Conversion between three conformational states of integrin I domains with a C-terminal pull spring studied with molecular dynamics

We test with molecular dynamics the hypothesis that interdomain forces in integrins, simulated with a spring attached to the C-terminal α7-helix of an integrin I domain, can allosterically stabilize alternative I domain conformations. Depending on the force applied and timecourse, in α_L and α_M I domains the β6-α7 loop moves successively between three ratchet positions; i.e. from closed to intermediate, and then to open. More distal, linked alterations in MIDAS loops and metal coordination closely resemble those seen when the MIDAS becomes ligated. Simulations show that the intermediate state is populated over a wider range of forces for α_L than α_M I domains. Simulations with mutant I domains suggest that specific ratchet residues regulate conformational equilibria. Simulations with α₁ and α₂ I domains reveal a lack of the intermediate conformation, owing to Phe to Glu substitution at the second ratchet residue. The findings have important implications for biological regulation of integrin adhesiveness.