The mechanism of amyloid fibril growth from Φ-value analysis

Amyloid fibrils are highly stable misfolded protein assemblies that play an important role in several neurodegenerative and systemic diseases. Although structural information of the amyloid state is now abundant, mechanistic details about the misfolding process remain elusive. Inspired by the Φ-value analysis of protein folding, we combined experiments and molecular simulations to resolve amino-acid contacts and determine the structure of the transition-state ensemble—the rate-limiting step—for fibril elongation of PI3K-SH3 amyloid fibrils. The ensemble was validated experimentally by Tanford β analysis and computationally by free energy calculations. Although protein folding proceeds on funnel-shaped landscapes, here we find that the energy landscape for the misfolding reaction consists of a large ‘golf course’ region, defined by a single energy barrier and transition state, accessing a sharply funnelled region. Thus, misfolding occurs by rare, successful monomer–fibril end collisions interspersed by numerous unsuccessful binding attempts. Taken together, these insights provide a quantitative and highly resolved description of a protein misfolding reaction.