Myotubularin-related protein 9 (MTMR9) is a catalytically inactive pseudophosphatase belonging to the myotubularin subfamily of the protein-tyrosine phosphatase family. MTMR9 lacks the critical catalytic cysteine residue in the conserved CX5R motif of the phosphatase domain, rendering it unable to dephosphorylate phosphoinositide substrates directly. Instead, it functions as a regulatory scaffold protein that heterodimerizes with catalytically active myotubularin family members (MTMR6, MTMR7, MTMR8) via its coiled-coil domain, modulating their activity, subcellular localization, and substrate specificity. In mammalian systems, MTMR9 recruits active partners to the ER-Golgi intermediate compartment and Golgi apparatus, where the heterodimeric complexes regulate phosphoinositide metabolism to maintain Golgi integrity and support ER-to-Golgi transport. MTMR9-containing complexes also participate in the regulation of autophagy and endolysosomal homeostasis. The protein contains a myotubularin phosphatase domain (inactive) and D8C_UMOD domains. No direct experimental data exist for this protein in Mytilus galloprovincialis; functional inferences are based on orthology to well-characterized mammalian and invertebrate MTMR9 proteins.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
|
GO:0005737
cytoplasm
|
IEA
GO_REF:0000118 |
ACCEPT |
Summary: Cytoplasmic localization is broadly consistent with MTMR9 orthologs in mammalian systems. MTMR9 localizes to cytoplasmic membrane compartments, particularly the ER-Golgi intermediate compartment and Golgi apparatus, where it recruits active MTMR binding partners. While not wrong, cytoplasm is a very general term; the more specific localization is to endomembrane compartments. Acceptable as a general localization for an IEA annotation based on TreeGrafter phylogenetic inference.
|
|
GO:0010507
negative regulation of autophagy
|
IEA
GO_REF:0000118 |
MARK AS OVER ANNOTATED |
Summary: The relationship between MTMR9 and autophagy regulation is complex and context-dependent. MTMR9 does not directly regulate autophagy; rather, it modulates the activity of active MTMR partners (MTMR6/7/8) that control phosphoinositide levels on autophagic membranes. In Drosophila, related myotubularin family members show condition-dependent effects on autophagy, sometimes promoting and sometimes inhibiting it. Studies show that knockdown of both MTMR8 and MTMR9 leads to p62 accumulation, suggesting a positive rather than negative role in autophagic flux. Annotating MTMR9 specifically as a negative regulator of autophagy is an over-simplification that conflates the indirect regulatory role of this pseudophosphatase with the direct activity of its binding partners, and the direction of effect is not clearly established for MTMR9 itself.
Reason: MTMR9 does not directly regulate autophagy. Its role is indirect, through modulation of active MTMR partners. The evidence for a specifically negative regulatory role is weak; some data suggest the opposite direction.
|
|
GO:0019903
protein phosphatase binding
|
IEA
GO_REF:0000118 |
ACCEPT |
Summary: This annotation is well-supported. MTMR9 is a catalytically inactive pseudophosphatase whose primary molecular function is to bind catalytically active myotubularin phosphatases (MTMR6, MTMR7, MTMR8) through coiled-coil domain interactions, forming regulatory heterodimers. This binding modulates the localization, stability, and substrate specificity of the active phosphatase partners. Protein phosphatase binding accurately captures this core molecular function.
|
|
GO:0046856
phosphatidylinositol dephosphorylation
|
IEA
GO_REF:0000118 |
MODIFY |
Summary: MTMR9 is a catalytically dead pseudophosphatase that cannot directly catalyze phosphatidylinositol dephosphorylation. It lacks the critical catalytic cysteine in the CX5R motif required for phosphatase activity. While MTMR9 participates in phosphatidylinositol metabolism indirectly by regulating the activity and localization of active MTMR phosphatases (MTMR6/7/8) that do catalyze this reaction, annotating MTMR9 as directly involved in this process is misleading. The more accurate annotation would be regulation of phosphatidylinositol dephosphorylation, reflecting its indirect modulatory role.
Reason: MTMR9 cannot catalyze phosphatidylinositol dephosphorylation due to absence of the catalytic cysteine. Its role is regulatory, modulating the activity of active MTMR partners. GO:0060304 (regulation of phosphatidylinositol dephosphorylation) better captures this function.
Proposed replacements:
regulation of phosphatidylinositol dephosphorylation
|
The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.
You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.
We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.
We are interested in where in or outside the cell the gene product carries out its function.
We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.
Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.
The protein A0A8B6GS20 from Mytilus galloprovincialis (Mediterranean mussel) is annotated as myotubularin-related protein 9 based on sequence homology. No direct experimental literature exists for this specific protein in M. galloprovincialis. Therefore, this report provides a comprehensive functional annotation based on extensive literature from the myotubularin (MTMR) protein family in model organisms, particularly focusing on MTMR9 orthologs in mammals and invertebrates.
A0A8B6GS20 belongs to the protein-tyrosine phosphatase family, specifically the myotubularin-related subfamily (saar2025themyotubularinrelated pages 1-2). The myotubularin family is highly conserved across eukaryotes, comprising 16 members in mammals (MTM1 and MTMR1-15), of which 9 are catalytically active phosphatases and 7 are catalytically inactive pseudophosphatases (allen2020aconservedmyotubularinrelated pages 1-2, manzeger2021conditiondependentfunctionalshift pages 1-4).
Based on its annotation as myotubularin-related protein 9, A0A8B6GS20 is predicted to be a catalytically inactive pseudophosphatase (wang2024recentadvancesof pages 2-3, liu2022theprogressof pages 3-5, reiterer2020thedeadphosphatases pages 25-28). Inactive myotubularins lack the catalytic cysteine residue in the conserved CX5R motif that is essential for phosphatase activity (allen2020aconservedmyotubularinrelated pages 1-2, manzeger2021conditiondependentfunctionalshift pages 1-4).
