Protein-protein binding usually involves structural changes that may extend beyond the rearrangements on a local scale, and cannot be explained by a classical lock-and-key mechanism. Several models have been advanced to explain the flexible binding of proteins such as the induced fit mechanism where the ligand is postulated to induce a conformational change at the interaction site upon binding, or the preexisting equilibrium hypothesis that assumes that protein samples an ensemble of conformations at equilibrium conditions and that the ligand binds selectively to an active conformation. We explored the equilibrium motions of proteins that exhibit relatively large (nonlocal) conformational changes upon protein binding using the Gaussian network model and the anisotropic network model of protein dynamics. For four complexes, LIR-1/HLA-A2, Actin/DNase I, CDK2/cyclin, and CDK6/p16(INK4a), the motions calculated for the monomer exhibiting the largest conformational change, in its unbound (free) form, correlate with the experimentally observed structural changes upon binding. This study emphasizes the preexisting equilibrium/conformational selection as a mechanism for protein-protein interaction and lends support the concept that proteins, in their native conformation, are predisposed to undergo conformational fluctuations that are relevant to, or even required for, their biological functions.
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