The C-terminal substrate-binding domian of LysR-type transcriptional regulator MdcR, which involved in the malonate catabolism contains the type 2 periplasmic binding fold.
This family includes the C-terminal substrate binding domain of LysR-type transcriptional regulator (LTTR) MdcR that controls the expression of the malonate decarboxylase (mdc) genes. Like other members of the LTTRs, MdcR is a positive regulatory protein for its target promoter and composed of two functional domains joined by a linker helix involved in oligomerization: an N-terminal HTH (helix-turn-helix) domain, which is responsible for the DNA-binding specificity, and a C-terminal substrate-binding domain, which is structurally homologous to the type 2 periplasmic binding proteins (PBP2). The PBP2 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.