Sequence-function relationships in MalG, an inner membrane protein from the maltose transport system in Escherichia coli

The malG gene encodes a hydrophobic cytoplasmic membrane protein which is required for the energy-dependent transport of maltose and maltodextrins in Escherichia coli. The MalG protein, together with MalF and MalK proteins, forms a multimeric complex in the membrane consisting of two MalK subunits for each MalF and MalG subunit. Fifteen mutations have been isolated in malG by random linker insertion mutagenesis. Two regions essential for maltose transport have been identified. In particular, a hydrophilic region containing the peptidic motif EAA---G---------I-LP, highly conserved among inner membrane proteins from binding protein-dependent transport systems, is essential for maltose transport. The results also show that several regions of MalG are not essential for function. A region (residues 30-50) encompassing the first predicted transmembrane segment and the first periplasmic loop in MalG may be modified extensively with little effect on maltose transport and no effect on the stability and the localization of the protein. A region located at the middle of the protein (residues 153-157) is not essential for the function of the protein. A region, essential for maltodextrin utilization but not for maltose transport, has been identified near the C-terminus of the protein.