Substrate binding domain of osmoregulatory ABC-type glycine betaine/choline/L-proline transport system and related proteins; the type 2 periplasmic binding protein fold
This family is part of a high affinity multicomponent binding-protein-dependent ATP-binding cassette 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 betaines, choline, and L-proline. 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.
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