Three-dimensional EM structure of the ectodomain of integrin αVβ3 in a complex with fibronectin

Integrins are alpha beta heterodimeric cell surface receptors that mediate transmembrane signaling by binding extracellular and cytoplasmic ligands. The ectodomain of integrin alpha V beta 3 crystallizes in a bent, genuflexed conformation considered to be inactive ( unable to bind physiological ligands in solution) unless it is fully extended by activating stimuli. We generated a stable, soluble complex of the Mn2+ -bound alpha V beta 3 ectodomain with a fragment of fibronectin ( FN) containing type III domains 7 to 10 and the EDB domain ( FN7-EDB-10). Transmission electron microscopy and single particle image analysis were used to determine the three-dimensional structure of this complex. Most alpha V beta 3 particles, whether unliganded or FN-bound, displayed compact, triangular shapes. A difference map comparing ligand-free and FN-bound alpha V beta 3 revealed density that could accommodate the RGD-containing FN10 in proximity to the ligand-binding site of beta 3, with FN9 just adjacent to the synergy site binding region of alpha V. We conclude that the ectodomain of alpha V beta 3 manifests a bent conformation that is capable of stably binding a physiological ligand in solution.