Clamshell dynamically accommodating
Bacterial genomes encode multiple distinct ABC importers to facilitate the acquisition of essential nutrients such as sugars, amino acids, vitamins, compatible solutes, and metal ions (Higgins, 1992; Davidson et al., 2008).Many ABC importers can transport more than one type of substrate molecule using high-affinity interactions between SBPs and transported ligands (herein termed cognate substrates) (Berntsson et al., 2010).Bending and unbending of the hinge region brings the two lobes together (closed conformation) or apart (open conformation), respectively.Crystallographic analyses show that the amount of opening varies between different SBPs; the lobe-movements observed range from small rearrangements as in the Type II SBP Btu F (Karpowich et al., 2003), to complete reorientation of both lobes by angles as large as 60° in the Type I SBP Liv J (Trakhanov et al., 2005).
Thus, it is assumed that the conformational switching of the SBPs enables the ABC transporter to allosterically sense the loading state of the SBP-ligand complex (‘translocation competency’), thereby contributing to transport specificity (Davidson et al., 2008; Quiocho and Ledvina, 1996).
ABC importers that employ SBPs can be subdivided as Type I or Type II based on structural and mechanistic distinctions (Locher, 2016; Swier et al., 2016).
A unifying feature of the transport mechanism of Type I and Type II ABC importers is the binding and delivery of substrate from a dedicated SBP to the translocator unit for import into the cytoplasm.
Intriguingly, in some cases, similar SBP conformations are formed by both transported and non-transported ligands.
In this case, the inability for transport arises from slow opening of the SBP or the selectivity provided by the translocator.
Search for clamshell dynamically accommodating:
Such findings suggest that only SBPs which adopt the closed conformation can productively interact with the translocator and initiate transport.