Posttranslational modifications on the PDZ ligands or PDZ domains modulate PDZ protein-protein interactions. (A) Phosphorylation of the PDZ ligand or PDZ domain inhibits PDZ interactions. The cross represents a reduced or abolished interaction. (B) Formation of intramolecular disulfide bond (symbol, 'ox') in the PDZ domain prevents binding of the other binding partner. (C, D) Phosphorylation or competitive binding changes the autoinhibited conformation.