The Chemokine Fractalkine Can Activate Integrins without CX3CR1 through Direct Binding to a Ligand-Binding Site Distinct from the Classical RGD-Binding Site

The chemokine domain of fractalkine (FKN-CD) binds to the classical RGD-binding site of αvβ3 and that the resulting ternary complex formation (integrin-FKN-CX3CR1) is critical for CX3CR1 signaling and FKN-induced integrin activation. However, only certain cell types express CX3CR1. Here we studied if FKN-CD can activate integrins in the absence of CX3CR1. We describe that WT FKN-CD activated recombinant soluble αvβ3 in cell-free conditions, but the integrin-binding defective mutant of FKN-CD (K36E/R37E) did not. This suggests that FKN-CD can activate αvβ3 in the absence of CX3CR1 through the direct binding of FKN-CD to αvβ3. WT FKN-CD activated αvβ3 on CX3CR1-negative cells (K562 and CHO) but K36E/R37E did not, suggesting that FKN-CD can activate integrin at the cellular levels in a manner similar to that in cell-free conditions. We hypothesized that FKN-CD enhances ligand binding to the classical RGD-binding site (site 1) through binding to a second binding site (site 2) that is distinct from site 1 in αvβ3. To identify the possible second FKN-CD binding site we performed docking simulation of αvβ3-FKN-CD interaction using αvβ3 with a closed inactive conformation as a target. The simulation predicted a potential FKN-CD-binding site in inactive αvβ3 (site 2), which is located at a crevice between αv and β3 on the opposite side of site 1 in the αvβ3 headpiece. We studied if FKN-CD really binds to site 2 using a peptide that is predicted to interact with FKN-CD in site 2. Notably the peptide specifically bound to FKN-CD and effectively suppressed integrin activation by FKN-CD. This suggests that FKN-CD actually binds to site 2, and this leads to integrin activation. We obtained very similar results in α4β1 and α5β1. The FKN binding to site 2 and resulting integrin activation may be a novel mechanism of integrin activation and of FKN signaling.