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      Molecular basis for SNX-BAR-mediated assembly of distinct endosomal sorting tubules

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          Abstract

          Sorting nexins (SNXs) are regulators of endosomal sorting. For the SNX-BAR subgroup, a Bin/Amphiphysin/Rvs (BAR) domain is vital for formation/stabilization of tubular subdomains that mediate cargo recycling. Here, by analysing the in vitro membrane remodelling properties of all 12 human SNX-BARs, we report that some, but not all, can elicit the formation of tubules with diameters that resemble sorting tubules observed in cells. We reveal that SNX-BARs display a restricted pattern of BAR domain-mediated dimerization, and by resolving a 2.8 Å structure of a SNX1-BAR domain homodimer, establish that dimerization is achieved in part through neutralization of charged residues in the hydrophobic BAR-dimerization interface. Membrane remodelling also requires functional amphipathic helices, predicted to be present in all SNX-BARs, and the formation of high order SNX-BAR oligomers through selective ‘tip–loop' interactions. Overall, the restricted and selective nature of these interactions provide a molecular explanation for how distinct SNX-BAR-decorated tubules are nucleated from the same endosomal vacuole, as observed in living cells. Our data provide insight into the molecular mechanism that generates and organizes the tubular endosomal network.

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          Most cited references 30

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          Structure validation by Calpha geometry: phi,psi and Cbeta deviation.

          Geometrical validation around the Calpha is described, with a new Cbeta measure and updated Ramachandran plot. Deviation of the observed Cbeta atom from ideal position provides a single measure encapsulating the major structure-validation information contained in bond angle distortions. Cbeta deviation is sensitive to incompatibilities between sidechain and backbone caused by misfit conformations or inappropriate refinement restraints. A new phi,psi plot using density-dependent smoothing for 81,234 non-Gly, non-Pro, and non-prePro residues with B < 30 from 500 high-resolution proteins shows sharp boundaries at critical edges and clear delineation between large empty areas and regions that are allowed but disfavored. One such region is the gamma-turn conformation near +75 degrees,-60 degrees, counted as forbidden by common structure-validation programs; however, it occurs in well-ordered parts of good structures, it is overrepresented near functional sites, and strain is partly compensated by the gamma-turn H-bond. Favored and allowed phi,psi regions are also defined for Pro, pre-Pro, and Gly (important because Gly phi,psi angles are more permissive but less accurately determined). Details of these accurate empirical distributions are poorly predicted by previous theoretical calculations, including a region left of alpha-helix, which rates as favorable in energy yet rarely occurs. A proposed factor explaining this discrepancy is that crowding of the two-peptide NHs permits donating only a single H-bond. New calculations by Hu et al. [Proteins 2002 (this issue)] for Ala and Gly dipeptides, using mixed quantum mechanics and molecular mechanics, fit our nonrepetitive data in excellent detail. To run our geometrical evaluations on a user-uploaded file, see MOLPROBITY (http://kinemage.biochem.duke.edu) or RAMPAGE (http://www-cryst.bioc.cam.ac.uk/rampage). Copyright 2003 Wiley-Liss, Inc.
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            BAR domains as sensors of membrane curvature: the amphiphysin BAR structure.

            The BAR (Bin/amphiphysin/Rvs) domain is the most conserved feature in amphiphysins from yeast to human and is also found in endophilins and nadrins. We solved the structure of the Drosophila amphiphysin BAR domain. It is a crescent-shaped dimer that binds preferentially to highly curved negatively charged membranes. With its N-terminal amphipathic helix and BAR domain (N-BAR), amphiphysin can drive membrane curvature in vitro and in vivo. The structure is similar to that of arfaptin2, which we find also binds and tubulates membranes. From this, we predict that BAR domains are in many protein families, including sorting nexins, centaurins, and oligophrenins. The universal and minimal BAR domain is a dimerization, membrane-binding, and curvature-sensing module.
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              Refinement of severely incomplete structures with maximum likelihood in BUSTER-TNT.

              BUSTER-TNT is a maximum-likelihood macromolecular refinement package. BUSTER assembles the structural model, scales observed and calculated structure-factor amplitudes and computes the model likelihood, whilst TNT handles the stereochemistry and NCS restraints/constraints and shifts the atomic coordinates, B factors and occupancies. In real space, in addition to the traditional atomic and bulk-solvent models, BUSTER models the parts of the structure for which an atomic model is not yet available ('missing structure') as low-resolution probability distributions for the random positions of the missing atoms. In reciprocal space, the BUSTER structure-factor distribution in the complex plane is a two-dimensional Gaussian centred around the structure factor calculated from the atomic, bulk-solvent and missing-structure models. The errors associated with these three structural components are added to compute the overall spread of the Gaussian. When the atomic model is very incomplete, modelling of the missing structure and the consistency of the BUSTER statistical model help structure building and completion because (i) the accuracy of the overall scale factors is increased, (ii) the bias affecting atomic model refinement is reduced by accounting for some of the scattering from the missing structure, (iii) the addition of a spatial definition to the source of incompleteness improves on traditional Luzzati and sigmaA-based error models and (iv) the program can perform selective density modification in the regions of unbuilt structure alone.
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                Author and article information

                Journal
                EMBO J
                EMBO J
                The EMBO Journal
                Nature Publishing Group
                0261-4189
                1460-2075
                28 November 2012
                19 October 2012
                19 October 2012
                : 31
                : 23
                : 4466-4480
                Affiliations
                [1 ]The Henry Wellcome Integrated Signalling Laboratories, School of Biochemistry, University of Bristol , Bristol, UK
                [2 ]Laboratory of Molecular Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health , Bethesda, MD, USA
                [3 ]Bio-Nanotechnology and Nanomedicine Laboratory, Department of Chemistry and Nano-Science Center, Lundbeck Foundation Center Biomembranes in Nanomedicine, University of Copenhagen , Copenhagen, Denmark
                [4 ]Department of Medical Biochemistry and Biophysics, Umea University , Umeå, Sweden
                Author notes
                [a ]The Henry Wellcome Integrated Signalling Laboratories, School of Biochemistry, University of Bristol , Bristol BS8 1TD, UK. Tel.:+44 117 3312193; Fax:+44 117 3312168; E-mail: pete.cullen@ 123456bristol.ac.uk
                [*]

                Present address: Center for Neurogenomics and Cognitive Research, Neuroscience Campus Amsterdam, V University, Amsterdam, The Netherlands

                [†]

                Present address: Syngenta, Bracknell, UK

                [‡]

                Present address: Novozymes A/S, 2880 Bagsværd, Denmark

                Article
                emboj2012283
                10.1038/emboj.2012.283
                3512392
                23085988
                75b56c88-5496-4c6f-8491-e88599e0380a
                Copyright © 2012, European Molecular Biology Organization

                This is an open-access article distributed under the terms of the Creative Commons Attribution Noncommercial Share Alike 3.0 Unported License, which allows readers to alter, transform, or build upon the article and then distribute the resulting work under the same or similar license to this one. The work must be attributed back to the original author and commercial use is not permitted without specific permission.

                Categories
                Article

                Molecular biology

                phosphoinositide, vps35, bar domain, sorting nexin, retromer

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