title: Ligand binding promotes the entropy-driven oligomerization of integrin alpha IIb beta 3.
abstract: Integrin alpha(IIb)beta(3) clusters on the platelet surface after binding adhesive proteins in a process that regulates signal transduction. However, the intermolecular forces driving integrin self-association are poorly understood. This work provides new insights into integrin clustering mechanisms by demonstrating how temperature and ligand binding interact to affect the oligomeric state of alpha(IIb)beta(3). The ligand-free receptor, solubilized in thermostable octyl glucoside micelles, exhibited a cooperative transition at approximately 43 degrees C, monitored by changes in intrinsic fluorescence and circular dichroism. Both signals changed in a direction opposite to that for global unfolding, and both were diminished upon binding the fibrinogen gamma-chain ligand-mimetic peptide cHArGD. Free and bound receptors also exhibited differential sensitivity to temperature-enhanced oligomerization, as measured by dynamic light scattering, sedimentation velocity, and sedimentation equilibrium. Van't Hoff analyses of dimerization constants for alpha(IIb)beta(3) complexed with cHArGD, cRGD, or eptifibatide yielded large, favorable entropy changes partly offset by unfavorable enthalpy changes. Transmission electron microscopy showed that ligand binding and 37 degrees C incubation enhanced assembly of integrin dimers and larger oligomers linked by tail-to-tail contacts. Interpretation of these images was aided by threading models for alpha(IIb)beta(3) protomers and dimers based on the ectodomain structure of alpha(v)beta(3). We propose that entropy-favorable nonpolar interactions drive ligand-induced integrin clustering and outside-in signaling.
authors.affiliation: Department of Biochemistry, Wake Forest University School of Medicine, Winston-Salem, North Carolina 27157, USA. rhantgan@wfubmc.edu
authors.forename: Roy R
authors.initials: RR
authors.lastname: Hantgan
authors.forename: Douglas S
authors.initials: DS
authors.lastname: Lyles
authors.forename: T Conn
authors.initials: TC
authors.lastname: Mallett
authors.forename: Mattia
authors.initials: M
authors.lastname: Rocco
authors.forename: Chandrasekaran
authors.initials: C
authors.lastname: Nagaswami
authors.forename: John W
authors.initials: JW
authors.lastname: Weisel
mesh_terms: D000328:Adult; D000595:Amino Acid Sequence; D001665:Binding Sites; D001792:Blood Platelets / Q000737:chemistry*; D002462:Cell Membrane / Q000648:ultrastructure; D019277:Entropy; D006358:Hot Temperature; D006801:Humans; D008024:Ligands; D008027:Light; D008958:Models, Molecular; D009842:Oligopeptides / Q000737:chemistry; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000737:chemistry* / Q000502:physiology / Q000648:ultrastructure; D011487:Protein Conformation; D012542:Scattering, Radiation; D013816:Thermodynamics
chemical_list: D008024:Ligands; D009842:Oligopeptides; D019039:Platelet Glycoprotein GPIIb-IIIa Complex