Processing of the Terminal Alpha-1,2-Linked Mannose Residues From Oligomannosidic N-Glycans Is Critical for Proper Root Growth

Christiane Veit,Friedrich Altmann,Richard Strasser,Julia König

doi:10.3389/fpls.2018.01807

Christiane Veit, Friedrich Altmann + Show 2 more

Open Access

https://doi.org/10.3389/fpls.2018.01807

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Abstract

N-glycosylation is an essential protein modification that plays roles in many diverse biological processes including protein folding, quality control and protein interactions. Despite recent advances in characterization of the N-glycosylation and N-glycan processing machinery our understanding of N-glycosylation related processes in plant development is limited. In Arabidopsis thaliana, failure of mannose trimming from oligomannosidic N-glycans in the endoplasmic reticulum (ER) and cis/medial-Golgi leads to a defect in root development in the mns123 triple mutant. Here, we show that the severe root phenotype of mns123 is restored in asparagine-linked glycosylation (ALG)-deficient plants with distinct defects in the biosynthesis of the lipid-linked oligosaccharide precursor. The root growth of these ALG-deficient plants is not affected by the α-mannosidase inhibitor kifunensine. Genetic evidence shows that the defect is uncoupled from the glycan-dependent ER-associated degradation (ERAD) pathway that removes misfolded glycoproteins with oligomannosidic N-glycans from the ER. Restoration of mannose trimming using a trans-Golgi targeted α-mannosidase suppresses the defect of mns123 roots. These data suggest that processing of terminal mannose residues from oligomannosidic N-glycans is important for an unknown late-Golgi or post-Golgi process that is implicated in proper root formation.

Highlights

N-glycosylation of proteins is an essential co- and posttranslational modification in eukaryotes
ALG3, ALG9, and ALG12deficient Arabidopsis display a wild-type like growth despite the fact that they have altered oligomannosidic N-glycans (Henquet et al, 2008; Hong et al, 2009, 2012)
Even under salt/osmotic stress conditions, the root growth of alg3 is comparable to wild-type (Kajiura et al, 2010) suggesting that the absence of mannose residues on the B- and C-branches of the assembled oligosaccharide is well tolerated by the plants and does not severely interfere with developmental processes

Summary

Introduction

N-glycosylation of proteins is an essential co- and posttranslational modification in eukaryotes. During N-glycosylation a preassembled lipid-linked oligosaccharide is transferred en bloc to an asparagine residue that is present in the consensus sequence motif Asn-X-Ser/Thr of a polypeptide (Aebi, 2013). Of the lipid-linked oligosaccharide occurs in an ordered stepwise manner by ALG (asparagine linked glycosylation) enzymes. The first steps of the lipid-linked oligosaccharide biosynthesis take place on the cytosolic side of the endoplasmic reticulum (ER) membrane. The synthesized Man5GlcNAc2-dolichol pyrophosphate is transported across the ER membrane and used by a series of different ALGs as an acceptor substrate. In the ER lumen, the first mannose residue is transferred by the α1,3-mannosyltransferase ALG3.

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