MTMR9 is classified as a pseudophosphatase because it lacks enzymatic activity toward phosphoinositides (liu2022theprogressof pages 3-5, reiterer2020thedeadphosphatases pages 25-28). Unlike catalytically active myotubularins that dephosphorylate phosphatidylinositol 3-phosphate [PI(3)P] and phosphatidylinositol 3,5-bisphosphate [PI(3,5)P2], MTMR9 cannot directly catalyze these reactions (wang2024recentadvancesof pages 2-3, reiterer2020thedeadphosphatases pages 25-28, manzeger2021conditiondependentfunctionalshift pages 1-4).
The primary function of MTMR9 is to serve as a regulatory adaptor and scaffold protein that modulates the activity, localization, and substrate specificity of catalytically active myotubularins (wang2024recentadvancesof pages 2-3, liu2022theprogressof pages 3-5, saar2025themyotubularinrelated pages 1-2). MTMR9 forms heterodimeric complexes with active myotubularins, specifically MTMR6, MTMR7, and MTMR8, through coiled-coil domain interactions (wang2024recentadvancesof pages 2-3, liu2022theprogressof pages 3-5, saar2025themyotubularinrelated pages 1-2).
Recent structural and biochemical studies demonstrate that MTMR9 can both homodimerize and heterodimerize with active partners (wang2024recentadvancesof pages 2-3, saar2025themyotubularinrelated pages 1-2). The coiled-coil domain of MTMR7 preferentially forms dimers, while MTMR9 coiled-coil domains can form trimers, indicating complex oligomerization states that may regulate function (saar2025themyotubularinrelated pages 1-2).
When MTMR9 forms heterodimeric complexes with active myotubularins, it can alter the substrate preference of the complex. For example, the MTMR8/MTMR9 complex preferentially acts on PI(3)P as substrate, while other MTMR complexes may have broader specificity toward both PI(3)P and PI(3,5)P2 (wang2024recentadvancesof pages 2-3, wang2024recentadvancesof pages 3-4). This partner-mediated substrate selectivity allows for fine-tuned regulation of phosphoinositide metabolism in different cellular compartments.
| Entity/group | Catalytic activity | Substrate specificity | Reaction catalyzed | Protein-protein interactions | Subcellular localization | Biological processes regulated | Notes for A0A8B6GS20 inference |
|---|---|---|---|---|---|---|---|
| Myotubularin family (general) | Family contains 16 MTMRs in humans; 9 are catalytically active and 7 are inactive pseudophosphatases (allen2020aconservedmyotubularinrelated pages 1-2, saar2025themyotubularinrelated pages 1-2, manzeger2021conditiondependentfunctionalshift pages 1-4) | Active MTMRs preferentially act on PI(3)P and PI(3,5)P2 (also written PtdIns3P and PtdIns(3,5)P2) (saar2025themyotubularinrelated pages 1-2, manzeger2021conditiondependentfunctionalshift pages 1-4) | PI(3)P → PI and PI(3,5)P2 → PI(5)P by removal of the 3-phosphate from the inositol ring (wang2024recentadvancesof pages 3-4, moschovakifilippidou2025lackofmyotubularin pages 1-5, manzeger2021conditiondependentfunctionalshift pages 1-4) | Active and inactive MTMRs form homo- and heterodimers via coiled-coil regions; inactive partners regulate localization, stability, substrate preference, and activity of active partners (wang2024recentadvancesof pages 2-3, liu2022theprogressof pages 3-5, saar2025themyotubularinrelated pages 1-2, manzeger2021conditiondependentfunctionalshift pages 1-4) | Typically associated with plasma membrane or intracellular membranes; different members localize to endosomes, Golgi/intermediate compartment, nuclear envelope, nucleus, and cytosol (liu2022theprogressof pages 3-5, saar2025themyotubularinrelated pages 1-2) | Membrane trafficking, endolysosomal homeostasis, autophagy, endocytosis, phagocytosis, signaling, cytoskeletal regulation, and lipid homeostasis (allen2020aconservedmyotubularinrelated pages 1-2, licheva2022phosphoregulationofthe pages 3-4, lange2024myotubularinrelatedproteinsregulate pages 1-5, allen2020identificationofa pages 1-9, manzeger2021conditiondependentfunctionalshift pages 1-4) | The mussel protein can be inferred to participate in conserved phosphoinositide-regulated membrane biology, but direct experimental evidence for A0A8B6GS20 is lacking (saar2025themyotubularinrelated pages 1-2, manzeger2021conditiondependentfunctionalshift pages 1-4) |
| Catalytically active MTMRs | Active phosphatases contain the catalytic CX5R motif (allen2020aconservedmyotubularinrelated pages 1-2, manzeger2021conditiondependentfunctionalshift pages 1-4) | PI(3)P and PI(3,5)P2 are the canonical substrates (saar2025themyotubularinrelated pages 1-2, manzeger2021conditiondependentfunctionalshift pages 1-4) | Generate PI and PI(5)P, thereby shaping phosphoinositide identity on membranes (wang2024recentadvancesof pages 3-4, moschovakifilippidou2025lackofmyotubularin pages 1-5) | Often pair with inactive MTMRs such as MTMR9, MTMR12, MTMR13, or MTMR5 to tune function (wang2024recentadvancesof pages 2-3, liu2022theprogressof pages 3-5) | Endosomes, autophagic membranes, plasma membrane-associated compartments, Golgi-related compartments, and other endomembranes depending on paralog (allen2020aconservedmyotubularinrelated pages 1-2, saar2025themyotubularinrelated pages 1-2, manzeger2021conditiondependentfunctionalshift pages 1-4) | Autophagosome formation/maturation, endosome-lysosome transport, lysosome homeostasis, receptor trafficking, and signaling outputs including AKT/KRAS-related effects (allen2020aconservedmyotubularinrelated pages 1-2, wang2024recentadvancesof pages 3-4, lange2024myotubularinrelatedproteinsregulate pages 1-5, allen2020identificationofa pages 1-9) | If A0A8B6GS20 retained catalytic residues it would suggest phosphatase activity, but its annotation as MTMR9-like argues instead for an inactive regulatory role (saar2025themyotubularinrelated pages 1-2) |
