Integrin α_v β₈ Adopts a High Affinity State for Soluble Ligands Under Physiological Conditions

It has been proposed that integrins adopt a low affinity conformation under physiological conditions. Integrin can either be activated through cytoplasm or by binding of cations such as Mn²⁺ to the head domain. The cytoplasmic activation pathway, that is, inside-out signaling has been regarded as the physiological pathway for integrin activation. Integrin β₈ is important for neuron vascular development. However, due to the highly divergent cytoplasmic domain, this integrin probably does not rely on inside-out signaling for affinity regulation. We therefore hypothesized that the β₈ integrin uniquely assumes a constitutively high affinity state under physiological conditions. We discovered that β₈ indeed exhibited high binding to soluble vitronectin in the presence of Ca²⁺ and the ligand binding could not be further enhanced by addition of Mn²⁺ . The lower ectodomain stalk of the integrin, which is comprised by the integrin epidermal growth factor-like (I-EGF) domains and βTD domain, is critical for this high affinity conformation. In addition, we found that unlike other integrins, Mg²⁺ at low concentration inhibited β₈ ligand binding. Mutagenesis studies indicated that β₈ integrin possesses a unique cation binding site which might contribute to the ligand binding affinity. Our study showed that both the β₈ lower ectodomain stalk and the head domain play an important role in its high affinity state under physiological conditions. J. Cell. Biochem. 118: 2044-2052, 2017. © 2016 Wiley Periodicals, Inc.