The C-terminal substrate binding domain of LysR-type transcriptional regulators involved in the catabolism of aromatic compounds and that of other related regulators, contains type 2 periplasmic binding fold.
This CD includes the C-terminal substrate binding domain of LTTRs involved in degradation of aromatic compounds, such as CbnR, BenM, CatM, ClcR and TfdR, as well as that of other transcriptional regulators clustered together in phylogenetic trees, including XapR, HcaR, MprR, IlvR, BudR, AlsR, LysR, and OccR. The structural topology of this substrate-binding domain is most similar to that of 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. Besides transport proteins, the PBP2 superfamily includes the substrate-binding domains from ionotropic glutamate receptors, LysR-like transcriptional regulators, and unorthodox sensor proteins involved in signal transduction.