| Catalytically inactive MTMRs (pseudophosphatases) | Inactive because catalytic-center cysteine is absent/substituted; cannot act as lipid phosphatases themselves (allen2020aconservedmyotubularinrelated pages 1-2, reiterer2020thedeadphosphatases pages 25-28, manzeger2021conditiondependentfunctionalshift pages 1-4) | No intrinsic phosphoinositide hydrolysis expected; instead they alter specificity or localization of active partners (manzeger2021conditiondependentfunctionalshift pages 1-4, liu2022theprogressof pages 3-5) | No direct dephosphorylation reaction catalyzed; regulatory/adaptor function (reiterer2020thedeadphosphatases pages 25-28, manzeger2021conditiondependentfunctionalshift pages 1-4) | Form regulatory heterodimers with active MTMRs; can stabilize complexes and direct compartment targeting (wang2024recentadvancesof pages 2-3, liu2022theprogressof pages 3-5, manzeger2021conditiondependentfunctionalshift pages 1-4) | Localization often follows binding partners and can include Golgi/intermediate compartment or other membranes (liu2022theprogressof pages 3-5, saar2025themyotubularinrelated pages 1-2) | Fine-tuning of autophagy, trafficking, signaling, and membrane identity through complex formation (liu2022theprogressof pages 3-5, manzeger2021conditiondependentfunctionalshift pages 1-4) | A0A8B6GS20 is most plausibly interpreted in this framework if it is truly MTMR9 orthologous (saar2025themyotubularinrelated pages 1-2) |
| MTMR9 specifically | Catalytically inactive pseudophosphatase; classified among inactive MTMRs (wang2024recentadvancesof pages 2-3, liu2022theprogressof pages 3-5, wang2024recentadvancesof pages 3-4) | Does not itself dephosphorylate PI(3)P/PI(3,5)P2; modulates active partners such as MTMR6/7/8. In one cited review, the MTMR8/MTMR9 complex prefers PI(3)P as substrate of the active partner complex (wang2024recentadvancesof pages 2-3, wang2024recentadvancesof pages 3-4) | Regulatory component rather than enzyme; supports partner-mediated hydrolysis of 3-phosphoinositides (wang2024recentadvancesof pages 2-3, liu2022theprogressof pages 3-5) | Homodimerizes and heterodimerizes with MTMR6, MTMR7, and MTMR8; recruits MTMR6 and MTMR8 to the intermediate compartment and Golgi and sustains Golgi integrity/ER-to-Golgi transport (wang2024recentadvancesof pages 2-3, liu2022theprogressof pages 3-5, saar2025themyotubularinrelated pages 1-2) | Intermediate compartment and Golgi apparatus when in complexes with MTMR6/8; broader localization likely depends on partner and coiled-coil/CTD interactions (liu2022theprogressof pages 3-5, saar2025themyotubularinrelated pages 1-2) | ER-to-Golgi transport, Golgi integrity, regulation of autophagy through partner complexes; human association studies also link MTMR9 variants/expression to metabolic traits such as obesity, hypertension, and HbA1c-related phenotypes (liu2022theprogressof pages 3-5, wang2024recentadvancesof pages 3-4) | Best-supported inference for A0A8B6GS20: a noncatalytic scaffold/adaptor in phosphoinositide-regulated membrane trafficking rather than a standalone lipid phosphatase (liu2022theprogressof pages 3-5, saar2025themyotubularinrelated pages 1-2, wang2024recentadvancesof pages 3-4) |
| MTMR6/7/8–MTMR9 regulatory module | MTMR6/7/8 are active; MTMR9 is inactive (wang2024recentadvancesof pages 2-3, liu2022theprogressof pages 3-5) | Active partner substrates are PI(3)P and/or PI(3,5)P2; substrate preference can change in complex, with MTMR8/MTMR9 noted to prefer PI(3)P (wang2024recentadvancesof pages 2-3, wang2024recentadvancesof pages 3-4) | Partner-mediated dephosphorylation of 3-phosphoinositides; MTMR9 acts as specificity/localization regulator (wang2024recentadvancesof pages 2-3, liu2022theprogressof pages 3-5) | MTMR6-MTMR9 and MTMR8-MTMR9 dimers are documented; MTMR7 also heterodimerizes with MTMR9 and MTMR9 can homodimerize/oligomerize (wang2024recentadvancesof pages 2-3, liu2022theprogressof pages 3-5, saar2025themyotubularinrelated pages 1-2) | Intermediate compartment/Golgi for MTMR6/8 complexes; MTMR6 and MTMR8 family members can also associate with nuclear envelope or other membrane systems depending on context (liu2022theprogressof pages 3-5, saar2025themyotubularinrelated pages 1-2) | ER-to-Golgi trafficking, Golgi maintenance, autophagy control, apoptosis modulation, and local phosphoinositide remodeling near channels or membrane trafficking machinery (wang2024recentadvancesof pages 2-3, liu2022theprogressof pages 3-5, saar2025themyotubularinrelated pages 1-2) | Strongest mechanistic template for annotating the mussel MTMR9-like protein (liu2022theprogressof pages 3-5, saar2025themyotubularinrelated pages 1-2) |
| MTMR6-family/invertebrate homologs | Drosophila Mtmr6 and human MTMR8 are active 3-phosphoinositide phosphatases (allen2020aconservedmyotubularinrelated pages 1-2, allen2020identificationofa pages 1-9, manzeger2021conditiondependentfunctionalshift pages 1-4) | PI(3)P and PI(3,5)P2 in autophagy-related contexts (manzeger2021conditiondependentfunctionalshift pages 1-4) | Dephosphorylation of PI(3)P/PI(3,5)P2 to regulate membrane flux and lysosome homeostasis (allen2020identificationofa pages 1-9, manzeger2021conditiondependentfunctionalshift pages 1-4) | Human MTMR8 has been studied alongside inactive binding partner MTMR9; Drosophila data support conserved functional modules (allen2020aconservedmyotubularinrelated pages 1-2, allen2020identificationofa pages 1-9) | Autophagic and endolysosomal compartments; effects seen on lysosome homeostasis and endocytic/phagocytic systems (allen2020aconservedmyotubularinrelated pages 1-2, allen2020identificationofa pages 1-9) | Maintains autophagic flux, lysosome homeostasis, fluid-phase endocytosis, phagocytosis, and development/viability in flies (allen2020aconservedmyotubularinrelated pages 1-2, allen2020identificationofa pages 1-9, manzeger2021conditiondependentfunctionalshift pages 1-4) | Supports the evolutionary conservation of MTMR-mediated membrane regulation in invertebrates, strengthening inference for mussel proteins (allen2020identificationofa pages 1-9, manzeger2021conditiondependentfunctionalshift pages 1-4) |
