DNAJB6b: a complete amyloid inhibitor

In Alzheimer's disease (AD), the aggregation of amyloid {beta} (A{beta}) peptides has been linked to disease pathology. Once aggregates have formed, existing fibrils catalyze the formation of new fibrils in a runaway process associated with neuro degeneration. One component of the cellular defense system against amyloid formation is the molecular chaperone DNAJB6b (JB6). JB6 is known to suppress both primary nucleation and fibril-catalyzed secondary nucleation. However, the extent of JB6 binding to A{beta} fibrils and whether this interaction leads to inhibition of fibril elongation also, have not been established. Here, we combine well-characterized aggregation kinetics of A{beta}42 and the knowledge of JB6 self-assembly equilibrium and kinetics, with high-sensitivity HPLC measurements of free JB6 at nanomolar concentrations to determine the affinity of JB6 for A{beta}42 fibrils. We show that JB6 binds to A{beta}42 fibrils with high affinity and that this binding can quantitatively explain the observed elongation inhibition. Diffusion-based techniques reveal that JB6 binds to amyloid intermediates, resulting in potent suppression of amyloid nucleation until the pool of free JB6 is depleted, after which rapid bulk aggregation ensues. Our results provide a mechanistic link between chaperone-fibril binding and amyloid suppression, offering a quantitative framework for understanding how molecular chaperones regulate amyloid formation.