Figure 1.

Proposed Scheme of Biosynthesis of Dol-PP–Linked Glc₃Man₉GlcNAc₂ in Arabidopsis.

Dol-PP-Man₅GlcNAc₂ is formed at the cytoplasmic side of the ER using cytoplasmic UDP-GlcNAc and GDP-Man as substrates and is flipped over into the ER lumen. Four Man and three Glc residues are sequentially added from Dol-P-Man and Dol-P-Glc, respectively, to form Dol-PP-Glc₃Man₉GlcNAc₂ that is transferred to nascent proteins OST. The high-mannose (H)-type glycans produced here are further processed in the Golgi into complex (C)-type glycans. The added glycoresidues, the corresponding glycosyltransferases, and three types of mannosyl bonds are shown. Three mannose branches are indicated on the enlarged Glc₃Man₉GlcNAc₂ structure with circled numbers representing the order by which each Man is added. Glycosidases involved in trimming Glc and Man residues (GI, GII, and α-1,2 mannosidase) are also shown.