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Comprehensive Research Report: MNN14 (YJR061W) — A Mannosylphosphorylation Enzyme in Saccharomyces cerevisiae

1. Gene/Protein Identity and Verification

MNN14 (systematic name YJR061W; UniProt P40355) encodes a 935-amino acid protein in Saccharomyces cerevisiae (strain ATCC 204508 / S288c). The protein is annotated as "Mannosyltransferase regulator 14" and belongs to the MNN4/fukutin protein family. Gene disruption and complementation experiments have established MNN14 as one of the main mannosylphosphorylation enzymes (MPEs) in S. cerevisiae (kang2021invitronglycan pages 2-3, kang2021invitronglycan pages 1-2). The gene was definitively characterized in the study by Kim et al. (2017, Appl. Microbiol. Biotechnol. 101:2979–2989), which demonstrated that double disruption of MNN4 and MNN14 abolished N-glycan mannosylphosphorylation in glyco-engineered S. cerevisiae (kang2021invitronglycan pages 7-8).

2. Protein Structure and Domain Architecture

Mnn14 is a type II membrane protein containing two principal functional regions (kang2021invitronglycan pages 3-5):

• Transmembrane (TM) domain (residues 24–46): A single-pass transmembrane segment characteristic of Golgi-resident glycosyltransferases. This domain anchors the protein in the Golgi membrane, with a short N-terminal cytoplasmic tail and a large C-terminal lumenal catalytic region.
• LicD domain (residues 470–720): The catalytic domain belonging to the nucleotidyltransferase fold protein family (Pfam PF04991). This domain is the hallmark of the fukutin/MNN4 family and is responsible for the phosphoryl-ligand transfer activity.

The fukutin family is defined by a conserved motif with the sequence G[TS]hhGhhx₄hhxaxxDxD, where 'h' denotes hydrophobic residues and 'a' aromatic residues (harnett2010howdonematodes pages 2-4). The characteristic DxD motif and a distal aspartic acid residue are predicted to coordinate a divalent cation, consistent with the experimentally demonstrated requirement for Mn²⁺ in Mnn14 catalysis (harnett2010howdonematodes pages 2-4, kang2021invitronglycan pages 2-3). On SDS-PAGE, the recombinant soluble form of Mnn14 (rMnn14₇₇₋₉₃₅) migrates at approximately 100 kDa (kang2021invitronglycan pages 3-5).

The following table summarizes the key properties of MNN14:

Table: This table summarizes the core biochemical, structural, and cellular properties of the yeast MNN14 (YJR061W) protein based on the available evidence. It is useful as a compact reference for its annotation, catalytic activity, localization, and pathway role.

3. Enzymatic Activity and Reaction Mechanism

3.1 Primary Function

Mnn14 functions as a mannosylphosphotransferase (mannosylphosphorylation enzyme, MPE). It catalyzes the transfer of mannosyl-phosphate from GDP-mannose (donor substrate) to mannose residues on high-mannose type N-glycans (acceptor substrate), creating mannose-1-phospho-6-mannose (ManP) phosphodiester linkages (kang2021invitronglycan pages 1-2, kang2021invitronglycan pages 3-5).

Mnn14 is a Golgi-resident mannosylphosphorylation enzyme that transfers mannosyl-phosphate from GDP-mannose to mannose residues on high-mannose type N-glycans, generating mannose-1-phospho-6-mannose (ManP) modifications on glycoprotein N-glycans (kang2021invitronglycan pages 3-5, kang2021invitronglycan pages 2-3)

In vitro, recombinant Mnn14 converts Man8GlcNAc2 into both mono-mannosyl-phosphorylated ManP-Man8GlcNAc2 and bis-mannosyl-phosphorylated Man2P2-Man8GlcNAc2 products, showing that the enzyme can install one or two phosphomannose caps on suitable high-mannose acceptors (kang2021invitronglycan pages 3-5)

