CMU1 (UMAG_05731) encodes a secreted chorismate mutase (EC 5.4.99.5) of the maize smut fungus Ustilago maydis (Mycosarcoma maydis). The 290-residue precursor carries an N-terminal signal peptide and an AroQ-class eukaryotic chorismate mutase fold, and forms homodimers. Unlike housekeeping chorismate mutases it is not allosterically regulated by tryptophan or tyrosine. CMU1 is strongly induced during biotrophic growth, is secreted by hyphae into the maize apoplast, is taken up by plant cells and spreads to neighbouring cells, where it acts in the host cytosol. There it converts chorismate to prephenate and heterodimerizes with host cytosolic chorismate mutase, which is thought to pull chorismate out of plastids and away from isochorismate-dependent salicylic acid synthesis, increasing phenylpropanoid flux. Maize infected with a cmu1 deletion strain accumulates about ten times more salicylic acid, and the mutant forms fewer tumours. The maize kiwellin ZmKWL1 binds an extended loop region specific to fungal secreted chorismate mutases and inhibits CMU1 catalytic activity, acting as a host counter-defence.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004106 chorismate mutase activity | EXP PMID:21976020 Metabolic priming by a secreted fungal effector. | ACCEPT | Summary: Djamei et al. (2011) characterized Cmu1 as an active secreted chorismate mutase; UniProt records catalytic activity and active-site mutations (R183A/K194A abolish activity) from this paper. Reason: Chorismate-to-prephenate isomerization is the enzyme's own catalysis and the molecular basis of its virulence function. Only the abstract is cached, but the activity is confirmed independently by the 2019 kinetic study and the curator read the full text. Supporting Evidence: PMID:21976020 Here we show that the chorismate mutase Cmu1 secreted by U. maydis is a virulence factor. PMID:21976020 Chorismate mutases are key enzymes of the shikimate pathway and catalyse the conversion of chorismate to prephenate |
| GO:0004106 chorismate mutase activity | IDA PMID:30651637 A kiwellin disarms the metabolic activity of a secreted fung... | ACCEPT | Summary: Han et al. (2019) measured Cmu1 chorismate mutase activity in vitro (UniProt: KM 0.8 mM, Vmax 40.8 umol/min/mg) and showed that maize kiwellin ZmKWL1 blocks it. Reason: Direct enzymatic assay of the purified protein; core molecular function. Supporting Evidence: PMID:30651637 one of the 20 maize-encoded kiwellins (ZmKWL1) specifically blocks the catalytic activity of Cmu1 |
| GO:0004106 chorismate mutase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Combined electronic annotation (InterPro AroQ eukaryotic CM superfamily, Rhea, EC 5.4.99.5). Reason: Consistent with the experimental annotations; the EC/Rhea mapping is correct for this enzyme. Supporting Evidence: PMID:30651637 specifically blocks the catalytic activity of Cmu1 |
| GO:0005576 extracellular region | EXP PMID:21976020 Metabolic priming by a secreted fungal effector. | ACCEPT | Summary: Cmu1 is secreted by U. maydis hyphae during biotrophic growth before uptake into maize cells. Reason: Signal-peptide-bearing secreted protein experimentally shown to be secreted (UniProt Secreted, ECO:0000269 from this paper). Supporting Evidence: PMID:21976020 the chorismate mutase Cmu1 secreted by U. maydis is a virulence factor |
| GO:0005576 extracellular region | IEA GO_REF:0000044 | ACCEPT | Summary: Mapping of the UniProt subcellular location "Secreted", itself experimentally supported. Reason: Correct mapping, consistent with the experimental annotation. Supporting Evidence: PMID:21976020 the chorismate mutase Cmu1 secreted by U. maydis is a virulence factor |
| GO:0009073 aromatic amino acid biosynthetic process | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: InterPro2GO mapping from the eukaryotic AroQ chorismate mutase superfamily, which is defined by housekeeping enzymes of the Phe/Tyr branch of the shikimate pathway. Reason: The domain-based inference assumes the housekeeping role of intracellular chorismate mutases. CMU1 is a secreted paralogue that acts in the host cytosol, is insensitive to the Trp/Tyr feedback that regulates biosynthetic chorismate mutases, and functions to redirect host chorismate away from salicylic acid synthesis, not to supply aromatic amino acids to the fungus. Its prephenate product does enter host phenylpropanoid flux, so the term is not strictly wrong, but it misrepresents the gene's biological role. Supporting Evidence: PMID:21976020 proteins unlikely to act in the fungal-host interface like a chorismate mutase PMID:34414650 These results suggest that Cmu1 increases the virulence of U. maydis by directing the metabolite flow into the phenylpropanoid pathway, reducing SA production. |
| GO:0044164 host cell cytosol | IEA GO_REF:0000044 | ACCEPT | Summary: Mapping of the UniProt location "Host cytoplasm, host cytosol", experimentally supported by Djamei et al. (2011). Reason: Cmu1 is taken up into plant cells and acts in the host cytosol together with host cytosolic chorismate mutase; the mapping is correct. Supporting Evidence: PMID:34414650 Cmu1 is secreted by U. maydis to the plant cytosol and nucleus, interacts with plant CMs, and is needed for full virulence of the pathogen. |
