The ABC transporter MalFGK2 sequesters the MalT transcription factor at the membrane in the absence of cognate substrate

Abstract

MalK, the cytoplasmic component of the maltose ABC transporter from Escherichia coli is known to control negatively the activity of MalT, the activator of the maltose regulon, through complex formation. Here we further investigate this regulatory process by monitoring MalT activity and performing fluorescence microscopy analyses under various conditions. We establish that, under physiological conditions, the molecular entity that interacts with MalT is not free MalK, but the maltose transporter, MalFGK2, which sequesters MalT to the membrane. Furthermore, we provide compelling evidence that the transporter's ability to bind MalT is not constitutive, but strongly diminished when MalFGK2 is engaged in sugar transport. Notably, the outward-facing transporter, i.e. the catalytic intermediate, is ineffective in inhibiting MalT compared to the inward-facing state, i.e. the resting form. Analyses of available genetic and structural data suggest how the interaction between one inactive MalT molecule and MalFGK2 would be sensitive to the transporter state, thereby allowing MalT release upon maltose entrance. A related mechanism may underpin signalling by other ABC transporters. © 2012 Blackwell Publishing Ltd.