Cell wall O-glycoproteins and N-glycoproteins: aspects of biosynthesis and function.

Eric Nguema-Ona,Maïté Vicré-Gibouin,Azeddine Driouich,Maxime Gotté,Barbara Plancot,Muriel Bardor,Patrice Lerouge

doi:10.3389/fpls.2014.00499

Abstract

Cell wall O-glycoproteins and N-glycoproteins are two types of glycomolecules whose glycans are structurally complex. They are both assembled and modified within the endomembrane system, i.e., the endoplasmic reticulum (ER) and the Golgi apparatus, before their transport to their final locations within or outside the cell. In contrast to extensins (EXTs), the O-glycan chains of arabinogalactan proteins (AGPs) are highly heterogeneous consisting mostly of (i) a short oligo-arabinoside chain of three to four residues, and (ii) a larger β-1,3-linked galactan backbone with β-1,6-linked side chains containing galactose, arabinose and, often, fucose, rhamnose, or glucuronic acid. The fine structure of arabinogalactan chains varies between, and within plant species, and is important for the functional activities of the glycoproteins. With regards to N-glycans, ER-synthesizing events are highly conserved in all eukaryotes studied so far since they are essential for efficient protein folding. In contrast, evolutionary adaptation of N-glycan processing in the Golgi apparatus has given rise to a variety of organism-specific complex structures. Therefore, plant complex-type N-glycans contain specific glyco-epitopes such as core β,2-xylose, core α1,3-fucose residues, and Lewisa substitutions on the terminal position of the antenna. Like O-glycans, N-glycans of proteins are essential for their stability and function. Mutants affected in the glycan metabolic pathways have provided valuable information on the role of N-/O-glycoproteins in the control of growth, morphogenesis and adaptation to biotic and abiotic stresses. With regards to O-glycoproteins, only EXTs and AGPs are considered herein. The biosynthesis of these glycoproteins and functional aspects are presented and discussed in this review.

Highlights

Plants synthesize glycoconjugates that are structurally diverse and complex reflecting the diversity of plant physiological functions
The glycomolecules are usually assembled and modified within the plant endomembrane system, including the endoplasmic reticulum (ER), the Golgi apparatus and secretory vesicles responsible for their transport to different cell compartments/organelles including the cell wall. Their synthesis involves a number of steps, beginning with the formation of activated nucleotide sugars such as NDP-sugars or NMP-sugars (Bar-Peled and O’Neill, 2011). After their synthesis in the cytosol, the nucleotide sugars are actively transported into the ER and Golgi stacks where they serve as donor substrates during glycan synthesis
The FRET photo-bleaching acceptor technique showed that both enzymes did not physically interact. These findings suggest that all the enzymes involved in arabinogalactan proteins (AGPs) glycan synthesis, probably co-regulated, are not necessarily part of a unique multi-protein complex

Summary

INTRODUCTION

Plants synthesize glycoconjugates that are structurally diverse and complex reflecting the diversity of plant physiological functions. The glycomolecules are usually assembled and modified within the plant endomembrane system, including the endoplasmic reticulum (ER), the Golgi apparatus and secretory vesicles responsible for their transport to different cell compartments/organelles including the cell wall Their synthesis involves a number of steps, beginning with the formation of activated nucleotide sugars such as NDP-sugars or NMP-sugars (Bar-Peled and O’Neill, 2011). The plant cell wall contains a significant percentage (∼10–15%) of N - and O-glycosylated proteins that are relatively less studied with regards to their biosynthesis and function Both the N - and the Oglycosylation of proteins has a significant impact on both their structural properties and biological activities (Varki, 1993). These include, for example, N-acetylglucosaminyltransferase I (GnT I; Bakker et al, 1999; Strasser et al, 1999a; Wenderoth and von Schaewen, 2000), Nacetylglucosaminyltransferase II (GnT II; Strasser et al, 1999b), core α-1,3-fucosyltransferase (α1,3-FuT; Leiter et al, 1999; Wilson et al, 2001a), β-1,2-xylosyltransferase

AGP glycan biosynthetic enzymes CAZy family Protein name Origin

Arabidopsis thaliana

Findings

CONCLUSION AND OUTLOOK