TRI5 (FGSG_03537) encodes trichodiene synthase (EC 4.2.3.6), a soluble class I sesquiterpene cyclase of the wheat head blight fungus Fusarium graminearum (teleomorph Gibberella zeae). Using Mg2+ bound by its conserved NSE/DTE motif, the enzyme ionizes and cyclizes (2E,6E)-farnesyl diphosphate to trichodiene plus diphosphate, the first committed step that diverts isoprenoid flux into trichothecene biosynthesis. Recombinant enzyme produces mainly trichodiene with minor related cyclization products. Trichodiene is subsequently oxygenated by the cytochrome P450 TRI4 and further modified by other TRI cluster enzymes to give the type B trichothecene deoxynivalenol (DON) and its acetylated derivatives in strain PH-1 (or nivalenol in other chemotypes). TRI5 is a cytosolic protein enriched next to the ER-derived toxisome where downstream, membrane-anchored TRI enzymes reside; independent of its catalytic activity, TRI5 protein is required for normal toxisome morphology and for production of several non-trichothecene sesquiterpenes such as culmorin. TRI5 expression is induced in infection structures on wheat, is controlled by the cluster regulators TRI6 and TRI10 and by a cis-acting promoter-derived long non-coding RNA. Because TRI5 deletion abolishes trichothecene production, tri5 mutants are widely used to show that DON acts as a virulence factor: they cause local lesions on inoculated wheat spikelets but fail to spread through the rachis, whereas virulence on barley, maize and Arabidopsis floral tissue is largely unaffected. The host-side effect is exerted by DON itself, an inhibitor of eukaryotic ribosomes, rather than by the TRI5 protein.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0016106 sesquiterpenoid biosynthetic process | IEA GO_REF:0000002 | MODIFY | Summary: InterPro2GO mapping from the ascomycete TRI5 family (IPR010458). TRI5 catalyses the cyclization of farnesyl diphosphate to trichodiene, the first step of trichothecene (sesquiterpenoid) biosynthesis, so the gene product directly performs a step of this process. Reason: Correct but broad. For the PH-1 protein (a DON-producing strain), the specific pathway is known: trichodiene is the precursor of deoxynivalenol (vomitoxin), and TRI5 deletion abolishes DON synthesis. GO:0106110 vomitoxin biosynthetic process is a descendant of GO:0016106 (via mycotoxin and sesquiterpenoid biosynthesis), and GO already applies it to the TRI5 gene product under another UniProt accession (A0A1I9FCV5, TAS from PMID:38877764). The family-level IEA remains appropriate for the InterPro mapping itself, since the family includes trichothecene producers of other chemotypes. Proposed replacements: vomitoxin biosynthetic process Supporting Evidence: PMID:30664933 The first step in DON synthesis is catalyzed by the sesquiterpene synthase (STS), Tri5 (trichodiene synthase), resulting in the cyclization of farnesyl diphosphate (FPP) to produce the sesquiterpene trichodiene. PMID:8589414 The disrupted gene, Tri5, encodes the enzyme trichodiene synthase, which catalyzes the first step in trichothecene biosynthesis. PMID:38877764 Deletion of TRI5 eliminates the ability of F. graminearum to synthesize DON |
| GO:0016838 carbon-oxygen lyase activity, acting on phosphates | IEA GO_REF:0000002 | ACCEPT | Summary: Trichodiene synthase (EC 4.2.3.6) is a carbon-oxygen lyase acting on phosphates (EC 4.2.3 subclass): it eliminates diphosphate from farnesyl diphosphate during cyclization. Reason: A correct, if general, parent of trichodiene synthase activity derived from the trichodiene synthase family signature. Broad IEA terms that are true are acceptable; the specific activity is captured by GO:0045482. Supporting Evidence: PMID:30664933 Tri5 was cloned and expressed in E. coli and shown to produce primarily trichodiene in addition to minor, related cyclization products. |
