Target: Horse (Equus caballus, NCBITaxon:9796) protein A0A9L0T3C1 (gene label MTMR9, 507 aa)
Hypothesis (focus type: function_assignment): The protein negatively regulates autophagy — GO:0010507 (negative regulation of autophagy, BP).
Human comparison lead: MTMR9 / Q96QG7.
Date: 2026-09-08
Verdict: Partially supported (via justified mammalian transfer, with a mandatory scoping caveat).
The hypothesis that horse MTMR9 (A0A9L0T3C1) negatively regulates autophagy is mechanistically grounded in primary literature, but MTMR9 does not perform this function as a direct enzyme. MTMR9 is a catalytically inactive pseudophosphatase. Its autophagy-relevant activity is entirely partner/complex-dependent: MTMR9 is an obligate activating subunit that binds active myotubularins (MTMR6/MTMR7/MTMR8), boosts their lipid-phosphatase activity, and redirects their substrate preference. In the MTMR8/MTMR9 complex, MTMR9 stimulates MTMR8 turnover of PtdIns(3)P and lowers cellular PtdIns(3)P — the essential autophagosome-nucleation lipid — thereby suppressing autophagy (PMID:22647598(https://pubmed.ncbi.nlm.nih.gov/22647598/)). This supports GO:0010507 for MTMR9 as a regulatory subunit, not as a standalone catalytic effector.
The identity basis for transferring human evidence to horse is strong. A Needleman–Wunsch global alignment of the supplied 507-residue horse sequence against human MTMR9 (Q96QG7, 549 aa) gave 90.3% global identity (496/549) and 97.8% identity across the horse protein's length, with an identical, catalytically dead phosphatase P-loop (LIHGTEGTDSTLQVT) in both species. The horse protein is a bona fide 1:1 MTMR9 ortholog; the ~42-residue length difference maps to a single contiguous N-terminal segment (likely a missing exon in the predicted horse gene model), while all functional domains are intact.
Three caveats dominate the curation decision. (i) All direct evidence is human/mammalian cell-based — there is no horse-specific experimental evidence; this is orthology transfer. (ii) The effect is context-dependent: in neurons, MTMR9 knockdown (unlike MTMR5/MTMR2) did not enhance autophagic substrate clearance (PMID:35580604(https://pubmed.ncbi.nlm.nih.gov/35580604/)). (iii) MTMR9's most directly documented cellular function is arguably ER-to-Golgi trafficking/secretion (PMID:31704058(https://pubmed.ncbi.nlm.nih.gov/31704058/)), competing with autophagy for "primary function" status, with autophagy being a downstream consequence of PtdIns(3)P depletion.
The supplied 507-amino-acid horse sequence was verified in full (length = 507, matching the frozen record). Its myotubularin protein-tyrosine-phosphatase (PTP) P-loop region reads ...ILIHGTEGTDSTLQ... at approximately residues 289–300. Critically, the canonical active-site cysteine of the CX₅R catalytic motif is absent from the phosphatase P-loop. The only CX₅R-pattern match anywhere in the protein (CVWQILR at ~360) lies outside the P-loop and is incidental — it is not positioned to act as a catalytic nucleophile. The domain architecture — an N-terminal PH-GRAM lipid-binding region, an inactive PTP-like domain, and a C-terminal coiled-coil (...RRQLAELETE) for dimerization — matches human MTMR9 (Q96QG7).
This is exactly what the myotubularin literature predicts. Human MTMR9 is one of the seven catalytically inactive myotubularins that lack the conserved active-site cysteine required for phosphatase activity: "Seven members are inactive because they lack the conserved cysteine residue in the CX(5)R motif required for activity" (PMID:22647598(https://pubmed.ncbi.nlm.nih.gov/22647598/)). The consequence for curation is decisive: MTMR9 cannot dephosphorylate PtdIns(3)P (or any phosphoinositide) on its own. Any autophagy phenotype must therefore be mediated through a partner, not through intrinsic catalysis.
The mechanistic basis for the autophagy claim is Zou et al. 2012 (PMID:22647598(https://pubmed.ncbi.nlm.nih.gov/22647598/)). MTMR9 dimerizes with the active myotubularins MTMR6/7/8; complex formation increases the partner's catalytic activity and changes its substrate specificity. The MTMR8/MTMR9 complex prefers PtdIns(3)P as substrate (MTMR9 raises MTMR8 activity toward PtdIns(3)P roughly fourfold) and, in cells, the complex reduces cellular PtdIns(3)P levels. The paper's title explicitly frames MTMR9 as the determinant of "the enzymatic activity, substrate specificity, and role in autophagy of MTMR8."
