The Molecular Mechanism of Dicarboxylic Acid Transport in Escherichia Coli K 12

It is the purpose of this communication to review the properties of the dicarboxylic acid transport system in Escherichia coli K 12, in particular the role of various dicarboxylate transport proteins, and the disposition of these components in the cytoplasmic membrane. The dicarboxylate transport system is an active process and is responsible for the uptake of succinate, fumarate, and malate. Membrane vesicles prepared from the EDTA, lysozyme, and osmotic shock treatment take up the dicarboxylic acids in the presence of an electron donor. Genetic analysis of various transport mutants indicates that there is only one dicarboxylic acid transport system present in Escherichia coli K 12, and that at least 3 genes, designated cbt, dct A, and dct B, are involved in this transport system. The products corresponding to the 3 genes are: a periplasmic binding protein (PBP) specified by cbt, and 2 membrane integral proteins, SBP 1 and SBP 2, specified by dct B and dct A, respectively. Components SBP 1 and SBP 2 appear to be exposed on both the inner and outer surfaces of the membrane, and lie in close proximity to each other. The substrate recognition sites of SBP 2 and SBP 1 are exposed on the outer and inner surfaces of the membrane respectively. The data presently available suggest that dicarboxylic acids may be translocated across the membrane via a transport channel. A tentative working model on the mechanism of translocation of dicarboxylic acids across the cell envelope by the periplasmic binding protein, and the 2 membrane carrier proteins is presented.