Sequence variation within the PBC and ligand specificity. (a) A schematic representation of the PBC showing the secondary structures and the consensus motif. (b) Families that contain a canonical and non-canonical PBC motif. Sequence alignment of the PBC region showing conserved and variable positions. Conserved residues are highlighted and Arg209 position is indicated by a black box. (c-f) The conformation of the PBC region in: the PKA regulatory subunit (PDB: 1RGS) (c); PDZ_GEF (PDB: 2D93) (d); cooA (PDB: 1FT9) (e); CprK (PDB: 2H6B) (f).