Relating conformation to function in integrin α 5 β 1

Whether β integrin ectodomains visit conformational states similarly to β and β integrins has not been characterized. Furthermore, despite a wealth of activating and inhibitory antibodies to β integrins, the conformational states that these antibodies stabilize, and the relation of these conformations to function, remain incompletely characterized. Using negative-stain electron microscopy, we show that the integrin α β ectodomain adopts extended-closed and extended-open conformations as well as a bent conformation. Antibodies SNAKA51, 8E3, N29, and 9EG7 bind to different domains in the α or β legs, activate, and stabilize extended ectodomain conformations. Antibodies 12G10 and HUTS-4 bind to the β βI domain and hybrid domains, respectively, activate, and stabilize the open headpiece conformation. Antibody TS2/16 binds a similar epitope as 12G10, activates, and appears to stabilize an open βI domain conformation without requiring extension or hybrid domain swingout. mAb13 and SG/19 bind to the βI domain and βI-hybrid domain interface, respectively, inhibit, and stabilize the closed conformation of the headpiece. The effects of the antibodies on cell adhesion to fibronectin substrates suggest that the extended-open conformation of α β is adhesive and that the extended-closed and bent-closed conformations are nonadhesive. The functional effects and binding sites of antibodies and fibronectin were consistent with their ability in binding to α β on cell surfaces to cross-enhance or inhibit one another by competitive or noncompetitive (allosteric) mechanisms.
NIH Grant HL-108248