  Single-molecule pulling experiments on unstructured proteins linked to
neurodegenerative diseases have measured rupture forces comparable to those for
stable folded proteins. To investigate the structural mechanisms of this
unexpected force resistance, we perform pulling simulations of the amyloid
{\beta}-peptide (A{\beta}) and {\alpha}-synuclein ({\alpha}S), starting from
simulated conformational ensembles for the free monomers. For both proteins,
the simulations yield a set of rupture events that agree well with the
experimental data. By analyzing the conformations right before rupture in each
event, we find that the mechanically resistant structures share a common
architecture, with similarities to the folds adopted by A{\beta} and {\alpha}S
in amyloid fibrils. The disease-linked Arctic mutation of A{\beta} is found to
increase the occurrence of highly force-resistant structures. Our study
suggests that the high rupture forces observed in A{\beta} and {\alpha}S
pulling experiments are caused by structures that might have a key role in
amyloid formation.