The enzyme also efficiently modifies protein-linked high-mannose glycans such as Man7-9GlcNAc2 on recombinant human acid α-glucosidase, and the dominant products can be bis-mannosyl-phosphorylated under optimized conditions (kang2021invitronglycan pages 5-7, kang2021invitronglycan pages 7-8)

Catalysis requires GDP-mannose as the donor substrate and Mn2+ as an activating divalent cation, with reported optimal assay conditions around pH 7.5 and 30°C (kang2021invitronglycan pages 2-3)

Available evidence further indicates that a minimal acceptor requirement is the presence of at least one α(1,2)-linked mannose outside the phosphorylation site, helping explain Mnn14 selectivity for high-mannose yeast-type N-glycans (kang2021invitronglycan pages 7-8)

Blockquote: This blockquote summarizes the core enzymatic activity of yeast Mnn14, including donor and acceptor specificity, reaction products, and catalytic requirements. It is useful as a concise functional annotation of the protein’s primary biochemical role.

3.2 Substrate Specificity

The enzyme acts on high-mannose type N-glycans, with Man₈GlcNAc₂ serving as a well-characterized substrate. It also efficiently modifies Man₇₋₉GlcNAc₂ glycans attached to proteins (kang2021invitronglycan pages 5-7, kang2021invitronglycan pages 7-8). The minimal substrate requirement for the enzyme is the presence of at least one α(1,2)-linked mannose residue outside the mannosyl-phosphorylation site (kang2021invitronglycan pages 7-8). In addition to N-linked glycans, MPEs in yeast are reported to add mannosyl-phosphate to O-mannose residues on glycoproteins (kang2021invitronglycan pages 7-8).

3.3 Reaction Products

From the Man₈GlcNAc₂ substrate, Mnn14 produces both mono-mannosyl-phosphorylated (ManP-Man₈GlcNAc₂) and bis-mannosyl-phosphorylated (Man₂P₂-Man₈GlcNAc₂) glycan products (kang2021invitronglycan pages 3-5). When applied to the high-mannose N-glycans on recombinant human lysosomal alpha-glucosidase (rhGAA) under optimized conditions, 74% of the glycans were converted to bis-mannosyl-phosphorylated forms (kang2021invitronglycan pages 5-7, kang2021invitronglycan pages 7-8).

3.4 Optimal Reaction Conditions

In vitro characterization of a recombinant soluble form (rMnn14₇₇₋₉₃₅) established the following optimal conditions: pH 7.5 in Tris-HCl buffer, 30°C, and 10 mM MnCl₂ with 2 mM GDP-mannose as the donor substrate (kang2021invitronglycan pages 5-7, kang2021invitronglycan pages 2-3). The buffer compound has a stronger effect on activity than pH itself; sodium phosphate buffer dramatically reduces activity at the same pH, possibly due to competition with the phosphotransferase reaction (kang2021invitronglycan pages 7-8, kang2021invitronglycan pages 3-5). The enzyme is absolutely dependent on divalent cations, with Mn²⁺ being far superior to other ions tested (kang2021invitronglycan pages 2-3).

4. Subcellular Localization

Mnn14 is a Golgi-resident enzyme. As a type II membrane protein with a single transmembrane segment (residues 24–46), it is anchored in the Golgi membrane with its catalytic LicD domain facing the Golgi lumen (kang2021invitronglycan pages 3-5). This localization is consistent with the broader N-glycan processing pathway in S. cerevisiae, in which sequential modifications of N-glycans occur as glycoproteins transit through different Golgi compartments: α-1,6-mannosylation in the cis-Golgi, α-1,2-mannosylation in the medial compartment, and α-1,3-mannosylation and phosphomannosylation in later compartments (fabre2014mannosylationoffungal pages 2-3).

5. Biological Pathway and Functional Context

5.1 N-Glycan Mannosylphosphate Modification Pathway

In S. cerevisiae, the modification of N-linked and O-linked glycans with mannosylphosphate is an important post-translational modification. This process creates negatively charged phosphodiester-linked mannose residues on cell wall mannoproteins, which contributes to the overall negative charge of the yeast cell surface and can be detected by Alcian blue staining (pakhomova2026alterationsinprotein pages 2-3). The mannosylphosphate modification is thought to strengthen the cell wall of the yeast (kang2021invitronglycan pages 7-8).

