The Rhesus blood-group antigens are defined by a complex association of membrane polypeptides that includes the non-glycosylated Rh proteins (RhD and RhCE) and the RHag glycoprotein, which is strictly required for cell surface expression of these antigens1. RhAG and the Rh polypeptides are erythroid-specific transmembrane proteins belonging to the same family (36% identity)2,3. Despite their importance in transfusion medicine, the function of RhAG and Rh proteins remains unknown, except that their absence in Rhnull individuals leads to morphological and functional abnormalities of erythrocytes, known as the Rh-deficiency syndrome. We recently found significant sequence similarity between the Rh family proteins, especially RhAG, and Mep/Amt ammonium transporters4,5. We show here that RhAG and also RhGK, a new human homologue expressed in kidney cells only, function as ammonium transport proteins when expressed in yeast. Both specifically complement the growth defect of a yeast mutant deficient in ammonium uptake. Moreover, ammonium efflux assays and growth tests in the presence of toxic concentrations of the analogue methylammonium indicate that RhAG and RhGK also promote ammonium export. Our results provide the first experimental evidence for a direct role of RhAG and RhGK in ammonium transport. These findings are of high interest, because no specific ammonium transport system has been characterized so far in human.
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We thank C. Hattab for help in preparing rabbit antibodies; R. Gaber for yeast strains; and C. Jauniaux and S. Lecomte for technical contributions. This research was supported by The Commission of the European Communities and the Communauté Française de Belgique, Direction de la Recherche Scientifique. A.-M.M. is Chargé de recherches du Fonds National belge de la Recherche Scientifique. G.M. is currently a fellow of the International Centre for Genetic Engineering and Biotechnology.
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Marini, AM., Matassi, G., Raynal, V. et al. The human Rhesus-associated RhAG protein and a kidney homologue promote ammonium transport in yeast. Nat Genet 26, 341–344 (2000). https://doi.org/10.1038/81656
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