Sequential Unfolding of Beta Helical Protein by Single-Molecule Atomic Force Microscopy

The parallel beta helix is a common fold among extracellular proteins, however its mechanical properties remain unexplored. In Gram-negative bacteria, extracellular proteins of diverse functions of the large 'TpsA' family all fold into long beta helices. Here, single-molecule atomic force microscopy and steered molecular dynamics simulations were combined to investigate the mechanical properties of a prototypic TpsA protein, FHA, the major adhesin of Bordetella pertussis. Strong extension forces were required to fully unfold this highly repetitive protein, and unfolding occurred along a stepwise, hierarchical process. Our analyses showed that the extremities of the beta helix unfold early, while central regions of the helix are more resistant to mechanical unfolding. In particular, a mechanically resistant subdomain conserved among TpsA proteins and critical for secretion was identified. This nucleus harbors structural elements packed against the beta helix that might contribute to stabilizing the N-terminal region of FHA. Hierarchical unfolding of the beta helix in response to a mechanical stress may maintain beta-helical portions that can serve as templates for regaining the native structure after stress. The mechanical properties uncovered here might apply to many proteins with beta-helical or related folds, both in prokaryotes and in eukaryotes, and play key roles in their structural integrity and functions.