Structural homology in the PA superfamily. The double beta-barrel that characterises the superfamily is highlighted in red. Shown ate representative structures from several families within the PA superfamily. Chymotrypsin (1gg6), tobacco etch virus protease (1lvm), calicivirin (1wqs), west nile virus protease (1fp7), exfoliative toxin (1exf), HtrA protease (1l1j), snake venom plasminogen activator (1bqy), chloroplast protease (4fln) and equine arteritis virus protease (1mbm).