3.A.1.1.1
Maltooligosaccharide porter. The 3-D structure has been reported by Oldham et al. (2007). An altering access mechanism has been suggested for the maltose transporter resulting from rigid-body rotations (Khare et al., 2009). The maltose-binding protein is open in the catalytic transition state for ATP hydrolysis during maltose transport (Austermuhle et al. 2004). Bordignon et al. (2010) and Schneider et al. (2012) reviewed the extensive knowledge available on MalEFGK₂, its mode of action and its regulatory interactions.  The transporter sequesters the MalT transcriptional activator at the cytoplasmic surface of the membrane in the absence of the transport substrate (Richet et al. 2012).  The crystal structures of the transporter complex MBP-MalFGK₂ bound with large malto-oligosaccharide in two different conformational states have also been determined. In the pretranslocation structure, Oldham et al. 2013 found that the transmembrane subunit MalG forms two hydrogen bonds with malto-oligosaccharide at the reducing end. In the outward-facing conformation, the transmrembrane subunit MalF binds three glucosyl units from the nonreducing end. These structural features explain why large modified malto-oligosaccharides are not transported by MalFGK₂ despite their high binding affinity to MBP. In the transport cycle, substrate is channeled from MBP into the transmembrane pathway with a polarity such that both MBP and MalFGK₂ contribute to the overall substrate selectivity of the system (Oldham et al. 2013).  Stabilization of the semi-open MalK2 conformation by maltose-bound MBP is key to the coupling of maltose transport to ATP hydrolysis in vivo, because it facilitates the progression of the MalK dimer from the open to the semi-open conformation, from which it can proceed to hydrolyze ATP (Alvarez et al. 2015). Both the binding of MalE to the periplasmic side of the transmembrane complex and binding of ATP to MalK₂ are necessary to facilitate the conformational change from the inward-facing state to the occluded state, in which MalK₂ is completely dimerized (Hsu et al. 2017). An integrated transport mechanism of the maltose ABC importer has been proposed (Mächtel et al. 2019).