A Pipeline towards the Biochemical Characterization of the Arabidopsis GT14 Family

Glycosyltransferases (GTs) catalyze the synthesis of glycosidic linkages and are essential in the biosynthesis of glycans, glycoconjugates (glycolipids and glycoproteins), and glycosides. Plant genomes generally encode many more GTs than animal genomes due to the synthesis of a cell wall and a wide variety of glycosylated secondary metabolites. The Arabidopsis thaliana genome is predicted to encode over 573 GTs that are currently classified into 42 diverse families. The biochemical functions of most of these GTs are still unknown. In this study, we updated the JBEI Arabidopsis GT clone collection by cloning an additional 105 GT cDNAs, 508 in total (89%), into Gateway-compatible vectors for downstream characterization. We further established a functional analysis pipeline using transient expression in tobacco (Nicotiana benthamiana) followed by enzymatic assays, fractionation of enzymatic products by reversed-phase HPLC (RP-HPLC) and characterization by mass spectrometry (MS). Using the GT14 family as an exemplar, we outline a strategy for identifying effective substrates of GT enzymes. By addition of UDP-GlcA as donor and the synthetic acceptors galactose-nitrobenzodiazole (Gal-NBD), β-1,6-galactotetraose (β-1,6-Gal₄) and β-1,3-galactopentose (β-1,3-Gal₅) to microsomes expressing individual GT14 enzymes, we verified the β-glucuronosyltransferase (GlcAT) activity of three members of this family (AtGlcAT14A, B, and E). In addition, a new family member (AT4G27480, 248) was shown to possess significantly higher activity than other GT14 enzymes. Our data indicate a likely role in arabinogalactan-protein (AGP) biosynthesis for these GT14 members. Together, the updated Arabidopsis GT clone collection and the biochemical analysis pipeline present an efficient means to identify and characterize novel GT catalytic activities.