title: NMR analysis of the alphaIIb beta3 cytoplasmic interaction suggests a mechanism for integrin regulation.
abstract: The integrin αIIbβ3 is a transmembrane (TM) heterodimeric adhesion receptor that exists in equilibrium between resting and active ligand binding conformations. In resting αIIbβ3, the TM and cytoplasmic domains of αIIb and β3 form a heterodimer that constrains αIIbβ3 in its resting conformation. To study the structure and dynamics of the cytoplasmic domain heterodimer, we prepared a disulfide-stabilized complex consisting of portions of the TM domains and the full cytoplasmic domains. NMR and hydrogen-deuterium exchange of this complex in micelles showed that the αIIb cytoplasmic domain is largely disordered, but it interacts with and influences the conformation of the β3 cytoplasmic domain. The β3 cytoplasmic domain consists of a stable proximal helix contiguous with the TM helix and two distal amphiphilic helices. To confirm the NMR structure in a membrane-like environment, we studied the β3 cytoplasmic domain tethered to phospholipid bilayers. Hydrogen-deuterium exchange mass spectrometry, as well as circular dichroism spectroscopy, demonstrated that the β3 cytoplasmic domain becomes more ordered and helical under these conditions, consistent with our NMR results. Further, these experiments suggest that the two distal helices associate with lipid bilayers but undergo fluctuations that would allow rapid binding of cytoplasmic proteins regulating integrin activation, such as talin and kindlin-3. Thus, these results provide a framework for understanding the kinetics and thermodynamics of protein interactions involving integrin cytoplasmic domains and suggest that such interactions act in a concerted fashion to influence integrin stalk separation and exposure of extracellular ligand binding sites.
authors.affiliation: Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, PA 19104, USA.
authors.forename: Douglas G
authors.initials: DG
authors.lastname: Metcalf
authors.forename: David T
authors.initials: DT
authors.lastname: Moore
authors.forename: Yibing
authors.initials: Y
authors.lastname: Wu
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Kielec
authors.forename: Kathleen
authors.initials: K
authors.lastname: Molnar
authors.forename: Kathleen G
authors.initials: KG
authors.lastname: Valentine
authors.forename: A Joshua
authors.initials: AJ
authors.lastname: Wand
authors.forename: Joel S
authors.initials: JS
authors.lastname: Bennett
authors.forename: William F
authors.initials: WF
authors.lastname: DeGrado
mesh_terms: D000818:Animals; D001665:Binding Sites; D002942:Circular Dichroism; D003593:Cytoplasm / Q000378:metabolism*; D041961:Deuterium Exchange Measurement; D004220:Disulfides / Q000737:chemistry / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008051:Lipid Bilayers / Q000737:chemistry / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008565:Membrane Proteins / Q000737:chemistry / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009363:Neoplasm Proteins / Q000737:chemistry / Q000378:metabolism; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding; D055503:Protein Multimerization; D020836:Protein Structure, Quaternary; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D016608:Talin / Q000737:chemistry / Q000378:metabolism; D013816:Thermodynamics
chemical_list: D004220:Disulfides; C477322:FERMT3 protein, human; D008051:Lipid Bilayers; D008565:Membrane Proteins; D009363:Neoplasm Proteins; D019039:Platelet Glycoprotein GPIIb-IIIa Complex; D011994:Recombinant Proteins; D016608:Talin