Sulfate ester binding protein-like, the type 2 periplasmic binding protein fold.
This subfamily includes the periplasmic component of putative ABC-type sulfonate transport system similar to SsuA. These domains are found in eubacterial SsuA proteins that serve as initial receptors in the ABC transport of bicarbonate, nitrate, taurine, or a wide range of aliphatic sulfonates, while other closest homologs are involved in thiamine (vitamin B1) biosynthetic pathway and desulfurization (DszB). After binding the ligand, SsuA interacts with a cognate membrane transport complex comprised of two integral membrane domains and two cytoplasmically located ATPase domains. This interaction triggers the ligand translocation across the cytoplasmic membrane energized by ATP hydrolysis. The SsuA proteins belong to the PBP2 superfamily of periplasmic binding proteins that differ in size and ligand specificity, but have similar tertiary structures consisting of two globular subdomains connected by a flexible hinge. They have been shown to 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.",