| Disease-/pathway-oriented functional summary | Family dysfunction alters phosphoinositide balance and downstream signaling (wang2024recentadvancesof pages 3-4, moschovakifilippidou2025lackofmyotubularin pages 1-5, lange2024myotubularinrelatedproteinsregulate pages 1-5) | PI(3)P accumulation is a recurrent consequence of lost active MTMR function (wang2024recentadvancesof pages 3-4, moschovakifilippidou2025lackofmyotubularin pages 1-5, lange2024myotubularinrelatedproteinsregulate pages 1-5) | Reduced MTMR activity elevates PI(3)P and perturbs PI4P/PtdSer/KRAS or mTOR/autophagy-linked pathways depending on context (wang2024recentadvancesof pages 3-4, moschovakifilippidou2025lackofmyotubularin pages 1-5, lange2024myotubularinrelatedproteinsregulate pages 1-5) | Complex formation is central to function and disease relevance (wang2024recentadvancesof pages 2-3, liu2022theprogressof pages 3-5) | Membrane systems most affected are endosomes, autophagic membranes, lysosomes, Golgi, and plasma membrane-associated lipid transfer sites (liu2022theprogressof pages 3-5, allen2020aconservedmyotubularinrelated pages 1-2, lange2024myotubularinrelatedproteinsregulate pages 1-5) | Autophagy dysregulation, membrane remodeling defects, receptor/signaling defects, neuropathy/myopathy associations, and altered KRAS membrane localization are documented across the family (allen2020aconservedmyotubularinrelated pages 1-2, moschovakifilippidou2025lackofmyotubularin pages 1-5, lange2024myotubularinrelatedproteinsregulate pages 1-5, manzeger2021conditiondependentfunctionalshift pages 1-4) | For A0A8B6GS20, the most defensible annotation is a conserved MTMR9-like regulator of phosphoinositide-dependent membrane trafficking and possibly autophagy/Golgi organization, not a directly demonstrated mussel-specific enzyme activity (liu2022theprogressof pages 3-5, saar2025themyotubularinrelated pages 1-2, wang2024recentadvancesof pages 3-4) |
Table: This table summarizes catalytic status, substrates, reactions, interactions, localization, and biological roles across the myotubularin family, with emphasis on MTMR9 as a pseudophosphatase. It is useful for inferring the likely function of the mussel protein A0A8B6GS20 when direct organism-specific literature is unavailable.
Active myotubularins catalyze the removal of the 3-phosphate group from phosphoinositides (saar2025themyotubularinrelated pages 1-2, moschovakifilippidou2025lackofmyotubularin pages 1-5, manzeger2021conditiondependentfunctionalshift pages 1-4). Specifically, they convert:
- PI(3)P → PI (phosphatidylinositol)
- PI(3,5)P2 → PI(5)P (phosphatidylinositol 5-phosphate)
These reactions are critical for regulating membrane identity and trafficking, as different phosphoinositides recruit distinct sets of effector proteins (allen2020aconservedmyotubularinrelated pages 1-2, licheva2022phosphoregulationofthe pages 3-4, manzeger2021conditiondependentfunctionalshift pages 1-4).
Myotubularins function antagonistically to Class III phosphatidylinositol 3-kinases (PI3K) such as VPS34/PIK3C3, which generate PI(3)P from PI (licheva2022phosphoregulationofthe pages 3-4, manzeger2021conditiondependentfunctionalshift pages 1-4). The balance between kinase-mediated phosphorylation and myotubularin-mediated dephosphorylation determines the steady-state levels of these signaling lipids on different membrane compartments (wang2024recentadvancesof pages 3-4, moschovakifilippidou2025lackofmyotubularin pages 1-5, lange2024myotubularinrelatedproteinsregulate pages 1-5).
Loss or reduction of myotubularin activity leads to accumulation of PI(3)P on membranes, which has profound consequences for downstream signaling pathways including mTOR/AKT signaling and autophagy regulation (wang2024recentadvancesof pages 3-4, moschovakifilippidou2025lackofmyotubularin pages 1-5, lange2024myotubularinrelatedproteinsregulate pages 1-5).
In mammalian cells, MTMR9 recruits its catalytically active binding partners MTMR6 and MTMR8 to specific subcellular compartments, particularly the intermediate compartment between the endoplasmic reticulum (ER) and Golgi apparatus (liu2022theprogressof pages 3-5, saar2025themyotubularinrelated pages 1-2). This recruitment function is essential for maintaining Golgi integrity and regulating ER-to-Golgi transport (liu2022theprogressof pages 3-5).
The MTMR6-MTMR9 and MTMR8-MTMR9 heterodimers are confirmed to localize to these membrane compartments where they regulate local phosphoinositide composition (wang2024recentadvancesof pages 2-3, liu2022theprogressof pages 3-5, saar2025themyotubularinrelated pages 1-2).
The broader myotubularin family shows diverse subcellular distributions, with different members localizing to:
- Endosomes (early and late)
- Autophagic membranes (phagophores, autophagosomes, autolysosomes)
- Lysosomes
- Nuclear envelope
- Plasma membrane-associated compartments
- Golgi apparatus and intermediate compartment
- Cytosol
This diversity reflects the specialized functions of different family members in regulating phosphoinositide composition at distinct cellular locations (liu2022theprogressof pages 3-5, allen2020aconservedmyotubularinrelated pages 1-2, saar2025themyotubularinrelated pages 1-2).
