Study Reveals Distinct Roles of Three Mannosidase Enzymes in Protein Quality Control and Maturation
A new preprint study reveals that three mammalian mannosidases — ManIA, ManIB, and ManIC — localize primarily to specialized quality control vesicles rather than the Golgi apparatus, and play distinct roles in both degrading defective glycoproteins and promoting the maturation of properly folded ones. The findings overturn the previous assumption that these enzymes were exclusively Golgi-resident maturation enzymes, showing instead that each occupies a separate vesicular compartment and exhibits a preference for trimming properly folded glycoproteins. This challenges the existing model of how cells distinguish between misfolded proteins destined for degradation and correctly folded proteins cleared for secretion.
Researchers posting to bioRxiv have re-examined the subcellular localization and function of three Class I α-1,2 mannosidases — ManIA, ManIB, and ManIC — which are involved in trimming mannose residues from N-linked glycoproteins, a step critical for both protein maturation and endoplasmic reticulum-associated degradation (ERAD). Contrary to the longstanding view that these enzymes reside primarily in the Golgi, the study finds they are predominantly found in quality control vesicles (QCVs), with only ManIC also maintaining a substantial Golgi population. Notably, each of the three enzymes is confined to a distinct vesicular population, suggesting a previously unrecognized compartmentalization of glycoprotein processing. All three enzymes were shown to promote ERAD targeting of misfolded glycoproteins, though with different substrate preferences, and they redundantly support the maturation and cell-surface delivery of a model glycoprotein. Crucially, in vitro experiments demonstrated that ManIA, ManIB, and ManIC preferentially process properly folded glycoproteins over denatured ones — the opposite preference from ERManI, EDEM1, and EDEM2, which selectively act on misfolded substrates. The authors propose a division-of-labor model: ERManI and the EDEMs tag misfolded glycoproteins for ERAD recognition, while ManIA, ManIB, and ManIC act on correctly folded glycoproteins to release them from lectin-mediated retention and facilitate forward trafficking through the secretory pathway.
What's missing
As a preprint, this study has not yet undergone peer review, so the findings should be treated as preliminary. The study relies heavily on model glycoproteins, and it remains unclear how broadly these findings generalize to endogenous glycoproteins in physiological contexts. The mechanisms by which each mannosidase is sorted into distinct vesicular populations are not yet explained, representing a key open question.
What different sources said
- bioRxivCenter
Mannosidases IA, IB and IC are in segregated vesicular structures and involved in both glycoprotein quality control and maturation
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