Schematic representation of class C GPCRs. Class C GPCRs share structural similarity with bacterial PBPs but have higher ligand-binding specificity. These receptors have a common structure consisting of an N-terminal extracellular VFTM, a transmembrane HD connected with VFTM via CRD, and a variable C-terminal intracellular tail. The bacterial PBPs have extensively ligand-binding properties, which are involved in transporting small nutritional molecules such as amino acids and vitamins. The class C GPCRs have a physiological function in shaping ligand-binding specificity. The GB2 group lacks the ability to directly bind ligands.