Cellular localization of the MalG protein from the maltose transport system in Escherichia coli K12.

In Escherichia coli five proteins are directly involved in the high affinity transport system for maltose and maltodextrins. Sequence data and genetic evidence indicate that three of them, MalF, MalG and MalK, are associated with the inner membrane. In order to characterize the MalG protein more thoroughly and to determine its subcellular localization under physiological conditions, we generated well-defined mutations induced in vitro that affect the size and the function of the malG gene product. Wild-type and mutant proteins were detected in total bacterial extracts and on purified subcellular fractions with the help of an antibody prepared against a synthetic peptide based on the predicted N-terminal sequence of MalG. The protein was solubilized from crude membranes by the detergent Triton X-100, indicating that it was localized in the inner membrane. A mutant carrying an in-phase insertion of four amino acids into a region conserved in several inner membrane proteins from periplasmic transport systems displayed a maltose-negative phenotype. This suggested that this region is very probably essential for MalG function.