N-glycan processing and endoglycosidase sensitivity.

(A) Schema of N-glycan biosynthesis: assembly and processing. Note the synthesis of high mannose, hybrid, and complex-type N-glycans and the sites of inhibition of N-glycan synthesis and processing by tunicamycin (1), deoxymannojirimycin (2), and swainsonine (3). GlcNAc-PhosT indicates N-acetylglucosamine-1-phosphotransferase; OST, oligosaccharyl transferase; Glcase I, α1,2 glucosidase I; Glcase II, α1,3 glucosidase II; ER Man I, ER-localized α1,2 mannosidase I; Gol Man I, Golgi-localized α1,2 mannosidase I; GlcNAc-TI, β1,2 N-acetylglucosaminyltransferase; Man II, α1,3/6 mannosidase II; GlcNAc-TII, β1,2 N-acetylglucosaminyltransferase. ● indicates mannose (man); ▴, glucose (Glc); ▪, N-acetylglucosamine (GlcNAc); ♦, fucose; ↑, potential sites of chain elongation; ◂, potential addition of β4 GlcNAc. (B) N-glycan branch specificity of endo-N-glycosidases. The specificities of N-glycan cleavage for N-glycosidase F (N-glycanase), endoglycosidase F2, and endoglycosidase H are as shown by arrows. ●, mannose; ▪, N-acetylglucosamine.