The β-tail domain (βTD) regulates physiologic ligand binding to integrin CD11b/CD18

Crystallographic and electron microscopy studies revealed genuflexed (bent) integrins in both unliganded (inactive) and physiologic ligandbound (active) states, suggesting that local conformational changes are sufficient for activation. Herein we have explored the role of local changes in the contact region between the membrane-proximal beta-tail domain (beta TD) and the ligand-binding RA domain of the bent conformation in regulating interaction of integrin CD11b/CD18 (alpha M beta 2) with its physiologic ligand iC3b. We replaced the beta TD CD loop residues D658GMD of the CD18 (beta 2) subunit with the equivalent D672SSG of the beta 3 subunit, with AGAA or with NGTD, expressed the respective heterodimeric receptors either transiently in epithelial HEK293T cells or stably in leukocytes (K562), and measured their ability to bind iC3b and to conformation-sensitive mAbs. In the presence of the physiologic divalent cations Ca2+ plus Mg2+ (at 1 mM each), the modified integrins showed increased (in HEK293) or constitutive (in K562) binding to iC3b compared with wild-type receptors. K562 expressing the beta TD-modified integrins bound in Ca2+Mg2+ to the beta A-directed high-affinity reporter mAb 24 but not to mAb KIM127, a reporter of the genu-straightened state. These data identify a role for the membrane proximal beta TD as an allosteric modulator of integrin activation.