Aspects of maltose transport in Escherichia coli: established facts and educated guesses.

The transport system can translocate maltose and maltodextrins (up to 7 glucose moieties) in chemically unmodified form against concentration gradients that can reach 1:10(5). To overcome the problems of diffusion of substrate across the outer membrane at substrate concentrations below 0.1 mM, the receptor for phage lambda has to be present in the outer membrane. The facilitated diffusion via the lambda receptor is accomplished by its interaction with maltose-binding protein. The maltose-binding protein-substrate complex, but not free substrates is recognized at the cytoplasmic membrane. This implies that the maltose-binding protein is essential for substrate translocation and that the cytoplasmic membrane has no second maltodextrin recognition site. Substrate translocation through the cytoplasmic membrane is tightly coupled to energy consumption and unidirectionally inward. The energy source used for substrate accumulation is not the electrochemical potential of protons, but phosphate-bound energy, likely to be ATP. In mutants that are blocked in maltose metabolism, exit of maltose occurs in chemically modified form as acetylmaltose. It is energy-dependent, blocked by uncouplers of the proton-conductive type and is stimulated by energy sources. It is not mediated by any malB-dependent function. The membrane-bound components of the system may establish a non-specific pore that is triggered by its interaction with the maltose-binding protein bound to its substrate. After translocation of one molecule substrate the pore has to be re-energized, possibly by ATP hydrolysis.