Conformational plasticity of the type I maltose ABC importer

ATP-binding cassette (ABC) transporters couple the translocation of solutes across membranes to ATP hydrolysis. Crystal structures of the Escherichia coli maltose importer (MalFGK ₂) in complex with its substrate binding protein (MalE) provided unprecedented insights in the mechanism of substrate translocation, leaving the MalE–transporter interactions still poorly understood. Using pulsed EPR and cross-linking methods we investigated the effects of maltose and MalE on complex formation and correlated motions of the MalK ₂ nucleotide-binding domains (NBDs). We found that both substrate-free (open) and liganded (closed) MalE interact with the transporter with similar affinity in all nucleotide states. In the apo-state, binding of open MalE occurs via the N -lobe, leaving the C-lobe disordered, but upon maltose binding, closed MalE associates tighter to the transporter. In both cases the NBDs remain open. In the presence of ATP, the transporter binds both substrate-free and liganded MalE, both inducing the outward-facing conformation trapped in the crystal with open MalE at the periplasmic side and NBDs tightly closed. In contrast to ATP, ADP–Mg ²⁺ alone is sufficient to induce a semiopen conformation in the NBDs. In this nucleotide-driven state, the transporter binds both open and closed MalE with slightly different periplasmic configurations. We also found that dissociation of MalE is not a required step for substrate translocation since a supercomplex with MalE cross-linked to MalG retains the ability to hydrolyze ATP and to transport maltose. These features of MalE–MalFGK ₂ interactions highlight the conformational plasticity of the maltose importer, providing insights into the ATPase stimulation by unliganded MalE.