Substrate-binding domain of ABC-type histidine transporter involves in betaine and proline uptake; the type 2 periplasmic-binding protein fold
This subfamily is part of a high affinity multicomponent binding-protein-dependent transport system specific to certain quaternary ammonium compounds for osmoregulation. The periplasmic substrate-binding domain, which is often fused to the permease component of the ATP-binding cassette transporter complex, is involved in uptake of osmoprotectants (also termed compatible solutes) such as glycine betaine, proline betaine, choline, and carnitine. Many microorganisms accumulate these compatible solutes in response to high osmolarity to offset the loss of cell water. This domain belongs to the type 2 periplasmic binding fold protein superfamily (PBP2). The PBP2 proteins are typically comprised of two globular subdomains connected by a flexible hinge and bind their ligand in the cleft between these domains in a manner resembling a Venus flytrap.
Jiyao W et al.(2023). "The conserved domain database in 2023.",