title: Regulation of integrin αIIbβ3 ligand binding and signaling by the metal ion binding sites in the β I domain.
abstract: The ability of αIIbβ3 to bind ligands and undergo outside-in signaling is regulated by three divalent cation binding sites in the β I domain. Specifically, the metal ion-dependent adhesion site (MIDAS) and the synergistic metal binding site (SyMBS) are thought to be required for ligand binding due to their synergy between Ca(2+) and Mg(2+). The adjacent to MIDAS (ADMIDAS) is an important ligand binding regulatory site that also acts as a critical link between the β I and hybrid domains for signaling. Mutations in this site have provided conflicting results for ligand binding and adhesion in different integrins. We have mutated the β3 SyMBS and ADMIDAS. The SyMBS mutant abolished ligand binding and outside-in signaling, but when an activating glycosylation mutation in the αIIb Calf 2 domain was introduced, the ligand binding affinity and signaling were restored. Thus, the SyMBS is important but not absolutely required for integrin bidirectional signaling. The ADMIDAS mutants showed reduced ligand binding affinity and abolished outside-in signaling, and the activating glycosylation mutation could fully restore integrin signaling of the ADMIDAS mutant. We propose that the ADMIDAS ion stabilizes the low-affinity state when the integrin headpiece is in the closed conformation, whereas it stabilizes the high-affinity state when the headpiece is in the open conformation with the swung-out hybrid domain.
authors.affiliation: Department of Biological Sciences, 202 Life Sciences Building, Louisiana State University, Baton Rouge, Louisiana 70803, United States.
authors.forename: Joel
authors.initials: J
authors.lastname: Raborn
authors.forename: Wei
authors.initials: W
authors.lastname: Wang
authors.forename: Bing-Hao
authors.initials: BH
authors.lastname: Luo
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002448:Cell Adhesion; D048430:Cell Shape; D005340:Fibrinogen / Q000378:metabolism; D057809:HEK293 Cells; D006801:Humans; D008024:Ligands; D008670:Metals / Q000737:chemistry* / Q000378:metabolism* / Q000494:pharmacology; D051379:Mice; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D015398:Signal Transduction
chemical_list: D008024:Ligands; D008670:Metals; D019039:Platelet Glycoprotein GPIIb-IIIa Complex; D005340:Fibrinogen