See1 (Seedling efficient effector 1; UMAG_02239) is a small (157-residue precursor) secreted effector protein of the maize smut fungus Ustilago maydis (Mycosarcoma maydis) with an N-terminal secretion signal and no recognised catalytic domain. It is strongly expressed during biotrophic growth in seedling leaves, is secreted by hyphae into the biotrophic interface and is translocated into maize cells, where it accumulates in the cytoplasm and prominently in the nucleus. Inside host cells See1 interacts with the maize co-chaperone SGT1 and interferes with MAPK-triggered phosphorylation of SGT1 at a monocot-specific threonine. See1 acts organ-specifically: it is required for the reactivation of DNA synthesis in differentiated maize leaf cells and for the growth of large vegetative leaf tumours, but is dispensable for tumour formation in tassel floral tissue, where host cells are already proliferating. The route from SGT1 phosphorylation to host cell-cycle re-entry, and any contribution to suppression of host immunity, remain unresolved.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005576 extracellular region | EXP PMID:25888589 A Secreted Effector Protein of Ustilago maydis Guides Maize ... | ACCEPT | Summary: Fungus-delivered See1-3xHA was detected by immunogold electron microscopy at the biotrophic interface surrounding intracellular hyphae, i.e. outside the fungal cell, before entry into maize cells. Reason: Experimentally demonstrated secretion of a signal-peptide-bearing effector. The extracellular (interface) pool is a transit compartment rather than the site of action, but the location is correct. Supporting Evidence: PMID:25888589 See1-3xHA was detected in the fungal hyphae, at the biotrophic interface, in plant cytoplasm, and prominently inside plant cell nuclei |
| GO:0005576 extracellular region | IEA GO_REF:0000044 | ACCEPT | Summary: Mapping of the UniProt subcellular location "Secreted", itself experimentally supported by Redkar et al. (2015). Reason: Correct mapping, consistent with the experimental annotation. Supporting Evidence: PMID:25888589 only the two proteins with N-terminal secretion signals were found in the biotrophic interface |
| GO:0030430 host cell cytoplasm | EXP PMID:25888589 A Secreted Effector Protein of Ustilago maydis Guides Maize ... | ACCEPT | Summary: See1 delivered by the fungus was detected in maize cytoplasm by immunogold TEM, while a secreted mCherry control remained at the interface; signal-peptide-less See1 also localized to maize cytoplasm after bombardment, and BiFC placed the See1-SGT1 interaction in the cytoplasm. Reason: Direct localization of the natively delivered effector inside host cells; one of the two sites of action (with the host nucleus) where See1 meets SGT1. Supporting Evidence: PMID:25888589 Only See1-3xHA was quantitatively detected inside host cells, with ∼20% of particles localizing to maize nuclei PMID:25888589 indicating an interaction of See1 and SGT1 in the cytoplasm and nucleus of maize cells |
| GO:0030430 host cell cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: Mapping of the UniProt location "Host cytoplasm", experimentally supported. Reason: Correct mapping, consistent with the experimental annotation. Supporting Evidence: PMID:25888589 See1 22–157 -mCherry localized to both the maize cytoplasm and nuclei |
| GO:0042025 host cell nucleus | EXP PMID:25888589 A Secreted Effector Protein of Ustilago maydis Guides Maize ... | ACCEPT | Summary: About 20% of immunogold particles of fungus-delivered See1-3xHA inside host cells were in maize nuclei; bombarded See1 also entered nuclei and BiFC showed nuclear See1-SGT1 complexes. Reason: Direct, quantified localization of the natively delivered effector; host nucleus is a site of See1 action. Supporting Evidence: PMID:25888589 See1-3xHA was detected in the fungal hyphae, at the biotrophic interface, in plant cytoplasm, and prominently inside plant cell nuclei |
| GO:0042025 host cell nucleus | IEA GO_REF:0000044 | ACCEPT | Summary: Mapping of the UniProt location "Host nucleus", experimentally supported. Reason: Correct mapping, consistent with the experimental annotation. Supporting Evidence: PMID:25888589 Only See1-3xHA was quantitatively detected inside host cells, with ∼20% of particles localizing to maize nuclei |
| GO:0141017 effector-mediated induction of cell cycle reactivation in host | IDA PMID:25888589 A Secreted Effector Protein of Ustilago maydis Guides Maize ... | ACCEPT | Summary: Leaves infected with a see1 deletion strain show a strong defect in host DNA synthesis (EdU incorporation in about 7% of colonized cells versus about 68% with the progenitor strain, and about 44% for an unrelated small-tumour mutant), and form only small leaf tumours; complementation restores virulence. Constitutive see1 expression increases tumour formation and EdU labelling in vegetative tassel-base tissue. Reason: This is the core biological role of See1 and a symbiont-side effector process term (definition: a secreted symbiont molecule reactivates the host cell cycle, resulting in DNA synthesis and host cell division and contributing to vegetative tumour formation), which matches the experimental findings closely. Participation passes: See1 is itself delivered into the host cells whose DNA synthesis is reactivated and acts there on a host cell-cycle/immunity regulator (SGT1), rather than merely being required for fungal colonization; the Tin3 control argues against an indirect small-tumour effect. The precise downstream steps from SGT1 to cell-cycle re-entry are unresolved, but the term does not assert a mechanism. Supporting Evidence: PMID:25888589 See1 is required for the reactivation of plant DNA synthesis, which is crucial for tumor progression in leaf cells. PMID:25888589 Quantification of EdU labeling showed that 67.5% ± 4.2% of the maize cells colonized by SG200 incorporated EdU at 4 DPI PMID:25888589 maydis ip locus fully restored virulence, demonstrating functional complementation of see1 PMID:25888589 See1 interferes with the MAPK-triggered phosphorylation of maize SGT1 at a monocot-specific phosphorylation site. |
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Download this section (compressed HTML)Q: See1 binds SGT1 and prevents its MAPK-triggered Thr phosphorylation. Does See1 act by masking the phosphosite, by excluding the MAPK from SGT1, or by inhibiting the kinase? The answer would determine whether an informative molecular function term (e.g. a kinase or enzyme inhibitor/regulator activity) can replace the currently empty molecular function.
Q: The authors propose that See1 also dampens host immunity through SGT1. Is there direct evidence (e.g. altered defence gene expression or ROS in see1 mutants independent of tumour size) that would justify a symbiont-mediated suppression of host innate immunity annotation in addition to the cell-cycle term?
Experiment: Express phosphonull (T150A) and phosphomimic (T150D/E) ZmSGT1 variants in maize (or test them in planta during infection with wild-type and see1 deletion strains) to determine whether loss of SGT1 Thr phosphorylation is sufficient and necessary for See1-dependent leaf DNA synthesis.
Experiment: In vitro reconstitution with purified See1, ZmSGT1 and a maize MAPK (e.g. ZmMPK6) to test whether See1 directly inhibits SGT1 phosphorylation and to map the See1-binding region on SGT1.
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