mtl3 (systematic name SPBC215.13; UniProt O94317) is a Schizosaccharomyces-specific, GPI-anchored, serine/threonine-rich plasma-membrane glycoprotein of the fungal Mid2-like cell-wall stress-sensor family (the name Mtl denotes "Mid Two-Like"; it is distinct from and unrelated to the Mtr4-like nuclear RNA helicase, which in S. pombe is the separately named mtl1/SPAC17H9.02). The 534-residue precursor has the canonical architecture of a single-pass fungal cell-surface sensor: an N-terminal secretion signal peptide and a single transmembrane segment, a long serine/threonine-rich, O-mannosylation-prone extracellular ectodomain that is intrinsically disordered/low-complexity, N-linked glycosylation, and a C-terminal GPI-anchor addition signal that tethers the mature protein to the outer face of the plasma membrane. It carries no enzymatic domain of any kind (no helicase, ATPase/P-loop, or other catalytic module). Genome-wide deletion phenotyping shows mtl3 loss alters sensitivity and resistance to cell-wall-perturbing and ionic stresses (e.g. altered responses to SDS combined with salts, ethanol, lithium, and oxidants) and reduces viability in stationary phase, consistent with a role at the cell surface in sensing envelope/membrane stress. By family membership its inferred role is to transmit cell-wall/membrane stress to intracellular signaling (as for the related fission-yeast sensors Wsc1 and Mtl2, which activate the Rho1 GTPase), but a direct molecular activity, ligand, signaling partner, or pathway output has not been experimentally established for mtl3 itself.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005783 endoplasmic reticulum | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: ER localization is supported by the systematic GFP-localization dataset cited by UniProt (ECO:0000269|PubMed:16823372) and is plausible as a secretory-pathway transit compartment for a signal-peptide-bearing, GPI-anchored membrane protein en route to the cell surface. It is not the primary functional site of the protein, which is the plasma membrane. Supporting Evidence: PMID:16823372 ORFeome cloning and global analysis of protein localization |
| GO:0005886 plasma membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Plasma-membrane localization is consistent with the protein's architecture (signal peptide, single transmembrane segment, and C-terminal GPI-anchor addition signal) and with its predicted GPI-anchored cell-surface status. This is a core localization, although the more specific term GO:0009897 (external side of plasma membrane) better captures where the GPI-anchored ectodomain resides. |
| GO:0003674 molecular_function | ND GO_REF:0000015 | KEEP AS NON CORE | Summary: No molecular function has been experimentally determined for mtl3, so the ND (No Data) root annotation is an honest and appropriate reflection of the current state of knowledge. Although family membership (a Mid2-like cell-wall sensor) suggests a stress-sensing/signal-transducing role, there is no direct assay of ligand binding, signaling output, or any catalytic activity for this protein, and its sequence lacks any enzymatic domain. The unknown molecular function is recorded as a knowledge gap rather than filled by an unsupported specific term. |
| GO:0008150 biological_process | ND GO_REF:0000015 | KEEP AS NON CORE | Summary: No biological process has been directly demonstrated for mtl3, so the ND root annotation is appropriate. Deletion phenotypes (altered sensitivity/resistance to cell-wall/membrane and ionic stressors; reduced stationary-phase viability) and membership in the Mid2-like sensor family point toward a role in cell-wall-integrity / stress signaling, but this is inferred, not established. A specific BP term should not be asserted from the current evidence; the gap is recorded below. |
| GO:0009897 external side of plasma membrane | TAS PMID:12845604 Genome-wide identification of fungal GPI proteins. | ACCEPT | Summary: This is the most specific and best-supported localization: a GPI-anchored, serine/threonine-rich ectodomain displayed on the outer (external) face of the plasma membrane, exactly as expected for a fungal cell-wall stress sensor. Supported by the genome-wide GPI-protein prediction study and consistent with the UniProt GPI-anchor and topology features. Supporting Evidence: PMID:12845604 only 33 GPI candidates were identified |
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Download this section (compressed HTML)Q: Does mtl3 function as a cell-wall/membrane stress sensor that activates Rho1 and/or the Pmk1 cell-wall-integrity MAP kinase pathway, and is this activity redundant with wsc1 and mtl2?
Q: What cue does the serine/threonine-rich, O-mannosylated ectodomain of mtl3 sense, and does the cytoplasmic/juxtamembrane region recruit any GEF or signaling partner?
Q: Why is mtl3 mRNA a target of the RNA-binding protein zfs1, and does this post-transcriptional control couple mtl3 levels to a specific stress condition?
Experiment: Construct single and combinatorial deletions (mtl3-delta alone and with wsc1-delta, mtl2-delta, mid2-delta) and assay growth and morphology under cell-wall/membrane stress (SDS, caspofungin/echinocandins, calcofluor, sorbitol osmotic support), scoring for synthetic phenotypes that reveal redundancy within the sensor family.
Experiment: Measure Rho1-GTP loading and Pmk1 MAPK phosphorylation in mtl3-delta versus wild type under cell-wall stress to test whether mtl3 feeds the Rho1/CWI signaling axis.
Experiment: Perform proximity-labeling or affinity-purification mass spectrometry on tagged mtl3 to identify cell-surface and cytoplasmic interaction partners, and biochemically confirm the predicted GPI anchor and O-mannosylated ectodomain.
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: mtl3 has no experimentally established molecular function, no identified binding partner or ligand, and no directly demonstrated biological process. Its assignment to the Mid2-like cell-wall stress-sensor family and its inferred role in transmitting cell-wall/membrane stress to intracellular signaling (e.g. via Rho1, as shown for the related sensors Wsc1 and Mtl2) are extrapolations from family membership, sequence architecture, and deletion phenotypes, not from direct study of the mtl3 protein.
OPEN BIOLOGY WHOLLY_DARK
What is known: Firmly established: subcellular localization (GPI-anchored, external face of the plasma membrane; ER in transit); protein architecture (signal peptide, single N-terminal transmembrane segment, long serine/threonine-rich disordered/low-complexity ectodomain, N-glycosylation, C-terminal GPI-anchor signal; no enzymatic domain); genome-wide deletion phenotypes (altered sensitivity/resistance to cell-wall/membrane and ionic stressors, reduced stationary-phase viability); post-transcriptional regulation of its mRNA (a zfs1 target); and a single S. japonicus ortholog (SJAG_00415). The gene is Schizosaccharomyces- specific.
Significance: Fission yeast maintains cell-wall integrity through a set of cell-surface sensors, but the specific contribution, redundancy, and signaling wiring of mtl3 within this system are unknown. mtl3 is a genuinely understudied ("dark") gene whose only high-confidence GO statement is its cell-surface localization; both its molecular function and biological process remain at the ND root.
What would resolve it: Direct functional study of mtl3: epistasis/redundancy analysis with wsc1, mtl2, mid2 and the Rho1/Pmk1 CWI machinery; sensor-activation assays under defined cell-wall/membrane stress; identification of interacting proteins; and structural/biochemical confirmation of the ectodomain modifications. Any positive result would move both the MF and BP aspects off the ND root.
Provenance (the field's own admissions):
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