Initiates complex N-linked carbohydrate formation. Essential for the conversion of high-mannose to hybrid and complex N-glycans. Required for normal root growth and morphology. N(4)-(alpha-D-Man-(1->3)-[alpha-D-Man-(1->3)-[alpha-D-Man-(1->6)]-alpha-D-Man-(1->6)]-beta-D-Man-(1->4)-beta-D-GlcNAc-(1->4)-beta-D-GlcNAc)-L-asparaginyl-[protein] (N-glucan mannose isomer 5A1,2) + UDP-N-acetyl-alpha-D-glucosamine = H(+) + N(4)-{beta-D-GlcNAc-(1->2)-alpha-D-Man-(1->3)-[alpha-D-Man-(1->3)-[alpha-D-Man-(1->6)]-alpha-D-Man-(1->6)]-beta-D-Man-(1->4)-beta-D-GlcNAc-(1->4)-beta-D-GlcNAc}-L-asparaginyl-[protein] + UDP The cofactor is mostly bound to the substrate. the apparent KM is >3.0 mM for Man(3)-octyl. Protein modification; protein glycosylation. A number of isoforms are produced. According to EST sequences. Expressed in roots, stems, leaves and flowers. Glycosylated. Plants have an increased salt-sensitivity resulting in growth inhibition, aberrant root-tip morphology and callose accumulation. Creation of a second N-glycosylation site in mutant cgl1 C5/cgl1-1 interferes with protein folding and sequesters the protein for degradation in the endoplasmic reticulum. Belongs to the glycosyltransferase 13 family.