Heterodimeric Alg13/Alg14 UDP-GlcNAc transferase (ALG13,14)

Protein N- glycosylation begins with the synthesis of a conserved lipid-linked oligosaccharide (LLO) precursor. After assembly, the oligosaccharide is transferred from the lipid to nascent proteins during their translation in the ER. Synthesis of this oligosaccharide is catalyzed in sequential steps by twelve endoplasmic reticulum (ER) membrane-associated Alg (A sparagine-l inked g lycosylation) glycosyltransferases. Seven sugars are added to dolichyl pyrophosphate on the cytoplasmic face of the ER, using nucleotide-sugar substrates UDP-GlcNAc and GDP-Man. After flipping into the ER lumen, seven more sugars, using dolichol-linked sugar substrates, are used to extend the Man₅GlcNAc₂-PP-Dol intermediate to produce Glc₃Man₇GlcNAc₂-PP-Dol (reviewed in: Helenius and Aebi 2004; Kelleher and Gilmore 2006; Weerapana and Imperiali 2006). The second step of LLO synthesis adds N-acetylglucosamine (GlcNAc) to GlcNAc-PP-Dol to produce GlcNAc₂-PP-Dol. This reaction is catalyzed by a glycosyltransferase (GTase) that consists of two subunits, Alg13 and Alg14 (Fig. 109.1). Most of what we know about this UDP-GlcNAc GTase comes from studies in yeast, in which both subunits are essential for viability. Alg13 and Alg14 are highly conserved in virtually all eukaryotes, from yeast to man, so insights from yeast are likely to be applicable to humans.