Elucidation of the Integrin Inside-Out Activation Mechanism

Abstract

Integrins are major cell surface receptors that are crucial for a variety of cell migration and adhesion events. They are ‘activated’ to a high affinity state by the intracellular protein talin, a process known as “inside-out activation”. In this process, complex formation between the talin head domain and the integrin β cytoplasmic tail, as well as talin/membrane interactions, are believed to play a crucial role. In this study, long multi-scale molecular dynamic simulations were used to probe the talin F2-F3/membrane and talin F3/β1D interactions in a POPC/POPG bilayer. A reorientation of the talin F2-F3 domain to optimize contacts with the negatively charged moieties in the membrane, followed by a large increase in the tilt angle of the β1D tail relative to the bilayer normal was observed. In addition, our simulations demonstrate that mutation of four basic residues in the F2 domain of talin, previously suggested to be involved in membrane interactions, reduces the affinity of talin F2-F3 for the membrane and changes its orientation relative to the bilayer surface. This perturbed orientation of talin relative to the membrane in the F2 mutant is expected, in turn, to perturb talin/integrin interactions. During the simulations, enrichment of the F2-F3 binding surface with anionic lipids reveals an important role for negatively charged moieties in the membrane. On the basis of these simulations, a model for disruption of the integrin α/β transmembrane (TM) interactions is proposed in which the large increase in the tilt angle to the β tail upon talin binding weakens the α/β TM association, destabilizes the α/β dimer thus leading to integrin tail separation.