F. Jacob-dubuisson et al., Channel formation by FhaC, the outer membrane protein involved in the secretion of the Bordetella pertussis filamentous hemagglutinin, J BIOL CHEM, 274(53), 1999, pp. 37731-37735
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.