title: Boron bridging of rhamnogalacturonan-II is promoted in vitro by cationic chaperones, including polyhistidine and wall glycoproteins.
abstract: Dimerization of rhamnogalacturonan-II (RG-II) via boron cross-links contributes to the assembly and biophysical properties of the cell wall. Pure RG-II is efficiently dimerized by boric acid (B(OH)3 ) in vitro only if nonbiological agents for example Pb(2+) are added. By contrast, newly synthesized RG-II domains dimerize very rapidly in vivo. We investigated biological agents that might enable this. We tested for three such agents: novel enzymes, borate-transferring ligands and cationic 'chaperones' that facilitate the close approach of two polyanionic RG-II molecules. Dimerization was monitored electrophoretically. Parsley shoot cell-wall enzymes did not affect RG-II dimerization in vitro. Borate-binding ligands (apiose, dehydroascorbic acid, alditols) and small organic cations (including polyamines) also lacked consistent effects. Polylysine bound permanently to RG-II, precluding electrophoretic analysis. However, another polycation, polyhistidine, strongly promoted RG-II dimerization by B(OH)3 without irreversible polyhistidine-RG-II complexation. Likewise, partially purified spinach extensins (histidine/lysine-rich cationic glycoproteins), strongly promoted RG-II dimerization by B(OH)3 in vitro. Thus certain polycations, including polyhistidine and wall glycoproteins, can chaperone RG-II, manoeuvring this polyanionic polysaccharide domain such that boron-bridging is favoured. These chaperones dissociate from RG-II after facilitating its dimerization, indicating that they act catalytically rather than stoichiometrically. We propose a natural role for extensin-RG-II interaction in steering cell-wall assembly.
authors: The Edinburgh Cell Wall Group, Institute of Molecular Plant Sciences, School of Biological Sciences, The University of Edinburgh, The King's Buildings, Mayfield Road, Edinburgh, EH9 3JH, UK.
authors: Dimitra
authors: D
authors: Chormova
authors: The Edinburgh Cell Wall Group, Institute of Molecular Plant Sciences, School of Biological Sciences, The University of Edinburgh, The King's Buildings, Mayfield Road, Edinburgh, EH9 3JH, UK.
authors: Stephen C
authors: SC
authors: Fry
mesh_terms: D001881:Borates / Q000378:metabolism; D001888:Boric Acids / Q000378:metabolism; D001895:Boron / Q000378:metabolism*; D002412:Cations / Q000378:metabolism; D002473:Cell Wall / Q000378:metabolism*; D019281:Dimerization; D006023:Glycoproteins / Q000378:metabolism*; D006639:Histidine / Q000378:metabolism; D018832:Molecular Chaperones / Q000378:metabolism*; D010368:Pectins / Q000378:metabolism*; D028528:Petroselinum / Q000737:chemistry / Q000201:enzymology*; D010940:Plant Proteins / Q000378:metabolism; D018520:Plant Shoots / Q000737:chemistry / Q000201:enzymology; D011134:Polysaccharides / Q000378:metabolism
chemical_list: D001881:Borates; D001888:Boric Acids; D002412:Cations; D006023:Glycoproteins; D018832:Molecular Chaperones; D010940:Plant Proteins; D011134:Polysaccharides; C042492:rhamnogalacturonan II; C033223:polyhistidine; D006639:Histidine; D010368:Pectins; D001895:Boron; C032688:boric acid
keywords: Boron; cell wall; cross-linking; dehydroascorbic acid; extension; pectic polysaccharides; polyhistidine; rhamnogalacturonan-II (RG-II)