Ligand-integrin α(v)β₃ interaction determined by photoaffinity cross-linking: A challenge to the prevailing model

Integrin α(v)β₃ plays a crucial role in angiogenesis, apoptosis, and bone remodeling, mainly by interacting with matrix proteins through recognition of an Arg-Gly-Asp (RGD) motif. Recently, a small cyclic RGD-containing α(v)β₃-ligand possessing a C-terminal photoreactive group was photo-cross-linked within β₃[99-118], in the N-terminus of the β₃ chain [Bitan G et al. (1999) Biochemistry 38, 3414-3420]. In this paper, a photoreactive group at the N-terminus of the RGD-ligand is shown to interact within β₃ [167-171], approximately 60 residues C-terminal to the previously identified domain. On the basis of these findings, a model of the putative I-like domain of the β₃ subunit, homologous to α(M)-, α(L)-, and α₂-I-domains, reveals that the β₃[99-118] and β₃[167-171] contact sites are close to each other and are on the opposite side relative to the metal ion-dependent adhesion site (MIDAS) motif. These observations contradict the prevailing model that proposes proximity between metal- and RGD-binding sites on the I-like domain. Our data suggest that either the I-like domain structure predicted for β₃ is incorrect, or there is no spatial proximity between the RGD-binding site and the MIDAS motif in the I-like domain. Our results indicate that the current models for ligand - Receptor interaction should be revisited.