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

L-Glutamate transport by the H+-glutamate and Na+-glutamate symport pr oteins of Escherichia coli K-12 (GltP(Ec) and GltS(Ec), respectively) and the Na+-H+-glutamate symport proteins of Bacillus stearothermophil us (GltT(Bs)) and Bacillus caldotenax (GltT(Bc)) was studied in membra ne vesicles derived from cells in which the proteins were either homol ogously or heterologously expressed. Substrate and inhibitor specifici ty 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 Gl tP(Ec), GltT(Bs) and GltT(Bc) catalysed an electrogenic symport of L-g lutamate with greater than or equal to two H+, i.e. when the proteins were expressed in E. coli. Surprisingly studies in membrane vesicles o f B. stearothermophilus and B. caldotenax indicated a Na+-H+-L-glutama te symport for both GltT(Bs) and GltT(Bc). The Na+ dependency of the G ltT transporters in the Bacillus strains increased with temperature. T hese observations suggest that the conformation of the transport prote ins in the E. coli and the Bacillus membranes differs, which influence s the coupling ion selectivity.