SGD applied the ND (No biological Data available) evidence code to MNN14, recording that no experimental data supported a GO assignment in the affected aspect at the time of curation. This has since been overtaken by the 2017 genetic and 2021 in-vitro enzymatic evidence.

PAINT/IBA annotations reach MNN14 through the LicD/fukutin-related nucleotidyltransferase family node (PANTHER PTHR15407). The family is a divalent-cation-dependent phosphoryl-transferase fold, so the inherited terms are fold-level and do not by themselves specify the mannosyl-phosphate donor/acceptor pair that the yeast experiments define.

The cellular-component annotations for MNN14 derive by vocabulary mapping from the UniProt SUBCELLULAR LOCATION line "Golgi apparatus membrane; Single-pass type II membrane protein", which is itself an ECO:0000305/ECO:0000255 inference from sequence topology rather than a direct localization experiment on MNN14.

MNN14, previously an MNN4 paralog of unknown function, is required for N-glycan mannosylphosphorylation in S. cerevisiae. This is the source of the SGD IGI annotation to GO:0006491.

MNN4 and MNN14 act partially redundantly: deleting either alone leaves residual mannosylphosphate, and only the double disruption eliminates N-glycan mannosylphosphorylation. The evidence is genetic, so it does not on its own distinguish a catalytic from a regulatory role for MNN14.

Mannosylphosphorylated glycans are fungal-specific, which is why removing this modification matters for yeast glyco-engineering of human-compatible glycoproteins.

Purified soluble recombinant Mnn14 is itself catalytic: it converts the Man8GlcNAc2 acceptor into mono- and bis-mannosyl-phosphorylated products in vitro. This establishes an intrinsic mannosylphosphate transferase activity for MNN14, not merely a requirement for the pathway.

The reaction uses GDP-mannose as the mannosyl-phosphate donor and a high-mannose N-glycan as acceptor, and is divalent-cation dependent with Mn2+ the preferred ion - consistent with the DXD motif and the LicD/fukutin-related fold.

Mnn14 acts on physiological high-mannose N-glycans on a real glycoprotein substrate: 74% of the Man7-9GlcNAc2 glycans of recombinant human lysosomal alpha-glucosidase were mannosyl-phosphorylated, mostly to the bis-phosphorylated form.

Mnn14 is described as one of the main mannosyl-phosphorylation enzymes (MPEs) of S. cerevisiae, based on the companion gene-disruption and complementation work.

Rim101p acts in vivo as a transcriptional repressor, and MNN14 is among the genes it represses (the source of the UniProt INDUCTION statement). This is regulatory-network context for MNN14 expression, not evidence of a molecular function or an ion-tolerance role for the MNN14 protein.

In the founding MNN4-family paradigm, catalysis and regulation are split between two proteins: MNN6 encodes the mannosylphosphate transferase and MNN4 its putative positive regulator, whose amount is rate-limiting. This is why MNN14's own transferase-versus-regulator contribution in vivo is treated here as an open question rather than assumed from the family name.

Mannosylphosphorylation is the major modification supplying negative charge to the S. cerevisiae cell wall, and its level is transcriptionally regulated (rising in late-log/stationary phase, repressed by the PKA pathway, enhanced by KCl) - placing the pathway MNN14 contributes to in a stress-responsive context.

UniProt records MNN14 as acting in N-glycan mannosylphosphorylation with partial redundancy to MNN4, citing the 2017 double-disruption study. Note that the entry name ("Mannosyltransferase regulator 14") predates the 2021 demonstration of intrinsic catalytic activity.

MNN14 is a single-pass type II membrane protein of the Golgi apparatus membrane, with a short cytoplasmic N-terminus (1-21), a signal-anchor transmembrane helix (22-42) and a large lumenal domain (43-935) carrying the catalytic region.

MNN14 carries a conserved DXD motif at 498-500, the divalent-cation coordinating motif typical of nucleotidyl/glycosyl transferases. UniProt annotates it by similarity to MNN4 and explicitly hedges its significance; the 2021 Mn2+-dependent in-vitro activity is consistent with it, but the motif has not been mutated in MNN14 itself.

MNN14 belongs to the MNN4 family and carries the LicD/fukutin-related nucleotidyltransferase fold (Pfam PF04991; InterPro IPR007074, IPR009644; PANTHER PTHR15407), a fold distinct from the KRE2/MNT1 (GT15) mannosyltransferases.

The UniProt entry for the paralog MNN4 is the source of the DXD-motif assignment transferred to MNN14 (MNN14 MOTIF 498-500 carries ECO:0000250|UniProtKB:P36044). It is cited here for that cross-attribution and for the family-level regulator framing against which MNN14's own catalytic activity is contrasted.