Structural features conserved in subclass of type II arabinogalactan

Abstract

Arabinogalactan-proteins (AGPs) are extracellular proteoglycans, which are presumed to participate in the regulation of cell shape, thus contributing to the excellent mechanical properties of plants. AGPs consist of a hydroxyprolinerich core-protein and large arabinogalactan (AG) sugar chains, called type II AGs. These AGs have a β-1,3-galactan backbone and β-1,6-galactan side chains, to which other sugars are attached. The structure of type II AG differs depending on source plant, tissue, and age. Type II AGs obtained from woody plants in large quantity as represented by gum arabic and larch AG, here designated gum arabic-subclass, have a β-1,3;1,6-galactan structure in which the β-1,3-galactan backbone is highly substituted with short β-1,6-galactan side chains. On the other hand, it is unclear whether type II AGs found as the glycan part of AGPs from herbaceous plants, here designated AGP-subclass, also have conserved β-1,3:1,6-galactan structural features. In the present study we explore similarities of type II AG structures in the AGP-subclass. Type II AGs in fractions obtained from spinach, broccoli, bok choy, komatsuna, and cucumber were hydrolyzed into galactose and β-1,6-galactooligosaccharides by specific enzymes. Based on the proportion of these sugars, the substitution ratio of the β-1,3-galactan backbone was calculated as 46-58% in the five vegetables, which is consistently lower than what is seen in gum arabic and larch AG. Although most side chains were short, long chains such as β-1,6-galactohexaose chains were also observed in these vegetables. The results suggest a conserved β-1,3;1,6-galactan structure in the AGP-subclass that distinguishes it from the gum arabic-subclass.

In vivo structural modification of type II arabinogalactans with fungal endo-β-1, 6-galactanase in Arabidopsis

“…Consistent with other plants such as spinach (Spinacia oleracea) and broccoli (Brassica oleracea var. italica) examined in a previous study (Ito et al, 2020), short side chains such as b-1,6-Gal and b-1,6-Gal 2 were very dominant over longer side chains including b-1,6-Gal 5 and b-1,6-Gal 6 in WT Arabidopsis (Figure 4A). Compared with WT plants, Dex-treated Dex:: Tv6GAL plants #5 and #6 had a significantly reduced proportion of long b-1,6-galactan side chains such as b-1,6-Gal 5 and b-1,6-Gal 6 (Figure 4B).…”

“…Although the structure of type II AG, in particular auxiliary sugars, is diverse, depending on plant species, tissues, and developmental stages, several common features can be found in the β-1,3:1,6-galactan ( Tsumuraya et al., 1987 ; Tsumuraya et al., 1988 ). First, the substitution ratio of β-1,3-galactan with β-1,6-galactosyl side chains in type II AGs ranges from 46 to 58% in many angiosperms ( Ito et al., 2020 ). Second, β-1,6-galactose (β-1,6-Gal) is the dominant side chain, but long side chains such as β-1,6-galactohexaose (β-1,6-Gal 6 ) also occur as minor side chains in type II AGs ( Ito et al., 2020 ).…”

“…First, the substitution ratio of β-1,3-galactan with β-1,6-galactosyl side chains in type II AGs ranges from 46 to 58% in many angiosperms ( Ito et al., 2020 ). Second, β-1,6-galactose (β-1,6-Gal) is the dominant side chain, but long side chains such as β-1,6-galactohexaose (β-1,6-Gal 6 ) also occur as minor side chains in type II AGs ( Ito et al., 2020 ).…”

Charge neutralization and β-elimination cleavage mechanism of family 42 L-rhamnose-α-1,4-D-glucuronate lyase revealed using neutron crystallography

“…AGPs consist of more than 90% type II arabinogalactan (a carbohydrate moiety) and a core protein rich in hydroxyproline. Type II arabinogalactan contains β-1,3-galactan as the main chain and β-1,6-galactooligosaccharide side chains, which are modified with various sugars, including L-rhamnose (Rha) and D-glucuronic acid (GlcA) (1, 4–6). The complicated polysaccharide structure of AGPs with many branches hampers the detailed analysis of their structure–function relationships (7, 8).…”

“…The complicated polysaccharide structure of AGPs with many branches hampers the detailed analysis of their structure–function relationships (7, 8). Gum arabic (GA) is a subclass of type II arabinogalactan that is sometimes regarded as a representative AGP (6). GA is a sticky exudate from Acacia trees that is produced during stress conditions, such as drought and mechanical injury (9).…”