A0A8B6GS20

UniProt ID: A0A8B6GS20
Organism: Mytilus galloprovincialis
Review Status: DRAFT
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Gene 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 Review

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.

Core Functions

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:
protein phosphatase binding
Directly Involved In:
regulation of phosphatidylinositol dephosphorylation
Cellular Locations:
cytoplasm
Supporting Evidence:
  • file:MYTGA/A0A8B6GS20/A0A8B6GS20-deep-research-falcon.md
    MTMR9 forms heterodimeric complexes with MTMR6, MTMR7, and MTMR8, modulating their localization and substrate specificity

References

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External Prediction Reviews

These computational predictions are reviewed separately from the GOA annotation set used for this review. The assessments below are from this project and do not constitute official GO annotations or endorsement by GO/UniProt. They are not included in the existing annotation review above.

ProtNLM2 External predictions

View prediction review YAML · A0A8B6GS20-protnlm-predictions-review.yaml · Review status: COMPLETE

The deposited myotubularin-like domain lacks the catalytic CX5R motif, arguing against intrinsic phosphatidylinositol-3-phosphate phosphatase activity.

Source documents: genes/MYTGA/A0A8B6GS20/A0A8B6GS20-uniprot.txt · genes/MYTGA/A0A8B6GS20/A0A8B6GS20-goa.tsv · publications/PMID_22647598.md · genes/MYTGA/A0A8B6GS20/A0A8B6GS20-hypotheses/prediction-pi3p-phosphatase/openscientist.md

Review score: 2 = concordant with evidence; 1 = uncertain; 0 = discordant with evidence. This is an assessment score, not a model probability.

GO:0004438 phosphatidylinositol-3-phosphate phosphatase activity GO_MF
NPI — Nonparalog incorrect Review score: 0/2
Prediction method: ProtNLM2 · Version: UniProt 2024_06 pilot
PSEUDOENZYME OVERANNOTATION
Review rationale: The target has a myotubularin-like phosphatase domain at residues 372–544, but this interval contains no CX5R catalytic motif. Experiments on MTMR complexes distinguish active phosphatases from inactive family members that lack the catalytic cysteine and regulate their partners (PMID:22647598). This sequence-level deficiency argues against the predicted intrinsic PI3P phosphatase activity even though the protein could influence phosphoinositide metabolism through a partner. The unusual N-terminal domain arrangement and exact MTMR paralog identity remain unresolved; the activity judgment does not depend on calling the target MTMR9.
Supporting Evidence:

Deep Research

Falcon

(A0A8B6GS20-deep-research-falcon.md)

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OpenScientist

(A0A8B6GS20-hypotheses/prediction-pi3p-phosphatase/openscientist.md)

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