| 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 (pqac-00000002, pqac-00000004, pqac-00000012) | Active MTMRs preferentially act on PI(3)P and PI(3,5)P2 (also written PtdIns3P and PtdIns(3,5)P2) (pqac-00000004, pqac-00000012) | PI(3)P → PI and PI(3,5)P2 → PI(5)P by removal of the 3-phosphate from the inositol ring (pqac-00000005, pqac-00000008, pqac-00000012) | Active and inactive MTMRs form homo- and heterodimers via coiled-coil regions; inactive partners regulate localization, stability, substrate preference, and activity of active partners (pqac-00000000, pqac-00000001, pqac-00000004, pqac-00000012) | Typically associated with plasma membrane or intracellular membranes; different members localize to endosomes, Golgi/intermediate compartment, nuclear envelope, nucleus, and cytosol (pqac-00000001, pqac-00000004) | Membrane trafficking, endolysosomal homeostasis, autophagy, endocytosis, phagocytosis, signaling, cytoskeletal regulation, and lipid homeostasis (pqac-00000002, pqac-00000007, pqac-00000010, pqac-00000011, pqac-00000012) | The mussel protein can be inferred to participate in conserved phosphoinositide-regulated membrane biology, but direct experimental evidence for A0A8B6GS20 is lacking (pqac-00000004, pqac-00000012) |
| Catalytically active MTMRs | Active phosphatases contain the catalytic CX5R motif (pqac-00000002, pqac-00000012) | PI(3)P and PI(3,5)P2 are the canonical substrates (pqac-00000004, pqac-00000012) | Generate PI and PI(5)P, thereby shaping phosphoinositide identity on membranes (pqac-00000005, pqac-00000008) | Often pair with inactive MTMRs such as MTMR9, MTMR12, MTMR13, or MTMR5 to tune function (pqac-00000000, pqac-00000001) | Endosomes, autophagic membranes, plasma membrane-associated compartments, Golgi-related compartments, and other endomembranes depending on paralog (pqac-00000002, pqac-00000004, pqac-00000012) | Autophagosome formation/maturation, endosome-lysosome transport, lysosome homeostasis, receptor trafficking, and signaling outputs including AKT/KRAS-related effects (pqac-00000002, pqac-00000005, pqac-00000010, pqac-00000011) | If A0A8B6GS20 retained catalytic residues it would suggest phosphatase activity, but its annotation as MTMR9-like argues instead for an inactive regulatory role (pqac-00000004) |
| Catalytically inactive MTMRs (pseudophosphatases) | Inactive because catalytic-center cysteine is absent/substituted; cannot act as lipid phosphatases themselves (pqac-00000002, pqac-00000003, pqac-00000012) | No intrinsic phosphoinositide hydrolysis expected; instead they alter specificity or localization of active partners (pqac-00000012, pqac-00000001) | No direct dephosphorylation reaction catalyzed; regulatory/adaptor function (pqac-00000003, pqac-00000012) | Form regulatory heterodimers with active MTMRs; can stabilize complexes and direct compartment targeting (pqac-00000000, pqac-00000001, pqac-00000012) | Localization often follows binding partners and can include Golgi/intermediate compartment or other membranes (pqac-00000001, pqac-00000004) | Fine-tuning of autophagy, trafficking, signaling, and membrane identity through complex formation (pqac-00000001, pqac-00000012) | A0A8B6GS20 is most plausibly interpreted in this framework if it is truly MTMR9 orthologous (pqac-00000004) |
| MTMR9 specifically | Catalytically inactive pseudophosphatase; classified among inactive MTMRs (pqac-00000000, pqac-00000001, pqac-00000005) | 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 (pqac-00000000, pqac-00000005) | Regulatory component rather than enzyme; supports partner-mediated hydrolysis of 3-phosphoinositides (pqac-00000000, pqac-00000001) | 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 (pqac-00000000, pqac-00000001, pqac-00000004) | Intermediate compartment and Golgi apparatus when in complexes with MTMR6/8; broader localization likely depends on partner and coiled-coil/CTD interactions (pqac-00000001, pqac-00000004) | 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 (pqac-00000001, pqac-00000005) | Best-supported inference for A0A8B6GS20: a noncatalytic scaffold/adaptor in phosphoinositide-regulated membrane trafficking rather than a standalone lipid phosphatase (pqac-00000001, pqac-00000004, pqac-00000005) |
| MTMR6/7/8–MTMR9 regulatory module | MTMR6/7/8 are active; MTMR9 is inactive (pqac-00000000, pqac-00000001) | 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 (pqac-00000000, pqac-00000005) | Partner-mediated dephosphorylation of 3-phosphoinositides; MTMR9 acts as specificity/localization regulator (pqac-00000000, pqac-00000001) | MTMR6-MTMR9 and MTMR8-MTMR9 dimers are documented; MTMR7 also heterodimerizes with MTMR9 and MTMR9 can homodimerize/oligomerize (pqac-00000000, pqac-00000001, pqac-00000004) | 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 (pqac-00000001, pqac-00000004) | ER-to-Golgi trafficking, Golgi maintenance, autophagy control, apoptosis modulation, and local phosphoinositide remodeling near channels or membrane trafficking machinery (pqac-00000000, pqac-00000001, pqac-00000004) | Strongest mechanistic template for annotating the mussel MTMR9-like protein (pqac-00000001, pqac-00000004) |
| MTMR6-family/invertebrate homologs | Drosophila Mtmr6 and human MTMR8 are active 3-phosphoinositide phosphatases (pqac-00000002, pqac-00000011, pqac-00000012) | PI(3)P and PI(3,5)P2 in autophagy-related contexts (pqac-00000012) | Dephosphorylation of PI(3)P/PI(3,5)P2 to regulate membrane flux and lysosome homeostasis (pqac-00000011, pqac-00000012) | Human MTMR8 has been studied alongside inactive binding partner MTMR9; Drosophila data support conserved functional modules (pqac-00000002, pqac-00000011) | Autophagic and endolysosomal compartments; effects seen on lysosome homeostasis and endocytic/phagocytic systems (pqac-00000002, pqac-00000011) | Maintains autophagic flux, lysosome homeostasis, fluid-phase endocytosis, phagocytosis, and development/viability in flies (pqac-00000002, pqac-00000011, pqac-00000012) | Supports the evolutionary conservation of MTMR-mediated membrane regulation in invertebrates, strengthening inference for mussel proteins (pqac-00000011, pqac-00000012) |
| Disease-/pathway-oriented functional summary | Family dysfunction alters phosphoinositide balance and downstream signaling (pqac-00000005, pqac-00000008, pqac-00000010) | PI(3)P accumulation is a recurrent consequence of lost active MTMR function (pqac-00000005, pqac-00000008, pqac-00000010) | Reduced MTMR activity elevates PI(3)P and perturbs PI4P/PtdSer/KRAS or mTOR/autophagy-linked pathways depending on context (pqac-00000005, pqac-00000008, pqac-00000010) | Complex formation is central to function and disease relevance (pqac-00000000, pqac-00000001) | Membrane systems most affected are endosomes, autophagic membranes, lysosomes, Golgi, and plasma membrane-associated lipid transfer sites (pqac-00000001, pqac-00000002, pqac-00000010) | Autophagy dysregulation, membrane remodeling defects, receptor/signaling defects, neuropathy/myopathy associations, and altered KRAS membrane localization are documented across the family (pqac-00000002, pqac-00000008, pqac-00000010, pqac-00000012) | 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 (pqac-00000001, pqac-00000004, pqac-00000005) |


*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.*