5.2 Relationship to MNN4 and MNN6

MNN14 is a paralog of MNN4 in the S. cerevisiae genome (pakhomova2026alterationsinprotein pages 2-3). Both belong to the fukutin protein family and presumably catalyze mannosylphosphate transfer from GDP-mannose (pakhomova2026alterationsinprotein pages 2-3). In the classical model of yeast mannosylphosphorylation, MNN4 was described as a putative positive regulator of MNN6 (also known as KTR6), which was identified as the actual mannosylphosphate transferase (jigami2008yeastglycobiologyand pages 4-6). MNN6 belongs to the KRE2/MNT1 family and is involved in mannosylphosphate transfer to α-1,2-mannose side chains (fabre2014mannosylationoffungal pages 7-7). However, more recent biochemical studies have demonstrated that Mnn14 itself possesses strong, direct mannosylphosphorylation activity, establishing it as a bona fide MPE rather than merely a regulator (kang2021invitronglycan pages 1-2, kang2021invitronglycan pages 3-5, kang2021invitronglycan pages 2-3).

Despite both MNN4 and MNN14 being paralogs, MNN4 appears to play the dominant role in mannosylphosphorylation under normal conditions: disruption of MNN4 alone is sufficient to abolish Alcian blue staining, suggesting that MNN14 may play a minor or partially redundant role in vivo (pakhomova2026alterationsinprotein pages 2-3). Nevertheless, double disruption of MNN4 and MNN14 was required to completely abolish N-glycan mannosylphosphorylation in glyco-engineered strains, confirming that MNN14 contributes to this modification (kang2021invitronglycan pages 7-8).

5.3 Evolutionary and Structural Context

The fukutin/LicD protein family to which MNN14 belongs encompasses a diverse group of phosphoryl-ligand transferases found across bacteria, fungi, and metazoans. In bacteria, LicD gene products (e.g., in Streptococcus pneumoniae and Haemophilus influenzae) transfer phosphorylcholine to carbohydrates on teichoic acid and lipopolysaccharide, respectively (harnett2010howdonematodes pages 2-4). In mammals, the fukutin (FKTN) and fukutin-related protein (FKRP) are involved in the post-translational modification of α-dystroglycan, and mutations in these genes cause forms of muscular dystrophy (dystroglycanopathies) (yoshidamoriguchi2015matriglycananovel pages 7-9). The conservation of the DxD motif and the divalent-cation-dependent phosphotransferase mechanism across these diverse family members supports the classification of MNN14 as a phosphoryl-ligand transferase (harnett2010howdonematodes pages 2-4).

6. Biotechnological Applications

A major area of applied interest for MNN14 is its use in glyco-engineering strategies for the production of therapeutic enzymes for lysosomal storage diseases (LSDs). Enzyme replacement therapy for LSDs requires recombinant enzymes containing mannose-6-phosphate (M6P) glycans for efficient cellular uptake via M6P receptors and lysosomal targeting (kang2021invitronglycan pages 1-2). The mannosyl-phosphorylated glycans produced by Mnn14 can be converted to M6P glycans through subsequent in vitro uncapping (removal of the outer mannose) and trimming reactions (kang2021invitronglycan pages 2-3, kang2021invitronglycan pages 1-2).

