Journal
NEW PHYTOLOGIST
Volume 209, Issue 1, Pages 241-251Publisher
WILEY-BLACKWELL
DOI: 10.1111/nph.13596
Keywords
Boron; cell wall; cross-linking; dehydroascorbic acid; extension; pectic polysaccharides; polyhistidine; rhamnogalacturonan-II (RG-II)
Categories
Funding
- BBSRC (UK) [BB/H000690/1]
- BBSRC [BB/H000690/1] Funding Source: UKRI
- Biotechnology and Biological Sciences Research Council [BB/H000690/1] Funding Source: researchfish
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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 Pb2+ 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.
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