The link to autophagy is lipid-mediated and well established at the pathway level: PtdIns(3)P is the essential autophagy-initiating lipid (produced by class III PI3K/VPS34) that marks the phagophore/omegasome and recruits the autophagy machinery for autophagosome nucleation. Depleting PtdIns(3)P suppresses autophagosome formation. Verbatim support: "the MTMR8/R9 complex prefers PtdIns(3)P" and "whereas the MTMR8/R9 complex reduces cellular PtdIns(3)P levels." Together these establish a coherent negative-regulation mechanism: MTMR9 (adaptor) → activates MTMR8 (enzyme) → depletes PtdIns(3)P (pro-autophagic lipid) → suppresses autophagy.
However, the direction and magnitude of the autophagy phenotype are not uniform across cell types. Chua et al. 2022 report that "knockdown of MTMR5 or MTMR2, but not the unrelated phosphatase MTMR9, significantly enhances neuronal degradation of TDP-43" (PMID:35580604(https://pubmed.ncbi.nlm.nih.gov/35580604/)). In neurons, removing MTMR9 did not enhance autophagic substrate clearance — the opposite of what a simple, universal autophagy-suppressor model would predict. This qualifies the hypothesis: the autophagy-suppressive role is context- and partner-dependent, contingent on which active myotubularin partner is expressed and active in a given tissue.
A Needleman–Wunsch global alignment (identity scoring, gap penalty = −1) was run between horse A0A9L0T3C1 (507 aa) and human MTMR9 Q96QG7 (549 aa, fetched from the UniProt REST API). Results: 496/549 identical residues = 90.3% global identity, and 97.8% identity across the 507-residue horse length. The ~42-residue difference reflects a shorter/truncated horse record rather than sequence divergence within the aligned region. The myotubularin PTP P-loop is identical in both species (both read LIHGTEGTDSTLQVT), and in both species it lacks the catalytic CX₅R cysteine required for phosphatase activity.
This conservation level — especially the identity of the functional signature motif and the shared loss of the catalytic cysteine — strongly supports treating the horse protein as a true functional ortholog. It is the empirical justification for transferring the human mechanistic evidence (Finding 2) to the horse annotation via ISS/ISO.
The alignment traceback (executed programmatically) shows that the 42 residues present in human MTMR9 but absent from the horse record form a single contiguous block at human residues ~92–139 (containing sequence such as IASS...ITLMYPFFYRPMFEV...EDGWHSFLPEQEF...LYSSA), located near/after the N-terminal PH-GRAM region. The PTP-like pseudophosphatase domain (P-loop LIHGTEGTDSTLQVT) and the C-terminal coiled-coil dimerization region are fully conserved (they fall within the 97.8% identity over the horse length).
The interpretation is that the horse difference is best explained as a single N-terminal gene-model/exon gap rather than a biologically meaningful divergence. The partner-binding and pseudophosphatase machinery that underpins the autophagy mechanism is intact. The gene-model artifact does not undermine functional transfer, but it should be flagged so curators know the horse record may be incomplete at its N-terminus (a truncated PH-GRAM could in principle alter membrane targeting).
The evidence converges on a single coherent model in which MTMR9 is an obligate, catalytically dead regulatory subunit, not an autophagy enzyme:
MTMR9 (A0A9L0T3C1, horse) = pseudophosphatase ADAPTOR
┌─────────────────────────────────────────────────────────┐
│ PH-GRAM │ inactive PTP-like domain │ coiled-coil │
│ (lipid/ │ (NO catalytic CX5R Cys — │ (dimerization │
│ membrane)│ cannot dephosphorylate) │ with partners) │
└─────────────────────────────────────────────────────────┘
│
│ heterodimerizes via coiled-coil
▼
MTMR6 / MTMR7 / MTMR8 (CATALYTICALLY ACTIVE partner)
│
MTMR9 raises partner activity (~4x for MTMR8)
and steers substrate specificity → PtdIns(3)P
▼
depletes cellular PtdIns(3)P
│
PtdIns(3)P = essential autophagosome-nucleation lipid
▼
↓ autophagosome formation = NEGATIVE
regulation of autophagy (GO:0010507)
— but CONTEXT-DEPENDENT
Two features dominate the curation decision. First, the activity is indirect. GO:0010507 is a regulation Biological Process node, so an indirect/regulatory contribution is not disqualifying in principle — regulation terms are appropriate for adaptors and scaffolds. But the direct molecular function of MTMR9 is enzyme regulator / phosphatase activator + protein heterodimerization, and the autophagy effect is a downstream pathway consequence of PtdIns(3)P depletion by the partner. Curators should not let GO:0010507 imply that MTMR9 is itself a lipid phosphatase acting on the autophagy pathway.