A recombinant soluble form of Mnn14 (rMnn14₇₇₋₉₃₅), engineered by deleting the N-terminal 76 amino acids including the transmembrane domain and part of the stem region, was successfully produced as a secreted protein in Pichia pastoris and demonstrated high mannosylphosphorylation activity (kang2021invitronglycan pages 1-2, kang2021invitronglycan pages 3-5). This recombinant enzyme was used to mannosyl-phosphorylate rhGAA with 74% conversion efficiency, predominantly yielding bis-mannosyl-phosphorylated glycans that can be converted to bis-M6P glycans with superior lysosomal targeting capability (kang2021invitronglycan pages 5-7, kang2021invitronglycan pages 7-8). This in vitro approach offers advantages over mammalian GlcNAc-1-phosphotransferase because Mnn14 is a single-chain enzyme (rather than a multi-subunit complex) and does not discriminate between lysosomal and non-lysosomal protein substrates (kang2021invitronglycan pages 7-8).

7. Summary

MNN14 (YJR061W) encodes a Golgi-resident, type II membrane mannosylphosphotransferase in S. cerevisiae that catalyzes the transfer of mannosyl-phosphate from GDP-mannose to high-mannose type N-glycans. The enzyme contains a LicD domain characteristic of the fukutin/MNN4 family of phosphoryl-ligand transferases, requires Mn²⁺ for catalysis, and produces both mono- and bis-mannosyl-phosphorylated glycan products. Its biological function is to add negatively charged phosphomannose modifications to cell wall glycoproteins, contributing to cell wall integrity. MNN14 is a paralog of MNN4 and plays a contributory role in the overall mannosylphosphorylation capacity of S. cerevisiae. Biotechnologically, recombinant Mnn14 has been developed as a tool for in vitro generation of mannose-6-phosphate glycans on therapeutic enzymes for the treatment of lysosomal storage diseases.

References

1. (kang2021invitronglycan pages 2-3): Ji-Yeon Kang, Hong-Yeol Choi, Dong-Il Kim, Ohsuk Kwon, and Doo-Byoung Oh. In vitro n-glycan mannosyl-phosphorylation of a therapeutic enzyme by using recombinant mnn14 produced from pichia pastoris. Journal of Microbiology and Biotechnology, 31:163-170, Jan 2021. URL: https://doi.org/10.4014/jmb.2010.10033, doi:10.4014/jmb.2010.10033. This article has 4 citations and is from a peer-reviewed journal.

2. (kang2021invitronglycan pages 1-2): Ji-Yeon Kang, Hong-Yeol Choi, Dong-Il Kim, Ohsuk Kwon, and Doo-Byoung Oh. In vitro n-glycan mannosyl-phosphorylation of a therapeutic enzyme by using recombinant mnn14 produced from pichia pastoris. Journal of Microbiology and Biotechnology, 31:163-170, Jan 2021. URL: https://doi.org/10.4014/jmb.2010.10033, doi:10.4014/jmb.2010.10033. This article has 4 citations and is from a peer-reviewed journal.

3. (kang2021invitronglycan pages 7-8): Ji-Yeon Kang, Hong-Yeol Choi, Dong-Il Kim, Ohsuk Kwon, and Doo-Byoung Oh. In vitro n-glycan mannosyl-phosphorylation of a therapeutic enzyme by using recombinant mnn14 produced from pichia pastoris. Journal of Microbiology and Biotechnology, 31:163-170, Jan 2021. URL: https://doi.org/10.4014/jmb.2010.10033, doi:10.4014/jmb.2010.10033. This article has 4 citations and is from a peer-reviewed journal.

4. (kang2021invitronglycan pages 3-5): Ji-Yeon Kang, Hong-Yeol Choi, Dong-Il Kim, Ohsuk Kwon, and Doo-Byoung Oh. In vitro n-glycan mannosyl-phosphorylation of a therapeutic enzyme by using recombinant mnn14 produced from pichia pastoris. Journal of Microbiology and Biotechnology, 31:163-170, Jan 2021. URL: https://doi.org/10.4014/jmb.2010.10033, doi:10.4014/jmb.2010.10033. This article has 4 citations and is from a peer-reviewed journal.

5. (harnett2010howdonematodes pages 2-4): William Harnett, Justyna Rzepecka, and Katrina M. Houston. How do nematodes transfer phosphorylcholine to carbohydrates? Trends in parasitology, 26 3:114-8, Mar 2010. URL: https://doi.org/10.1016/j.pt.2009.12.003, doi:10.1016/j.pt.2009.12.003. This article has 25 citations and is from a domain leading peer-reviewed journal.

