MNN14 (systematic name YJR061W) encodes a 935-residue single-pass type II membrane protein of the Golgi apparatus in Saccharomyces cerevisiae. It is a paralog of MNN4 and belongs to the MNN4 family within the LicD/fukutin-related nucleotidyltransferase superfamily (Pfam PF04991 LicD; InterPro IPR007074 and IPR009644; PANTHER PTHR15407 fukutin-related), a divalent-cation-dependent phosphoryl-transferase fold distinct from the KRE2/MNT1 (GT15) mannosyltransferases. MNN14 is a mannosylphosphorylation enzyme: recombinant soluble MNN14 transfers mannosyl-phosphate from GDP-mannose to high-mannose N-glycans (e.g. Man8GlcNAc2 and the Man7-9GlcNAc2 glycans of a therapeutic protein), producing mono- and bis-mannosyl- phosphorylated glycans in an Mn2+-dependent reaction. It acts partially redundantly with MNN4 in vivo: single deletions leave residual mannosylphosphate, whereas simultaneous deletion of MNN4 and MNN14 abolishes N-glycan mannosylphosphorylation. Mannosylphosphorylation adds mannose-1-phosphate to the mannans of fungal glycoproteins, contributing negative surface charge, and is fungal-specific, so loss of MNN4 plus MNN14 is used in yeast glyco-engineering to make human-compatible glycoproteins. The protein carries a conserved DXD motif (498-500) typical of divalent-cation- dependent transferases. The in-vivo division of labour between the intrinsic catalytic activity of MNN14 and MNN4 (curated as a regulator of the Mnn6/Ktr6 mannosylphosphate transferase), the acceptor-position specificity of MNN14, and the basis of the MNN4/MNN14 redundancy remain to be resolved.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0009101 glycoprotein biosynthetic process | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetically inferred (IBA) biological-process annotation from the MNN4-family tree, whose panel includes the paralog MNN4 (SGD:S000001684). MNN14 acts in N-glycan mannosylphosphorylation, which adds mannose-1-phosphate to glycoprotein-linked mannans and is part of glycoprotein biosynthesis, so the BP is correct. It is more general than the experimentally supported N-glycan-processing annotation below and is retained as a valid but non-core parent process. Reason: Correct but generic BP consistent with the gene's role; superseded for specificity by the experimental GO:0006491 annotation. Retained per GO conventions as a valid IBA inference. Supporting Evidence: PMID:28101612 essential for N-glycan mannosylphosphorylation |
| GO:0000139 Golgi membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Subcellular-location annotation supported by the UniProt SUBCELLULAR LOCATION (Golgi apparatus membrane) and by the protein's single-pass type II membrane topology (cytoplasmic 1-21; signal-anchor TM 22-42; lumenal 43-935). Golgi localization is expected for an enzyme/regulator acting on outer-chain mannan maturation and is consistent with the MNN4 paralog. Reason: Domain/topology- and SubCell-supported cellular component; the correct and informative localization for this Golgi mannosylphosphorylation factor. Supporting Evidence: UniProt:P40355 Golgi apparatus membrane UniProt:P40355 Signal-anchor for type II membrane protein |
| GO:0006491 N-glycan processing | IGI PMID:28101612 Abolishment of N-glycan mannosylphosphorylation in glyco-eng... | ACCEPT | Summary: Experimental (IGI) annotation curated by SGD from the genetic interaction with MNN4 (with/from SGD:S000001684). Kim et al. 2017 showed that MNN14 is required for full N-glycan mannosylphosphorylation and that the MNN4+MNN14 double deletion eliminates it, placing MNN14 in N-glycan outer-chain maturation. GO:0006491 (conversion of N-linked glycan to a mature form by glycosidases/glycosyltransferases) appropriately captures this outer-chain modification step. This is the best-supported process annotation and the core biological role. Reason: Experimentally grounded (IGI) core biological process directly supported by the double-deletion phenotype; the single defensible functional placement for this otherwise dark gene. Supporting Evidence: PMID:28101612 essential for N-glycan mannosylphosphorylation PMID:28101612 Double disruption of MNN4 and MNN14 genes was enough to eliminate N-glycan |
