MNT4 (systematic name YNR059W) is a putative alpha-1,3-mannosyltransferase of the Mnn1p (MNN1/MNT) family in the budding yeast Saccharomyces cerevisiae. It is a 580-residue single-pass type II membrane protein: a short N-terminal cytoplasmic tail, a signal-anchor transmembrane helix (residues ~11-29), and a large lumenal C-terminal domain that adopts the nucleotide-diphospho-sugar (GT-A-like) glycosyltransferase fold characteristic of the family (Pfam Mannosyl_trans3; CAZy family GT71). This topology and fold are shared with the biochemically characterized family members Mnn1p, Mnt2p and Mnt3p, which are Golgi enzymes that add terminal alpha-1,3-linked mannose residues to N- and O-linked oligosaccharides. The candidate catalytic two-aspartate (DxD) motif that is essential for Mnn1p activity appears to be conserved in MNT4, consistent with a probable mannosyltransferase activity. However, no enzyme assay, acceptor substrate, reaction product, subcellular-localization determination, or deletion phenotype has been reported for MNT4 itself, and genetic analysis of the family showed that MNT4 is not required for O-linked glycan synthesis. Its specific molecular activity, acceptor substrate, site of action, and in-vivo biological role therefore remain undetermined.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000033 alpha-1,3-mannosyltransferase activity | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Family-level phylogenetic (IBA) transfer from the characterized Mnn1p-family mannosyltransferases (PANTHER PTHR31392; MNN1/MNT2/MNT3). MNT4 is a bona fide member of this type II membrane glycosyltransferase family and retains the candidate catalytic DxD motif (D300-S-D302), so a family-level alpha-1,3-mannosyltransferase activity is domain-defensible as a probable/putative activity. However there is no enzyme assay or acceptor substrate for MNT4, and it is not required for O-glycan synthesis, so this should be read as a probable activity with an undetermined acceptor rather than an established function. Reason: Homology-supported probable activity (intact family DxD motif) but no direct biochemical evidence for MNT4 and no known acceptor substrate; retain as the best family-level MF hypothesis, not as an established core function. |
| GO:0005794 Golgi apparatus | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetic (IBA) inference that MNT4 acts in the Golgi, by analogy to Mnn1p/Mnt2p/ Mnt3p which are Golgi mannosyltransferases. Consistent with the predicted type II Golgi-type membrane topology, but the actual compartment of MNT4 is not experimentally established; high-throughput fluorescent-tag screens report MNT4 in the ER and vacuole rather than a clean Golgi signal, so the site of action is uncertain. Reason: Family-level localization inference plausible from topology but not verified for MNT4; retain as non-core given conflicting high-throughput localization and no MNT4-specific confirmation. |
| GO:0006493 protein O-linked glycosylation | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: Phylogenetic (IBA) transfer of the family's O-mannosylation process. For MNT4 specifically this process is not supported: the only genetic test of MNT4 (Romero et al. 1999) found MNT4 is not required for O-glycan synthesis, in contrast to its paralogs MNT2/MNT3. This is a family-default over-annotation for MNT4. Reason: The specific O-linked glycosylation role is a family transfer that is contradicted by the one MNT4-specific genetic result (MNT4 not required for O-glycan synthesis; PMID:10521541); do not present it as an established MNT4 process. Propagation Review Root cause: PROPAGATION BAD Failure modes: FUNCTIONAL DIVERGENCE Sources checked: PANTHER:PTN001264810 · MNN1-related node (PTHR31392) SUPPORTS SOURCE BUT NOT TARGET The O-mannosylation role is established for the MNN1/MNT2/MNT3 members but Romero et al. 1999 genetically showed MNT4 is not required for O-glycan synthesis, so the process term should not transfer to MNT4. SGD:S000000803 · MNN1 SUPPORTS SOURCE BUT NOT TARGET Characterized Golgi O/N-mannosyltransferase; sound source, unsafe transfer to MNT4. |
| GO:0000030 mannosyltransferase activity | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: Electronic (ARBA) generic mannosyltransferase activity, the parent of GO:0000033. True at the family level and consistent with the fold/DxD motif, but less informative than the alpha-1,3 child term. Reason: Correct but generic parent term; domain-defensible as a probable activity, not core. |
| GO:0005794 Golgi apparatus | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: Electronic (ARBA) Golgi localization, duplicating the IBA Golgi assignment. Same caveat: family-level inference, not experimentally verified for MNT4, and in tension with ER/vacuole high-throughput localization. Reason: Redundant electronic family-level localization; retain as non-core pending MNT4-specific evidence. |
