Tests of the Extension and Deadbolt Models of Integrin Activation

• Published in: The Journal of biological chemistry Vol. 282; no. 16; pp. 11914 - 11920
• Main Authors: Zhu, Jieqing, Boylan, Brian, Luo, Bing-Hao, Newman, Peter J., Springer, Timothy A.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 20.04.2007; American Society for Biochemistry and Molecular Biology
• Subjects: Animals; Cell Membrane - metabolism; CHO Cells; Cricetinae; Cricetulus; Fibronectins - metabolism; Humans; Integrin alphaV - metabolism; Integrin beta3 - metabolism; Integrins - metabolism; Ligands; Mutation; Platelet Membrane Glycoprotein IIb - chemistry; Protein Binding; Protein Structure, Tertiary
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M700249200

Despite extensive evidence that integrin conformational changes between bent and extended conformations regulate affinity for ligands, an alternative hypothesis has been proposed in which a “deadbolt” can regulate affinity for ligand in the absence of extension. Here, we tested both the deadbolt and the extension models. According to the deadbolt model, a hairpin loop in the β3 tail domain could act as a deadbolt to restrain the displacement of the β3 I domain β6-α7 loop and maintain integrin in the low affinity state. We found that mutating or deleting the β3 tail domain loop has no effect on ligand binding by either αIIbβ 3 or αVβ3 integrins. In contrast, we found that mutations that lock integrins in the bent conformation with disulfide bonds resist inside-out activation induced by cytoplasmic domain mutation. Furthermore, we demonstrated that extension is required for accessibility to fibronectin but not smaller fragments. The data demonstrate that integrin extension is required for ligand binding during integrin inside-out signaling and that the deadbolt does not regulate integrin activation.