The most well-characterized function of MTMR9 is its role in ER-to-Golgi membrane trafficking (liu2022theprogressof pages 3-5). MTMR9 recruits MTMR6 and MTMR8 to the intermediate compartment and Golgi apparatus, forming functional dimers that regulate the transport of cargo from the ER to the Golgi and maintain Golgi structural integrity (liu2022theprogressof pages 3-5, saar2025themyotubularinrelated pages 1-2).
This function requires precise regulation of phosphoinositide levels, particularly PI(3)P, at ER-Golgi contact sites. Disruption of MTMR9 function impairs these trafficking pathways (liu2022theprogressof pages 3-5).
Myotubularins play critical roles in autophagy, the cellular self-degradation pathway essential for protein and organelle quality control (allen2020aconservedmyotubularinrelated pages 1-2, chua2022myotubularinrelatedphosphatase5 pages 1-4, licheva2022phosphoregulationofthe pages 3-4, allen2020identificationofa pages 1-9, manzeger2021conditiondependentfunctionalshift pages 1-4). The regulation of autophagy by myotubularins is complex and context-dependent:
Active myotubularins regulate autophagic flux by controlling PI(3)P and PI(3,5)P2 levels on autophagic membranes (allen2020aconservedmyotubularinrelated pages 1-2, licheva2022phosphoregulationofthe pages 3-4, allen2020identificationofa pages 1-9). PI(3)P is required for autophagosome formation and maturation, while its timely removal is necessary for autophagosome-lysosome fusion and autolysosome reformation (allen2020aconservedmyotubularinrelated pages 1-2, licheva2022phosphoregulationofthe pages 3-4).
Studies in Drosophila demonstrate that myotubularin-related phosphatases (including MTMR6 orthologs) are essential for maintaining autophagic flux and lysosome homeostasis (allen2020aconservedmyotubularinrelated pages 1-2, allen2020identificationofa pages 1-9, manzeger2021conditiondependentfunctionalshift pages 1-4). Decreased function of these phosphatases results in autophagic vesicle accumulation and impaired cargo degradation (allen2020aconservedmyotubularinrelated pages 1-2, allen2020identificationofa pages 1-9).
MTMR9 complexes modulate autophagy through their effects on partner myotubularin activity. The heterodimer MTMR6-MTMR9 and MTMR8-MTMR9 complexes have been shown to regulate autophagy activity, with MTMR9 expression affecting autophagosome levels (wang2024recentadvancesof pages 2-3, wang2024recentadvancesof pages 3-4). Knockdown of both MTMR8 and MTMR9 results in increased levels of p62, a protein that accumulates when autophagy is impaired (liu2022theprogressof pages 3-5).
Studies in neurons reveal that MTMR5 (which interacts with MTMR2) suppresses autophagy by dephosphorylating phosphoinositides critical for autophagy initiation and autophagosome maturation, and knockdown of MTMR5 or MTMR2 (but not MTMR9) significantly enhances neuronal degradation of autophagy substrates (chua2022myotubularinrelatedphosphatase5 pages 1-4). This highlights the specificity of different MTMR complexes in autophagy regulation.
Myotubularins regulate multiple aspects of membrane trafficking and endocytosis beyond autophagy (allen2020aconservedmyotubularinrelated pages 1-2, lange2024myotubularinrelatedproteinsregulate pages 1-5, allen2020identificationofa pages 1-9). Studies in Drosophila show that loss of myotubularin function impairs fluid-phase endocytosis in fat body cells and phagocytosis in embryonic macrophages (allen2020aconservedmyotubularinrelated pages 1-2, allen2020identificationofa pages 1-9).
The regulation of PI(3)P levels by myotubularins is critical for endosome maturation, receptor trafficking, and cargo sorting through the endolysosomal system (allen2020aconservedmyotubularinrelated pages 1-2, allen2020identificationofa pages 1-9).
Myotubularins influence multiple signaling pathways through their effects on phosphoinositide metabolism:
PI3K/AKT Pathway: Myotubularins regulate the PI3K/AKT signaling pathway, which is critical for cell growth, survival, and metabolism (wang2024recentadvancesof pages 3-4). Silencing of active myotubularins (MTM1, MTMR2, MTMR3, MTMR4, MTMR7) leads to abnormal accumulation of PI(3)P, which can suppress AKT phosphorylation and downstream signaling (wang2024recentadvancesof pages 3-4).
KRAS Signaling: Recent studies demonstrate that myotubularins regulate KRAS function by controlling plasma membrane levels of phosphoinositides and phosphatidylserine (PtdSer) (lange2024myotubularinrelatedproteinsregulate pages 1-5). Silencing MTMR2/3/4/7 disrupts KRAS plasma membrane localization by depleting plasma membrane PI4P levels, which in turn affects PtdSer enrichment at the plasma membrane (lange2024myotubularinrelatedproteinsregulate pages 1-5). This reveals an indirect mechanism by which myotubularin-mediated PI(3)P dephosphorylation generates PI needed for PI4P synthesis, which is essential for KRAS membrane binding and oncogenic signaling (lange2024myotubularinrelatedproteinsregulate pages 1-5).
Apoptosis Regulation: The MTMR6-MTMR9 heterodimer has been shown to ameliorate cellular apoptosis both in vivo and in vitro (wang2024recentadvancesof pages 2-3).
Human genetic studies have identified associations between MTMR9 variants and metabolic traits. A single nucleotide polymorphism (SNP) in the MTMR9 gene intron is associated with increased HbA1c levels (wang2024recentadvancesof pages 3-4). MTMR9 expression in the murine hypothalamic region is elevated after fasting and decreased after a high-fat diet, suggesting involvement in metabolic regulation, obesity, and hypertension (liu2022theprogressof pages 3-5, wang2024recentadvancesof pages 3-4).
