UBA1

TgUba1 is a cytoplasmic E1-like ubiquitin-activating enzyme in Toxoplasma gondii that functions in the ubiquitin-like modification pathway centered on the modifier TgUrm1 (PMID:36142209). It physically interacts with TgUrm1 through its E1-like AD domain, which is required for the interaction (PMID:36142209), and it promotes TgUrm1 conjugation (urmylation) of target proteins specifically under oxidative stress, since loss of TgUba1 sharply reduces TgUrm1 conjugate formation upon diamide challenge (PMID:36142209). Genetically, TgUba1 is essential for parasite proliferation and for virulence in mice but is dispensable for host-cell invasion and egress (PMID:36142209). Proximity-based interaction mapping links TgUba1 to ubiquitin-like modification, anti-oxidative stress, and metabolic regulation processes (PMID:36142209). Beyond these findings, no further mechanistic detail — including direct catalytic characterization or defined urmylation substrates — has been characterized in the available corpus.

1. 2022 Medium

   Establishing where TgUba1 acts in the cell provided the first spatial context for its role in an activating-enzyme pathway.

   Evidence Subcellular localization experiment in T. gondii

   PMID:36142209

   Open questions at the time

   • No functional consequence tied specifically to cytoplasmic localization
   • No information on dynamic relocalization under stress
2. 2022 Medium

   Whether TgUba1 engages the ubiquitin-like modifier TgUrm1 and through which domain was resolved, defining it as the activating partner in an Urm1 conjugation system.

   Evidence Pulldown/co-IP with AD (E1-like) domain-deletion mapping in T. gondii

   PMID:36142209

   Open questions at the time

   • Direct catalytic (thioester/adenylation) activity toward TgUrm1 not demonstrated
   • Reciprocal or in vitro reconstitution of the interaction not shown
3. 2022 Medium

   The physiological output of TgUba1 was defined by showing it is required for TgUrm1 conjugation specifically under oxidative stress, connecting the enzyme to a stress-responsive modification.

   Evidence Western blot of TgUrm1 conjugate band in TgUba1 knockout parasites under diamide stimulation

   PMID:36142209

   Open questions at the time

   • Urmylation substrate proteins not identified
   • Biochemical mechanism of stress-triggered enhancement unresolved
4. 2022 Medium

   The importance of TgUba1 for the parasite lifecycle was established by genetic ablation, distinguishing which processes depend on it.

   Evidence CRISPR/Cas9 knockout with complementation, in vitro proliferation assays, and mouse virulence assay

   PMID:36142209

   Open questions at the time

   • Molecular basis linking loss of urmylation to the proliferation/virulence defect not established
   • No mechanistic explanation for why invasion and egress are unaffected
5. 2022 Low

   A candidate interactome was generated to place TgUba1 within broader cellular processes.

   Evidence Proximity-based (BioID-type) labeling with mass spectrometry in T. gondii

   PMID:36142209

   Open questions at the time

   • Individual interactors not independently validated
   • Functional relevance of the identified proteins to TgUba1 activity untested

• The direct enzymatic mechanism of TgUba1 and the identity of urmylated substrates driving proliferation and virulence remain unresolved.
• No in vitro reconstitution of TgUrm1 activation/conjugation
• No defined urmylation substrate set
• Causal link between urmylation loss and virulence attenuation undefined