A systematic exploration of the influence of the protein stability on amyloid fibril formation in vitro.

There are a number of diseases in which normally soluble proteins associate into regular, insoluble amyloid fibrils. The development of in vitro model systems in which detailed structural, kinetic, and thermodynamic characterization are feasible is of critical importance to our understanding of the amyloid fibril phenomenon. The formation of amyloid fibrils by proteins that are not associated with disease has been recently described, suggesting that this may be a common property of many proteins and not only of the few proteins associated with amyloidoses. The B1 Ig-binding domain of protein G (beta1) is an extremely well-characterized model system. We have found that under certain experimental conditions, some variants of beta1 form fibrils with high reproducibility. By controlling the stability of the protein-either by mutations or by changing experimental conditions-we are able to modulate the ability of the protein to form fibrils. For all of the variants, we find that the key requirement for fibril formation is to choose conditions in which the population of intermediate conformations present during the unfolding transition is maximized.