Molecular mechanism of energy coupling in active transport

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Molecular mechanism of energy coupling in active transport of solutes across biological membranes was studied from biochemical and genetic points of view.First we analyzed the melibiose transport system of Escherichia coli, Salmonella typhimurium and Enterobacter aerogenes, the glucose transport system of Vibrio parahaemolyticus, and so on. We characterized these transport systems, and energy coupling in these systems were revealed. We constructed chimeric transport proteins from, for example, the melibiose transporter of E.coli and that of S.typhimurium Although the melibiose transporters of these two organisms are highly homologous in the primary structure, they showed different cation coupling. Analysis of the chimeric proteins revealed domains imvolved in the cation coupling, The glucose transporter of V.parahaemolyticus (Na+/glucose symporter) showed similarity in the primary structure with human SGLT (Na+/glucose symporter). Thus, several amino acid residues which seem to be important for cation recogonition or substrate (or inhibitor) recognition have been intentified.The sugar phosphate/inorganic phosphate antiporter of E.coli has been overexpressed and purified using several genetic techniques. The biochemical properties and energy coupling in the antiporter was characterized. The techniques utilized in the studies were revealed to be very useful in the study of membrane proteins, in general.Energy coupling in the Na+/H+ antiporters in E.coli, V.parahaemolyticus and Pseudomonas aeruginosa has been also investigated. We found that several amino acid residues in the Na+/H+ antiporter of E.coli are important for pH sensing or its activity. Membrane topology of the antiporter was revealed.Collaboration and discussion with foreign Scientists were very valuable.