Myotubularins are evolutionarily conserved across eukaryotes, including invertebrates (allen2020identificationofa pages 1-9, manzeger2021conditiondependentfunctionalshift pages 1-4). Studies in Drosophila melanogaster provide important insights into myotubularin function in invertebrate systems:
Drosophila MTMR6 orthologs (dMtmr6/CG3530) function as regulators of autophagic flux and are required for development and viability (allen2020aconservedmyotubularinrelated pages 1-2, allen2020identificationofa pages 1-9, manzeger2021conditiondependentfunctionalshift pages 1-4). These proteins maintain lysosome homeostasis and regulate endocytosis and phagocytosis, demonstrating conserved functions across metazoans (allen2020aconservedmyotubularinrelated pages 1-2, allen2020identificationofa pages 1-9).
The condition-dependent regulation of autophagy by Drosophila myotubularins (EDTP and Mtmr6) reveals complex, context-specific functions: EDTP inhibits basal autophagy but does not influence stress-induced autophagy, while Mtmr6 promotes autophagy under nutrient-rich conditions but blocks its hyperactivation in response to stress (manzeger2021conditiondependentfunctionalshift pages 1-4). This demonstrates the sophisticated regulatory mechanisms employed by myotubularin family members.
The conservation of myotubularin structure and function from invertebrates to mammals strongly supports the inference that the mussel protein A0A8B6GS20 likely participates in similar phosphoinositide-regulated membrane biology processes (saar2025themyotubularinrelated pages 1-2, allen2020identificationofa pages 1-9, manzeger2021conditiondependentfunctionalshift pages 1-4).
Myotubularin family members share conserved domain architecture (saar2025themyotubularinrelated pages 1-2):
Recent work reveals that myotubularin C-terminal domains are largely disordered and contain SLiMs important for targeting to subcellular compartments, interactions with diverse proteins including transcription factors, and involvement in signaling and phase separation (saar2025themyotubularinrelated pages 1-2). The coiled-coil domains mediate the critical protein-protein interactions between active and inactive myotubularins (wang2024recentadvancesof pages 2-3, saar2025themyotubularinrelated pages 1-2).
Based on the comprehensive literature review of myotubularin family proteins, the following functional predictions can be made for A0A8B6GS20:
A0A8B6GS20 is most likely a regulatory scaffold protein rather than a catalytically active phosphoinositide phosphatase (liu2022theprogressof pages 3-5, reiterer2020thedeadphosphatases pages 25-28, saar2025themyotubularinrelated pages 1-2). As an MTMR9 ortholog, it probably functions by forming heterodimeric complexes with catalytically active myotubularins present in M. galloprovincialis, modulating their localization, substrate specificity, and activity.
The protein likely regulates phosphoinositide metabolism indirectly through its effects on partner enzymes, rather than directly catalyzing dephosphorylation reactions (wang2024recentadvancesof pages 2-3, liu2022theprogressof pages 3-5, reiterer2020thedeadphosphatases pages 25-28).
A0A8B6GS20 is predicted to localize to membrane trafficking compartments, particularly those involved in secretory pathway organization (ER-Golgi intermediate compartment, Golgi apparatus) and/or endolysosomal compartments, depending on its binding partners (liu2022theprogressof pages 3-5, saar2025themyotubularinrelated pages 1-2).
The protein is likely involved in:
- Membrane trafficking and secretory pathway function (ER-to-Golgi transport, Golgi organization)
- Autophagy regulation (potentially modulating autophagic flux)
- Endolysosomal homeostasis
- Cellular signaling through effects on phosphoinositide-dependent pathways
These predictions are based on the conserved functions of MTMR9 orthologs in mammals and invertebrates (liu2022theprogressof pages 3-5, allen2020aconservedmyotubularinrelated pages 1-2, saar2025themyotubularinrelated pages 1-2, allen2020identificationofa pages 1-9, manzeger2021conditiondependentfunctionalshift pages 1-4).
Critical limitation: No direct experimental data exists for A0A8B6GS20 in Mytilus galloprovincialis. All functional annotations are inferred from orthologous proteins in model organisms.
Recommended experimental approaches to validate these predictions:
1. Biochemical characterization of phosphatase activity (or lack thereof)
2. Identification of binding partners in mussel cells
3. Subcellular localization studies
4. Functional knockdown/knockout studies examining effects on membrane trafficking and autophagy
5. Analysis of phosphoinositide levels in cells with altered A0A8B6GS20 expression
A0A8B6GS20 from Mytilus galloprovincialis is annotated as myotubularin-related protein 9 based on sequence homology. Drawing from extensive literature on the myotubularin family, this protein is predicted to function as a catalytically inactive regulatory scaffold that modulates the activity of catalytically active myotubularin phosphatases through protein-protein interactions. Its primary role is likely in membrane trafficking, particularly ER-to-Golgi transport and Golgi organization, with potential involvement in autophagy regulation and endolysosomal homeostasis. Rather than directly catalyzing phosphoinositide dephosphorylation, A0A8B6GS20 likely regulates these processes by controlling the localization and substrate specificity of active myotubularin partners. These predictions are strongly supported by evolutionary conservation of myotubularin function from invertebrates to mammals, but await direct experimental validation in the mussel system.
References
(saar2025themyotubularinrelated pages 1-2): Daniel Saar, Caroline L. E. Lennartsson, Philip Weidner, Elke Burgermeister, and Birthe B. Kragelund. The myotubularin related proteins and the untapped interaction potential of their disordered c‐terminal regions. Proteins, 93:831-854, Nov 2025. URL: https://doi.org/10.1002/prot.26774, doi:10.1002/prot.26774. This article has 5 citations.
(allen2020aconservedmyotubularinrelated pages 1-2): Elizabeth A. Allen, Clelia Amato, Tina M. Fortier, Panagiotis Velentzas, Will Wood, and Eric H. Baehrecke. A conserved myotubularin-related phosphatase regulates autophagy by maintaining autophagic flux. The Journal of Cell Biology, Sep 2020. URL: https://doi.org/10.1083/jcb.201909073, doi:10.1083/jcb.201909073. This article has 33 citations.
