The first glucose is added to the N-glycan precursor, mediated by ALG6. Defects in ALG6 are associated with CDG-Ic disorder (Imbach T et al, 1999; Sun L et al, 2005). The donor is a dolichol-phosphate-glucose (synthesized by ALG5).
Van Hove, JL
dolichyl-phosphate-glucose-glycolipid alpha-glucosyltransferase activity of ALG6 [endoplasmic reticulum membrane]