On the Potential Function of Type II Arabinogalactan O-Glycosylation in Regulating the Fate of Plant Secretory Proteins

Abstract

In a plant-specific mode of protein glycosylation, various sugars and glycans are attached to hydroxyproline giving rise to a variety of diverse O-glycoproteins. The sub-family of arabinogalactan proteins is implicated in a multitude of biological functions, however, the mechanistic role of O-glycosylation on AGPs by type II arabinogalactans is largely elusive. Some models suggest roles of the O-glycans such as in ligand-receptor interactions and as localized calcium ion store. Structurally different but possibly analogous types of protein O-glycosylation exist in animal and yeast models and roles for O-glycans were suggested in determining the fate of O-glycoproteins by affecting intracellular sorting or proteolytic activation and degradation. At present, only few examples exist that describe how the fate of artificial and endogenous arabinogalactan proteins is affected by O-glycosylation with type II arabinogalactans. In addition to other roles, these glycans might act as a molecular determinant for cellular localization and protein lifetime of many endogenous proteins.

Arabinogalactan Proteins in Plant Roots – An Update on Possible Functions

“…However, this remains to be conclusively proven. The cleavage of the anchor may also release the plasma membrane from the cell wall matrix, influencing membrane dynamics, including the trafficking of membrane receptors between the plasmalemma and inner compartments (Seifert, 2020). AGPs might act as a cargo linkage/receptor during the FIGURE 1 | Schematic summary of the involvement of arabinogalactan proteins (AGPs) in root processes.…”

N- and O-Glycosylation Pathways in the Microalgae Polyphyletic Group

“…Some of them start in the ER and continue in the Golgi apparatus, while others occur exclusively in the Golgi apparatus. O-glycosylation involves an oxygen-carbon bond between the hydroxyl group of a Ser or a Thr residue of the protein and the oligosaccharide chain in mammals (Bennett et al, 2012) while in plants, O-glycosylation occurs essentially in hydroxyproline residue (Hyp;Nguema-Ona et al, 2014;Seifert, 2020). In most eukaryotes including humans, O-glycans do not present a common structure or a consensus sequence.…”

“…They consist in a superfamily of plant cell wall proteins that are divided into three major multigene families: the highly glycosylated arabinogalactan proteins (AGPs), the moderately glycosylated extensins (EXTs) and the low glycosylated proline-rich proteins. The O-glycosylation of HRGPs results from two consecutive post-translational modifications involving the hydroxylation of Pro (Hyp) residues by prolyl 4-hydroxylases in the ER and the subsequent O-glycosylation in the Golgi apparatus of some, but not all, Hyp residues by glycosyltransferases before being transported to their final location within or outside the cell (Nguema-Ona et al, 2014;Seifert, 2020). Overall, O-glycan cores in plants present a Gal residue attached to a Ser or an unique arabinose (Ara) residue attached to an Hyp.…”

Plant Protein O-Arabinosylation

“…Not all structural variations of arabinogalactans are shown. The representative Type-II structures were assembled from Tryfona et al (2012) and Seifert (2020) . Type-III structures and the dicot pollen allergens were derived from Leonard et al (2005) and Leonard et al (2010) .…”

“…The glycans are β-1,3-galactans featuring β-1,6-linked galactan side-chains and further decorated with rhamnose, (Me-)glucuronic acid, arabinose, and fucose ( Figure 2 ; Tryfona et al, 2012 ). AGPs were reviewed recently ( Seifert, 2020 ). One of the subfamilies, referred to as hybrid in a newly updated HRGP classification scheme ( Liu et al, 2020 ), comprises both sites for clustered non-contiguous Hyp O -galactosylation and sites for contiguous Hyp O -arabinosylation and is thus included here.…”