The evolution of N-glycan-dependent endoplasmic reticulum quality control factors for glycoprotein folding and degradation

*Department of Molecular and Cell Biology, Boston University Goldman School of Dental Medicine, Boston, MA 02118;
^†Graduate Program in Bioinformatics, Boston University, Boston, MA 02215; and
^‡Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, MA 01605-2324

Contributed by Phillips W. Robbins, May 25, 2007 (received for review April 4, 2007)

Abstract

Asn-linked glycans (N-glycans) play important roles in the quality control (QC) of glycoprotein folding in the endoplasmic reticulum (ER) lumen and in ER-associated degradation (ERAD) of proteins by cytosolic proteasomes. A UDP-Glc:glycoprotein glucosyltransferase glucosylates N-glycans of misfolded proteins, which are then bound and refolded by calreticulin and/or calnexin in association with a protein disulfide isomerase. Alternatively, an α-1,2-mannosidase (Mns1) and mannosidase-like proteins (ER degradation-enhancing α-mannosidase-like proteins 1, 2, and 3) are part of a process that results in the dislocation of misfolded glycoproteins into the cytosol, where proteins are degraded in the proteasome. Recently we found that numerous protists and fungi contain 0–11 sugars in their N-glycan precursors versus 14 sugars in those of animals, plants, fungi, and Dictyostelium. Our goal here was to determine what effect N-glycan precursor diversity has on N-glycan-dependent QC systems of glycoprotein folding and ERAD. N-glycan-dependent QC of folding (UDP-Glc:glycoprotein glucosyltransferase, calreticulin, and/or calnexin) was present and active in some but not all protists containing at least five mannose residues in their N-glycans and was absent in protists lacking Man. In contrast, N-glycan-dependent ERAD appeared to be absent from the majority of protists. However, Trypanosoma and Trichomonas genomes predicted ER degradation-enhancing α-mannosidase-like protein and Mns1 orthologs, respectively, each of which had α-mannosidase activity in vitro. Phylogenetic analyses suggested that the diversity of N-glycan-dependent QC of glycoprotein folding (and possibly that of ERAD) was best explained by secondary loss. We conclude that N-glycan precursor length has profound effects on N-glycan-dependent QC of glycoprotein folding and ERAD.

Footnotes

^§To whom correspondence should be addressed at:
Boston University Goldman School of Dental Medicine, 715 Albany Street, Boston, MA 02118.
E-mail: robbinsp{at}bu.edu
Author contributions: R.G., P.W.R., and J.S. designed research; S.B. and D.J.K. performed research; P.V., J.C., R.G., P.W.R., and J.S. analyzed data; and R.G., P.W.R., and J.S. wrote the paper.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/cgi/content/full/0704862104/DC1.
Abbreviations:

ER,

endoplasmic reticulum;

CNX,

calnexin;

CRT,

calreticulin;

EDEM,

ER degradation-enhancing α-mannosidase-like protein;

ERAD,

ER-associated degradation;

QC,

quality control;

UGGT,

UDP-Glc:glycoprotein glucosyltransferase;

PNGase,

peptide-N-glycanase;

Glc,

glucose;

Man,

mannose.
Freely available online through the PNAS open access option.