Alg13p, the Catalytic Subunit of the ER UDP-GlcNAc glycosyltransferase, Is a Target for Proteasomal Degradation

Nicole Averbeck,* Xiao-Dong Gao, ${dagger}$ Shin-Ichiro Nishimura, ${dagger}$ and Neta Dean*

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

Monitoring Editor: Reid Gilmore

The second step of dolichol-linkedoligosaccharide synthesis in the N-linked glycosylation pathwayat the ER membrane is catalyzed by an unusual hetero-oligomericUDP-N-acetylglucosamine transferase that in most eukaryotesis comprised of at least two subunits, Alg13p and Alg14p. Alg13pis the cytosolic and catalytic subunit that is recruited tothe ER by the membrane protein Alg14p. We show that in S. cerevisiae,cytosolic Alg13p is very short lived while membrane associatedAlg13 is relatively stable. Cytosolic Alg13p is a target forproteasomal degradation, and the failure to degrade excess Alg13pleads to glycosylation defects. Alg13p degradation does notrequire ubiquitin but instead, requires a C-terminal domainwhose deletion results in Alg13p stability. Conversely, appendingthis sequence onto normally long-lived ${beta}$ -galactosidase causesit to undergo rapid degradation, demonstrating that this C-terminaldomain represents a novel and autonomous degradation motif.These data lead to the model that proteasomal degradation ofexcess unassembled Alg13p is an important quality control mechanismthat ensures proper protein complex assembly and correct N-linkedglycosylation.

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