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      Real-time Analysis of Conformation-sensitive Antibody Binding Provides New Insights into Integrin Conformational Regulation*S⃞

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          Abstract

          Integrins are heterodimeric adhesion receptors that regulate immune cell adhesion. Integrin-dependent adhesion is controlled by multiple conformational states that include states with different affinity to the ligand, states with various degrees of molecule unbending, and others. Affinity change and molecule unbending play major roles in the regulation of cell adhesion. The relationship between different conformational states of the integrin is unclear. Here we have used conformationally sensitive antibodies and a small LDV-containing ligand to study the role of the inside-out signaling through formyl peptide receptor and CXCR4 in the regulation of α 4β 1 integrin conformation. We found that in the absence of ligand, activation by formyl peptide or SDF-1 did not result in a significant exposure of HUTS-21 epitope. Occupancy of the ligand binding pocket without cell activation was sufficient to induce epitope exposure. EC 50 for HUTS-21 binding in the presence of LDV was identical to a previously reported ligand equilibrium dissociation constant at rest and after activation. Furthermore, the rate of HUTS-21 binding was also related to the VLA-4 activation state even at saturating ligand concentration. We propose that the unbending of the integrin molecule after guanine nucleotide-binding protein-coupled receptor-induced signaling accounts for the enhanced rate of HUTS-21 binding. Taken together, current results support the existence of multiple conformational states independently regulated by both inside-out signaling and ligand binding. Our data suggest that VLA-4 integrin hybrid domain movement does not depend on the affinity state of the ligand binding pocket.

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          Rapid planetesimal formation in turbulent circumstellar discs

          The initial stages of planet formation in circumstellar gas discs proceed via dust grains that collide and build up larger and larger bodies (Safronov 1969). How this process continues from metre-sized boulders to kilometre-scale planetesimals is a major unsolved problem (Dominik et al. 2007): boulders stick together poorly (Benz 2000), and spiral into the protostar in a few hundred orbits due to a head wind from the slower rotating gas (Weidenschilling 1977). Gravitational collapse of the solid component has been suggested to overcome this barrier (Safronov 1969, Goldreich & Ward 1973, Youdin & Shu 2002). Even low levels of turbulence, however, inhibit sedimentation of solids to a sufficiently dense midplane layer (Weidenschilling & Cuzzi 1993, Dominik et al. 2007), but turbulence must be present to explain observed gas accretion in protostellar discs (Hartmann 1998). Here we report the discovery of efficient gravitational collapse of boulders in locally overdense regions in the midplane. The boulders concentrate initially in transient high pressures in the turbulent gas (Johansen, Klahr, & Henning 2006), and these concentrations are augmented a further order of magnitude by a streaming instability (Youdin & Goodman 2005, Johansen, Henning, & Klahr 2006, Johansen & Youdin 2007) driven by the relative flow of gas and solids. We find that gravitationally bound clusters form with masses comparable to dwarf planets and containing a distribution of boulder sizes. Gravitational collapse happens much faster than radial drift, offering a possible path to planetesimal formation in accreting circumstellar discs.
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              Structural basis for allostery in integrins and binding to fibrinogen-mimetic therapeutics.

              Integrins are important adhesion receptors in all Metazoa that transmit conformational change bidirectionally across the membrane. Integrin alpha and beta subunits form a head and two long legs in the ectodomain and span the membrane. Here, we define with crystal structures the atomic basis for allosteric regulation of the conformation and affinity for ligand of the integrin ectodomain, and how fibrinogen-mimetic therapeutics bind to platelet integrin alpha(IIb)beta3. Allostery in the beta3 I domain alters three metal binding sites, associated loops and alpha1- and alpha7-helices. Piston-like displacement of the alpha7-helix causes a 62 degrees reorientation between the beta3 I and hybrid domains. Transmission through the rigidly connected plexin/semaphorin/integrin (PSI) domain in the upper beta3 leg causes a 70 A separation between the knees of the alpha and beta legs. Allostery in the head thus disrupts interaction between the legs in a previously described low-affinity bent integrin conformation, and leg extension positions the high-affinity head far above the cell surface.
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                Author and article information

                Journal
                J Biol Chem
                jbc
                The Journal of Biological Chemistry
                American Society for Biochemistry and Molecular Biology
                0021-9258
                1083-351X
                22 May 2009
                22 May 2009
                : 284
                : 21
                : 14337-14346
                Affiliations
                []Department of Pathology, [§ ]Cancer Research and Treatment Center, and []Department of Biochemistry and Molecular Biology, University of New Mexico Health Sciences Center, Albuquerque, New Mexico 87131 and the []Institute of Chemistry, Romanian Academy, Timisoara 300223, Romania
                Author notes

                1 To whom correspondence may be addressed: MSC08 4630, 915 Camino de Salud, Albuquerque, NM 87131. Fax: 505-272-6995; E-mail: achigaev@ 123456salud.unm.edu . 2 To whom correspondence may be addressed: MSC08 4630, 915 Camino de Salud, Albuquerque, NM 87131. Fax: 505-272-6995; E-mail: lsklar@ 123456salud.unm.edu .

                Article
                14337
                10.1074/jbc.M901178200
                2682882
                19251697
                62634998-5a27-4033-8c06-31ae2ba8640b
                Copyright © 2009, The American Society for Biochemistry and Molecular Biology, Inc.

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                Creative Commons Attribution Non-Commercial License applies to Author Choice Articles

                History
                : 19 February 2009
                Categories
                Molecular Basis of Cell and Developmental Biology

                Biochemistry
                Biochemistry

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