Protein design is a key factor for subunit-subunit association

Fundamental questions about role of the quaternary structures are addressed by using a statistical mechanics off-lattice model of a dimer protein. The model, in spite of its simplicity, captures key features of the monomer-mo nomer interactions revealed by atomic force experiments. Force curves durin g association and dissociation processes are characterized by sudden jumps followed by smooth behavior and form hysteresis loops. Furthermore, the pro cess is reversible in a finite range of temperature stabilizing the dimer, and the width of the hysteresis loop increases as the design procedure impr oves: i.e., stabilizes the dimer more. It is shown that, in the interface b etween the two monomeric subunits, the design procedure naturally favors th ose amino acids whose mutual interaction is stronger.