The function of ALG8 (ALG8 alpha-1,3-glucosyltransferase, ENSG00000159063) is as follows. Dolichyl pyrophosphate Glc1Man9GlcNAc2 alpha-1,3- glucosyltransferase that operates in the biosynthetic pathway of dolichol-linked oligosaccharides, the glycan precursors employed in protein asparagine (N)-glycosylation. The assembly of dolichol-linked oligosaccharides begins on the cytosolic side of the endoplasmic reticulum membrane and finishes in its lumen. The sequential addition of sugars to dolichol pyrophosphate produces dolichol-linked oligosaccharides containing fourteen sugars, including two GlcNAcs, nine mannoses and three glucoses. Once assembled, the oligosaccharide is transferred from the lipid to nascent proteins by oligosaccharyltransferases. In the lumen of the endoplasmic reticulum, adds the second glucose residue from dolichyl phosphate glucose (Dol-P- Glc) onto the lipid-linked oligosaccharide intermediate Glc(1)Man(9)GlcNAc(2)-PP-Dol to produce Glc(2)Man(9)GlcNAc(2)-PP-Dol. Glc(2)Man(9)GlcNAc(2)-PP-Dol is a substrate for ALG10, the following enzyme in the biosynthetic pathway (PubMed:12480927, PubMed:15235028). Required for PKD1/Polycystin-1 maturation and localization to the plasma membrane of the primary cilia (By similarity). {ECO:0000250|UniProtKB:Q6P8H8, ECO:0000269|PubMed:12480927, ECO:0000269|PubMed:15235028}.