Focal adhesions are sites of integrin extension

Integrins undergo global conformational changes that specify their activation state. Current models portray the inactive receptor in a bent conformation that upon activation converts to a fully extended form in which the integrin subunit leg regions are separated to enable ligand binding and subsequent signaling. To test the applicability of this model in adherent cells, we used a fluorescent resonance energy transfer (FRET)-based approach, in combination with engineered integrin mutants and monoclonal antibody reporters, to image integrin alpha 5 beta 1 conformation. We find that restricting leg separation causes the integrin to adopt a bent conformation that is unable to respond to agonists and mediate cell spreading. By measuring FRET between labeled alpha 5 beta 1 and the cell membrane, we find extended receptors are enriched in focal adhesions compared with adjacent regions of the plasma membrane. These results demonstrate definitely that major quaternary rearrangements of beta-integrin subunits occur in adherent cells and that conversion from a bent to extended form takes place at focal adhesions.