Formation of a Chloride-conducting State in the Maltose ATP-binding Cassette (ABC) Transporter

Abstract

ATP-binding cassette transporters use an alternating access mechanism to move substrates across cellular membranes. This mode of transport ensures the selective passage of molecules while preserving membrane impermeability. The crystal structures of MalFGK 2 , inward-and outward-facing, show that the transporter is sealed against ions and small molecules. It has yet to be determined whether membrane impermeability is maintained when MalFGK 2 cycles between these two conformations. Through the use of a mutant that resides in intermediate conformations close to the transition state, we demonstrate that not only is chloride conductance occurring, but also to a degree large enough to compromise cell viability. Introduction of mutations in the periplasmic gate lead to the formation of a channel that is quasi-permanently open. MalFGK 2 must therefore stay away from these ion-conducting conformations to preserve the membrane barrier; otherwise, a few mutations that increase access to the ion-conducting states are enough to convert an ATP-binding cassette transporter into a channel.

An integrated transport mechanism of the maltose ABC importer

“…In this configuration, open MalE also participates in the stimulation of ATPase activity by stabilizing the transition state for ATP hydrolysis [69], [72], [95], [112]. In agreement with this idea, several studies suggested that the maltose transporter binding-protein independent (BPI) mutants may shift the equilibrium from the inward-facing state to the outward-facing state [66], [110], [113], [114], [115], [116], thereby explaining how these mutants hydrolyze ATP in the absence of MalE [71], [73], [117]. Accordingly, most of the primary BPI mutations were found at the interface between MalF and MalG and were predicted to destabilize the inward-facing conformation [84].…”