The requirement for DsbA in pullulanase secretion is independent of disulphide bond formation in the enzyme.

Results from previous studies have suggested that an intramolecular disulphide bond in the exoprotein pullulanase is needed for its recognition and transport across the outer membrane. This interpretation of the data is shown here to be incorrect: pullulanase devoid of all potential disulphide bonds is secreted with apparently the same efficiency as the wild-type protein. Furthermore, the periplasmic disulphide bond, oxidoreductase DsbA, previously shown to catalyse the formation of a disulphide bond in pullulanase and to decrease its transit time in the periplasm, is shown here to be required for the rapid secretion of pullulanase devoid of disulphide bonds. Several possible explanations for the role of DsbA in pullulanase secretion are discussed.