| GO:0003674 molecular_function | ND GO_REF:0000015 | MODIFY | Summary: Root-level placeholder assigned by SGD with the ND (No biological Data) code, reflecting that no molecular-function had been curated for MNN14. This is now superseded by direct biochemical evidence: recombinant soluble MNN14 transfers mannosyl-phosphate from GDP-mannose to high-mannose N-glycans in vitro (Kang et al. 2021), establishing a mannosylphosphate transferase activity (GO:0000031). The ND root should be replaced by that specific MF (proposed as a NEW annotation below and used as the core molecular function). Reason: The ND placeholder is no longer accurate: an experimental (in-vitro) MF for MNN14 now exists. GO:0000031 (mannosylphosphate transferase activity) is the specific, evidence-supported molecular function and should replace the uninformative root term. Proposed replacements: mannosylphosphate transferase activity Supporting Evidence: PMID:33144549 in vitro mannosyl-phosphorylation of high-mannose type N-glycans that utilizes a recombinant Mnn14 protein PMID:33144549 2 mM GDP-mannose (donor substrate) |
| GO:0000031 mannosylphosphate transferase activity | IDA PMID:33144549 In Vitro N-Glycan Mannosyl-Phosphorylation of a Therapeutic ... | NEW | Summary: Proposed NEW molecular-function annotation not currently in GOA. Recombinant soluble MNN14 (rMnn14 77-935) directly transfers mannosyl-phosphate from the donor GDP-mannose to high-mannose N-glycan acceptors, producing mono- and bis-mannosyl-phosphorylated glycans in an Mn2+-dependent reaction optimal at pH 7.5 / 30 C (Kang et al. 2021). GO:0000031 exactly matches this reaction (GDP-mannose + mannose-acceptor -> phosphorylated glycan + GMP) and is the intrinsic catalytic activity of MNN14. The assay used the soluble catalytic domain expressed in P. pastoris, consistent with an intrinsic (not merely regulatory) activity. Reason: Experimentally supported (in-vitro biochemical, IDA-equivalent) molecular function for MNN14 that is missing from GOA; this is the core catalytic function of the gene product. Supporting Evidence: PMID:33144549 a strategy is established here for the in vitro mannosyl-phosphorylation of high-mannose type N-glycans PMID:33144549 a recombinant Mnn14 protein derived from Saccharomyces cerevisiae PMID:33144549 the main MPEs in Saccharomyces cerevisiae |
| GO:0005575 cellular_component | ND GO_REF:0000015 | KEEP AS NON CORE | Summary: Root-level placeholder assigned by SGD with the ND code. It is superseded by the more informative Golgi membrane (GO:0000139) annotation above, which is supported by SubCell and the type II membrane topology. Reason: Uninformative root placeholder retained per GO conventions; the specific localization is already captured by the Golgi membrane annotation. |
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Download this section (compressed HTML)Q: Given that MNN14 has intrinsic mannosylphosphate transferase activity in vitro while MNN4 is curated as a regulator of Mnn6/Ktr6, how are catalysis and regulation partitioned between MNN14 and MNN4 in vivo?
Q: Does the conserved DXD motif of MNN14 support catalysis, and is it required for MNN14 function in vivo?
Experiment: Introduce DXD-motif catalytic point mutations into chromosomal MNN14 (and MNN4) and score the N-glycan mannosylphosphate profile in single and double mutants; complement mnn4 mnn14 double deletions with wild-type versus DXD-mutant alleles; and test whether MNN14/MNN4 stimulate purified Mnn6/Ktr6 activity in a reconstituted reaction.
Hypothesis: The DXD motif is required for the in-vivo mannosylphosphate transferase activity of MNN14, and MNN14 contributes catalysis (not only regulation) to cellular mannosylphosphorylation.
Type: structure-function / reconstitution
Experiment: Perform quantitative N-glycan and mannan structural analysis (mass spectrometry) of cell-wall mannoproteins and a secreted reporter glycoprotein from wild-type, mnn4, mnn14, and mnn4 mnn14 strains to map which mannosylphosphate positions depend on each paralog.
