Mannose trimming reactions in the early stages of the N-glycan processing pathway

In all eukaryotes, asparagine-linked glycosylation is one of the most common co- and posttranslational modifications of secretory and membrane proteins. In mammals, N-glycosylation is involved in several cellular processes including protein folding, protein stability, intracellular trafficking and interactions with other proteins. Until recently, the functional importance of protein N-glycosylation in plants has been widely elusive. We have now identified class I α-mannosidase mutants (mns) impaired in de-mannosylation of N-glycans.(1) The mns mutants accumulate high amounts of oligo-mannosidic N-glycans and display striking cell wall alterations and defects in root development. Especially the mns1 mns2 mns3 triple mutant forms short roots with radially swollen cortical cells and displays alterations in development of aerial plant parts. Our data indicate that the N-glycan processing defects in mns mutants affect one or several glycoproteins involved in cell wall formation, which could be linked to the potential impact of mannose trimming on glycoprotein quality control in the endoplasmic reticulum (ER) and ER-associated degradation of misfolded glycoproteins.