Channel formation by FhaC, the outer membrane protein involved in the secretion of the Bordetella pertussis filamentous hemagglutinin

Many virulence factors of pathogenic microorganisms are presented at the ce ll surface, However, protein secretion across the outer membrane of Gram-ne gative bacteria remains poorly understood. Here we used the extremely effic ient secretion of the Bordetella pertussis filamentous hemagglutinin (FHA) to decipher this process. FHA secretion requires a single specific accessor y protein, FhaC, the prototype of a family of proteins necessary for the ex tracellular localization of various virulence proteins in Gram-negative bac teria. We show that FhaC is heat-modifiable and localized in the outer memb rane. Circular dichroism spectra indicated that FhaC is rich in beta-strand s, in agreement with structural predictions for this protein. We further de monstrated that FhaC forms pores in artificial membranes, as evidenced by s ingle-channel conductance measurements through planar lipid bilayers, as we ll as by liposome swelling assays and patch-clamp experiments using proteol iposomes, Single-channel conductance appeared to fluctuate very fast, sugge sting that the FhaC channels frequently assume a closed conformation. We th us propose that FhaC forms a specific beta-barrel channel in the outer memb rane for the outward translocation of FHA.