title: The Sodium Glucose Cotransporter SGLT1 Is an Extremely Efficient Facilitator of Passive Water Transport.
abstract: The small intestine is void of aquaporins adept at facilitating vectorial water transport, and yet it reabsorbs ∼8 liters of fluid daily. Implications of the sodium glucose cotransporter SGLT1 in either pumping water or passively channeling water contrast with its reported water transporting capacity, which lags behind that of aquaporin-1 by 3 orders of magnitude. Here we overexpressed SGLT1 in MDCK cell monolayers and reconstituted the purified transporter into proteoliposomes. We observed the rate of osmotic proteoliposome deflation by light scattering. Fluorescence correlation spectroscopy served to assess (i) SGLT1 abundance in both vesicles and plasma membranes and (ii) flow-mediated dilution of an aqueous dye adjacent to the cell monolayer. Calculation of the unitary water channel permeability, pf, yielded similar values for cell and proteoliposome experiments. Neither the absence of glucose or Na(+), nor the lack of membrane voltage in vesicles, nor the directionality of water flow grossly altered pf Such weak dependence on protein conformation indicates that a water-impermeable occluded state (glucose and Na(+) in their binding pockets) lasts for only a minor fraction of the transport cycle or, alternatively, that occlusion of the substrate does not render the transporter water-impermeable as was suggested by computational studies of the bacterial homologue vSGLT. Although the similarity between the pf values of SGLT1 and aquaporin-1 makes a transcellular pathway plausible, it renders water pumping physiologically negligible because the passive flux would be orders of magnitude larger.
authors.affiliation: From the Institute of Biophysics, Johannes Kepler University Linz, 4020 Linz, Austria.
authors.forename: Liudmila
authors.initials: L
authors.lastname: Erokhova
authors.affiliation: From the Institute of Biophysics, Johannes Kepler University Linz, 4020 Linz, Austria.
authors.forename: Andreas
authors.initials: A
authors.lastname: Horner
authors.affiliation: From the Institute of Biophysics, Johannes Kepler University Linz, 4020 Linz, Austria.
authors.forename: Nicole
authors.initials: N
authors.lastname: Ollinger
authors.affiliation: From the Institute of Biophysics, Johannes Kepler University Linz, 4020 Linz, Austria.
authors.forename: Christine
authors.initials: C
authors.lastname: Siligan
authors.affiliation: From the Institute of Biophysics, Johannes Kepler University Linz, 4020 Linz, Austria peter.pohl@jku.at.
authors.forename: Peter
authors.identifier: http://orcid.org/0000-0002-1792-2314
authors.initials: P
authors.lastname: Pohl
mesh_terms: D000818:Animals; D051398:Aquaporin 1 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D001692:Biological Transport / Q000502:physiology; D002462:Cell Membrane / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002463:Cell Membrane Permeability; D004285:Dogs; D005947:Glucose / Q000737:chemistry / Q000378:metabolism; D006801:Humans; D061985:Madin Darby Canine Kidney Cells; D012964:Sodium / Q000737:chemistry / Q000378:metabolism; D051269:Sodium-Glucose Transporter 1 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D014867:Water / Q000737:chemistry / Q000378:metabolism*
chemical_list: C495674:SLC5A1 protein, human; D051269:Sodium-Glucose Transporter 1; D014867:Water; D051398:Aquaporin 1; D012964:Sodium; D005947:Glucose
keywords: fluorescence correlation spectroscopy (FCS); glucose transport; membrane reconstitution; membrane transport; sodium transport; transporter; water channel