The CD9, CD81, and CD151 EC2 domains bind to the classical RGD-binding site of integrin αvβ3.

Tetraspanins play important roles in normal (e.g., cell adhesion, motility, activation, and proliferation), and pathological conditions (e.g., metastasis and viral infection). Tetraspanins interact with integrins and regulate integrin functions, but the specifics of tetraspanin-integrin interaction are unclear. Using co-immunoprecipitation with integrins as a sole method to detect interaction between integrins and full-length tetraspanins, it has been proposed that the variable region (helices D and E) of the extracellular-2 (EC2) of tetraspanins laterally associate with non-ligand-binding site of integrins. We describe that, using adhesion assays, the EC2 domain of CD81, CD9, and CD151 bound to integrin αvβ3 and this binding was suppressed by cRGDfV, a specific inhibitor of αvβ3, and antibody 7E3 that is mapped to the ligand-binding site of β3. We also present evidence that the specificity loop of β3 directly bound to the EC2 domains. This suggests that the EC2 domains specifically binds to the classical ligand-binding site of αvβ3. αvβ3 was a more effective receptor for the EC2 domains than previously known tetraspanin receptors α3β1, α4β1, and α6β1. Docking simulation predicted that the helices A and B of CD81 EC2 binds to the RGD-binding site of αvβ3. Substituting Lys residues at position 116, 144/148 of CD81 EC2 in the predicted integrin-binding interface reduced the binding of CD81 EC2 to αvβ3, consistent with the docking model. These findings suggest that, in contrast to previous models, the ligand-binding site of integrin αvβ3, a new tetraspanin receptor, binds to the constant region (helices A and B) of the EC2 domain.