The capricious electrostatic force: Revealing the signaling pathway in integrin α 2-I domain

Abstract

Integrins are cellular adhesion proteins located on cell surface. They are known to have open and closed conformations that correspond to high and low binding affinity to ligands, respectively. Integrin α2 binds to the ligands via the ligand binding domain, α2-I domain, which also has open and closed conformations. Experimentally, the closed to open conformation change is shown to be triggered by pulling the C-terminal away from the ligand binding site, but how the signal propagates from the distant C-terminal to the binding site is unknown. To explain the mechanisms of the conformation change, we built models of the α2-I domain open and closed conformations in ligand free and ligand bound states, respectively. We found that the signaling pathway consists of F313-I280-V252 residues that connect the C-terminal and the ligand binding site. The pathway is highly conserved as revealed by a protein sequence analysis among 55 species. Furthermore, MM/PBSA energy calculations on the stabilities and ligand binding affinities of the closed and open conformations are consistent with experimental measurements. The open conformation is more favorable for ligand binding, and the closed conformation is more stable in unbound state. Energy analysis also revealed the "hot spots" for ligand binding, and most residues that contribute strongly to ligand binding free energy are highly conserved in evolution. In addition, the electrostatic analysis showed that the closed conformation has stronger long-range electrostatic attraction to the ligand compared with the open conformation. The difference is caused by the rearrangement of several charged residues during the binding. These observations make us suggest that the integrin α2-I domain binding process involves the two-step "dock-lock" mechanism. The closed conformation first attracts the ligand from a long distance and afterwards, the open conformation locks the ligand at the binding site with high binding affinity.