Abstract
Free N-glycans (FNGs) are present at micromolar concentrations in plant cells during their differentiation, growth, and maturation stages. It has been postulated that these FNGs are signaling molecules involved in plant development or fruit ripening. However, the hypothetical biochemical and molecular function of FNGs has not been yet established. The structure of FNGs found ubiquitously in plant tissues such as hypocotyls, leaves, roots, developing seeds, or fruits can be classified into two types: high-mannose type and plant complex type; the former, in most cases, has only one GlcNAc residue at the reducing end (GN1 type), while the latter has the chitobiosyl unit at the reducing end (GN2 type). These findings suggest that endo-β-N-acetylglucosaminidase (ENGase) must be involved in the production of GN1 type FNGs, whereas only peptide:N-glycanase (PNGase) is involved in the production of GN2 type FNGs. It has been hypothesized that cytosolic PNGase (cPNGase) and ENGase in animal cells are involved in the production of high-mannose type FNGs in order to release N-glycans from the misfolded glycoproteins in the protein quality control systems. In the case of plants, it is well known that another type of PNGase, the acidic PNGase (aPNGase) is involved in the production of plant complex type FNGs in an acidic organelle, suggesting the de-N-glycosylation mechanism in plants is different from that in animal cells. To better understand the role of these FNGs in plants, the genes encoding these N-glycan releasing enzymes (ENGase and PNGase) were first identified, and then structure of FNGs in ENGase knocked-out plants were analyzed. These transgenic plants provide new insight into the plant-specific de-N-glycosylation mechanism and putative physiological functions of FNGs. In this review, we focus on the structural features of plant FNGs, as well as functional features of cPNGase/ENGase and plant specific PNGase, and putative functions of FNGs are also discussed.
Highlights
Free N-glycans (FNGs) are present at micromolar concentrations in plant cells during their differentiation, growth, and maturation stages
The structural features of plant N-glycans are as follows: (1) the occurrences of β1,2-xylosyl (Xyl) residue linked to β1,4mannosyl (Man) residue and α1,3-fucosyl (Fuc) residue linked to the reducing end N-acetylglucosaminyl (GlcNAc) residue, (2) the occurrence of Lewis a (Lea) epitope [Galβ1-3(Fucα1-4)GlcNAc] at the non-reducing end, and (3) the lack of N -acetylneuraminic www.frontiersin.org
The disposal of misfolded/miss-associated proteins is known as endoplasmic reticulum (ER)-associated degradation (ERAD), and the resulting FNGs produced by cytosolic PNGase (cPNGase) always belong to highmannose type structure with the N-acetylchitobiosyl unit at their reducing ends (GN2 type FNGs)
Summary
Free N-glycans (FNGs) are present at micromolar concentrations in plant cells during their differentiation, growth, and maturation stages. Plant specific aPNGase involved in the production of plant complex type FNGs is genetically different from cPNGase localized in the cytosol and has the optimum activity in the acidic environment.
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