Integrins are multifunctional cell surface receptors that play critical roles in normal physiological processes such as proliferation, survival, migration, and cell fate transitions. Integrins are implicated in several pathological conditions, including inflammation, tumor metastasis, lung fibrosis, and autoimmune disorders. Recent studies show that apart from their endogenous ligands, αVβ3 and α5β1 integrins bind directly to the hydroxylated form of cholesterol (25HC and 24HC) to upregulate pro-inflammatory response. Among the integrins, αVβ3 integrin has been considered a viable target for treating many diseases. Despite enormous efforts, currently available RGD-based anti-integrin drugs lack specificity, exhibit partial agonism, and cause serious side effects, including tumor growth and angiogenesis. The development of safe and efficient inhibitors requires a thorough understanding of the structural basis for outside-in activation and inhibition mechanism of integrin αvβ3. In this study, we performed a comprehensive residue-residue contact and network analysis of the αvβ3 integrin headpiece using all available experimental structures. Surprisingly, our analyses show that specific orientation and interactions of residues from the specificity-determining loop (SDL) are critical in the molecular recognition and regulation of integrin αvβ3. In addition, network analysis reveals that residues from the allosteric site (site II) connect to the primary RGD-binding site via SDL, which acts as an interface for the allosteric modulation. The obtained results provide a comprehensive structural perspective, useful insights into the distinct features of active and inactive conformations, and the role of SDL in the ligand recognition process and allostery.
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