| GO:0045482 trichodiene synthase activity | IEA GO_REF:0000120 | ACCEPT | Summary: TRI5 catalyses (2E,6E)-farnesyl diphosphate = trichodiene + diphosphate (RHEA:12052, EC 4.2.3.6). Recombinant F. graminearum TRI5 produces primarily trichodiene, and mutation of the Mg2+-binding motif (N225D/S229T) abolishes activity. This is the core molecular function. Reason: Precise activity term, supported for this protein by heterologous expression and by loss of trichothecene production in deletion and catalytic-site mutants. Supporting Evidence: PMID:30664933 Tri5 was cloned and expressed in E. coli and shown to produce primarily trichodiene in addition to minor, related cyclization products. PMID:30664933 This shared precursor is produced by the cyclization of farnesyl pyrophosphate (FPP) by a sesquiterpene synthase (STS) called trichodiene synthase or Tri5 |
| GO:0045482 trichodiene synthase activity | IMP PMID:18179606 Fusarium graminearum gene deletion mutants map1 and tri5 rev... | ACCEPT | Summary: PHI-base annotation from a pathogenicity study of a PH-1 tri5 deletion mutant, which is unable to produce DON; the mutant causes only local eye-shaped lesions on wheat spikelets and fails to infect the rachis, but is fully pathogenic on Arabidopsis floral tissue. Reason: The loss of DON production in the mutant is consistent with loss of the first enzyme of the pathway, and the activity itself is well established biochemically (PMID:30664933). The paper's main findings concern virulence, which is an indirect consequence of the toxin's action on host ribosomes rather than a function of TRI5 protein; GOA appropriately did not attach a host-interaction process term to TRI5 from this study. Only the abstract is cached, so the curator's full reading is deferred to. Supporting Evidence: PMID:18179606 The tri5 mutant, which is unable to produce DON, exhibited reduced pathogenicity on wheat ears, causing only discrete eye-shaped lesions on spikelets which failed to infect the rachis. |
| GO:0005829 cytosol | TAS PMID:30664933 Expression of the Fusarium graminearum terpenome and involve... | NEW | Summary: TRI5 is a soluble cytosolic enzyme (no signal peptide or transmembrane segment), enriched in the cytosol between ER cisternae of the toxisome, adjacent to the ER-anchored downstream enzymes TRI1, TRI4 and TRI11. Reason: No cellular component annotation exists for TRI5. Its location is stated as established in the toxisome literature and is consistent with the absence of membrane-targeting features in the sequence. The toxisome is ER-derived, so an ER term would be inappropriate for the soluble enzyme. Supporting Evidence: PMID:30664933 While it is established that Tri5 is a cytosolic enzyme (22,55,71) it also is enriched in the vicinity of the toxisome, seemingly within the layers of cytosol separating stacks of ER cisternae |
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Download this section (compressed HTML)Q: Does TRI5 physically associate with the cytosolic faces of the ER-anchored TRI1, TRI4 and TRI11 enzymes or with toxisome membranes, and is this association what makes TRI5 protein (independent of catalysis) necessary for toxisome morphology and culmorin synthesis?
Q: Should a virulence or host-interaction process be attached to TRI5 at all, given that the host-side effect (ribosome inhibition, cell-to-cell invasion through the rachis) is exerted by DON rather than by the enzyme? GOA currently carries none, which seems correct.
Experiment: Proximity labelling (TurboID) or split-fluorescent-protein assays with TRI5 and the ER-anchored TRI1/TRI4/TRI11 under toxin-inducing conditions, comparing wild-type and the catalytically inactive N225D/S229T allele, to test the proposed scaffolding role of TRI5 at the toxisome.
Experiment: Chemical complementation of a PH-1 tri5 deletion with trichodiene or DON applied in planta at the rachis node to separate the toxin's contribution to spread from any other contribution of TRI5 protein.
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