Htm1p-Pdi1p is a folding-sensitive mannosidase that marks N-glycoproteins for ER-associated protein degradation.

Our understanding of how the endoplasmic reticulum (ER)-associated protein degradation (ERAD) machinery efficiently targets terminally misfolded proteins while avoiding the misidentification of nascent polypeptides and correctly folded proteins is limited. For luminal N-glycoproteins, demannosylation of their N-glycan to expose a terminal α1,6-linked mannose is necessary for their degradation via ERAD, but whether this modification is specific to misfolded proteins is unknown. Here we report that the complex of the mannosidase Htm1p and the protein disulfide isomerase Pdi1p (Htm1p-Pdi1p) acts as a folding-sensitive mannosidase for catalyzing this first committed step in Saccharomyces cerevisiae We reconstitute this step in vitro with Htm1p-Pdi1p and model glycoprotein substrates whose structural states we can manipulate. We find that Htm1p-Pdi1p is a glycoprotein-specific mannosidase that preferentially targets nonnative glycoproteins trapped in partially structured states. As such, Htm1p-Pdi1p is suited to act as a licensing factor that monitors folding in the ER lumen and preferentially commits glycoproteins trapped in partially structured states for degradation.