Bug, for Boardetella Uptake Gene, proteins are a large family of periplasmic solute-binding proteins abundant in beta-proteobacteria. They adopt a characteristic Venus flytrap fold with two globular domains bisected by a ligand-binding cleft PMID:16403514. These proteins are specific for carboxylated solutes and show a characteristic mode of binding which involves two highly conserved beta strand-beta turn-alpha helix motifs originating from each domain. Structural domains comprising this superfamily share the structure of domain 1, involved in ligand recognition. However, stabilization of the closed conformation requires proper interactions to be established with domain 2, and thus domain 2 discriminates between productively and non-productively bound ligands PMID:17870093.