The maltose transport system of Escherichia coli: Protein-protein and protein-ligand interactions required for substrate transport across the cytoplasmic membrane

The transport of maltose and maltodextrins across the cytoplasmic membrane of Escherichia coli is dependent upon the presence of the maltose binding protein (MBP). This periplasmic binding protein, which binds both maltose and maltodextrins with high affinity, mediates the transport of these sugar substrates from the periplasm into the cytoplasm through its interaction with the membrane-associated transport (MalFGK;MBP undergoes a conformational change upon binding its substrates, and the nature of this conformational change is dependent upon the particular substrate MBP complexes with. Previous studies have shown that two types of MBP complexes can be formed, one in which MBP binds to the substrate via the end-on mode and another in which the substrate binds to MBP by the middle mode. Using fluorescence emission and ultraviolet (UV) differential spectroscopy, the manner by which maltose, maltodextrins and their derivatives, and ;Fluorescence and UV differential spectroscopy show only the local environment of aromatic residues. To correlate the two binding modes with global conformational changes of MBP, electron spin resonance (ESR) spectroscopy was used. It was found that all substrates which bound to MBP via the end-on mode caused the N- and C-terminal domains of MBP to come together by approximately 6 A. This domain closure did not occur when MBP complexed with a substrate by the middle mode. These results strongly suggest that the middle binding mode is a physiologically inactive binding mode because it does not allow MBP to undergo the necessary conformational change required for its productive interaction with the MalFGK;The interaction of liganded MBP with the MalFGK...