Htm1 protein generates the N-glycan signal for glycoprotein degradation in the endoplasmic reticulum.
To maintain protein homeostasis in secretory compartments, eukaryotic cells harbor a quality control system that monitors protein folding and protein complex assembly in the endoplasmic reticulum (ER). Proteins that do not fold properly or integrate into cognate complexes are degraded by ER-associated degradation (ERAD) involving retrotranslocation to the cytoplasm and proteasomal peptide hydrolysis. N-linked glycans are essential in glycoprotein ERAD; the covalent oligosaccharide structure is used as a signal to display the folding status of the host protein. In this study, we define the function of the Htm1 protein as an alpha1,2-specific exomannosidase that generates the Man(7)GlcNAc(2) oligosaccharide with a terminal alpha1,6-linked mannosyl residue on degradation substrates. This oligosaccharide signal is decoded by the ER-localized lectin Yos9p that in conjunction with Hrd3p triggers the ubiquitin-proteasome-dependent hydrolysis of these glycoproteins. The Htm1p exomannosidase activity requires processing of the N-glycan by glucosidase I, glucosidase II, and mannosidase I, resulting in a sequential order of specific N-glycan structures that reflect the folding status of the glycoprotein.
Clerc
Simone
S
Department of Biology, Institute of Microbiology, Eidgenössische Technische Hochschule Zürich, Zürich, Switzerland.
Hirsch
Christian
C
Oggier
Daniela Maria
DM
Deprez
Paola
P
Jakob
Claude
C
Sommer
Thomas
T
Aebi
Markus
M
D002352:Carrier Proteins / Q000378:metabolism / Q000502:physiology; D004721:Endoplasmic Reticulum / Q000378:metabolism* / Q000648:ultrastructure; D006023:Glycoproteins / Q000378:metabolism*; D006868:Hydrolysis; D008351:Mannans / Q000378:metabolism; D008361:Mannosidases / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D009844:Oligosaccharides / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex / Q000502:physiology; D019704:Protein Disulfide-Isomerases; D017510:Protein Folding; D025941:Protein Interaction Mapping; D012441:Saccharomyces cerevisiae / Q000378:metabolism* / Q000648:ultrastructure; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D013379:Substrate Specificity; D025801:Ubiquitin / Q000502:physiology
D002352:Carrier Proteins; D006023:Glycoproteins; D008351:Mannans; D009844:Oligosaccharides; C080576:PDI1 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D025801:Ubiquitin; C495027:Yos9 protein, S cerevisiae; C434527:MNL1 protein, S cerevisiae; D008361:Mannosidases; D046988:Proteasome Endopeptidase Complex; D019704:Protein Disulfide-Isomerases