Schematic structure of the leukocyte integrin. (A) The integrin's primary structure, including divalent cation-binding sites (Mg²⁺ as red stars, and Ca²⁺ as gray stars). (B,C) Schematic representations of the bent (inactive) and straightened (active) conformations of the integrin, respectively. The arrangement of domains is based on the 3-dimensional crystal structure of α_Vβ₃-integrin, with an I domain added between the second and third β-propeller repeats. Each domain is colored as in panel A. I-d indicates I domain; I-EGF, integrin-epidermal growth factor domain; PSI, plexin/semaphorin/integrin; and βTM, β-tail domain.