Alternating Access in Maltose Transporter Mediated by Rigid-Body Rotations

ATP-binding cassette transporters couple ATP hydrolysis to substrate translocation through an alternating access mechanism, but the nature of the conformational changes in a transport cycle remains elusive. Previously we reported the structure of the maltose transporter MalFGK(2) in an outward-facing conformation in which the transmembrane (TM) helices outline a substrate-binding pocket open toward the periplasmic surface and ATP is poised for hydrolysis along the closed nucleotide-binding dimer interface. Here we report the structure of the nucleotide-free maltose transporter in which the substrate binding pocket is only accessible from the cytoplasm and the nucleotide-binding interface is open. Comparison of the same transporter crystallized in two different conformations reveals that alternating access involves rigid-body rotations of the TM subdomains that are coupled to the closure and opening of the nucleotide-binding domain interface. The comparison also reveals that point mutations enabling binding protein-independent transport line dynamic interfaces in the TM region.