Protein hot spots: the islands of stability

Understanding the structural basis of protein-protein interactions (PPIs) may shed light on the organization and functioning of signal transduction and metabolic networks and may assist in structure-based design of ligands (drugs) targeting protein-protein interfaces. The residues at the bimolecular interface, designated as the hot spots, contribute most of the free binding energy of PPI. To date, there is no conclusive atomistic explanation for the unique functional properties of the hot spots. We hypothesized that backbone compliance may play a role in protein-protein recognition and in the mechanism of binding of small-molecule compounds to protein surfaces. We used a steered molecular dynamics simulation to explore the compliance properties of the backbone of surface-exposed residues in several model proteins: interleukin-2, mouse double minute protein 2 and proliferating cell nuclear antigen. We demonstrated that protein surfaces exhibit distinct patterns in which highly immobile residues form defined clusters ("stability patches") alternating with areas of moderate to high mobility. These "stability patches" tend to localize in functionally important regions involved in protein-protein recognition. We propose a mechanism by which the distinct structural organization of the hot spots may contribute to their role in mediating PPI and facilitating binding of structurally diverse small-molecule compounds to protein surfaces.