The free energy folding penalty accompanying binding of intrinsically disordered α-helical motifs.

Intrinsically disordered proteins (IDPs) are abundant in eukaryotic proteomes and preform critical roles in many cellular processes, most often through the association with globular proteins. Despite lacking a stable three-dimensional structure by themselves, they may acquire a defined conformation upon binding globular targets. The most common type of secondary structure acquired by these binding motifs entails formation of an α-helix. It has been hypothesized that such disorder-to-order transitions are associated with a significant free energy penalty due to IDP folding, which reduces the overall IDP-target affinity. However, the exact magnitude of IDP folding penalty in α-helical binding motifs has not been systematically estimated. Here, we report the folding penalty contributions for 30 IDPs undergoing folding-upon-binding and find that the average IDP folding penalty is +2.0 kcal/mol and ranges from 0.7 to 3.5 kcal/mol. We observe that the folding penalty scales approximately linearly with the change in IDP helicity upon binding, which provides a simple empirical way to estimate folding penalty. We analyze to what extent do pre-structuring and target-bound IDP dynamics (fuzziness) reduce the folding penalty and find that these effects combined, on average, reduce the folding cost by around half. Taken together, the presented analysis provides a quantitative basis for understanding the role of folding penalty in IDP-target interactions and introduces a method estimate this quantity. Estimation and reduction of IDP folding penalty may prove useful in the rational design of helix-stabilized inhibitors of IDP-target interactions. STATEMENT: The α-helical binding motifs are ubiquitous among the intrinsically disordered proteins (IDPs). Upon binding their targets, they undergo a disorder-to-order transition, which is accompanied by a significant folding penalty whose magnitude is generally not known. Here, we use recently developed statistical-thermodynamic model to estimate the folding penalties for 30 IDPs and clarify the roles of IDP pre-folding and bound-state dynamics in reducing the folding penalty.