Editing ABC transporter (section)

=== Small ABC importers ===
Structures of the ModBC-A and MalFGK<sub>2</sub>-E, which are in complex with their binding protein, correspond to small (Type I) ABC importers. The TMDs of ModBC-A and MalFGK<sub>2</sub>-E have only six helices per subunit. The homodimer of ModBC-A is in a conformation in which the TM subunits (ModB) orient in an inverted V-shape with a cavity accessible to the cytoplasm. The ABC subunits (ModC), on the other hand, are arranged in an open, nucleotide-free conformation, in which the P-loop of one subunit faces but is detached from the LSGGQ motif of the other. The binding protein ModA is in a closed conformation with substrate bound in a cleft between its two lobes and attached to the extracellular loops of ModB, wherein the substrate is sitting directly above the closed entrance of the transporter. The MalFGK<sub>2</sub>-E structure resembles the catalytic [[transition state]] for ATP hydrolysis. It is in a closed conformation where it contains two ATP molecules, sandwiched between the Walker A and B motifs of one subunit and the LSGGQ motif of the other subunit. The maltose binding protein (MBP or MalE) is docked on the periplasmic side of the TM subunits (MalF and MalG) and a large, occluded cavity can be found at the interface of MalF and MalG. The arrangement of the TM helices is in a conformation that is closed toward the cytoplasm but with an opening that faces outward. The structure suggests a possibility that MBP may stimulate the [[ATPase]] activity of the transporter upon binding.<ref name=davidson/><ref name=rees/><ref name=oldham2008/>