MAN1B1

MAN1B1 is a Golgi-resident alpha-1,2-mannosidase that functions as a glycoprotein quality control gatekeeper through both catalytic and non-catalytic mechanisms. Its enzymatic activity trims terminal mannose residues from N-glycans to generate ERAD signals for misfolded glycoproteins such as alpha1-antitrypsin NHK, while a separate catalysis-independent mechanism controlled by its evolutionarily extended N-terminal cytoplasmic tail accelerates proteasomal degradation of misfolded substrates independently of N-glycans (PMID:32958677, PMID:24627495). MAN1B1 is recruited by Membralin (TMEM259) into a MAN1B1–VCP complex that directs densely glycosylated viral envelope proteins to lysosomes via ER-to-lysosome-associated degradation, and its loss markedly enhances pseudoviral infectivity (PMID:41324484). Biallelic loss-of-function mutations cause a congenital disorder of glycosylation (MAN1B1-CDG) characterized by accumulation of hybrid-type N-glycans, Golgi fragmentation, and intellectual disability (PMID:21763484, PMID:24566669, PMID:24348268).

1. 2011 High

   Establishing that MAN1B1 is an enzymatically active alpha-1,2-mannosidase whose catalytic function is essential, as disease-associated missense mutations either collapsed kcat by ~1300-fold or abolished stable protein expression, linking enzymatic loss to congenital glycosylation defects.

   Evidence Sanger sequencing of patient families, recombinant enzyme kinetic assays, and mammalian cell expression of disease mutants

   PMID:21763484

   Open questions at the time

   • Physiological substrates in vivo not identified
   • Whether catalytic-dead protein retains any non-enzymatic function was not tested
2. 2011 High

   Revising the assumed ER localization, this work showed endogenous MAN1B1 predominantly resides in the Golgi where it undergoes O-glycosylation, establishing the Golgi rather than the ER as the compartment where ERAD substrate tagging by mannose trimming occurs.

   Evidence Subcellular fractionation, immunofluorescence, O-glycosylation analysis, and COPI-motif chimera pulse-chase assays in human cells

   PMID:21697506

   Open questions at the time

   • Mechanism of Golgi retention was undefined
   • Whether a minor ER pool contributes to function was not excluded
3. 2013 Medium

   Patient cell analysis confirmed Golgi localization and revealed that MAN1B1 deficiency causes Golgi dilatation and fragmentation, demonstrating a structural role of the enzyme in maintaining Golgi integrity.

   Evidence Immunofluorescence and electron microscopy in patient-derived fibroblasts with confirmed MAN1B1 mutations

   PMID:24348268

   Open questions at the time

   • Whether Golgi disruption is a direct effect or secondary to glycan processing defects was not resolved
   • Molecular mechanism of Golgi fragmentation unknown
4. 2014 High

   Systematic domain dissection revealed that MAN1B1 possesses a catalysis-independent quality control function: neither mannosidase activity nor the catalytic domain was required for retention and degradation of misfolded NHK, with a conserved luminal stem decapeptide controlling substrate fate, fundamentally expanding the protein's role beyond its enzymatic activity.

   Evidence Active-site mutagenesis, catalytic domain deletions, and pulse-chase degradation assays in human cells

   PMID:24627495

   Open questions at the time

   • Binding partners of the non-enzymatic decapeptide not identified
   • Whether the dual mechanism operates on the same or distinct substrate pools was unclear
5. 2014 High

   Mass spectrometry glycoprofiling of a large patient cohort defined the in vivo enzymatic product of MAN1B1 deficiency as accumulation of hybrid-type N-glycans, pinpointing the enzyme's activity to trimming the terminal mannose from the middle branch of N-glycans in the Golgi.

   Evidence High-resolution mass spectrometry of intact plasma transferrin and serum glycoproteins from 12 MAN1B1-CDG patients

   PMID:24566669

   Open questions at the time

   • Whether all hybrid glycan species are direct substrates versus secondary effects was not distinguished
6. 2015 High

   MAN1B1 was placed in the ERAD pathway for HIV-1 envelope glycoprotein, showing that TSPO-mediated Env degradation requires MAN1B1 and that the catalytic domain mediates direct Env interaction, extending its quality control role to viral glycoprotein disposal.

