An Arabidopsis thaliana arabinogalactan-protein (AGP31) and several cationic AGP fragments catalyse the boron bridging of rhamnogalacturonan-II. Issue 18 (30th September 2022)

• Record Type: Journal Article
• Authors: Sanhueza, Dayan; Begum, Rifat Ara; Albenne, Cécile; Jamet, Elisabeth; Fry, Stephen C.
• Is Part Of: Biochemical journal. Volume 479:Issue 18(2022)
• Page Start: 1967
• Page End: 1984
• Publication Date: 2022-09-30
• Subjects: Arabidopsis thaliana -- arabinogalactan-protein -- borate diester bridges -- pectic polysaccharide -- plant cell wall -- rhamnogalacturonan-II; Biochemistry -- Periodicals; 572
• DOI: 10.1042/BCJ20220340
• Languages: English
• ISSNs: 0264-6021

Abstract

Abstract : Rhamnogalacturonan-II (RG-II) is a complex pectic domain in plant primary cell walls. In vivo, most RG-II domains are covalently dimerised via borate diester bridges, essential for correct cell-wall assembly, but the dimerisation of pure RG-II monomers by boric acid in vitro is extremely slow. Cationic 'chaperones' can promote dimerisation, probably by overcoming the mutual repulsion between neighbouring anionic RG-II molecules. Highly effective artificial chaperones include Pb 2+ and polyhistidine, but the proposed natural chaperones remained elusive. We have now tested cationic peptide fragments of several Arabidopsis thaliana arabinogalactan-proteins (AGPs) as candidates. Fragments of AGP17, 18, 19 and 31 were effective, typically at ∼25 µg/ml (9–19 µM), promoting the boron bridging of 16–20 µM monomeric RG-II at pH 4.8 in vitro . Native AGP31 glycoprotein was also effective, and hexahistidine was moderately so. All chaperones tested interacted reversibly with RG-II and were not consumed during the reaction; thus they acted catalytically, and may constitute the first reported boron-acting enzyme activity, an RG-II borate diesterase. Many of the peptide chaperones became less effective catalysts at higher concentration, which we interpret as due to the formation of RG-II–peptide complexes with a net positive charge, as mutually repulsive as negatively charged pure RG-II molecules. The four unique AGPs studied here may serve an enzymic role in the living plant cell, acting on RG-II within Golgi cisternae and/or in the apoplast after secretion. In this way, RG-II and specific AGPs may contribute to cell-wall assembly and hence plant cell expansion and development.