Second, the phenotype's direction is not universal. The Zou model (autophagy suppression via PtdIns(3)P depletion) is established in one system; the Chua neuronal data show no autophagy enhancement upon MTMR9 knockdown. This is consistent with a model in which MTMR9's net effect on autophagy depends on which active partner (MTMR6/7/8) it is paired with, the relative abundance of PtdIns(3)P pools, and tissue context. Quantitatively, MTMR9's boost to MTMR8/PtdIns(3)P activity is modest (~4-fold) compared to its large effect on MTMR6/PtdIns(3,5)P₂ (~30-fold), so autophagy is not obviously the dominant output of the module.
A competing/parallel function complicates the "primary function" question: MTMR9 localizes to the ER–Golgi intermediate compartment and Golgi, recruits its active partners MTMR6/MTMR8 there, co-localizes with and regulates RAB1A/WHAMM, and modulates protein secretion (including WNT3A) (PMID:31704058(https://pubmed.ncbi.nlm.nih.gov/31704058/)). This trafficking role may be at least as central to MTMR9's biology as autophagy regulation.
| Citation | Evidence type | Supports / Refutes / Qualifies / Competing | Claim tested | Key finding | Organism / context | Confidence & limitations |
|---|---|---|---|---|---|---|
| PMID:22647598(https://pubmed.ncbi.nlm.nih.gov/22647598/) (Zou et al. 2012, JBC) | Direct biochemical assay + cell PI3P measurement | Supports | MTMR9 lowers PtdIns(3)P via MTMR8 → suppresses autophagy | MTMR9 dimerizes with MTMR6/7/8, increases activity, alters specificity; MTMR8/R9 complex prefers PtdIns(3)P and reduces cellular PtdIns(3)P; title: MTMR9 "determines the … role in autophagy of MTMR8" | Human cell lines; in vitro enzymology + cellular lipid levels | High for mechanism; autophagy readout is downstream of lipid change; horse not tested; MTMR9 indirect/non-catalytic |
| PMID:22647598(https://pubmed.ncbi.nlm.nih.gov/22647598/) | Sequence/structural annotation | Qualifies | MTMR9 is catalytically dead | 7 myotubularins (incl. MTMR9) lack the CX₅R catalytic cysteine | Human/family-wide | High; confirms activity is indirect |
| PMID:35580604(https://pubmed.ncbi.nlm.nih.gov/35580604/) (Chua et al. 2022) | Loss-of-function (knockdown) phenotype | Qualifies / partial refute of universality | Does MTMR9 loss enhance autophagy? | MTMR5/MTMR2 knockdown enhanced neuronal TDP-43 autophagic degradation; MTMR9 knockdown did NOT | iPSC-derived neurons | Moderate–high; shows cell-type/partner-context dependence |
| PMID:31704058(https://pubmed.ncbi.nlm.nih.gov/31704058/) (Doubravská et al. 2020) | Localization + interaction + LOF phenotype | Competing (alternative core function) | Primary cellular role of MTMR9 | MTMR9 localizes to ERGIC/Golgi, recruits MTMR6/MTMR8, regulates RAB1A/WHAMM; loss disrupts Golgi and reduces WNT3A secretion | Human cells | High for trafficking role; autophagy not the assayed endpoint |
| This report (sequence analysis) | Structural/evolutionary (computational) | Supports mechanism (pseudophosphatase) | Is horse MTMR9 catalytically active? | 507-aa horse seq verified; P-loop ILIHGTEGTDSTLQ lacks catalytic CX₅R cysteine → inactive; PH-GRAM + inactive PTP + coiled-coil architecture matches MTMR9 |
Equus caballus supplied sequence | High; horse protein must act through partners |
| This report (NW alignment vs Q96QG7) | Structural/evolutionary (computational) | Supports orthology transfer | Is A0A9L0T3C1 a true MTMR9 ortholog? | 496/549 = 90.3% global; 97.8% over horse length; identical P-loop LIHGTEGTDSTLQVT, both catalytically dead |
Horse vs human MTMR9 | High; 1:1 ortholog; horse record ~42 aa shorter (single N-terminal block) |
Literature notes:
Immediate molecular function of MTMR9: a pseudophosphatase adaptor/activator. It heterodimerizes with catalytically active myotubularins and (a) increases their catalytic rate and (b) shifts substrate preference. In the MTMR8/R9 complex, the net cellular effect is depletion of PtdIns(3)P. MTMR9 has no intrinsic phosphatase activity (dead P-loop).
