Activation of Leukocyte β2 Integrins by Conversion from Bent to Extended Conformations

SummaryWe used negative stain electron microscopy (EM) to examine the conformational changes in the ectodomains required for activation of the leukocyte integrins αXβ2 and αLβ2. They transitioned between a bent conformation and two extended conformations in which the headpiece was in either a closed or an open state. Extended integrins exhibited marked flexibility at the α subunit genu and between integrin epidermal growth factor-like (I-EGF) domains 1 and 2. A clasp to mimic juxtamembrane association between the integrin α and β subunits stabilized the bent conformation strongly for αXβ2 and less so for αLβ2. A small molecule allosteric antagonist induced the extended, open headpiece conformation. A Fab known to activate β2 integrins on leukocytes induced extension, and a Fab reporter of activation bound only after extension had been induced. The results establish an intimate relationship between extension of β2 integrins and their activation in immune responses and leukocyte trafficking.