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

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Abstract
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+ transport, the Mep1 protein carrying an amino acid substitution in its cytoplasmic C‐terminus trans‐inhibits 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+ transport defect of strain 26972c, has been described as an NH4+ 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+ 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.