The C-terminal substrate binding domain of LysR-type transcriptional regulator TrpI, which is involved in control of tryptophan synthesis, contains type 2 periplasmic binding fold.
TrpI and indoleglycerol phosphate (InGP), are required to activate transcription of the trpBA, the genes for tryptophan synthase. The trpBA is induced by the InGp substrate, rather than by tryptophan, but the exact mechanism of the activation event is not known. This substrate-binding domain of TrpI shows significant homology to the type 2 periplasmic binding proteins (PBP2), which are responsible for the uptake of a variety of substrates such as phosphate, sulfate, polysaccharides, lysine/arginine/ornithine, and histidine. The PBP2 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 cytoplasmically located ATPase domains. This interaction triggers the ligand translocation across the cytoplasmic membrane energized by ATP hydrolysis.