A 50-Å Separation of the Integrin αvβ3 Extracellular Domain C Termini Reveals an Intermediate Activation State

• Published in: The Journal of biological chemistry 2004-12, Vol.279 (52), p.54567-54572
• Main Authors: Gline, Stephanie E., Cambier, Stephanie, Govaerts, Cedric, Nishimura, Stephen L.
• Format: Article
• Language: English
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M406582200

The integrin αvβ3 has been shown to exist in low and high affinity conformations. Activation to the high affinity state is thought to depend on the “switchblade-like” opening, from a low affinity bent conformation with a closed headpiece to an extended form of the integrin with an open headpiece. Activation has been shown to depend on separation of the cytoplasmic domains. How cytoplasmic domain separation is related to separation of the transmembrane domains is unknown, and the distance of separation of the transmembrane domains required for activation has not been defined. A constrained secreted form of αvβ3 was engineered that introduced a 50-Å separation of the integrin C-terminal tails of the extracellular domains of the αv and β3 subunits. Receptor binding and recognition by ligand-induced binding state (LIBS) monoclonal antibodies demonstrated that the mutant receptor was locked into a low affinity state that was likely in a partially extended conformation but with a closed headpiece. In the presence of RGD peptide, the constrained receptor was able to fully extend, as determined by full exposure of LIBS epitopes. In the presence of the appropriate LIBS antibody, high affinity ligand binding of the constrained receptor was achieved. The results support the existence of transient intermediate activation states of secreted αvβ3. Furthermore, these results with the secreted αvβ3 receptor support a model for the full-length membrane-bound form of αvβ3, whereby a 50-Å lateral separation of the integrin αv and β3 transmembrane domains would be sufficient to enforce the switchblade-like opening to the extended conformation but insufficient for full receptor activation.