Substrate binding domain of ABC-type histidine/lysine/arginine/ornithine transporters and related proteins; the type 2 periplasmic-binding protein fold
This family comprises the periplasmic substrate-binding proteins, including the lysine-, arginine-, ornithine-binding protein (LAO) and the histidine-binding protein (HisJ), which serve as initial receptors for active transport. HisJ and LAO proteins belong to the type 2 periplasmic-binding fold protein (PBP2) superfamily, whose members are involved in chemotaxis and uptake of nutrients and other small molecules from the extracellular space as a primary receptor. PBP2 typically comprises 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. After binding their specific ligand with high affinity, they can interact with a cognate membrane transport complex comprised of two integral membrane domains and two receptor cytoplasmically-located ATPase domains. This interaction triggers the ligand translocation across the cytoplasmic membrane energized by ATP hydrolysis.
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