It has been proposed that synthesis of beta-1,6-glucan, one of Saccharomyces cerevisiae cell wall components, is initiated by a uridine diphosphate (UDP)-glucose-dependent reaction in the lumen of the endoplasmic reticulum (ER). Because this sugar nucleotide is not synthesized in the lumen of the ER, we have examined whether or not UDP-glucose can be transported across the ER membrane. We have detected transport of this sugar nucleotide into the ER in vivo and into ER-containing microsomes in vitro. Experiments with ER-containing microsomes showed that transport of UDP-glucose was temperature dependent and saturable with an apparent Km of 46 microM and a Vmax of 200 pmol/mg protein/3 min. Transport was substrate specific because UDP-N-acetylglucosamine did not enter these vesicles. Demonstration of UDP-glucose transport into the ER lumen in vivo was accomplished by functional expression of Schizosaccharomyces pombe UDP-glucose:glycoprotein glucosyltransferase (GT) in S. cerevisiae, which is devoid of this activity. Monoglucosylated protein-linked oligosaccharides were detected in alg6 or alg5 mutant cells, which transfer Man9GlcNAc2 to protein; glucosylation was dependent on the inhibition of glucosidase II or the disruption of the gene encoding this enzyme. Although S. cerevisiae lacks GT, it contains Kre5p, a protein with significant homology and the same size and subcellular location as GT. Deletion mutants, kre5Delta, lack cell wall beta-1,6 glucan and grow very slowly. Expression of S. pombe GT in kre5Delta mutants did not complement the slow-growth phenotype, indicating that both proteins have different functions in spite of their similarities.