Hypothesis: MNN14 and MNN4 act on distinct acceptor positions or substrate classes, explaining their partial redundancy.
Type: comparative glycomics
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The in-vivo division of labour between MNN14 and its paralog MNN4 is unresolved. MNN14 has intrinsic mannosylphosphate transferase activity in vitro, yet MNN4 is curated as a positive regulator (enzyme activator) of the Mnn6/Ktr6 mannosylphosphate transferase, not a catalyst. It is unknown whether, in the cell, MNN14 acts primarily as a catalyst, MNN4 primarily as a regulator, and how the two models are reconciled for two closely related paralogs.
NARROWING BIOLOGY RESIDUAL_SUBGAP
What is known: Firmly established: recombinant soluble MNN14 catalyzes mannosyl-phosphate transfer from GDP-mannose to high-mannose N-glycans in vitro (Mn2+-dependent), so MNN14 has intrinsic transferase activity (GO:0000031); and genetically both MNN4 and MNN14 are required for full N-glycan mannosylphosphorylation. Also established, classically, is that Mnn6/Ktr6 (KRE2/MNT1 family) is a mannosylphosphate transferase and that MNN4 is a positive regulator whose transcript/protein level is rate-limiting; MNN4's curated molecular function is enzyme activator activity (GO:0008047).
Significance: Clarifying whether MNN14 (and MNN4) contribute catalysis, regulation, or both in vivo would resolve an apparent contradiction between the in-vitro enzymology and the classical regulator model, and would sharpen the mannosylphosphorylation pathway model that underpins yeast glyco-engineering.
What would resolve it: In-vivo structure-function analysis (DXD-motif catalytic mutants of MNN14 and MNN4 assayed for the glycan phenotype), reconstitution tests of whether MNN14/MNN4 stimulate Mnn6/Ktr6, and quantitative comparison of the catalytic contribution of each paralog in single and double mutants.
Provenance (the field's own admissions):
Gap: The acceptor-position specificity of MNN14 in vivo is unknown: which mannose residues/positions on N-linked (and possibly O-linked) glycans MNN14 phosphorylates in the cell, and whether MNN4 and MNN14 act on the same or different positions.
OPEN BIOLOGY MF_DARK
What is known: In vitro, MNN14 mannosyl-phosphorylates high-mannose N-glycans (Man8GlcNAc2 and Man7-9GlcNAc2 on rhGAA), producing mono- and bis-mannosyl-phosphorylated products; genetically, N-glycan mannosylphosphorylation is abolished only when both MNN4 and MNN14 are deleted. The precise in-vivo acceptor positions and whether the two paralogs are position-selective have not been mapped.
Significance: Mapping acceptor positions would convert the pathway placement into a fully defined molecular reaction and explain why two paralogs are maintained.
What would resolve it: Structural mannan/N-glycan analysis (e.g. by mass spectrometry) of defined acceptors in wild-type versus mnn14 single and mnn4 mnn14 double mutants, combined with in-vitro reactions on candidate acceptor substrates.
Provenance (the field's own admissions):
Gap: The basis of the functional redundancy between MNN14 and MNN4, and any distinct or condition-specific role of MNN14, are unknown. There is no described standalone loss-of-function phenotype for an mnn14 single mutant beyond the residual mannosylphosphate profile.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: Established: single deletion of either MNN4 or MNN14 leaves residual N-glycan mannosylphosphorylation, and only the double deletion eliminates it, so the two paralogs have overlapping but individually non-essential activities. MNN14 is a non-essential gene, and its expression is repressed by RIM101.
Significance: Understanding whether MNN14 and MNN4 differ in acceptor specificity, substrate class (N- vs O-linked; core vs outer chain), or expression context would explain why two paralogs are maintained and would sharpen the mannosylphosphorylation model relevant to glyco-engineering.
What would resolve it: Comparative glycan profiling and genetic-interaction/expression analyses of mnn4 and mnn14 single mutants under varied growth conditions, and tests of RIM101-dependent regulation of MNN14 relative to MNN4.
Provenance (the field's own admissions):
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