The assembly of transcriptional apparatus – coupled folding and binding studies with several intrinsically disordered proteins (#36)
Intrinsically disordered proteins (IDPs), which represent around a third of eukaryotic proteins, perform functions despite lacking well-defined structures. A subset of these actually fold on binding to a partner molecule, such as another protein or nucleic acid. One much-studied key hub protein is the coactivator CBP/p300, whose folded KIX domain binds a number of different intrinsically disordered transcription factors at two separate sites on its surface. In the cell the interplay between these proteins decides the expression profile.
We have performed comparative kinetic experiments to determine the association and dissociation rates for six separate disordered ligands with KIX, shedding light on the mechanism of these coupled folding and binding reactions. These are shown to be the fastest protein-protein interactions yet reported (without long-range electrostatic attraction), though they are still not diffusion-limited. We further describe how the behavior of the system is modified in the presence of other binding partners, and the general mechanistic basis for the positive allostery between the two binding sites of this prototypical transcription coactivator.