A Glucuronosyltransferase from Arabidopsis Thaliana Involved In
Total Page：16
File Type：pdf, Size：1020Kb
The Plant Journal (2013) 76, 1016–1029 doi: 10.1111/tpj.12353 A b–glucuronosyltransferase from Arabidopsis thaliana involved in biosynthesis of type II arabinogalactan has a role in cell elongation during seedling growth Eva Knoch1,†, Adiphol Dilokpimol1,†, Theodora Tryfona2, Christian P. Poulsen1, Guangyan Xiong3, Jesper Harholt1, Bent L. Petersen1, Peter Ulvskov1, Masood Z. Hadi4, Toshihisa Kotake5, Yoichi Tsumuraya5, Markus Pauly3, Paul Dupree2 and Naomi Geshi1,* 1Department of Plant and Environmental Sciences, Faculty of Science, University of Copenhagen, Thorvaldsensvej 40, Frederiksberg C 1871, Denmark, 2Department of Biochemistry, University of Cambridge Cambridge CB2 1QW, UK, 3Energy Biosciences Building 212C, 2151 Berkeley Way, Berkeley, CA 94720-5230, USA, 4Joint BioEnergy Institute, Lawrence Berkeley National Laboratory, 717 Potter Street, Berkeley CA 94720, USA, and 5Division of Life Science, Graduate School of Science and Engineering, Saitama University, 255 Shimo–okubo, Sakura–ku, Saitama 338–8570, Japan Received 11 September 2013; revised 30 September 2013; accepted 8 October 2013; published online 15 October 2013. *For correspondence (e-mail [email protected]). †These authors contributed equally to this work. SUMMARY We have characterized a b–glucuronosyltransferase (AtGlcAT14A) 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 pas- toris transferred glucuronic acid (GlcA) to b–1,6–galactooligosaccharides with degrees of polymerization (DP) ranging from 3–11, and to b–1,3–galactooligosaccharides of DP5 and 7, indicating that the enzyme is a glucuronosyltransferase that modifies both the b–1,6- and b–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 GlcA substitution on Gal–b– 1,6–Gal and b–1,3–Gal, indicating an in vivo role of AtGlcAT14A 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 AtGlcAT14A results in a rela- tive increase of the longer and branched b–1,3- and b–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 AtGlcAT14A and galactosyltransferases in synthesis of type II AG, and thus addition of GlcA may ter- minate Gal chain extension. We discuss a role for the glucuronosyltransferase in the biosynthesis of type II AG, with a biological role during seedling growth. Keywords: glycosyltransferase family 14, glucuronosyltransferase, arabinogalactan protein, type II arabino- galactan, plant cell wall, Golgi apparatus, Arabidopsis thaliana. INTRODUCTION The arabinogalactan proteins (AGPs, AG proteins) belong processes has been reported, including somatic embryo- to a highly diverse class of glycoproteins present on cell genesis, cell–cell interactions and cell elongation (Seifert surfaces of plants (Seifert and Roberts, 2007; Ellis et al., and Roberts, 2007). Most of these studies involved use of 2010; Tan et al., 2012). AGPs consist mainly of glycans monoclonal antibodies raised against the AG polysaccha- (>90% w/w), and are synthesized by post-translational rides, and a temporal and spatial appearance of specific modification in the secretory pathway. The importance of AG epitopes during development has been reported. the carbohydrate moieties of AGPs in various cellular However, as the precise epitope structure for most of the 1016 © 2013 The Authors The Plant Journal © 2013 John Wiley & Sons Ltd This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. b–glucuronosyltransferase involved in AGP biosynthesis 1017 antibodies is not known, the molecular function of the AG an extensive reduction in Ara moieties in beta-glucosyl glycan structures remains to be determined. The appear- Yariv-precipited AGPs (Gille et al., 2013). This alteration in ance of different AG glycans is probably controlled by a arabinosylation leads to shortened primary roots, but glycosylation process rather than redundancy of protein lateral root growth is not affected. cores, as a developmentally controlled pattern of glycosyl- We have characterized an Arabidopsis GT encoded by ation was observed on a single synthetic peptide At5g39990 that belongs to the GT14 family in the Carbo- expressed in Arabidopsis (Estevez et al., 2006). Therefore, hydrate Active Enzyme database (CAZy, www.cazy.org; elucidation of the enzymes involved in the biosynthesis of Cantarel et al., 2009). The GT14 family contains several AG glycans is expected to facilitate our understanding of mammalian GTs involved in protein glycosylation, e.g. the function of AGPs. b–1,6-N–acetyl glucosaminyltransferases catalyzing b–1,6- The structure of AG polysaccharides is very heteroge- linked N–acetylglucosaminylation in core 2 or