| GO:0044164 host cell cytosol | TAS PMID:30651637 A kiwellin disarms the metabolic activity of a secreted fung... | ACCEPT | Summary: PHI-base traceable author statement that Cmu1 acts in the host cytosol. Reason: The primary localization evidence is Djamei et al. (2011); the 2019 paper restates it in the context of Cmu1 channelling host chorismate. Location is correct and is the site of action. Supporting Evidence: PMID:21976020 The enzyme is taken up by plant cells, can spread to neighbouring cells PMID:30651637 presumably affects biosynthesis of the plant immune signal salicylic acid by channelling chorismate into the phenylpropanoid pathway |
| GO:0046417 chorismate metabolic process | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: InterPro2GO mapping from the chorismate mutase II superfamily. Reason: Accurate in the literal sense, since CMU1 consumes chorismate, but it is a general metabolic term that adds nothing beyond the activity annotation and does not capture the host-manipulation role. Supporting Evidence: PMID:21976020 catalyse the conversion of chorismate to prephenate |
| GO:0140502 effector-mediated suppression of host salicylic acid-mediated innate immune signaling | IDA PMID:30651637 A kiwellin disarms the metabolic activity of a secreted fung... | ACCEPT | Summary: Cmu1 lowers host salicylic acid: maize infected with a cmu1 deletion strain accumulates about tenfold more SA, and the enzyme is proposed to divert chorismate from plastidic SA synthesis. ZmKWL1 inhibition of Cmu1 activity (Han et al. 2019) reinforces that the enzymatic activity is the virulence mechanism. Reason: A symbiont-side effector term, appropriately used instead of a plant-side defense term. Participation passes: CMU1 itself catalyses the step (chorismate to prephenate) that drains the SA precursor pool, so it does work in the process rather than being required for or acted upon by it. Strictly the documented effect is on host SA accumulation (biosynthesis) rather than on SA signalling components, and GO has no symbiont-side term for suppression of host SA biosynthesis, so this is the closest available term. The primary SA data are from Djamei et al. (2011); the PHI-base curator read the 2019 full text. Supporting Evidence: PMID:30651637 The secreted fungal chorismate mutase Cmu1 presumably affects biosynthesis of the plant immune signal salicylic acid by channelling chorismate into the phenylpropanoid pathway PMID:34414650 Infecting plants with a Cmu1 deletion mutant of U. maydis resulted in a 10‐fold increase of SA compared to infection with the wild type (Djamei et al., 2011). PMID:34414650 It was proposed that Cmu1 acts in conjunction with a cytosolic plant CM, thereby extracting more chorismate from the plastids, leading to lower substrate availability for plastidic SA biosynthesis. file:MYCMD/CMU1/CMU1-deep-research-falcon.md Cmu1 is an enzyme that affects SA accumulation, not an SA receptor or a demonstrated direct modifier of SA-signalling proteins. |
| GO:0140593 host apoplast | TAS PMID:30651637 A kiwellin disarms the metabolic activity of a secreted fung... | ACCEPT | Summary: PHI-base author statement placing Cmu1 in the maize apoplast, where it is secreted from hyphae and where the secreted kiwellin ZmKWL1 is proposed to intercept it before host-cell entry. Reason: A secreted effector of an intercellularly growing biotroph necessarily transits the host apoplast before uptake into plant cells, and the 2019 model of kiwellin interception places the Cmu1-ZmKWL1 encounter there. More specific than extracellular region and correct for the pre-translocation pool. Supporting Evidence: PMID:30651637 ZmKWL1 hinders substrate access to the active site of Cmu1 file:MYCMD/CMU1/CMU1-deep-research-falcon.md ZmKWL1 has a secretion signal and can inhibit Cmu1, supporting a model in which it captures Cmu1 in the apoplast before host-cell entry. |
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Download this section (compressed HTML)Q: CMU1 and other secreted chorismate mutases and isochorismatases (e.g. Verticillium VdIsc1, Phytophthora PsIsc1, nematode Mi-CM-3) lower host salicylic acid by acting on its biosynthetic precursors, not on SA signalling components. Should GO add a symbiont-side term such as "symbiont-mediated suppression of host salicylic acid biosynthetic process" so these enzymes are not annotated to a signalling term?
Q: Is the heterodimer with host cytosolic chorismate mutase ZmCM2 required for the virulence function, or does CMU1 catalysis alone suffice? This determines whether a modulation-of-host-enzyme activity term is warranted in addition to the catalytic activity.
Experiment: Isotope-labelled chorismate or shikimate flux tracing in maize leaves infected with wild-type, cmu1 deletion and catalytically dead (R183A/K194A) strains, to measure directly the partitioning of chorismate between prephenate/phenylpropanoids and isochorismate/salicylic acid.
Experiment: Complement the cmu1 deletion with a CMU1 variant lacking the extended loop region (no ZmKWL1 binding but catalytically active) and test virulence on wild-type and ZmKWL1-silenced maize, to separate escape from kiwellin inhibition from intrinsic catalytic contribution.
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