A β-glucuronosyltransferase from Arabidopsis thaliana involved in biosynthesis of type II arabinogalactan has a role in cell elongation during seedling growth.

We have characterized a β-glucuronosyltransferase ( At Glc AT14 A) from Arabidopsis thaliana that is involved in the biosynthesis of type II arabinogalactan ( AG). This enzyme belongs to the Carbohydrate Active Enzyme database glycosyltransferase family 14 ( GT14). The protein was localized to the Golgi apparatus when transiently expressed in Nicotiana benthamiana. The soluble catalytic domain expressed in Pichia pastoris transferred glucuronic acid ( Glc A) to β-1,6-galactooligosaccharides with degrees of polymerization ( DP) ranging from 3-11, and to β-1,3-galactooligosaccharides of DP5 and 7, indicating that the enzyme is a glucuronosyltransferase that modifies both the β-1,6- and β-1,3-galactan present in type II AG. Two allelic T- DNA insertion mutant lines showed 20-35% enhanced cell elongation during seedling growth compared to wild-type. Analyses of AG isolated from the mutants revealed a reduction of Glc A substitution on Gal-β-1,6-Gal and β-1,3-Gal, indicating an in vivo role of At Glc AT14A in synthesis of those structures in type II AG. Moreover, a relative increase in the levels of 3-, 6- and 3,6-linked galactose (Gal) and reduced levels of 3-, 2- and 2,5-linked arabinose (Ara) were seen, suggesting that the mutation in AtGlc AT14A results in a relative increase of the longer and branched β-1,3- and β-1,6-galactans. This increase of galactosylation in the mutants is most likely caused by increased availability of the O6 position of Gal, which is a shared acceptor site for At Glc AT14 A and galactosyltransferases in synthesis of type II AG, and thus addition of Glc A may terminate Gal chain extension. We discuss a role for the glucuronosyltransferase in the biosynthesis of type II AG, with a biological role during seedling growth. [ABSTRACT FROM AUTHOR]