Toxin secretion in Bordetella pertussis: breaking the Gram-negative barrier

Abstract

Unlike cells of any other living creature, Gram-negative bacteria have two membranes. This presents a severe barrier to moving compounds in and out of the cell. Pathogenic Gram-negative bacteria in particular have devised unique strategies to secrete the potent toxins and virulence factors they find necessary to subvert the host defenses. Bordetella pertussis, the causative agent of human whooping cough is no exception. This microorganism produces three potent protein toxins: dermonecrotic toxin, pertussis toxin and adenylate cyclase toxin (Weiss & Hewlett, 1986), each of which localizes to a different cellular compartment. The dermonecrotic toxin is cytoplasmic, and appears to be incapable of crossing any membranes. Both pertussis toxin and adenylate cyclase toxin, however, are found outside of the bacterial cell. Pertussis toxin is secreted into the culture supernatant, while adenylate cyclase toxin remains bound to the outer membrane. These two toxins appear to utilize entirely different secretion pathways. The adenylate cyclase toxin utilizes a protein-specific, Sec-independent pathway. In contrast, each of the five pertussis toxin subunits has a secretion signal-sequence which should allow the protein to be secreted past the first membrane and into the periplasm by the non-specific Sec-dependent pathway. It was thought that secretion outside of the second membrane would occur spontaneously once the protein made it to the periplasm. We have identified new bacterial mutants which suggest that this is incorrect, and that in addition to the generalized Sec-dependent pathway, a novel secretion apparatus is required for pertussis toxin secretion. This pertussis toxin secretion pathway shares homology with post-periplasmic secretion pathways present in other Gram-negative pathogens.