What stabilizes pre-folded structures in the intrinsically disordered α-helical binding motifs?
Many examples are known of regions of intrinsically disordered proteins (IDPs) that fold into -helices upon binding their globular protein targets. In their unbound state these regions possess a small amount of residual helicity, referred to as pre-folded structure, which has been studied on case by case basis. In order to investigate what determines these pre-folded structures we compiled a database of peptides that fold-upon-binding, and experimentally characterized their helicity in the unbound and target-bound state. These regions are more hydrophobic and lack proline residues compared to IDPs in general. On average they possess about 17% helicity in the pre-folded state and gain 40% of helicity upon target binding. We observe that the locations of pre-folded helical regions strongly overlap with those in the targetbound IDPs. To understand this correlation, we analyzed per-residue energetic contributions stabilizing helical structure and found that target-interacting IDP have higher helix propensity. Notably, leucine is the most common residue involved in IDP-target interactions and, due to its high helix propensity, it strongly stabilizes pre-folded helical structures. For many IDP binding motifs, particularly those enriched in leucine, we observe that they not only mediate target-interactions but also confer stability to the pre-folded structure. Collectively, this shows that the formation of pre-folded helical elements is coupled to the IDP-target interactions, explaining why such elements are a common feature of -helical binding motifs. Moreover, it probably explains the preference for leucine among IDP-target hotspots, even though this residue is underrepresented among hotspots in the interfaces between globular proteins.