ULP1

Toxoplasma gondii ULP1 is an active SUMO protease that couples desumoylation activity to the parasite's secretory pathway and lytic cycle (PMID:39345210, PMID:38352341, PMID:30005881). Recombinant TgUlp1 removes SUMO from a sumoylated substrate in vitro and functionally complements a yeast Ulp1 knockout, establishing it as a key enzyme in T. gondii sumoylation/desumoylation (PMID:30005881). The protein localizes to the Golgi apparatus, is structurally similar to the trafficking factor p115/Uso1, and physically associates with the conserved oligomeric Golgi (COG) complex (PMID:39345210, PMID:38352341); conditional depletion impairs microneme secretion, host cell invasion, intracellular replication, and egress, defining an essential role in the secretory pathway (PMID:39345210, PMID:38352341). TgUlp1 abundance and protease activity are set post-transcriptionally: the microRNA Tg-miR-60 restrains its expression and activity (PMID:30005881), and a natural antisense transcript, TgUlp1-NAT1, down-regulates TgUlp1 mRNA through the RNAi pathway in a TgDicer-dependent manner (PMID:33011210).

1. 2018 High

   Whether TgUlp1 is a functional SUMO protease was unknown; demonstrating in vitro desumoylation and yeast complementation established it as the enzyme driving T. gondii desumoylation.

   Evidence In vitro desumoylation assay with recombinant protein and functional complementation of a yeast Ulp1 knockout

   PMID:30005881

   Open questions at the time

   • Endogenous SUMO substrates in T. gondii not identified
   • Subcellular site of desumoylation not linked to enzymatic activity in this study
2. 2018 Medium

   How TgUlp1 levels are controlled was unknown; showing that Tg-miR-60 inhibition raises TgUlp1 expression, protease activity, and growth fitness established post-transcriptional regulation by a parasite microRNA.

   Evidence Transgenic epitope-tagged strain with miR-60 inhibitor transfection, enzymatic activity and growth assays

   PMID:30005881

   Open questions at the time

   • Direct binding of Tg-miR-60 to the TgUlp1 transcript not demonstrated
   • Physiological conditions triggering miR-60 regulation unknown
3. 2020 Medium

   A second layer of TgUlp1 control was defined by identifying a natural antisense transcript (TgUlp1-NAT1) that lowers TgUlp1 mRNA through the RNAi pathway, revealing Dicer-dependent locus-encoded regulation.

   Evidence Dual luciferase reporter assay, in vitro RNase III processing, and TgDicer-knockout strain analysis

   PMID:33011210

   Open questions at the time

   • Quantitative contribution of NAT1 to endogenous TgUlp1 protein levels not established
   • Interplay between NAT1 and Tg-miR-60 regulation unresolved
4. 2024 High

   The cellular role of TgUlp1 was undefined; Golgi localization, structural similarity to p115/Uso1, COG-complex interaction, and conditional depletion phenotypes established it as essential for the secretory pathway and the lytic cycle.

   Evidence Conditional knockdown with imaging, plaque/invasion/egress and microneme secretion assays, plus TurboID proximity labeling and co-IP/MS

   PMID:39345210 PMID:38352341

   Open questions at the time

   • Mechanistic link between SUMO protease activity and Golgi trafficking not established
   • Whether COG-complex components are desumoylation substrates unknown
   • How ULP1 depletion mechanistically blocks microneme secretion unresolved

• It remains unknown how TgUlp1's catalytic SUMO protease activity is mechanistically integrated with its Golgi/COG-associated trafficking function and its endogenous substrates.
• No endogenous SUMO substrate identified at the Golgi
• No structural model connecting protease and trafficking roles
• Catalytic-dead depletion complementation to separate the two functions not reported