Arabinogalactan-Proteins as Boron-Acting Enzymes, Cross-Linking the Rhamnogalacturonan-II Domains of Pectin

Abstract

Most pectic rhamnogalacturonan-II (RG-II) domains in plant cell walls are borate-bridged dimers. However, the sub-cellular locations, pH dependence, reversibility and biocatalyst involvement in borate bridging remain uncertain. Experiments discussed here explored these questions, utilising suspension-cultured plant cells. In-vivo pulse radiolabelling showed that most RG-II domains dimerise extremely quickly (3 required the simultaneous presence of RG-II-binding ‘chaperones’: co-ordinately binding metals and/or ionically binding cationic peptides. Natural chaperones of the latter type include highly basic arabinogalactan protein fragments, e.g., KHKRKHKHKRHHH, which catalyse a reaction [2 RG-II + B(OH)3 → RG-II–B–RG-II], suggesting that plants can ‘enzymically’ metabolise boron.

Keywords

• acid-growth
• arabinogalactan-proteins
• borate diesterase
• boron
• Ca²⁺
• chaperones