Conformational Dynamics in the Binding-Protein-Independent Mutant of the Escherichia coli Maltose Transporter, MalG511, and Its Interaction with Maltose Binding Protein

MalG511 is a genetically selected binding-protein-independent mutant of the Escherichia coli maltose transporter MalFGK2, which retains specificity for maltose and shows a high basal ATPase activity in the absence of maltose binding protein (MBP). It shows an intriguing biphasic behavior in maltose transport assays in the presence of MBP, with low levels of MBP stimulating the activity and higher levels (>50 μM) inhibiting the transport activity. Remarkably, the rescuing effect of the MBP suppressor mutant, MBPG13D, turns it into an attractive model for studying regulatory mechanisms in the ABC transporter superfamily. It is hypothesized that the special characteristics of MalG511 result from mutations that shift its equilibrium toward the transition state of MalFGK2. We tested this hypothesis by using site-directed spin labeling in combination with electron paramagnetic resonance spectroscopy, which showed conformational changes in MalG511 and its interaction with MBP and MBPG13D during its catalytic cycle. We found that MalG511 utilizes the same alternate access mechanism as MalFGK2, including all three open, semi-open, and closed states of the MalK dimer, to transport maltose across the membrane. However, the equilibrium of this mutant is shifted toward the semi-open state in its resting state and interacts with MBP with high affinity, providing an explanation for the inhibition of MalG511 by MBP at higher concentrations. In contrast, the mutant binding protein, MBPG13D, interacts with lower affinity and could restore MalG511 to a normal catalytic cycle.