Abstract
Arabinogalactan-proteins (AGP) are proteoglycans found in cell walls and plasma membranes, and involved in various functions. The carbohydrate moieties of AGPs comprise beta-(1->3)-galactan backbones, branched with (1->6)-linked beta-Gal residues, to which other sugars such as L-Ara, GlcA, and 4-O-methyl-GlcA (4-Me-GlcA) residues are attached. We investigated the structures of L-Ara-containing oligosaccharides released from the sugar moieties by the action of exo-beta-(1->3)-galactanase. The oligosaccharides were characterized based on MALDI-TOF/MS, MS/MS, NMR, methylation analysis, and sequential enzymatic degradation with several AGP-specific enzymes.
We observed an acidic tetrasaccharide, beta-4-Me-GlcA-(1->6)[alpha-L-Araf-(1->3)]-beta-Gal-(1->6)-Gal, as a major L-Ara-containing oligosaccharide, implying that single L-Ara groups locate on Gal residues at non-reducing sites through alpha-(1->3)-linkages. A pentasaccharide consists of additional L-Ara groups attached at the L-Ara in the tetrasaccharide through alpha-(1->5)-linkages. We also observed oligosaccharides substituted with single L-Ara residues on different Gal residues along beta-(1->6)-linked Gal chains.
