The courtship of proteins: understanding the encounter complex.

The formation of protein complexes involves an encounter complex, in which proteins show few specific interactions and assume many orientations. Recent kinetic and structural studies have shed light on this elusive state. It is generally dominated by electrostatic interactions, although hydrophobic interactions can play a role. During the encounter phase the proteins remain largely solvated. In extreme cases, the proteins only form an encounter complex, and in many other complexes, the encounter state constitutes a significant amount (5% or more), indicating that the energy difference between encounter and productive complexes is small. Thus, the encounter complex represents an essential part of protein complexes.