I–branched neous even on a single peptide (Estevez et al., 2006; Tan O–glycosylation and protein O–b-xylosyltransferases et al., 2010), but commonly consists of a b–1,3-galactan (Bierhuizen et al., 1993; Yu et al., 2001; Wilson, 2002). In backbone with substitution at the O6 position with b–1,6- contrast, none of the putative plant GT14s [e.g. 11 from galactan side chains (type II AG, Tan et al., 2012); a model Arabidopsis, 12 from rice (Oryza sativa)] have been char- structure is shown in Figure 3(a). The side chains are acterized. Plant GT14s are phylogenetically related to the typically further substituted by arabinose (Ara) and less protein family containing Domain of Unknown Function frequently with other sugars such as glucuronic acid or 266 (DUF266; Ye et al., 2011), and a mutation in a 4–O–methyl glucuronic acid (collectively referred to as DUF266 protein in rice (brittle culm 10, BC10) caused a GlcA), rhamnose (Rha) and fucose (Fuc) (Tsumuraya et al., severe alteration in the mechanical strength of the stem 1988; Tan et al., 2010; Tryfona et al., 2010, 2012). The gly- and the AG quantity and structure (Zhou et al., 2009). cosylation of AGPs is catalyzed by glycosyltransferases The authors concluded that BC10 is probably a GT, but (GTs) that are located mainly in the Golgi apparatus. GTs involvement in the AG glycosylation pathway is not act in a regio- and stereo-specific manner (Lairson et al., clear. 2008), and it is expected that at least ten functionally dis- In this paper, we provide evidence for GlcAT activity of tinct GTs are required for the biosynthesis of type II AG. At5g39990 and its role in the biosynthesis of type II AG So far, two fucosyltransferases (AtFUT4 and AtFUT6; Wu structures and cell elongation during seedling growth. et al., 2010), two galactosyltransferases (AtGALT2; Basu et al., 2013; AtGALT31A; Geshi et al., 2013) and a putative RESULTS arabinosyltransferase (AtRAY1; Gille et al., 2013) from CAZy family GT14 Arabidopsis have been reported in the AG glycosylation pathway. AG fucosyltransferase activity was demonstrated Little is known about plant GTs in CAZy family GT14. As by gain of function of fucosylated AGs by heterologous plants do not have the same type of glycoconjugates pro- expression of Arabidopsis AtFUT4 and AtFUT6 in tobacco duced by mammalian GT14 enzymes (Bierhuizen et al., BY2 cells (Wu et al., 2010). Fucose on AGPs is important 1993; Yu et al., 2001; Wilson, 2002) and the plant GT14 for root development (Van Hengel and Roberts, 2002), but enzymes are phylogenetically distantly related to their a role for AtFUT4 and AtFUT6 in vivo remains to be eluci- mammalian counterparts (Figure 1), a distinct activity is dated. Galactosyltransferase (GalT) activity towards expected for the plant enzymes. We hypothesized that hydroxyproline in the synthetic peptide was demonstrated plant GT14 enzymes may be involved in the AG glycosyla- for Arabidopsis AtGALT2 expressed in Pichia pastoris tion pathway for three reasons: (i) plant GT14 members (Basu et al., 2013). The atgalt2 mutants demonstrated are related to DUF266 (Ye et al., 2011), and mutation in lower GalT activity and a reduced level of b–galactosyl one of the DUF266 proteins in rice (BC10) caused severe Yariv-precipited AGPs, but no apparent morphological phe- alteration in AG quantity and structure (Zhou et al., 2009), notype was reported (Basu et al., 2013). The GalT activity (ii) some GT14s are co-expressed with genes encoding the in elongation of the b1,6-galactan side chains of AG was protein backbone of AGPs (Showalter et al., 2010), and (iii) demonstrated by Arabidopsis AtGALT31A expressed in some GT14 genes are co-expressed with AtGALT31A Escherichia coli and Nicotiana benthamiana (Geshi et al., (At1g32930), whose heterologously expressed protein 2013). A mutation in AtGALT31A caused aberrant asym- demonstrated GT activity in elongation of the b–1,6-galac- metric formative divisions in the hypophysis during tan side chains of AG (Geshi et al., 2013). An Arabidopsis embryogenesis, and embryo development was arrested at gene, At5g39990 (indicated by an asterisk in Figure 1), is the globular stage, indicating an essential role for AG co-expressed with AtGALT31A during stem elongation glycan in the normal development of embryo (Geshi et al., (Figure S1), thus we selected the protein encoded by this 2013). Although GT activity of RAY1 has not yet been gene for further characterization regarding its involvement demonstrated, mutations in this GT family 77 gene led to in AG biosynthesis. © 2013 The Authors The Plant Journal © 2013 John Wiley & Sons Ltd, The Plant Journal, (2013), 76, 1016–1029 1018 Eva Knoch et al.