The initial steps in N-linked glycosylation involve the synthesis of a lipid-linked core oligosaccharide followed by the transfer of the core glycan to nascent polypeptides in the endoplasmic reticulum (ER). Here, we describe alg11, a new yeast glycosylation mutant that is defective in the last step of the synthesis of the Man(5)GlcNAc(2)-PP-dolichol core oligosaccharide on the cytosolic face of the ER. A deletion of the ALG11 gene leads to poor growth and temperature-sensitive lethality. In an alg11 lesion, both Man(3)GlcNAc(2)-PP-dolichol and Man(4)GlcNAc(2)-PP-dolichol are translocated into the ER lumen as substrates for the Man-P-dolichol-dependent sugar transferases in this compartment. This leads to a unique family of oligosaccharide structures lacking one or both of the lower arm alpha1,2-linked Man residues. The former are elongated to mannan, whereas the latter are poor substrates for outerchain initiation by Ochlp (Nakayama, K.-I., Nakanishi-Shindo, Y., Tanaka, A., Haga-Toda, Y., and Jigami, Y. (1997) FEBS Lett. 412, 547-550) and accumulate largely as truncated biosynthetic end products. The ALG11 gene is predicted to encode a 63.1-kDa membrane protein that by indirect immunofluorescence resides in the ER. The Alg11 protein is highly conserved, with homologs in fission yeast, worms, flies, and plants. In addition to these Alg11-related proteins, Alg11p is also similar to Alg2p, a protein that regulates the addition of the third mannose to the core oligosaccharide. All of these Alg11-related proteins share a 23-amino acid sequence that is found in over 60 proteins from bacteria to man whose function is in sugar metabolism, implicating this sequence as a potential sugar nucleotide binding motif.