Biological significance of complex N-glycans in plants and their impact on plant physiology

Abstract

Asparagine (N)-linked protein glycosylation is a ubiquitous co- and post-translational modification which can alter the biological function of proteins and consequently affects the development, growth, and physiology of organisms. Despite an increasing knowledge of N-glycan biosynthesis and processing, we still understand very little about the biological function of individual N-glycan structures in plants. In particular, the N-glycan-processing steps mediated by Golgi-resident enzymes create a structurally diverse set of protein-linked carbohydrate structures. Some of these complex N-glycan modifications like the presence of β1,2-xylose, core α1,3-fucose or the Lewis a-epitope are characteristic for plants and are evolutionary highly conserved. In mammals, complex N-glycans are involved in different cellular processes including molecular recognition and signaling events. In contrast, the complex N-glycan function is still largely unknown in plants. Here, in this short review, I focus on important recent developments and discuss their implications for future research in plant glycobiology and plant biotechnology.

Keywords: Golgi apparatus; N-acetylglucosaminyltransferase; N-glycosylation; endoplasmic reticulum; glycoprotein; protein glycosylation.

FIGURE 1

(A) Examples for two characteristic types of N-glycans linked to the Asn–X–Ser/Thr sequence of proteins: oligomannosidic (e.g., Man8) and complex-type (e.g., GnGnXF) N-glycans. (B) Possible route in the formation of complex N-glycans in plants. Upon transfer of the preassembled oligosaccharide, the first N-glycan trimming reactions are catalyzed by α-glucosidases I (GCSI)/ II (GCSII) and α-mannosidase 3 (MNS3). Complex N-glycan formation is initiated in the Golgi apparatus by β1,2-N-acetylglucosaminyltransferase I (GNTI, highlighted in red). MNS1/2, Golgi α-mannosidase I (two forms with largely redundant function are present in A. thaliana); GMII, Golgi α-mannosidase II; GNTII, β1,2-N-acetylglucosaminyltransferase II; XYLT, β1,2-xylosyltransferase; FUT11/12, core α1,3-fucosyltransferases (two forms with largely redundant function are present in A. thaliana); GALT1, Lewis-type β1,3-galactosyltransferase; FUT13, α1,4-fucosyltransferase. Structural analysis of N-glycans from different A. thaliana mutants and in vitro enzyme activity assays revealed that downstream of GNTI the substrate specificity of the processing enzymes is less stringent (Strasser et al., 2006, 2007b). Not shown: the possible removal of terminal GlcNAc residues by β-hexosaminidases (HEXO proteins) which generates paucimannosidic N-glycans in post-Golgi compartments or in the extracellular space (Liebminger et al., 2011). (C) The phenotypes of characteristic N-glycan-processing mutants are shown. While an N-glycan-processing defect (mns1 mns2 mns3) upstream of GNTI results in a severe root and shoot phenotype in A. thaliana (Liebminger et al., 2009), cgl1 (or gntI, the allelic A. thaliana T-DNA knockout mutant) does not display any growth or developmental phenotype under normal growth conditions (von Schaewen et al., 1993; Kang et al., 2008). In contrast, rice gnt1 displays a severe growth phenotype resulting in early lethality (Fanata et al., 2013). The major N-glycan structures of the mutants are indicated.