Almost all chromatin regulators are present as members of complexes, which is decidedly different from signaling proteins, which are often monomeric. Why does nature use proteins in complexes rather than leave them to perform their functions as monomers? Most likely, protein complexes have evolved to allow an ordered series of biochemical reactions and interactions to occur on the surface of the complex. The same thing might be accomplished if all the proteins were monomers, but the reactions would be very slow and perhaps the biological functions would not be performed with sufficient efficiency. The best example is, again, the ribosome: protein translation is an ordered series of events involving many subunits working simultaneously and in tandem. Almost certainly, chromatin regulation by the large ATP-dependent chromatin remodelers and perhaps other chromatin regulators involves an ordered series of specific biochemical reactions that must be spatially coordinated. Thus, a critical question in the study of ATP-dependent chromatin remodeling is the nature of the spatial relationships within the complex and the order of reactions occurring on the surfaces (as well as within the hydrophobic
