title: Tests of the extension and deadbolt models of integrin activation.
abstract: 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 beta3 tail domain could act as a deadbolt to restrain the displacement of the beta3 I domain beta6-alpha7 loop and maintain integrin in the low affinity state. We found that mutating or deleting the beta3 tail domain loop has no effect on ligand binding by either alphaIIbbeta 3 or alphaVbeta3 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.
authors.affiliation: The CBR Institute for Biomedical Research, Harvard Medical School, Boston, Massachusetts 02115, USA.
authors.forename: Jieqing
authors.initials: J
authors.lastname: Zhu
authors.forename: Brian
authors.initials: B
authors.lastname: Boylan
authors.forename: Bing-Hao
authors.initials: BH
authors.lastname: Luo
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Newman
authors.forename: Timothy A
authors.initials: TA
authors.lastname: Springer
mesh_terms: D000818:Animals; D016466:CHO Cells; D002462:Cell Membrane / Q000378:metabolism; D006224:Cricetinae; D003412:Cricetulus; D005353:Fibronectins / Q000378:metabolism; D006801:Humans; D039564:Integrin alphaV / Q000378:metabolism; D039661:Integrin beta3 / Q000378:metabolism; D016023:Integrins / Q000378:metabolism*; D008024:Ligands; D009154:Mutation; D040201:Platelet Membrane Glycoprotein IIb / Q000737:chemistry; D011485:Protein Binding; D017434:Protein Structure, Tertiary
chemical_list: D005353:Fibronectins; D039564:Integrin alphaV; D039661:Integrin beta3; D016023:Integrins; D008024:Ligands; D040201:Platelet Membrane Glycoprotein IIb