| GO:0009101 glycoprotein biosynthetic process | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: Electronic (ARBA/InterPro) generic glycoprotein-biosynthesis process, consistent with a glycosyltransferase of the secretory pathway. Broad and not experimentally grounded for MNT4; less problematic than the specific O-linked glycosylation term because it does not commit MNT4 to the O-mannosylation pathway that genetics exclude. Reason: Generic, homology-level process term; acceptable as non-core context but not an established MNT4 function. |
| GO:0016020 membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic subcellular-location mapping to membrane, matching the UniProt prediction of a single-pass type II membrane protein (signal-anchor TM 11-29). This is the most defensible localization statement for MNT4 (it rests on its own predicted topology rather than a family transfer). Reason: Directly supported by the protein's own predicted transmembrane topology; correct, though generic. |
| GO:0016757 glycosyltransferase activity | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: Electronic (InterPro IPR022751) generic glycosyltransferase activity, the top-level parent of the mannosyltransferase terms. Correct given the fold but uninformative. Reason: Correct but very general parent term; subsumed by the mannosyltransferase MF. |
| GO:0000033 alpha-1,3-mannosyltransferase activity | ISS PMID:10521541 Mnt2p and Mnt3p of Saccharomyces cerevisiae are members of t... | KEEP AS NON CORE | Summary: Sequence-similarity (ISS) transfer of alpha-1,3-mannosyltransferase activity to MNT4 from the characterized paralogs MNN1/MNT2/MNT3 (with-from SGD:S000000803/001276/003226), based on the family paper. Same substance as the IBA MF row: domain-defensible as a probable activity (intact DxD) but with no MNT4 enzyme assay and an unknown acceptor. Note the ISS is drawn from the very paper whose genetics show MNT4 is not required for O-glycan synthesis. Reason: Homology (ISS) inference of the family activity; retain as a probable non-core MF, not an experimentally established function. |
| GO:0006493 protein O-linked glycosylation | IC PMID:10521541 Mnt2p and Mnt3p of Saccharomyces cerevisiae are members of t... | MARK AS OVER ANNOTATED | Summary: Curator inference (IC) of the O-linked glycosylation process from the MF annotation (GO:0000033), not from independent MNT4 experimental data. Because it is derived from the family MF and the same paper's genetics show MNT4 is not required for O-glycan synthesis, the O-glycosylation process assignment is not supported for MNT4. Reason: IC process derived from a homology-based MF; contradicted for MNT4 by the paper's own genetic result that MNT4 is not required for O-glycan synthesis (PMID:10521541). |
| GO:0005575 cellular_component | ND GO_REF:0000015 | KEEP AS NON CORE | Summary: Root cellular_component with ND (no data) evidence - a placeholder indicating no curated localization at the time. Superseded by the membrane (IEA) annotation and by the acknowledged localization uncertainty for MNT4. Reason: ND root placeholder; carries no functional information. |
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Download this section (compressed HTML)Q: Does purified MNT4 have any mannosyltransferase activity in vitro, and if so, what is its nucleotide-sugar donor and acceptor?
Q: Is the candidate catalytic DxD motif (D300-S-D302) required for any MNT4 activity, or is MNT4 a catalytically inactive (pseudo-enzyme) family member?
Q: Where does MNT4 localize when tagged in a topology-preserving way - Golgi (like its paralogs) or ER/vacuole (as some high-throughput screens suggest)?
Q: Does MNT4 act redundantly with MNN1/MNT2/MNT3 on any glycan, such that a phenotype only appears in higher-order mutants?
Experiment: Express and purify the MNT4 lumenal domain and assay mannosyltransferase activity against a panel of acceptors (mannooligosaccharides, glycoproteins, glycolipids) with GDP-mannose and dolichol-phosphate-mannose donors; include DxD (D300A/D302A) catalytic-dead controls.
Hypothesis: MNT4 is a catalytically active alpha-1,3-mannosyltransferase whose acceptor differs from the O-linked mannan elaborated by Mnt2p/Mnt3p.
Experiment: Build mnt4 single and higher-order deletions (with mnn1, mnt2, mnt3) and analyze O- and N-linked mannan by mass spectrometry / beta-elimination, plus cell-wall and drug-sensitivity phenotyping.
Hypothesis: MNT4 contributes to mannan biosynthesis redundantly with other MNN1/MNT family members.
Experiment: Determine endogenous MNT4 localization by co-imaging a topology-preserving tagged allele with Golgi (Sec7/Mnn1), ER (Sec63), and vacuole markers.
Hypothesis: MNT4 functions in the Golgi like its paralogs (or, alternatively, in the ER/vacuole).
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