Upon mechanical pulling at either terminal end, β barrel outer membrane proteins stepwise unfold β strands or β hairpins until entirely extracted from the membrane. This unique unfolding pathway has been described for β barrels comprising 8, 14, or 22 β strands. Here we mechanically unfold the 18-stranded β barrel outer membrane protein LamB from Escherichia coli. We find that its mechanical unfolding pathway is shaped by the stepwise unfolding of β hairpins. However, we also observe that β hairpins can unfold groupwise. Thereby, β hairpins unfolding at higher pulling forces show a higher probability to unfold collectively, whereas β hairpins unfolding at lower forces tend to unfold individually. This result suggests that the collective unfolding of β hairpins resembles a far-from-equilibrium process, whereas the unfolding of individual β hairpins describes a closer-to-equilibrium process. Our findings support a direct link between outer membrane protein structure and the unfolding pathway and contribute to a better understanding of their unfolding in response to mechanical stress.
Keywords: Bacterial outer membrane proteins; LamB; SMFS; atomic force microscopy; high-resolution AFM; maltoporin; membrane protein unfolding; single-molecule force spectroscopy; β barrels; β hairpins.
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