  Intrinsically disordered proteins (IDPs) abound in cellular regulation. Their
interactions are often transitory and highly sensitive to salt concentration
and posttranslational modifications. However, little is known about the effect
of macromolecular crowding on the kinetics and stability of the interactions of
IDPs with their cellular targets. Here, we investigate the influence of
crowding on the coupled folding and binding between two IDPs, using
polyethylene glycol as a crowding agent across a broad size range.
Single-molecule F\"orster resonance energy transfer allows us to quantify
several key parameters simultaneously: equilibrium dissociation constants,
kinetic association and dissociation rates, and translational diffusion
coefficients resulting from changes in microviscosity. We find that the
stability of the IDP complex increases not only with the concentration but also
with the size of the crowders, in contradiction to scaled-particle theory.
However, both the equilibrium and the kinetic results can be explained
quantitatively by depletion interactions if the polymeric properties of
proteins and crowders are taken into account. This approach thus provides an
integrated framework for addressing the complex interplay between depletion
effects and polymer physics on IDP interactions in a crowded environment.
