Folding and binding of an intrinsically disordered protein: Fast, but not 'diffusion-limited'

Coupled folding and binding of intrinsically disordered proteins (IDPs) is prevalent in biology. As the first step toward understanding the mechanism of binding, it is important to know if a reaction is 'diffusion-limited' as, if this speed limit is reached, the association must proceed through an induced fit mechanism. Here, we use a model system where the 'BH3 region' of PUMA, an IDP, forms a single, contiguous α-helix upon binding the folded protein Mcl-1. Using stopped-flow techniques, we systematically compare the rate constant for association (k₊) under a number of solvent conditions and temperatures. We show that our system is not 'diffusion-limited', despite having a k₊ in the often-quoted 'diffusion-limited' regime (10 ⁵-10⁶ M^-1 s^-1 at high ionic strength) and displaying an inverse dependence on solvent viscosity. These standard tests, developed for folded protein-protein interactions, are not appropriate for reactions where one protein is disordered.