UDP-glucose:dolichyl-phosphate glucosyltransferase is a transmembrane-bound enzyme of the endoplasmic reticulum involved in protein N-linked glycosylation. This enzyme catalyzes the transfer of glucose from UDP-glucose to dolichyl phosphate. The structural gene encoding this transferase from Saccharomyces cerevisiae was isolated by complementation of an alg5-1 mutation. DNA sequencing of ALG5 revealed an open-reading frame of 1002 bases encoding a transmembrane protein of molecular mass 38.3 kDa. Overexpression of Alg5p in both yeast and Escherichia coli results in an increase of UDP-glucose:dolichyl-phosphate glucosyltransferase activity, whereas a deletion of the yeast gene leads to a loss of this activity and a concomitant underglycosylation of carboxypeptidase Y. The ALG5 protein has sequence similarity to the GDP-mannose:dolichyl-phosphate mannosyltransferase (Dpm1p) from S. cerevisiae. Topological studies indicate that UDP-glucose:dolichyl-phosphate glucosyltransferase is a transmembrane protein that spans the membrane several times.