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 (MalE), and a membrane-bound complex (MalFGK(2)), comprising the pore-forming hydrophobic subunits, MalF and MalG, and two copies of the ABC subunit, MalK. We report on the isolation of suppressor mutations within malFG that partially restore transport of a maltose-negative mutant carrying the malK809 allele (MalKQ140K). 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 MalFG. Cross-linking data revealed proximity of these regions to MalE. In particular, as demonstrated in vitro and in vivo, Gly-13 of substrate-free and substrate-loaded MalE is in close contact to Pro-78 of MalG. These data suggest that MalE is permanently in close contact to MalG-P1 via its N-terminal domain. Together, our results are interpreted in favour of the notion that substrate availability is communicated from MalE to the MalK dimer via extracytoplasmic loops of MalFG, and are discussed with respect to a current transport model.