Subunit interactions in ABC transporters: a conserved sequence in hydrophobic membrane proteins of periplasmic permeases defines an important site of interaction with the ATPase subunits.

The cytoplasmic membrane proteins of bacterial binding protein-dependent transporters belong to the superfamily of ABC transporters. The hydrophobic proteins display a conserved, at least 20 amino acid EAA---G---------I-LP region exposed in the cytosol, the EAA region. We mutagenized the EAA regions of MalF and MalG proteins of the Escherichia coli maltose transport system. Substitutions at the same positions in MalF and MalG have different phenotypes, indicating that EAA regions do not act symmetrically. Mutations in malG or malF that slightly affect or do not affect transport, determine a completely defective phenotype when present together. This suggests that EAA regions of MalF and MalG may interact during transport. Maltose-negative mutants fall into two categories with respect to the cellular localization of the MalK ATPase: in the first, MalK is membrane-bound, as in wild-type strains, while in the second, it is cytosolic, as in strains deleted in the malF and malG genes. From maltose-negative mutants of the two categories, we isolated suppressor mutations within malK that restore transport. They map mainly in the putative helical domain of MalK, suggesting that EAA regions may constitute a recognition site for the ABC ATPase helical domain.