Structure and permeation mechanism of a mammalian urea transporter

By Elena J. Levin, Yu Cao, Giray Enkavi, Matthias Quick, Yaping Pan, Emad Tajkhorshid, Ming Zhou.

Published in Proceedings of the National Academy of Sciences USA 109(28): 11194-9, on July 10, 2012. PMID: 22733730. PMCID: PMC3396522. Link to publication page.

Core Facility: Computational Modeling

Figure 1: 2.36 Å crystal structure of bovine UT-B. (A) The UT-B trimer viewed from the extracellular side of the membrane, with the threefold symmetry axis (black triangle) labeled. (B) The structure of a UT-B protomer as viewed from within the plane of the membrane (Left) and from the extracellular side of the membrane (Right). The transmembrane helices are colored in pseudo-symmetry-related pairs. (C) The urea permeation pathway is shown viewed from two perpendicular orientations, with cross-sections of the wide (Left) and narrow (Right) dimensions of the pore marked in beige. Insets show residues lining the pore within the regions marked by black rectangles.

Abstract

As an adaptation to infrequent access to water, terrestrial mammals produce urine that is hyperosmotic to plasma. To prevent osmotic diuresis by the large quantity of urea generated by protein catabolism, the kidney epithelia contain facilitative urea transporters (UTs) that allow rapid equilibration between the urinary space and the hyperosmotic interstitium. Here we report the first X-ray crystal structure of a mammalian UT, UT-B, at a resolution of 2.36 Å. UT-B is a homotrimer and each protomer contains a urea conduction pore with a narrow selectivity filter. Structural analyses and molecular dynamics simulations showed that the selectivity filter has two urea binding sites separated by an approximately 5.0 kcal/mol energy barrier. Functional studies showed that the rate of urea conduction in UT-B is increased by hypoosmotic stress, and that the site of osmoregulation coincides with the location of the energy barrier.