Molecular exaptation by the integrin αI domain.

Integrins bind ligands between their alpha (α) and beta (β) subunits and transmit signals through conformational changes. Early in chordate evolution, some α subunits acquired an "inserted" (I) domain that expanded integrin's ligand-binding repertoire but obstructed the ancestral ligand pocket, seemingly blocking conventional integrin activation. Here, we compare cryo-electron microscopy structures of apo and ligand-bound states of the I domain-containing αEβ integrin and the I domain-lacking αβ integrin to illuminate how the I domain intrinsically mimics an extrinsic ligand to preserve integrin function. We trace the I domain's evolutionary origin to an ancestral collagen-collagen interaction domain, identifying an ancient molecular exaptation that facilitated integrin activation immediately upon I domain insertion. Our analyses reveal the evolutionary and biochemical basis of expanded cellular communication in vertebrates.