Extensin and Arabinogalactan-Protein Biosynthesis: Glycosyltransferases, Research Challenges, and Biosensors

Abstract

Recent research, mostly in Arabidopsis thaliana, has led to the identification and characterization of the glycosyltransferases responsible for the biosynthesis of two of the most functionally important and abundant families of plant cell wall proteins, extensins, and arabinogalactan-proteins. Extensin glycosylation involves monogalactosylation of serine residues by O-α-serine galactosyltransferase and the addition of oligoarabinosides one to five arabinose units in length to contiguous hydroxyproline residues by a set of specific arabinosyltransferase enzymes, which includes hydroxyproline O-β-arabinosyltransferases, β-1,2-arabinosyltransferases, and at least one α-1,3-arabinosyltransferase. AGP glycosylation, however, is much more complex and involves the addition of large arabinogalactan polysaccharide chains to non-contiguous hydroxyproline residues. These arabinogalactan chains are composed of β-1,3-galactan backbones decorated with β-1,6-galactose side chains that are further modified with α-arabinose as well as other sugars, including β-(methyl)glucuronic acid, α-rhamnose, and α-fucose. Specific sets of hydroxyproline O-β-galactosyltransferases, β-1,3-galactosyltransferases, β-1,6-galactosyltransferases, α-arabinosyltransferases, β-glucuronosyltransferases, α-rhamnosyltransferases, and α-fucosyltransferases are responsible for the synthesis of these complex structures. This mini-review summarizes the EXT and AGP glycosyltransferases identified and characterized to date along with corresponding genetic mutant data, which addresses the functional importance of EXT and AGP glycosylation. In one case, genetic mutant data indicate that the carbohydrate moiety of arabinogalactan-proteins may serve as an extracellular biosensor or signal for normal cellular growth. Finally, future research challenges with respect to understanding the function of these enzymes more completely and discovering and characterizing additional glycosyltransferases responsible for extensin and arabinogalactan-protein biosynthesis are also discussed.

Engineering ‘designer’ glycomodules for boosting recombinant protein secretion in tobacco hairy root culture and studying hydroxyproline‐O‐glycosylation process in plants

“…These results indicated the 42 kDa band corresponds to the (SP) 32 ‐EGFP polypeptide backbone with further modification on Ser residues, most likely O ‐galactosylation with a single galactose. Ser‐ O ‐monogalactosylation is a plant‐specific modification of proteins that has been found in both extensins and AGPs (Saito et al ., ; Showalter and Basu, 2016a). In fact, the (SP4) 18 ‐EGFP products might be Ser‐ O ‐galactosylated, as ~4 mol% Gal was detected in the secreted (SP4) 18 ‐EGFP (Figure b).…”

Insights into the Evolution of Hydroxyproline-Rich Glycoproteins from 1000 Plant Transcriptomes

“…In contrast, relatively little is known about the origin and evolution of the hydroxyproline-rich glycoproteins (HRGPs), a major group of wall glycoproteins, despite their widespread occurrence (Supplemental Table S1) and importance in plant growth and development (for review, see Fincher et al, 1983;Kieliszewski and Lamport, 1994;MajewskaSawka and Nothnagel, 2000;Ellis et al, 2010;Lamport et al, 2011;Draeger et al, 2015;Velasquez et al, 2015;Showalter and Basu, 2016). Examination of these glycoproteins in a wider range of plant species should provide valuable insights into how they have evolved in parallel with the polysaccharide components in plant walls.…”

KNS4/UPEX1: A Type II Arabinogalactan β-(1,3)-Galactosyltransferase Required for Pollen Exine Development

“…Among the members of this family, At1g77810, one of the closest homologs of KNS4, has been shown to encode a b-(1,3)- GalT (Qu et al, 2008). An enzyme with this activity is required for the biosynthesis of the b-(1,3)-galactan core structure of type II AGs, the glycan moieties that decorate AGPs (Ellis et al, 2010;Showalter and Basu 2016), as well as the type II AG side chains on RG-I (Mohnen, 2008). Furthermore, it was recently reported that the gene product of another close homolog, AtGALT31A (At1g32930), possesses b-(1,6)-GalT activity, which is also required for the formation of side chains in type II AGs (Geshi et al, 2013).…”