Type 1 periplasmic binding fold superfamily. This model and hierarchy represent the ligand binding domains of the LacI family of transcriptional regulators, periplasmic binding proteins of the ABC-type transport systems, the family C G-protein couples receptors (GPCRs), membrane bound guanylyl cyclases including the family of natriuretic peptide receptors (NPRs), and the N-terminal leucine/isoleucine/valine- binding protein (LIVBP)-like domains of the ionotropic glutamate receptors (iGluRs). In LacI-like transcriptional regulator and the bacterial periplasmic binding proteins the ligands are monosaccharides including lactose, ribose, fructose, xylose, arabinose, galactose/glucose, and other sugars, with a few exceptions. Periplasmic sugar binding proteins are one of the components of ABC transporters and are involved in the active transport of water-soluble ligands. The LacI family of proteins consists of transcriptional regulators related to the lac repressor. In this case, the sugar binding domain binds a sugar which changes the DNA binding activity of the repressor domain. The periplasmic binding proteins are the primary receptors for chemotaxis and transport of many sugar based solutes. The core structures of periplasmic binding proteins are classified into two types, and they differ in number and order of beta strands: type 1 has six beta strands, while type 2 has five beta strands per sub-domain. These two structural folds are thought to be distantly related via a common ancestor. Notably, while the N-terminal LIVBP-like domain of iGluRs belongs to the type 1 periplasmic-binding fold protein superfamily, the glutamate-binding domain of the iGluR is structurally similar to the type 2 periplasmic-binding fold.