The formation of N-glycosidic linkages of glycoproteins involves the ordered assembly of the common Glc3Man9GlcNAc2 core-oligosaccharide on the lipid carrier dolichyl diphosphate. Early mannosylation steps occur on the cytoplasmic side of the endoplasmic reticulum with GDP-Man as donor, the final reactions from Man5GlcNAc2-PP-Dol to Man9Glc-NAc2-PP-Dol on the lumenal side use dolichyl beta-D-mannosyl phosphate. ALG9 mannosyltransferase catalyses the addition of two different alpha-1,2-mannose residues: the addition of alpha-1,2-mannose to Man6GlcNAc2-PP-Dol (EC 188.8.131.529) and the addition of alpha-1,2-mannose to Man8GlcNAc2-PP-Dol (EC 184.108.40.2061).
congenital disorders of glycosylation, a deficiency of the ALG9 alpha1,2 mannosyltransferase enzyme, causes an accumulation of lipid-linked-GlcNAc2Man6 and -GlcNAc2Man8 structures, which is paralleled by the transfer of incomplete oligosaccharide precursors to protein. A homozygous point-mutation E523K is detected in the ALG9 gene. The ALG9 defect found in the patient with congenital disorders of glycosylation, who presents with developmental delay, hypotonia, seizures, and hepatomegaly, shows that efficient lipid-linked oligosaccharide synthesis is required for proper human development and physiology
an ALG9-defective (congenital disorders of glycosylation type IL) patient who is homozygous for the p.Y286C (c.860A>G) mutation. This patient presents with psychomotor retardation, axial hypotonia, epilepsy, failure to thrive, inverted nipples, hepatomegaly, and pericardial effusion. Due to the ALG9 deficiency, the cells of this patient accumulated the lipid-linked oligosaccharides Man6GlcNAc2-PP-dolichol and Man8GlcNAc2-PP-dolichol. Lipid-linked Man6GlcNAc2 and Man8GlcNAc2 are transferred onto proteins with the same efficiency. In addition, glycoproteins bearing these Man6GlcNAc2 and Man8GlcNAc2 structures efficiently enter in the glucosylation/deglucosylation cycle of the quality control system to assist in protein folding. In comparison with control cells, patient’s cells degrade misfolded glycoproteins at an increasing rate. The Man8GlcNAc2 isomer C on the patient’s glycoproteins is found to promote the degradation of misfolded glycoproteins
residue R100 is located near the end of the largest luminal loop between the first two predicted transmembrane segments and is absolutely conserved in all known ALG9 enzymes. Mutation suppresses a dwarf mutant, bri1-9, the phenotypes of which are caused by endoplasmic reticulum retention and endoplasmic reticulum-associated degradation of a brassinosteroid receptor, BRASSINOSTEROID-INSENSITIVE 1, BR1. The mutation prevents the Glc3Man9GlcNAc2 assembly and inhibits the endoplasmic reticulum-associated degradation of bri1-9. Overexpression of EBS4 in the R100W bri1-9 mutant, which encodes the Arabidopsis ortholog of the yeast ALG12 catalyzing the ER luminal alpha1,6 Man addition, adds an alpha1,6 Man to the truncated N-glycan precursor accumulated in R100W bri1-9, promotes the bri1-9 endoplasmic reticulum-associated degradation, and neutralizes the R100W suppressor phenotype
patient, who is homozygous for the ALG9 mutation p.Y286C, deleterious effect Y286C on the ALG9 function. Compared the complementation efficiency of the wild-type and mutant ALG9 cDNA in yeast cells deficient for alg9. In an assay, the growth efficiency of the transformed yeast double mutant alg9 wbp1-2 is tested. Deficiency in lipid-linked oligosaccharide biosynthesis (alg9) in combination with reduced oligosaccharyltransferase activity (wbp1-2) results in a temperature-sensitive phenotype at 30°C. At this restrictive temperature, both the normal and mutant ALG9 cDNAs are able to restore growth. The HsALG9 transformants perform similar to the yeast alg9. The complementation with the mutant construct (HsALG9 (Y286C)) is less efficient, resulting is a reduced growth restoration. This difference becomes more prominent when the transformants are grown at 32°C. The hypoglycosylation of the alg9 yeast strain is reflected in the presence of CPY glycoforms lacking one or two N-linked oligosacharides. The human ALG9 cDNA complements the yeast mutation partially, as shown by the improved glycosylation of CPY. The complementation efficiency of the HsALG9 (Y286C) is less efficient and almost comparable to the empty vector
homozygous splice variant NM_024740.2: c.1173+2T4A in the ALG9 gene causes rare lethal autosomal recessive Gillessen-Kaesbach-Nishimura skeletal dysplasia. Skipping of exon 10 leads to shorter RNA and results in an increase in monoglycosylated transferrin