FIGURE 4.

Simplified representation of the AChR pentamer and the N-glycosylation pathway of proteins and CMS-related genes (red color). (A) Adult (red) and fetal (green) acetylcholine receptors and glycosylation sites. The AChR is made up of five subunits arranged around a central pore. Each subunit is composed of an extracellular domain, four transmembrane domains (M1–M4) and a large cytoplasmic loop linking M3 and M4. Each subunit also contains a conserved N-glycosylation consensus sequence (Asn-X-Ser/Thr) located at the N-terminal region (Gehle et al., 1997), which is necessary for the correct assembly of AChR pentamers and efficient export to the cell surface. (B) The N-linked glycosylation pathway takes place in the endoplasmic reticulum (ER). The first step is the assembly of the core glycan (N-acetylglucosamine, glucose and mannose) on the lipid dolichol. Next, a number of cytosolic glycosyltransferases proceed to dolichol glycosylation on the cytoplasmic face of the ER: GFPT1 synthesizes UDP-GlcNAc (Uridine diphosphate N-acetylglucosamine); DPAGT1 and the ALG13/14 complex add the first and second N-acetylglucosamine to dolichol. Additional sugar residues are incorporated by ALG2 and other enzymes. Then RFT1 flips the resulting product is flipped into the ER lumen where further sugar moieties are included until the glycan is transferred to asparagine residues of nascent proteins by the multimeric oligosaccharyl transferase complex (OST). DOLK, dolichol kinase; DPM, dolichol-phosphate mannose synthase; Fru-6-P, fructose-6-phosphate; GlcN-6-P, glucosamine-6-phosphate; Glu-6-P, glucose-6-phosphate; GMPPB, GDP-mannose pyrophosphrylase B.