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 (105?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.