The molecular mechanisms of dicarboxylic acid transport in Escherichia coli K12. The role and orientation of the two membrane-bound dicarboxylate binding proteins.

Previous communications from this laboratory have indicated that dicarboxylic acid transport in Escherichia coli is an active process, and that at least three genes are responsible for this transport system. In attempts to identify the transport components, one periplasmic binding protein and two membrane integral proteins (SBP 1 and SBP 2) were implicated to participate in the transport system in vivo. In the present communication, we demonstrate, through biochemical analysis of the transport mutants, that the two membrane transport genes, dctA and dctB, are responsible for the two membrane-bound dicarboxylate binding proteins, SBP 2 and SBP 1, respectively. We also find that the substrate recognition sites of SBP 1 and SBP 2 are exposed to the inner and outer surfaces of the membrane, respectively. This may have important implications for the role of SBP 1 and SBP 2 in the translocation process.