TCB Publications - Abstract

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.


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