Filamentous haemagglutinin (FHA) is the major adhesin of B. pertussis, the bacterium that causes whooping cough. It is the prototypical member of the Two-Partner Secretion pathway family, a class of proteins associated with virulence in Gram-negative bacteria. Such proteins are large yet efficiently exported across the bacterial outer membrane without an obvious energy source, suggesting the hypothesis that translocation is driven by folding. Here, we use magnetic tweezers to apply stable and constant forces to single molecules corresponding to the N-terminal 480 amino acids of FHA (which initiate outer membrane translocation) and observe equilibrium unfolding and refolding in multiple discrete steps. This distributed (rather than cooperative) folding of isolated FHA provides evidence for processive, vectorial folding in vivo.
