N-glycan LLO assembly I — cytoplasmic face (to Man5GlcNAc2-PP-dolichol); DPAGT1/ALG13/ALG14/ALG1/ALG2/ALG11

N-linked glycosylation begins with the stepwise assembly of the dolichol-linked oligosaccharide (LLO, or dolichol-PP-oligosaccharide) on the cytoplasmic face of the endoplasmic-reticulum membrane. Using nucleotide-sugar donors from the cytosol, the pathway builds a Man5GlcNAc2 glycan on the polyprenol carrier dolichyl phosphate before the intermediate is flipped into the ER lumen. DPAGT1 (GlcNAc-1-phosphotransferase, the first and committed step, and the tunicamycin target) transfers GlcNAc-1-P from UDP-GlcNAc onto dolichyl phosphate to give GlcNAc-PP-dolichol; the bipartite UDP-GlcNAc transferase ALG13/ALG14 (ALG13 catalytic, ALG14 membrane-anchoring) adds the second GlcNAc to form the chitobiose core GlcNAc2-PP-dolichol; ALG1 (beta-1,4-mannosyltransferase) adds the first mannose; the bifunctional ALG2 (alpha-1,3 and alpha-1,6 mannosyltransferase) adds the second and third mannoses to give Man3GlcNAc2-PP-dolichol; and ALG11 (alpha-1,2-mannosyltransferase) adds the fourth and fifth mannoses to complete Man5GlcNAc2-PP-dolichol, the final cytoplasmic-face intermediate (subsequently flipped to the lumen by the RFT1 flippase, and extended by the lumenal ALG enzymes). All the mannose-addition steps in this segment use GDP-mannose. Inherited defects in each step cause a congenital disorder of glycosylation (CDG type I): DPAGT1-CDG (and a congenital myasthenic syndrome), ALG13-CDG (an X-linked developmental/epileptic encephalopathy), ALG14-CDG (congenital myasthenic syndrome), ALG1-CDG, ALG2-CDG (and CMS14) and ALG11-CDG.