Critical sequence motifs. (a) Aligned sequences of 5 representative PDZ domains: α1-syntrophin(1/1) (PDB ID:2pdz), NHERF1(1/2) (PDB ID:1i92), Harmonin(2/3) (PDB ID:2kbs), Pick1(1/1) (PDB ID:2pku) and PTP-BL(2/5) (PDB ID:1vj6). While first row indicates the aligned sequence of corresponding PDZ domain, second row represents the sequence in seven class amino acid types. Secondary structure positions of the PDZ sequences are represented graphically at the top (αA, Αb, βA-βF). Three sequence motifs ("12", "16", "25") proposed to account for ligand specificity are indicated by yellow highlight. (b) Cartoon diagrams of these PDZ domains, motifs "12", "16" and "25" are colored in red and shown in stick form.