The C-terminal substrate-binding domain of LysR-type transcriptional regulator HvrB, an activator of S-adenosyl-L-homocysteine hydrolase expression, contains the type 2 periplasmic binding fold.
The transcriptional regulator HvrB of the LysR family is required for the light-dependent activation of both ahcY, which encoding the enzyme S-adenosyl-L-homocysteine hydrolase (AdoHcyase) that responsible for the reversible hydrolysis of AdoHcy to adenosine and homocysteine, and orf5, a gene of unknown. The topology of this C-terminal domain of HvrB 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.