Identification of the erythrocyte Rh blood group glycoprotein as a mammalian ammonium transporter

The Rh blood group proteins are well known as the erythrocyte targets of the potent antibody response that causes hemolytic disease of the newborn. These proteins have been described in molecular detail; however, little is known about their function. A transport function is suggested by their predicted structure and from phylogenetic analysis. To obtain evidence for a role in solute transport, we expressed Rh proteins in Xenopus oocytes and now demonstrate that the erythroid Rh-associated glycoprotein mediates uptake of ammonium across cell membranes. Rh-associated glycoprotein carrier-mediated uptake, characterized with the radioactive analog of ammonium [(14)C]methylamine (MA), had an apparent EC(50) of 1.6 mm and a maximum uptake rate (V(max)) of 190 pmol/oocyte/min. Uptake was independent of the membrane potential and the Na(+) gradient. MA transport was stimulated by raising extracellular pH or by lowering intracellular pH, suggesting that uptake was coupled to an outwardly directed H(+) gradient. MA uptake was insensitive to additions of amiloride, amine-containing compounds tetramethyl- and tetraethylammonium chloride, glutamine, and urea. However, MA uptake was significantly antagonized by ammonium chloride with inhibition kinetics (IC(50) = 1.14 mm) consistent with the hypothesis that the uptake of MA and ammonium involves a similar H(+)-coupled counter-transport mechanism.