Htm1 protein generates the N-glycan signal for glycoprotein degradation in the endoplasmic reticulum

Abstract

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 α1,2-specific exomannosidase that generates the Man7GlcNAc2 oligosaccharide with a terminal α1,6-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.

Analysis of antibody production in Saccharomyces cerevisiae: effects of ER protein quality control disruption

“…Htm1p (Clerc et al 2009) and Ubc7p (Hiller et al 1996). From a biotechnologists' perspective, it is paramount to study these processes under more natural conditions and using real protein substrates.…”

“…In addition, IgG secretion was increased in the Δalg3 strains, a genotype which is considered not to produce the glycan signal for ERAD. It has been shown that removal of a mannose by Htm1p is required for efficient binding of ERAD targeted proteins to Yos9p (Szathmary et al 2005;Clerc et al 2009). Therefore, this could imply that the slowly folding IgG protein escape from degradation through ERAD in the Δhtm1 background, giving it more time to fold and be secreted.…”

“…The resulting Man8GlcNAc2 glycan, when present on a still unfolded protein, is recognized by the Pdi1p/Htm1p complex (Gauss et al 2011). The Htm1p mannosidase of this complex further trims down the glycan to the Man7GlcNAc2 form by removing the terminal mannose at the C-branch of the glycan, in order to create the substrate for HRD ligase-mediated glycoprotein degradation (Clerc et al 2009). The lectin Yos9p is part of this HRD ligase complex and recognizes glycans containing the now available terminal α1,6 linked mannose residues (Quan et al 2008).…”

Genome‐wide screen of Saccharomyces cerevisiae for killer toxin HM‐1 resistance

“…A mannosidaselike protein Mnl1p recognizes a single isomer of Man 8 GlcNAc 2 glycan of misfolded glycoprotein, and cuts off a terminal 1,2-alpha-mannose residue from the C-chain to generate Man 7 GlcNAc 2 glycan (Figure 2 et Jakob et al, 2001;Nakatsukasa et al, 2001). Lectin Yos9p recognizes the generated 1,6-alpha-linked mannose terminal of Man 7 GlcNAc 2 glycan and then accelerates the ERAD (Buschhorn et al, 2004;Clerc et al, 2009;Friedmann et al, 2002;Gauss et al, 2006;Kanehara et al, 2007;Szathmary et al, 2005). In mns1 cells, Man 9 -GlcNAc 2 glycan is not processed to Man 8 GlcNAc 2 because of lack of Mns1p and is not recognized by Mnl1p.…”

“…In mns1 cells, Man 9 -GlcNAc 2 glycan is not processed to Man 8 GlcNAc 2 because of lack of Mns1p and is not recognized by Mnl1p. The 1,6-alpha-linked mannose terminal in N -glycan of alg3 mutant can be recognized by Yos9p (Clerc et al, 2009). Therefore, we assumed that HM-1 resistance of alg3 and mns1 mutants result from abnormal ERAD acceleration of some glycoproteins.…”