Ligand-binding domain of the LacI-GalR family of transcription regulators and the sugar-binding domain of ABC-type transport systems
Ligand-binding domain of the LacI-GalR family of transcription regulators and the sugar-binding domain of ABC-type transport systems, all of which contain the type I periplasmic binding protein-like fold. Their specific ligands include lactose, ribose, fructose, xylose, arabinose, galactose/glucose, and other sugars. The LacI family of proteins consists of transcriptional regulators related to the lac repressor; in general the sugar binding domain in this family binds a sugar, which in turn changes the DNA binding activity of the repressor domain. The core structure of the periplasmic binding proteins is classified into two types and they differ in number and order of beta strands in each domain: type I, which has six beta strands, and type II, which has five beta strands. These two distinct structural arrangements may have originated from a common ancestor.