Two Arabidopsis thaliana Golgi α-mannosidase I enzymes are responsible for plant N-glycan maturation

Abstract

N-Glycosylation is an important post-translational modification that occurs in many secreted and membrane proteins in eukaryotic cells. Golgi α-mannosidase I hydrolases (MANI) are key enzymes that play a role in the early Nglycan modification pathway in the Golgi apparatus. In Arabidopsis thaliana, two putative MANI genes, AtMANIa (At3g21160) and AtMANIb (At1g51590), were identified. Biochemical analysis using bacterially produced recombinantAtMANIisoforms revealed that bothAtMANIisoforms encode 1-deoxymannojirimycin-sensitive α-mannosidase I and act on Man8GlcNAc2 and Man9GlcNAc2 structures to yield Man5GlcNAc2. Structures of hydrolytic intermediates accumulated in the AtMANI reactions indicate that AtMANIs employ hydrolytic pathways distinct from those of mammalian MANIs. In planta, AtMANI-GFP/DsRed fusion proteins were detected in the Golgi stacks. Arabidopsis mutant lines manIa-1, manIa-2, manIb-1, and manIb-2 showed N-glycan profiles similar to that of wild type. On the other hand, the manIa manIb double mutant lines produced Man8GlcNAc2 as the predominant N-glycan and lacked plant-specific complex and hybrid N-glycans. These data indicate that either AtMANIa or AtMANIb can function as the Golgi α-mannosidase I that produces the Man5GlcNAc2 N-glycan structure necessary for complex N-glycan synthesis. © The Author 2009. Published by Oxford University Press.