The human Rhesus-associated RhAG protein and a kidney homologue promote ammonium transport in yeast

The Rhesus blood-group antigens are defined by a complex association of mem brane polypeptides that includes the nonglycosylated Rh proteins (RhD and R hCE) and the RHag glycoprotein, which is strictly required for cell surface expression of these antigens', 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 Rh AG and Rh proteins remains unknown, except that their absence in Rh-null in dividuals leads to morphological and functional abnormalities of erythrocyt es, known as the Rh-deficiency syndrome. We recently found significant sequ ence similarity between the Rh family proteins, especially RhAG, and Mep/Am t ammonium transporters(4,5). We show here that RhAG and also RhGK, a new h uman homologue expressed in kidney cells only, function as ammonium transpo rt proteins when expressed in yeast. Both specifically complement the growt h 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 ammoni um export. Our results provide the first experimental evidence for a direct role of RhAG and RhGK in ammonium transport. These findings are of high in terest, because no specific ammonium transport system has been characterize d so far in human.