TCB Publications - Abstract

R. Ryan Geyer, Raif Musa-Aziz, Giray Enkavi, Paween Mahinthichaichan, Emad Tajkhorshid, and Walter F. Boron. Movement of NH3 through the human urea transporter B: a new gas channel. American Journal of Physiology - Renal Physiology, 304:F1447-F1457, 2013. (PMC: PMC3680674)

GEYE2013-ET Aquaporins and Rh proteins can function as gas (CO$_2$ and NH$_3$) channels. The present study explores the urea, H$_2$O, CO$_2$, and NH$_3$ permeability of the human urea transporter B (UT-B) (SLC14A1), expressed in Xenopus oocytes. We monitored urea uptake using [$^{14}$C]urea and measured osmotic water permeability ($P_f$) using video microscopy. To obtain a semiquantitative measure of gas permeability, we used microelectrodes to record the maximum transient change in surface pH ($\Delta$pH$_{\textrm{S}}$) caused by exposing oocytes to 5% CO$_2$/33 mM HCO$_3^-$ (pHS increase) or 0.5 mM NH$_3$/NH$_4^+$ (pHS decrease). UT-B expression increased oocyte permeability to urea by $\greaterthan$ 20-fold, and $P_f$ by 8-fold vs. H$_2$O-injected control oocytes. UT-B expression had no effect on the CO$_2$-induced $\Delta$pHS but doubled the NH$_3$-induced $\Delta$pHS. Phloretin reduced UT-B-dependent urea uptake (J $^*_{\textrm{urea}}$) by 45%, P$^*_f$ by 50%, and (- $\Delta$pHS)$NH_3$ by 70%. p-Chloromercuribenzene sulfonate reduced J $^*_{\textrm{urea}}$ by 25%, P$^*_f$ by 30%, and ($\Delta$pH $^*_{\textrm{S}}$)NH$_3$ by 100%. Molecular dynamics (MD) simulations of membrane-embedded models of UT-B identified the monomeric UT-B pores as the main conduction pathway for both H$_2$O and NH$_3$ and characterized the energetics associated with permeation of these species through the channel. Mutating each of two conserved threonines lining the monomeric urea pores reduced H$_2$O and NH$_3$ permeability. Our data confirm that UT-B has significant H$_2$O permeability and for the first time demonstrate significant NH$_3$ permeability. Thus the UTs become the third family of gas channels. Inhibitor and mutagenesis studies and results of MD simulations suggest that NH$_3$ and H$_2$O pass through the three monomeric urea channels in UT-B.

Download Full Text

The manuscripts available on our site are provided for your personal use only and may not be retransmitted or redistributed without written permissions from the paper's publisher and author. You may not upload any of this site's material to any public server, on-line service, network, or bulletin board without prior written permission from the publisher and author. You may not make copies for any commercial purpose. Reproduction or storage of materials retrieved from this web site is subject to the U.S. Copyright Act of 1976, Title 17 U.S.C.

Download full text: Journal, Request a Copy