Extracellular transport of cholera toxin B subunit using Neisseria IgA protease beta-domain: conformation-dependent outer membrane translocation.

The beta-domain of the Neisseria IgA protease precursor (Iga) provides the essential transport function for the protease across the outer membrane. To investigate the secretion function of the beta-domain (Iga beta), we engineered hybrid proteins between Iga beta and the non-toxic 12 kd cholera toxin B subunit (CtxB) and examined their targeting behaviour in Salmonella typhimurium. We show that CtxB-Iga beta hybrid proteins integrate into the outer membrane, leading to the exposition of the CtxB moiety on the cell surface. Exposed CtxB can be degraded by externally added proteases like trypsin, but can also be specifically cleaved off from membrane-associated Iga beta by purified IgA protease. We further demonstrate that folding of the CtxB moiety at the periplasmic side of the outer membrane interferes with its translocation. Prevention of disulphide-induced folding in periplasmic CtxB renders the protein moiety competent for outer membrane transport. Iga beta may be of general interest as an export vehicle for even larger proteins from Gram-negative bacteria.