Mannosidases IA, IB and IC are in segregated vesicular structures and involved in both glycoprotein quality control and maturation

N-linked glycoprotein processing critically depends on the trimming of -1,2 mannose residues, a key step required both for glycoprotein maturation along the secretory pathway and for targeting defective glycoproteins to endoplasmic reticulum-associated degradation (ERAD). Mammalian cells express seven Class I -1,2 mannosidases, yet their individual roles remain poorly defined, particularly for ManIA, ManIB, and ManIC, which were originally considered Golgi-resident maturation enzymes. Here, we re-evaluated the subcellular localization and functional contributions of these three mannosidases to glycoprotein quality control and maturation. We found that ManIA, ManIB, and ManIC localize predominantly to quality control vesicles (QCVs), previously identified by our group, whereas only ManIC displays a substantial Golgi population. Surprisingly, each enzyme is confined to a different vesicular population. All three enzymes promote ERAD targeting of misfolded model glycoproteins, albeit with different substrate preferences. In addition, they redundantly support the maturation and cell-surface delivery of a model glycoprotein. Most strikingly, in vitro analyses revealed that ManIA, ManIB, and ManIC preferentially trim a properly folded model glycoprotein rather than its denatured form. This is the opposite of the substrate preference that we previously observed for ERManI, EDEM1, and EDEM2. These findings support a model in which ERManI and the EDEMs selectively process misfolded glycoproteins to promote their recognition by the proximal lectin OS-9 and subsequent ERAD. In contrast, ManIA, ManIB and ManIC can slowly process misfolded glycoproteins but act rapidly on properly folded glycoprotein molecules, releasing them from ER-Golgi lectin-mediated retention or retrieval pathways, thereby promoting forward trafficking and maturation in the Golgi.