The Patterned Structure of Galactoglucomannan Suggests It May Bind to Cellulose in Seed Mucilage.

Abstract

The interaction between mannan polysaccharides and cellulose microfibrils is thought to be important for cell wall properties in some vascular plants; however, the molecular arrangement of mannan in the cell wall and the nature of the molecular bonding between mannan and cellulose remain unknown. Previous studies have shown that mannan is important in maintaining Arabidopsis seed mucilage architecture, and that Cellulose Synthase Like A2 (CSLA2) synthesizes a glucomannan backbone, which Mannan α-Galactosyl Transferase 1 (MAGT1)/MUCI10/GTL6 might decorate with single α-galactose (Gal) branches. Here, we investigated the ratio and sequence of mannose (Man) and glucose (Glc), and the arrangement of galactose (Gal) residues, in Arabidopsis mucilage mannan using enzyme sequential digestion, PACE, and mass spectrometry. We found that seed mucilage galactoglucomannan has a backbone consisting of the repeating disaccharide [4)-β-Glc-(1,4)-β-Man-(1,], and most of the Man residues in the backbone are substituted by single α-1,6-Gal. CSLA2 is responsible for the synthesis of this patterned glucomannan backbone and MAGT1 catalyses the addition of α-Gal. In vitro activity assays revealed that MAGT1 can transfer α-Gal from UDP-Gal only to Man residues within the CSLA2 patterned glucomannan backbone acceptor. These results indicate that CSLAs and galactosyltransferases are able to make precisely defined galactoglucomannan structures. Molecular dynamics simulations suggested this patterned galactoglucomannan is able to bind stably to some hydrophilic faces and to hydrophobic faces of cellulose microfibrils.