MNN14 (YJR061W, P40355) — curation notes

Journal for the AI GO-annotation review of Saccharomyces cerevisiae MNN14.
Understudied ("dark") gene. Primary deliverable is a rigorous knowledge_gaps section.
Every assertion below carries inline provenance.

Identity (verified)

• UniProt: P40355 (MNN14_YEAST), 935 aa, MW 108,427.
• SGD: S000003822; systematic/ordered-locus name YJR061W; ORF name J1736.
• RefSeq NP_012595.1.
• Protein name in UniProt: "Mannosyltransferase regulator 14"
  [ECO:0000303|PubMed:28101612]. The name was coined in the 2017 glyco-engineering
  paper; it does NOT itself establish a mannosyltransferase catalytic activity.

Family / domains — IMPORTANT attribution point

The task brief anticipated a GT15/MNN1 alpha-1,3-mannosyltransferase. This is not what
the record shows. MNN14 is NOT in the MNN1/GT15 family.

• UniProt SIMILARITY: "Belongs to the MNN4 family." [UniProt P40355, "SIMILARITY: Belongs to the MNN4 family."]
• InterPro: IPR009644 (FKTN/MNN-like).
  [UniProt P40355, "InterPro; IPR009644; FKTN/MNN-like."].
• PANTHER: PTHR15407 (FUKUTIN-RELATED)
  [UniProt P40355, "PANTHER; PTHR15407; FUKUTIN-RELATED; 1."].

So MNN14 sits in the fukutin-related/MNN-like family shared by MNN4 and the metazoan
fukutin (FKTN)/FKRP ribitol-phosphate transferases, distinct from the
KRE2/MNT1/GT15 mannosyltransferases.

Topology (type II Golgi membrane protein)

• TOPO_DOM 1..21 Cytoplasmic; TRANSMEM 22..42 (signal-anchor, type II); TOPO_DOM 43..935 Lumenal.
  [UniProt P40355 FT lines]. Consistent with a Golgi-lumen-acting glycan-modifying protein.
• KW: Golgi apparatus; Membrane; Signal-anchor; Transmembrane.

DXD motif (catalytic-motif reasoning)

• MOTIF 498..500 "DXD" [ECO:0000250|UniProtKB:P36044] — inferred by similarity to MNN4 (P36044).
• UniProt DOMAIN comment: "The conserved DXD motif is essential for the function, which could
  be an indication that MNN14 has transferase activity." [UniProt P40355,
  "The conserved DXD motif is essential for the function, which could be an indication that MNN14 has transferase activity."]
• DXD motifs coordinate a divalent cation and the nucleotide-sugar in many GT-A / phosphotransferase
  enzymes. Its presence is consistent with, but does NOT prove, a catalytic transferase role for
  MNN14 — see knowledge gap. The same wording is used for MNN4, whose curated MF is regulator,
  not transferase (below).

Function — KNOWN vs NOT known

KNOWN (experimentally, PMID:28101612 — abstract-only in cache; IGI in GOA)

• MNN14 is an MNN4 paralog required for full N-glycan mannosylphosphorylation in
  S. cerevisiae. PMID:28101612
• Partial redundancy with MNN4: single deletions leave residual mannosylphosphate; the
  MNN4+MNN14 double deletion abolishes N-glycan mannosylphosphorylation.
  PMID:28101612
• Biotechnology relevance: eliminating mannosylphosphate (an och1Δmnn1Δmnn4Δmnn14Δ strain) is a
  step toward human-compatible glycoproteins in yeast. [PMID:28101612 abstract]
• INDUCTION: expression is repressed by RIM101 [UniProt P40355, "Expression is repressed by RIM101."],
  from PMID:12509465 (Lamb & Mitchell 2003; Rim101 represses NRG1/SMP1). This is a regulatory-network
  observation, not a molecular function.
• Localization: Golgi apparatus membrane (ECO:0000305; by SubCell + topology; IEA in GOA).
  [UniProt P40355, "SUBCELLULAR LOCATION: Golgi apparatus membrane"]

NOT known (the real gaps)

• In-vivo division of labour with MNN4 is unresolved. MNN14 has intrinsic
  mannosylphosphate transferase activity in vitro PMID:33144549, whereas the close paralog
  MNN4 is curated as an enzyme activator/positive regulator of the Mnn6/Ktr6
  mannosylphosphate transferase and is rate-limiting for mannosylphosphorylation
  PMID:9459307.
• Direct acceptor positions for MNN14 in vivo are unknown. Recombinant MNN14 transfers
  mannosyl-phosphate to high-mannose N-glycans in vitro, but which mannose residues/positions
  depend on MNN14 versus MNN4 in the cell remains unresolved.
• Basis of the MNN4/MNN14 redundancy is unknown (paralog sub-/neo-functionalization;
  condition-, substrate-, or acceptor-position specificity).
• Standalone loss-of-function phenotype beyond the glyco-profile is uncharacterized; MNN14 is
  non-essential and there is no described growth/stress phenotype for mnn14Δ alone.