   Evidence CRISPR/Cas9 MAN1B1 knockout, Co-IP, domain deletion/chimera, and active-site mutagenesis in human cells with HIV-1 Env

   PMID:26205822

   Open questions at the time

   • Mechanism by which TSPO and MAN1B1 cooperate was not resolved
   • Whether catalytic trimming or lectin-like binding mediates Env recognition was unclear
7. 2020 High

   The dual mechanism of MAN1B1 was formally delineated: a conventional catalytic pathway generating N-glycan ERAD signals, and an unconventional pathway driven by the N-terminal cytoplasmic tail that accelerates proteasomal degradation independently of N-glycans, with each system operating on overlapping ERAD substrates.

   Evidence MAN1B1 knockout HEK293T cells with domain-specific rescue constructs, pulse-chase labeling, and proteasome inhibitor studies across multiple AAT variants

   PMID:32958677

   Open questions at the time

   • Cytoplasmic interactors mediating the non-catalytic pathway not identified
   • Relative physiological contribution of each pathway unclear
8. 2022 Medium

   MAN1B1 function was extended to neuronal development: knockdown in mouse neurons disrupted axon growth, dendrite formation, and spine maturation, while in utero electroporation revealed impaired neural stem cell proliferation and cortical neuron migration, providing a cellular basis for intellectual disability in MAN1B1-CDG patients.

   Evidence shRNA knockdown in cultured mouse excitatory neurons and in utero electroporation in mouse cortex with morphometric analysis

   PMID:40869158

   Open questions at the time

   • No rescue with wild-type MAN1B1 was performed
   • Whether neuronal phenotypes are glycan-dependent or mediated by the non-catalytic arm is unknown
   • Specific neuronal substrates not identified
9. 2025 High

   Membralin (TMEM259) was identified as the adaptor that recruits MAN1B1 into a MAN1B1–VCP complex, directing densely glycosylated viral class I fusion proteins (SARS-CoV-2 spike, Ebola GP, influenza HA, HIV-1 Env) to lysosomes via ERLAD, establishing MAN1B1 as a broad innate antiviral effector.

   Evidence Reciprocal Co-IP, domain mapping, MAN1B1 knockout, pseudoviral infectivity assays, LIR mutagenesis, and lysosomal delivery tracking across multiple viral glycoproteins

   PMID:41324484

   Open questions at the time

   • Whether catalytic activity or lectin-like recognition drives viral substrate selection is unresolved
   • Structural basis of the Membralin–MAN1B1–VCP complex unknown
10. 2025 Medium

    Post-translational regulation of MAN1B1 was uncovered: ERK activation stabilizes MAN1B1 by modulating its interaction with the E3 ligase HRD1 that normally ubiquitinates MAN1B1 for degradation, linking MAPK signaling to glycoprotein quality control capacity and, in bladder cancer, to CD47-mediated immune evasion.

    Evidence Co-IP of MAN1B1–HRD1 interaction, MAN1B1 knockout in vitro and in vivo, phagocytosis assays, ERK pathway inhibition in bladder cancer cells

    PMID:40493414

    Open questions at the time

    • Direct ERK phosphorylation site on MAN1B1 or HRD1 not mapped
    • Mechanism not reconstituted with purified components
    • Whether HRD1-mediated turnover occurs in non-cancer contexts is untested

• Key unresolved questions include the structural basis of the MAN1B1 dual mechanism, the identity of cytoplasmic interactors of the N-terminal tail, the molecular determinants distinguishing substrates routed to proteasomal ERAD versus lysosomal ERLAD, and the precise localization determinant that partitions MAN1B1 between the Golgi and quality control vesicles.
• No crystal or cryo-EM structure of MAN1B1
• Cytoplasmic tail interactome unmapped
• ERAD versus ERLAD substrate sorting logic unknown