Are protein-protein interfaces special regions on a protein's surface?

Protein-protein interactions (PPIs) are involved in many cellular processes. Experimentally obtained protein quaternary structures provide the location of protein-protein interfaces, the surface region of a given protein that interacts with another. These regions are termed half-interfaces (HIs). Canonical HIs cover roughly one third of a protein's surface and were found to have more hydrophobic residues than the non-interface surface region. In addition, the classical view of protein HIs was that there are a few (if not one) HIs per protein that are structurally and chemically unique. However, on average, a given protein interacts with at least a dozen others. This raises the question of whether they use the same or other HIs. By copying HIs from monomers with the same folds in solved quaternary structures, we introduce the concept of geometric HIs (HIs whose geometry has a significant match to other known interfaces) and show that on average they cover three quarters of a protein's surface. We then demonstrate that in some cases, these geometric HI could result in real physical interactions (which may or may not be biologically relevant). The composition of the new HIs is on average more charged compared to most known ones, suggesting that the current protein interface database is biased towards more hydrophobic, possibly more obligate, complexes. Finally, our results provide evidence for interface fuzziness and PPI promiscuity. Thus, the classical view of unique, well defined HIs needs to be revisited as HIs are another example of coarse-graining that is used by nature.