(manzeger2021conditiondependentfunctionalshift pages 1-4): Anna Manzéger, Kinga Tagscherer, Péter Lőrincz, Henrik Szaker, Tamás Lukácsovich, Petra Pilz, Regina Kméczik, George Csikós, Miklós Erdélyi, Miklós Sass, Tibor Kovács, Tibor Vellai, and Viktor A. Billes. Condition-dependent functional shift of two drosophila mtmr lipid phosphatases in autophagy control. Autophagy, 17:4010-4028, Mar 2021. URL: https://doi.org/10.1080/15548627.2021.1899681, doi:10.1080/15548627.2021.1899681. This article has 17 citations and is from a domain leading peer-reviewed journal.
(wang2024recentadvancesof pages 2-3): Jia Wang, Wei Guo, Qiang Wang, Yongjian Yang, and Xiongshan Sun. Recent advances of myotubularin-related (mtmr) protein family in cardiovascular diseases. Frontiers in Cardiovascular Medicine, Mar 2024. URL: https://doi.org/10.3389/fcvm.2024.1364604, doi:10.3389/fcvm.2024.1364604. This article has 16 citations and is from a peer-reviewed journal.
(liu2022theprogressof pages 3-5): Deqiang Liu, Yiming Zhang, Hui Fang, Jinxiang Yuan, and Lizhen Ji. The progress of research into pseudophosphatases. Frontiers in Public Health, Aug 2022. URL: https://doi.org/10.3389/fpubh.2022.965631, doi:10.3389/fpubh.2022.965631. This article has 4 citations.
(reiterer2020thedeadphosphatases pages 25-28): Veronika Reiterer, Krzysztof Pawłowski, Guillaume Desrochers, Arnim Pause, Hayley J. Sharpe, and Hesso Farhan. The dead phosphatases society: a review of the emerging roles of pseudophosphatases. The FEBS Journal, 287:4198-4220, Jun 2020. URL: https://doi.org/10.1111/febs.15431, doi:10.1111/febs.15431. This article has 37 citations.
(wang2024recentadvancesof pages 3-4): Jia Wang, Wei Guo, Qiang Wang, Yongjian Yang, and Xiongshan Sun. Recent advances of myotubularin-related (mtmr) protein family in cardiovascular diseases. Frontiers in Cardiovascular Medicine, Mar 2024. URL: https://doi.org/10.3389/fcvm.2024.1364604, doi:10.3389/fcvm.2024.1364604. This article has 16 citations and is from a peer-reviewed journal.
(moschovakifilippidou2025lackofmyotubularin pages 1-5): Foteini Moschovaki-Filippidou, Christine Kretz, David Reiss, Gaëtan Chicanne, Bernard Payrastre, and Jocelyn Laporte. Lack of myotubularin phosphatase activity is the main cause of x-linked myotubular myopathy. JCI Insight, Oct 2025. URL: https://doi.org/10.1172/jci.insight.189286, doi:10.1172/jci.insight.189286. This article has 1 citations and is from a domain leading peer-reviewed journal.
(licheva2022phosphoregulationofthe pages 3-4): Mariya Licheva, Babu Raman, Claudine Kraft, and Fulvio Reggiori. Phosphoregulation of the autophagy machinery by kinases and phosphatases. Autophagy, 18:104-123, May 2022. URL: https://doi.org/10.1080/15548627.2021.1909407, doi:10.1080/15548627.2021.1909407. This article has 87 citations and is from a domain leading peer-reviewed journal.
(lange2024myotubularinrelatedproteinsregulate pages 1-5): Taylor E. Lange, Ali Naji, Ransome van der Hoeven, Hong Liang, Yong Zhou, Gerald R.V. Hammond, John F. Hancock, and Kwang-jin Cho. Myotubularin-related proteins regulate kras function by controlling plasma membrane levels of polyphosphoinositides and phosphatidylserine. bioRxiv, Jan 2024. URL: https://doi.org/10.1101/2024.01.22.576612, doi:10.1101/2024.01.22.576612. This article has 1 citations.
(allen2020identificationofa pages 1-9): Elizabeth Allen. Identification of a myotubularin-related phosphatase that regulates autophagic flux and lysosome homeostasis. ArXiv, 2020. URL: https://doi.org/10.13028/ag58-jw39, doi:10.13028/ag58-jw39. This article has 0 citations.
(chua2022myotubularinrelatedphosphatase5 pages 1-4): Jason P. Chua, Karan Bedi, Michelle T. Paulsen, Mats Ljungman, Elizabeth M. H. Tank, Erin S. Kim, Jennifer M. Colón-Mercado, Michael E. Ward, Lois S. Weisman, and Sami J. Barmada. Myotubularin-related phosphatase 5 is a critical determinant of autophagy in neurons. Current Biology, 32:2581-2595.e6, Jul 2022. URL: https://doi.org/10.1101/2021.07.20.453106, doi:10.1101/2021.07.20.453106. This article has 34 citations and is from a highest quality peer-reviewed journal.
id: A0A8B6GS20
gene_symbol: A0A8B6GS20
product_type: PROTEIN
status: DRAFT
taxon:
id: NCBITaxon:29158
label: Mytilus galloprovincialis
description: >-
Myotubularin-related protein 9 (MTMR9) is a catalytically inactive
pseudophosphatase belonging to the myotubularin subfamily of the
protein-tyrosine phosphatase family. MTMR9 lacks the critical catalytic
cysteine residue in the conserved CX5R motif of the phosphatase domain,
rendering it unable to dephosphorylate phosphoinositide substrates directly.
