Maltose binding protein (MalE) interacts with periplasmic loops P2 and P1 respectively of the MalFG subunits of the maltose ATP binding cassette transporter (MalFGK2) from Escherichia coli Salmonella during the transport cycle

The ATP binding cassette (ABC-) transporter mediating the uptake of maltose/maltodextrins in Escherichia coli/Salmonella enterica serovar Typhimurium is one of the best characterized systems and serves as a model for studying the molecular mechanism by which ABC importers exert their functions. The transporter is composed of a periplasmic maltose binding protein (MaIE), and a membrane-bound complex (MaIFGK(2)), comprising the pore-forming hydrophobic subunits, MaIF and MalG, and two copies of the ABC subunit, MalK. We report on the isolation of suppressor mutations within maIFG that partially restore transport of a maltose-negative mutant carrying the maIK809 allele (MaIKQ140K). The mutation affects the conserved LSGGQ motif that is involved in ATP binding. Three out of four suppressor mutations map in periplasmic loops P2 and P1 respectively of MaIFG. Cross-linking data revealed proximity of these regions to MaIE. In particular, as demonstrated in vitro and in vivo, Gly-13 of substrate-free and substrate-loaded MaIE is in close contact to Pro-78 of MaIG. These data suggest that MalE is permanently in close contact to MaIG-P1 via its N-terminal domain. Together, our results are interpreted in favour of the notion that substrate availability is communicated from MaIE to the MaIK dimer via extracytoplasmic loops of MaIFG, and are discussed with respect to a current transport model.