GOA annotations to review (5 live rows + 1 proposed molecular-function row)

1. GO:0009101 glycoprotein biosynthetic process — IBA (GO_REF:0000033); IBA panel includes
   SGD:S000001684 (MNN4). BP is correct (mannosylphosphorylation is glycoprotein biosynthesis);
   generic but defensible. Not the most specific but IBA-appropriate. -> KEEP_AS_NON_CORE / ACCEPT.
2. GO:0000139 Golgi membrane — IEA (SubCell). Supported by topology + SubCell. -> ACCEPT.
3. GO:0006491 N-glycan processing — IGI (PMID:28101612), with SGD:S000001684 (MNN4). This is the
   experimental genetic-interaction annotation matching the double-deletion result. -> ACCEPT (core BP).
4. GO:0003674 molecular_function — ND (root). Superseded by the separate NEW
   GO:0000031 row carrying the 2021 in-vitro catalytic evidence. -> REMOVE.
5. GO:0005575 cellular_component — ND (root). Superseded by Golgi membrane. -> REMOVE.
6. GO:0000031 mannosylphosphate transferase activity — NEW from the 2021 recombinant
   Mnn14 biochemical assay.

Note: GO:0006491 "N-glycan processing" is defined as conversion of N-linked glycan to mature form by
glycosidases/glycosyltransferases [OLS GO:0006491]. Mannosylphosphorylation is an N-glycan
outer-chain maturation/modification, so this is an appropriate (if slightly generic) BP.

Molecular-function term triage

• GO:0000031 mannosylphosphate transferase activity — asserted as a NEW row because Kang
  et al. 2021 directly showed that recombinant soluble MNN14 can transfer mannosyl-phosphate
  from GDP-mannose to high-mannose N-glycan acceptors PMID:33144549.
• GO:0008047 enzyme activator activity — the MF of the paralog MNN4 (IMP:SGD); still not
  asserted for MNN14 because MNN14 itself has intrinsic catalytic activity, and whether it
  also activates Mnn6/Ktr6 in vivo has not been tested.

References gathered

• PMID:28101612 — Kim et al. 2017, Appl Microbiol Biotechnol. Primary experimental (abstract-only cache).
  Genetic evidence linking MNN14 to N-glycan mannosylphosphorylation; source of IGI
  GO:0006491.
• PMID:12509465 — Lamb & Mitchell 2003 (RIM101 represses NRG1/SMP1). Source of the INDUCTION note;
  MNN14 mentioned as a Rim101-repressed target. Secondary/regulatory context.
• PMID:9459307 — Odani et al. 1997, FEBS Lett. MNN4 = positive regulator of mannosylphosphorylation;
  establishes the MNN4-family regulator paradigm (background for the MF gap).
• UniProt:P40355 — domain/family/topology/DXD evidence.
• UniProt:P36044 — MNN4 paralog record (regulator MF; DXD ambiguity) for attribution.

Web verification log

• rest.uniprot.org P40355 (full record downloaded to MNN14-uniprot.txt): family=MNN4,
  InterPro IPR009644, PANTHER PTHR15407, DXD 498-500, type II Golgi, FUNCTION =
  "role in N-glycan mannosylphosphorylation... partially redundant with MNN4."
• rest.uniprot.org P36044 (MNN4): MF enzyme activator activity (IMP:SGD); "seems to have a
  regulatory role... transferase activity cannot be ruled out."
• WebSearch (Odani 1997 PMID:9459307; Wang MNN6=KTR6): MNN6/Ktr6 = the mannosylphosphate transferase;
  MNN4 = its positive regulator, Mnn4p amount rate-limiting.
• OLS: GO:0006491, GO:0009101, GO:0000031, GO:0008047 definitions confirmed.

2026-09-28 — IBA re-review

The single IBA row derives from current PANTHER data:

PTHR15407  PTN001034988  GO:0009101  P  IBD  CGD:CAL0000174110|CGD:CAL0000175223|MGI:MGI:2179507|SGD:S000001684|SGD:S000003822|UniProtKB:O75072  taxon:33154  20251219

This is not a stale or circular propagation. PTN001034988 still exists, MNN14's own
SGD:S000003822 evidence in the WITH/FROM list is an experimentally characterized
descendant used to place the ancestral IBD, and GO:0009101 glycoprotein biosynthetic process is broad but valid for the mannosylphosphorylation activity defined by
PMID:28101612 and PMID:33144549.

Newer literature search turned up no post-2021 direct S. cerevisiae MNN14 papers that
change the GO calls. Pakhomova et al. 2026 is about Ogataea polymorpha
phosphomannosylation mutants and only mentions MNN14 as a S. cerevisiae MNN4 paralog;
2024-2026 reviews on heterologous protein glycosylation likewise use MNN14 as background
for mannosylphosphate removal or M6P glyco-engineering.

2026-10-01 current-GOA refresh

Forced a current GOA/UniProt refresh and re-fetched all cached PMIDs. Current GOA still has
the same five exact rows and no retired assertions are needed: one live IBA row, one live
SubCell row, one live SGD IGI row, and the two SGD ND placeholders all remain represented
in the review. The refresh backfilled current supporting_entities for the three non-ND
GOA rows.

Re-checked PANTHER PTHR15407 after the refresh. PTN001034988 is still the only ancestral
PAINT node that transfers to MNN14, and it carries only the deliberately broad
GO:0009101 glycoprotein biosynthetic process assertion. The metazoan
GO:0000139 Golgi membrane IBD at PTN000395622 does not propagate to MNN14. The 2026
literature search found newer glyco-engineering reviews and an orthologous
Ogataea polymorpha phosphomannosylation paper, but no post-2021 direct
S. cerevisiae MNN14 study that changed the MNN14 action set.