Link to autophagy (downstream): PtdIns(3)P produced by VPS34 at the phagophore is required for autophagosome nucleation and maturation. Lowering PtdIns(3)P via MTMR8/R9 therefore reduces autophagy — the mechanistic basis for GO:0010507. The autophagy phenotype is a downstream consequence of a lipid-level change, not a direct activity of MTMR9.
Direction of phenotype: ↑ MTMR8/R9 complex activity → ↓ PtdIns(3)P → ↓ autophagy (negative regulation). Loss of the complex would be expected to ↑ autophagy — but this was not observed for MTMR9 knockdown in neurons (PMID:35580604(https://pubmed.ncbi.nlm.nih.gov/35580604/)), underscoring context dependence.
Effects inferred only from loss of function: the neuronal TDP-43 clearance data are knockdown phenotypes; MTMR9 knockdown did not change autophagic clearance, so loss-of-function evidence does not support a universal autophagy-suppressor role.
| Gap | What was checked | Why it matters | What would resolve it |
|---|---|---|---|
| No horse-specific data | PubMed for MTMR9/autophagy — all hits human | Annotation rests entirely on transfer | Confirm MTMR6/7/8 orthologs & co-expression in horse tissues; a horse-cell autophagy assay |
| Directionality of the autophagy phenotype for MTMR9 specifically | Zou (suppress) vs Chua (no effect in neurons) | GO:0010507 asserts a negative direction; may not hold in all tissues | Tissue-panel autophagy-flux with defined MTMR partners |
| Primary vs downstream | Compared autophagy vs trafficking literature | Determines whether GO:0010507 is core or non-core | Comparative loss-of-function phenotyping (autophagy vs secretion) |
| Partner requirement | Human data implicate MTMR6/7/8; horse partners not examined | Mechanism requires a specific active heterodimer | Co-IP/interaction proteomics + RNA-seq of partner expression in horse |
| Horse record completeness | NW alignment traceback: one N-terminal ~42-aa gap (human ~92–139) | Sequence-based annotation may use an incomplete model; truncated PH-GRAM could alter targeting | Inspect horse genomic locus/RefSeq model & RNA-seq exon coverage |
| Quantitative MTMR9-only autophagy readout | Zou measured PtdIns(3)P & MTMR8 activity, not MTMR9-only flux | Strength of the negative regulation claim | LC3-II/p62 flux & autophagosome counts on MTMR9-specific perturbation |
All leads require curator verification.
ILIHGTEGTDSTLQ (~res 289–300) lacks the catalytic CX₅R cysteine (only CX₅R match CVWQILR@360 lies outside the P-loop) → inactive pseudophosphatase, consistent with human MTMR9.LIHGTEGTDSTLQVT in both species (both catalytically dead). Confirms A0A9L0T3C1 is a bona fide 1:1 MTMR9 ortholog and validates human→horse evidence transfer.IASS…LYSSA), i.e., one missing N-terminal segment near the PH-GRAM region — consistent with a missing exon in the predicted horse model. The PTP-like domain, P-loop and C-terminal coiled-coil are intact.Horse A0A9L0T3C1 is a genuine MTMR9 ortholog, and human evidence shows MTMR9 does contribute to negative regulation of autophagy — but indirectly, as a catalytically dead activating subunit of a PtdIns(3)P-depleting myotubularin complex, and in a context-dependent manner. GO:0010507 is a defensible inferred (ISS/ISO) annotation for horse MTMR9 if scoped as a complex-mediated, non-catalytic, partner-dependent regulatory role, complemented by more informative molecular-function terms and with clear caveats about the absence of horse-specific data and a competing ER-to-Golgi trafficking function.