient, mediating 65% of intestinal water uptake (Zeuthen, Gorraitz, Her, Wright, & Loo, 2016), and have been shown to be associated with the majority of perisynaptic glutamate clearance (MacAulay, Hamann, & Zeuthen, 2004). Interestingly, certain water co-transporters, including the Na 1 /K 1 /2Clco-transporter-1 and volume-regulated anion channels, mediate cell volume changes in astrocytes (Nilius, 2004;Kimelberg, Macvicar, & Sontheimer, 2006;Jayakumar and Norenberg, 2010;Zeuthen, 2010).…”

Claudin‐15 forms a water channel through the tight junction with distinct function compared to claudin‐2

“…In recent years, it was shown that the SGLT1 behaves as a water channel . The passive SGLT1‐mediated water transport was independent of the presence of ligands, such as Na + and glucose, but the addition of glucose increased the transport of water into the cell and the rate of sugar‐coupled water transport was directly proportional to the rate of Na + /glucose cotransport.…”

Water Flow from Different Solutions During Perfusion of the Functioning Portion of the Small Intestine of the Rats In vivo

“…Also here there is a significant absorption of water, and in the proximal fragment this process is more intense than in the rest of the intestine [3,10]. The absorption of water accompanies the transport of monosaccharides (mainly glucose) [4] and amino acids [3,11]. Water absorption in the gut can be of different nature: the transport of water together with the ions, the absorption of water by osmotic pressure of chyme, influenced by various humoral factors or changes in the functional state.…”

“…Experimental studies of processes in biological systems typically involve the evaluation of various concentrations of liquid components [1][2][3][4]. Changing of the concentration of these components can occur not only due to the transformation and movement of solutes, but also due to the change of the volume of the solvent.…”