6. (pakhomova2026alterationsinprotein pages 2-3): Maria Pakhomova, Azamat Karginov, Maria Kulakova, Polina Vladimirova, Olga Mitkevich, and Michael Agaphonov. Alterations in protein n-glycosylation confer vanadate resistance in ogataea polymorpha mutants defective in phosphomannosylation. Frontiers in Molecular Biosciences, Jan 2026. URL: https://doi.org/10.3389/fmolb.2026.1741711, doi:10.3389/fmolb.2026.1741711. This article has 0 citations.

7. (kang2021invitronglycan pages 5-7): Ji-Yeon Kang, Hong-Yeol Choi, Dong-Il Kim, Ohsuk Kwon, and Doo-Byoung Oh. In vitro n-glycan mannosyl-phosphorylation of a therapeutic enzyme by using recombinant mnn14 produced from pichia pastoris. Journal of Microbiology and Biotechnology, 31:163-170, Jan 2021. URL: https://doi.org/10.4014/jmb.2010.10033, doi:10.4014/jmb.2010.10033. This article has 4 citations and is from a peer-reviewed journal.

8. (fabre2014mannosylationoffungal pages 2-3): Emeline Fabre, Thomas Hurtaux, and Chantal Fradin. Mannosylation of fungal glycoconjugates in the golgi apparatus. Current opinion in microbiology, 20:103-10, Aug 2014. URL: https://doi.org/10.1016/j.mib.2014.05.008, doi:10.1016/j.mib.2014.05.008. This article has 24 citations and is from a peer-reviewed journal.

9. (jigami2008yeastglycobiologyand pages 4-6): Yoshifumi JIGAMI. Yeast glycobiology and its application. Bioscience, Biotechnology, and Biochemistry, 72:637-648, Mar 2008. URL: https://doi.org/10.1271/bbb.70725, doi:10.1271/bbb.70725. This article has 54 citations.

10. (fabre2014mannosylationoffungal pages 7-7): Emeline Fabre, Thomas Hurtaux, and Chantal Fradin. Mannosylation of fungal glycoconjugates in the golgi apparatus. Current opinion in microbiology, 20:103-10, Aug 2014. URL: https://doi.org/10.1016/j.mib.2014.05.008, doi:10.1016/j.mib.2014.05.008. This article has 24 citations and is from a peer-reviewed journal.

11. (yoshidamoriguchi2015matriglycananovel pages 7-9): T. Yoshida-Moriguchi and K. P. Campbell. Matriglycan: a novel polysaccharide that links dystroglycan to the basement membrane. Glycobiology, 25:702-713, Apr 2015. URL: https://doi.org/10.1093/glycob/cwv021, doi:10.1093/glycob/cwv021. This article has 262 citations and is from a peer-reviewed journal.

Artifacts

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• Edison artifact artifact-01

Citations

1. kang2021invitronglycan pages 7-8
2. kang2021invitronglycan pages 3-5
3. harnett2010howdonematodes pages 2-4
4. kang2021invitronglycan pages 1-2
5. kang2021invitronglycan pages 2-3
6. pakhomova2026alterationsinprotein pages 2-3
7. fabre2014mannosylationoffungal pages 2-3
8. jigami2008yeastglycobiologyand pages 4-6
9. fabre2014mannosylationoffungal pages 7-7
10. yoshidamoriguchi2015matriglycananovel pages 7-9
11. kang2021invitronglycan pages 5-7
12. TS
13. https://doi.org/10.4014/jmb.2010.10033,
14. https://doi.org/10.1016/j.pt.2009.12.003,
15. https://doi.org/10.3389/fmolb.2026.1741711,
16. https://doi.org/10.1016/j.mib.2014.05.008,
17. https://doi.org/10.1271/bbb.70725,
18. https://doi.org/10.1093/glycob/cwv021,