| Topic | Key points | Representative evidence/source | Citation IDs |
|---|---|---|---|
| Identity/domains | • Human **MAN1B1** encodes **ERManI/ER α1,2-mannosidase I** linked to UniProt **Q9UKM7**.
• It is a **type II membrane** glycosidase in the **GH47/class I α1,2-mannosidase** family.
• Protein architecture includes an **N-terminal cytosolic tail**, **TM segment**, and **luminal catalytic domain**. | Pan 2011; Sun 2020 | (pqac-00000005, pqac-00000023) |
| Catalytic reaction | • Catalyzes removal of **α1,2-linked mannose** residues from high-mannose N-glycans.
• Canonical reaction highlighted for MAN1B1 is **Man9GlcNAc2 → Man8GlcNAc2 isomer B**.
• Overexpression studies also support further trimming toward **Man5-7GlcNAc2** in cells. | Rymen 2013; Pan 2011; Zhang 2021 review | (pqac-00000001, pqac-00000006, pqac-00000014) |
| Substrate specificity | • Shows preference for the **B-branch terminal mannose** of **Man9GlcNAc2**.
• Substrates are mainly **N-glycosylated secretory pathway glycoproteins**, especially misfolded ERAD clients.
• Class I mannosidases cleave **only α1,2-linked mannose** residues; kifunensine blocks this processing and causes **Man9** accumulation. | Benyair 2015; Munteanu 2023 | (pqac-00000002, pqac-00000003, pqac-00000015) |
| Localization | • Localization is **debated/dynamic**: endogenous MAN1B1 was reported in the **Golgi/cis-Golgi** by some studies.
• Other work places it in **ER-derived quality control vesicles (QCVs)** and the **ER quality control compartment (ERQC)**, distinct from Golgi under optimized imaging/fractionation.
• Trafficking/localization may be stress-sensitive and influenced by **COPI/COPII-related cycling**. | Pan 2011; Rymen 2013; Benyair 2015 | (pqac-00000005, pqac-00000007, pqac-00000002, pqac-00000003) |
| ERAD roles | • Mannose trimming by MAN1B1 helps generate the **glycan timer/sorting signal** that routes terminally misfolded glycoproteins from folding cycles to **ER-associated degradation (ERAD)**.
• WT MAN1B1 accelerates degradation of model ERAD substrates such as **NHK** and influences degradation of **ATZ/PIZ**-related clients.
• Inhibition or knockdown of class I mannosidase activity impairs ERAD, supporting a causal role. | Pan 2011; Zhang 2021 review; Munteanu 2023 | (pqac-00000000, pqac-00000006, pqac-00000014, pqac-00000012) |
| Catalysis-independent role | • MAN1B1 has a second, **catalysis-independent** quality-control activity mediated by the **N-terminal cytoplasmic tail (aa 1-54)**.
• This pathway can promote **proteasomal degradation** of misfolded AAT variants even when MAN1B1 catalytic activity is disabled.
• Tail-dependent activity is **independent of N-glycans on the client substrate**, implying a noncanonical recruitment/sorting role. | Sun 2020 | (pqac-00000021, pqac-00000022, pqac-00000024, pqac-00000025) |
| Disease association | • **Biallelic MAN1B1 variants** cause **MAN1B1-CDG / Rafiq syndrome**, a **CDG-II** processing defect.
• Core phenotype includes **developmental delay/intellectual disability**, **facial dysmorphism**, and often **truncal obesity**.
• Patient cells also show **Golgi fragmentation/dilatation**, linking enzyme deficiency to secretory-pathway dysfunction. | Rymen 2013; Baz 2024; Yalcintepe 2023 | (pqac-00000001, pqac-00000007, pqac-00000028, pqac-00000030) |
| Diagnostic biomarkers | • Serum transferrin testing shows a **type II pattern** with markedly increased **trisialotransferrin**.
• Serum N-glycan MS reveals distinctive **hybrid glycans**, including **NeuAc1Hex6HexNAc3** and **NeuAc1Hex5HexNAc3**, often with **fucosylated counterparts**.
• A 2025 diagnostic review highlights **hybrid glycans on transferrin** as a characteristic clue for MAN1B1-CDG. | Rymen 2013; Wada 2025 | (pqac-00000009, pqac-00000010, pqac-00000008) |
| Experimental tools/applications | • **Kifunensine** is a widely used **class I mannosidase inhibitor** to probe MAN1B1-class trimming and ERQC/ERAD function.
• Modern workflows combine **lectin enrichment, Endo H digestion, and LC-MS/MS glycoproteomics** to identify class I mannosidase-dependent substrates.
• Clinical implementation includes **exome sequencing** plus **transferrin glycoform/MS glycan profiling** for diagnosis. | Munteanu 2023; Wada 2025; Baz 2024 | (pqac-00000011, pqac-00000012, pqac-00000015, pqac-00000008, pqac-00000034) |
| Recent (2023-2024) developments | • **2023 glycoproteomics** work expanded practical mapping of class I mannosidase-sensitive glycoproteins and early secretory pathway cargo under **kifunensine** perturbation.
• **2024 clinical exome** data show real-world diagnostic utility, including a patient diagnosed with **Rafiq syndrome** due to homozygous **MAN1B1 p.His691Arg**.
• Recent disease-focused literature emphasizes improved **molecular diagnostics** and phenotypic recognition of glycosylation disorders, including MAN1B1-CDG. | Munteanu 2023; Baz 2024 | (pqac-00000011, pqac-00000012, pqac-00000028, pqac-00000029) |
*Table: This table summarizes the core functional annotation of human MAN1B1/ERManI, including catalytic activity, localization, ERAD roles, disease links, and recent diagnostic/experimental developments. It is useful as a compact evidence map for interpreting the gene’s molecular function and clinical relevance.*