ALG1 encodes an essential beta-1,4-mannosyltransferas that adds the first mannose moiety to the growing dolichol-linked oligosaccharide (DLO) precursor on the cytosolic face of the endoplasmic reticulum (ER).Alg1 protein physically interacts with Alg2 and Alg11 manosyltransferases, which together catalyze the five mannose additions on the ER cytosolic face.To further understanding of how these mannosyltransferases function, we have undertaken a structural and functional study on eukaryotic Alg1 proteins.The Saccharomyces cerevisiae Alg1p contains 449 amino acids with a hydrophobic region near the N-terminus.