Paper Citing NAMD - Abstract
Geyer, R. Ryan; Musa-Aziz, Raif; Enkavi, Giray; Mahinthichaichan, P.; Tajkhorshid, Emad; Boron, Walter F.
Movement of NH3 through the human urea transporter B: a new gas channel
AMERICAN JOURNAL OF PHYSIOLOGY-RENAL PHYSIOLOGY, 304:F1447-F1457, JUN 2013
Aquaporins and Rh proteins can function as gas (CO2 and NH3) channels. The present study explores the urea, H2O, CO2, and NH3 permeability of the human urea transporter B (UT-B) (SLC14A1), expressed in Xenopus oocytes. We monitored urea uptake using [C-14]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(S)) caused by exposing oocytes to 5% CO2/33 mM HCO3- (pHS increase) or 0.5 mM NH3/NH4- (pH(S) decrease). UT-B expression increased oocyte permeability to urea by >20-fold, and P-f by 8-fold vs. H2O-injected control oocytes. UT-B expression had no effect on the CO2-induced Delta pHS but doubled the NH3-induced Delta pH(S). Phloretin reduced UT-B-dependent urea uptake (J*(urea) ) by 45%, P-f* by 50%, and (- Delta pH(S)*)NH3 by 70%. p-Chloromercuribenzene sulfonate reduced J*urea by 25%, P*(f) by 30%, and (Delta pH(S)*)(NH3) 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 H2O and NH3 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 H2O and NH3 permeability. Our data confirm that UT-B has significant H2O permeability and for the first time demonstrate significant NH3 permeability. Thus the UTs become the third family of gas channels. Inhibitor and mutagenesis studies and results of MD simulations suggest that NH3 and H2O pass through the three monomeric urea channels in UT-B.
