Roles of Magnesium and Simvastatin in Structural Changes, Stability, and Affinity of the Ligand Binding Domain of Integrin αXβ2

Myeloid leukocytes contribute to inflammatory responses in immune dysregulations such as atherosclerosis, autoimmune encephalomyelitis, and arthritis. Leukocyte integrin, αXβ2 plays a pivotal role in their recruitment from circulation to lesions. Integrin αXβ2, known as a dendritic cell marker and complement receptor 4, is a heterodimeric cell surface receptor that bi-directionally relays cellular signals and functions in cellular trafficking, phagocytosis, and T-cell proliferation. In the regulation of these events, the ligand-binding domain of αXβ2, called the αX I-domain, acts like the gatekeeper of the bidirectional signals and relays them through affinity maturation and conformational changes from the closed to open state. A divalent cation is essential for the αX I-domain interaction with an acidic residue of a ligand. The biophysical approaches discussed here reveal that the addition of Mg2+ results in an alteration of the structure and stability of the αX I-domain in-solution. The multiplicity of the αX I-domain conformations existing in-solution is associated with its affinity regulation to physiological ligands such as fibrinogen and iC3b. The hydrophobic environment in the Metal Ion Dependent Adhesion site (MIDAS), the Mg2+-binding motif, appears to determine the ionization states of two critical MIDAS aspartates, Mg2+ creates a link from MIDAS motif to the allosteric sites, all of which contribute to the integrin activation. The local rearrangements and cation-dependent flexibility of the αX I-domain facilitate affinity maturation. These features support the rapid equilibrium of the αX I-domain between the closed and the open conformations required during rapid leukocyte activation, important for regulating leukocyte adhesiveness and migration.