Lactose Transport System of Streptococcus thermophilus. Functional Reconstitution of the Protein and Characterization of the Kinetic Mechanism of Transport

The kinetic mechanism of the lactose transport system of Streptococcus thermophilus was studied in membrane vesicles fused with cytochrome c oxidase containing liposomes and in proteoliposomes in which cytochrome c oxidase was coreconstituted with the lactose transport protein. Selective manipulation of the components of the proton (and sodium) motive force indicated that both a membrane potential and a PH gradient could drive transport. The galactoside/proton stoichiometry was close to unity. Experiments which discriminate between the effects of internal pH and DELTApH as driving force on galactoside/proton symport showed that the carrier is highly activated at alkaline internal pH values, which biases the transport system kinetically toward the pH component of the proton motive force. Galactoside efflux increased with increasing pH with a pK(a) of about 8, whereas galactoside exchange (and counterflow) exhibited a pH optimum around 7 with pK(a) values of 6 and 8, respectively. Imposition of DELTApH (interior alkaline) retarded the rate of efflux at any pH value tested, whereas the rate of exchange was stimulated by an imposed DELTApH at pH 5.8, not affected at pH 7.0, and inhibited at pH 8.0 and 9.0. The results have been evaluated in terms of random and ordered association/dissociation of galactoside and proton on the inner surface of the membrane. Imposition of DELTAPSI (interior negative) decreased the rate of efflux but had no effect on the rate of exchange, indicating that the unloaded transport protein carries a net negative charge and that during exchange and counterflow the carrier recycles in the protonated form.</p

Characterization and functional expression in Escherichia coli of the sodium/proton/glutamate symport proteins of Bacillus stearothermophilus and Bacillus caldotenax

The genes encoding the Na+/H+/L-glutamate symport proteins of the thermophilic organisms Bacillus stearothermophilus (gltT(Bs)) and Bacillus caldotenax (gltT(Bc)) were cloned by complementation of Escherichia coli JC5412 for growth on glutamate as sole source of carbon, energy and nitrogen. The nucleotide sequences of the gltT(Bs) and gltT(Bc), genes were determined. In both cases the translated sequences corresponded with proteins of 421 amino acid residues (96.7% amino acid identity between GltT(Bs) and GltT(Bc)). Putative promoter, terminator and ribosome-binding-site sequences were found in the flanking regions. These expression signals were functional in E coli. The hydropathy profiles indicate that the proteins are hydrophobic and could form 12 membrane-spanning regions. The Na+/H+ CoUpled L-glutamate symport proteins GltT(Bs) and GltT(Bc) are homologous to the strictly H+ coupled L-glutamate transport protein of E. coli K-12 (overall 57.2% identity). Functional expression of glutamate transport activity was demonstrated by uptake of glutamate in whole cells and membrane vesicles. In accordance with previous observations (de Vrij et al., 1989; Heyne et al., 1991), glutamate uptake was driven by the electrochemical gradients of sodium ions and protons