Figure 1.

Two pathways to complex protein N-glycosylation in mammals. Complex-type N-glycans are produced in the Golgi apparatus and are the predominant forms among extracellular compartments. Each pathway depends on a separate α-mannosidase activity to produce the hybrid N-glycan substrate for the GlcNAcT-II glycosyltransferase. Differential use of each pathway among glycoprotein substrates indicates additional controls in N-glycan repertoire expression. Black square, N-acetylglucosamine; open triangle, fucose; black circles, galactose; open circles, mannose. Anomeric linkage states are denoted. The α1–6 linkage of fucose to the asparagine-proximal N-acetylglucosamine (dashed lines) can be found on both hybrid and complex N-glycans.