Instead, it functions as a regulatory scaffold protein that heterodimerizes
with catalytically active myotubularin family members (MTMR6, MTMR7, MTMR8)
via its coiled-coil domain, modulating their activity, subcellular
localization, and substrate specificity. In mammalian systems, MTMR9 recruits
active partners to the ER-Golgi intermediate compartment and Golgi apparatus,
where the heterodimeric complexes regulate phosphoinositide metabolism to
maintain Golgi integrity and support ER-to-Golgi transport. MTMR9-containing
complexes also participate in the regulation of autophagy and endolysosomal
homeostasis. The protein contains a myotubularin phosphatase domain (inactive)
and D8C_UMOD domains. No direct experimental data exist for this protein in
Mytilus galloprovincialis; functional inferences are based on orthology to
well-characterized mammalian and invertebrate MTMR9 proteins.
existing_annotations:
- term:
id: GO:0005737
label: cytoplasm
evidence_type: IEA
original_reference_id: GO_REF:0000118
qualifier: located_in
review:
summary: >-
Cytoplasmic localization is broadly consistent with MTMR9 orthologs in
mammalian systems. MTMR9 localizes to cytoplasmic membrane compartments,
particularly the ER-Golgi intermediate compartment and Golgi apparatus,
where it recruits active MTMR binding partners. While not wrong,
cytoplasm is a very general term; the more specific localization is to
endomembrane compartments. Acceptable as a general localization for an
IEA annotation based on TreeGrafter phylogenetic inference.
action: ACCEPT
- term:
id: GO:0010507
label: negative regulation of autophagy
evidence_type: IEA
original_reference_id: GO_REF:0000118
qualifier: involved_in
review:
summary: >-
The relationship between MTMR9 and autophagy regulation is complex and
context-dependent. MTMR9 does not directly regulate autophagy; rather, it
modulates the activity of active MTMR partners (MTMR6/7/8) that control
phosphoinositide levels on autophagic membranes. In Drosophila, related
myotubularin family members show condition-dependent effects on autophagy,
sometimes promoting and sometimes inhibiting it. Studies show that
knockdown of both MTMR8 and MTMR9 leads to p62 accumulation, suggesting
a positive rather than negative role in autophagic flux. Annotating MTMR9
specifically as a negative regulator of autophagy is an over-simplification
that conflates the indirect regulatory role of this pseudophosphatase with
the direct activity of its binding partners, and the direction of effect
is not clearly established for MTMR9 itself.
action: MARK_AS_OVER_ANNOTATED
reason: >-
MTMR9 does not directly regulate autophagy. Its role is indirect, through
modulation of active MTMR partners. The evidence for a specifically
negative regulatory role is weak; some data suggest the opposite direction.
- term:
id: GO:0019903
label: protein phosphatase binding
evidence_type: IEA
original_reference_id: GO_REF:0000118
qualifier: enables
review:
summary: >-
This annotation is well-supported. MTMR9 is a catalytically inactive
pseudophosphatase whose primary molecular function is to bind catalytically
active myotubularin phosphatases (MTMR6, MTMR7, MTMR8) through
coiled-coil domain interactions, forming regulatory heterodimers. This
binding modulates the localization, stability, and substrate specificity
of the active phosphatase partners. Protein phosphatase binding accurately
captures this core molecular function.
action: ACCEPT
- term:
id: GO:0046856
label: phosphatidylinositol dephosphorylation
evidence_type: IEA
original_reference_id: GO_REF:0000118
qualifier: involved_in
review:
summary: >-
MTMR9 is a catalytically dead pseudophosphatase that cannot directly
catalyze phosphatidylinositol dephosphorylation. It lacks the critical
catalytic cysteine in the CX5R motif required for phosphatase activity.
While MTMR9 participates in phosphatidylinositol metabolism indirectly by
regulating the activity and localization of active MTMR phosphatases
(MTMR6/7/8) that do catalyze this reaction, annotating MTMR9 as
directly involved in this process is misleading. The more accurate
annotation would be regulation of phosphatidylinositol dephosphorylation,
reflecting its indirect modulatory role.
action: MODIFY
reason: >-
MTMR9 cannot catalyze phosphatidylinositol dephosphorylation due to
absence of the catalytic cysteine. Its role is regulatory, modulating
the activity of active MTMR partners. GO:0060304 (regulation of
phosphatidylinositol dephosphorylation) better captures this function.
proposed_replacement_terms:
- id: GO:0060304
label: regulation of phosphatidylinositol dephosphorylation
references:
- id: GO_REF:0000118
title: TreeGrafter-generated GO annotations
findings: []
- id: file:MYTGA/A0A8B6GS20/A0A8B6GS20-deep-research-falcon.md
title: Deep research report on MTMR9 in Mytilus galloprovincialis
findings:
- statement: >-
MTMR9 is a catalytically inactive pseudophosphatase that lacks the
catalytic cysteine in the CX5R motif
- statement: >-
MTMR9 forms heterodimeric complexes with MTMR6, MTMR7, and MTMR8,
modulating their localization and substrate specificity
- statement: >-
MTMR9 recruits MTMR6 and MTMR8 to the intermediate compartment and
Golgi apparatus, sustaining Golgi integrity and ER-to-Golgi transport
- statement: >-
No direct experimental data exist for A0A8B6GS20 in Mytilus
galloprovincialis; all functional annotations are inferred from
orthologous proteins in model organisms
core_functions:
- description: >-
MTMR9 functions as a regulatory scaffold that binds catalytically active
myotubularin phosphatases (MTMR6/7/8), modulating their localization,
substrate specificity, and enzymatic activity. This is the primary molecular
function of this pseudophosphatase. By heterodimerizing with active MTMRs,
MTMR9 indirectly regulates phosphoinositide metabolism at endomembrane
compartments including the ER-Golgi intermediate compartment.
molecular_function:
id: GO:0019903
label: protein phosphatase binding
directly_involved_in:
- id: GO:0060304
label: regulation of phosphatidylinositol dephosphorylation
locations:
- id: GO:0005737
label: cytoplasm
supported_by:
- reference_id: file:MYTGA/A0A8B6GS20/A0A8B6GS20-deep-research-falcon.md
supporting_text: >-
MTMR9 forms heterodimeric complexes with MTMR6, MTMR7, and MTMR8,
modulating their localization and substrate specificity