Cross-talk between ammonium transporters in yeast and interference by the soybean SAT1 protein

Summary Ammonium uptake in the yeast Saccharomyces cerevisiae involves three membrane transporters (Mep1, 2 and -3) belonging to an evolutionarily conserved protein family that also includes the rhesus (Rh) blood group polypeptides of erythrocytes. We show here that, in the 26972c mutant defective in NH4 a transport, the Mep1 protein carrying an amino acid substitution in its cytoplasmic C-terminus transinhibits the closely related Mep3 protein. The same mutation introduced into Mep3 leads to loss of transport activity and this inactive form also trans-inhibits native Mep3. Inhibition of Mep3 is post-translational and can be overcome by overexpression. These results are consistent with a direct interaction between Mep proteins, as is the case for the Rh polypeptides. The soybean GmSAT1 gene, recently cloned for its ability to complement the NH4 a transport defect of strain 26972c, has been described as an NH4 a channel protein involved in the transfer of fixed nitrogen from the bacteroid to the host plant. We show here that GmSAT1 contains a sequence homologous to the DNA-binding domain of basic helix‐loop‐helix (bHLH) transcription factors. We also show that GmSAT1 restores NH4 a uptake in the yeast mutant by interfering with the inhibition of Mep3. Our results are not consistent with a direct role of GmSAT1 in ammonium transport.