CATION-SELECTIVITY OF THE L-GLUTAMATE TRANSPORTERS OF ESCHERICHIA-COLI, BACILLUS-STEAROTHERMOPHILUS AND BACILLUS-CALDOTENAX - DEPENDENCE ON THE ENVIRONMENT IN WHICH THE PROTEINS ARE EXPRESSED

L-Glutamate transport by the H+-glutamate and Na+-glutamate symport proteins of Escherichia coli K-12 (GltP(Ec) and GltS(Ec), respectively) and the Na+-H+-glutamate symport proteins of Bacillus stearothermophilus (GltT(Bs)) and Bacillus caldotenax (GltT(Bc)) was studied in membrane vesicles derived from cells in which the proteins were either homologously or heterologously expressed. Substrate and inhibitor specificity studies indicate that GltP(Ec), GltT(Bs) and GltT(Bc) fall into the same group of transporters, whereas GltS(Ec) is distinctly different from the others. Also, the cation specificity of GltS(Ec) is different; GltS(Ec) transported L-glutamate with (at least) two Na+, whereas GltP(Ec), GltT(Bs) and GltT(Bc) catalysed an electrogenic symport of L-glutamate with greater than or equal to two H+, i.e. when the proteins were expressed in E. coli. Surprisingly studies in membrane vesicles of B. stearothermophilus and B. caldotenax indicated a Na+-H+-L-glutamate symport for both GltT(Bs) and GltT(Bc). The Na+ dependency of the GltT transporters in the Bacillus strains increased with temperature. These observations suggest that the conformation of the transport proteins in the E. coli and the Bacillus membranes differs, which influences the coupling ion selectivity.