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Vol. 19, Issue 5, 2169-2178, May 2008

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Alg13p, the Catalytic Subunit of the Endoplasmic Reticulum UDP-GlcNAc Glycosyltransferase, Is a Target for Proteasomal Degradation

Nicole Averbeck^*, Xiao-Dong Gao, Shin-Ichiro Nishimura, and Neta Dean^*

*Department of Biochemistry and Cell Biology, Stony Brook University, Stony Brook, NY 11794-5215; and Graduate School of Advanced Life Science, Frontier Research Center for Post-Genomic Science and Technology, Hokkaido University, Sapporo 001-0021, Japan

Submitted October 25, 2007; Revised February 20, 2008; Accepted February 28, 2008
Monitoring Editor: Reid Gilmore

The second step of dolichol-linked oligosaccharide synthesis in the N-linked glycosylation pathway at the endoplasmic reticulum (ER) membrane is catalyzed by an unusual hetero-oligomeric UDP-N-acetylglucosamine transferase that in most eukaryotes is comprised of at least two subunits, Alg13p and Alg14p. Alg13p is the cytosolic and catalytic subunit that is recruited to the ER by the membrane protein Alg14p. We show that in Saccharomyces cerevisiae, cytosolic Alg13p is very short-lived, whereas membrane-associated Alg13 is relatively stable. Cytosolic Alg13p is a target for proteasomal degradation, and the failure to degrade excess Alg13p leads to glycosylation defects. Alg13p degradation does not require ubiquitin but instead, requires a C-terminal domain whose deletion results in Alg13p stability. Conversely, appending this sequence onto normally long-lived β-galactosidase causes it to undergo rapid degradation, demonstrating that this C-terminal domain represents a novel and autonomous degradation motif. These data lead to the model that proteasomal degradation of excess unassembled Alg13p is an important quality control mechanism that ensures proper protein complex assembly and correct N-linked glycosylation.

Address correspondence to: Neta Dean (Neta.Dean{at}stonybrook.edu)

Abbreviations used: ER, endoplasmic reticulum; GlcNAc, N-acetylglucosamine; LLO, lipid-linked oligosaccharide; v-ATPase, vacuolar ATPase; GTase, glycosyltransferase.