FBXL2

TgFBXL2 is a Toxoplasma gondii F-box protein that couples SCF-type ubiquitin ligase activity to the control of developmental gene expression and cell division (PMID:38814984). It functions as the substrate-recognition subunit of a stably assembled SCF-E3 ubiquitin ligase complex (SKP1–Cullin-1–F-box) that recruits substrates for poly-ubiquitination, and this complex is further associated with the COP9 signalosome, a known negative regulator of SCF-E3 activity (PMID:38814984). The protein concentrates at distinct perinucleolar sites within the tachyzoite nucleus (PMID:38814984). Conditional depletion establishes two essential roles: TgFBXL2 represses sexual-stage (feline-restricted) gene programs in tachyzoites, since its loss de-represses a large cohort of sexual-commitment genes, and it is required for centrosome replication and daughter cell development (PMID:38814984). The direct ubiquitination substrates through which TgFBXL2 exerts transcriptional repression and cell-division control have not been identified in the available corpus.

1. 2024 Medium

   Establishing where TgFBXL2 acts pinpointed a nuclear, perinucleolar site of function, framing it as a candidate regulator of nuclear/transcriptional processes rather than a cytoplasmic factor.

   Evidence Subcellular localization in T. gondii tachyzoites

   PMID:38814984

   Open questions at the time

   • Localization does not define what TgFBXL2 does at perinucleolar sites
   • Mechanism of nuclear/perinucleolar targeting unknown
2. 2024 Medium

   Biochemical isolation showed TgFBXL2 is stably engaged in an SKP1–Cullin-1–F-box SCF complex, defining it as a substrate-recruiting subunit of an E3 ubiquitin ligase.

   Evidence Co-immunoprecipitation / biochemical complex isolation in T. gondii

   PMID:38814984

   Open questions at the time

   • No ubiquitination substrate identified
   • Ligase activity not reconstituted in vitro
3. 2024 Medium

   Detection of COP9 signalosome association with the TgFBXL2 SCF complex identified a potential regulatory brake on its ubiquitination activity.

   Evidence Co-immunoprecipitation / biochemical complex isolation in T. gondii

   PMID:38814984

   Open questions at the time

   • Functional consequence of COP9 signalosome association not tested
   • Whether the signalosome activates or restrains TgFBXL2 in vivo unresolved
4. 2024 Medium

   Conditional depletion revealed that TgFBXL2 is required for centrosome replication and daughter cell development, linking this F-box protein to the parasite cell-division machinery.

   Evidence Conditional knockdown with phenotypic readout of centrosome and daughter cell morphology in T. gondii

   PMID:38814984

   Open questions at the time

   • Substrate driving the centrosome phenotype unknown
   • Whether the division defect is direct or secondary to transcriptional derepression not resolved
5. 2024 High

   Depletion coupled to transcriptome profiling showed loss of TgFBXL2 de-represses sexual-stage (feline-restricted) genes, establishing it as a repressor of the sexual-commitment program in tachyzoites.

   Evidence Conditional knockdown followed by RNAseq transcriptome profiling in T. gondii

   PMID:38814984

   Open questions at the time

   • Repression mechanism (direct ubiquitination of a transcription factor vs indirect) not defined
   • Ubiquitination substrate mediating repression unidentified

• The direct poly-ubiquitination substrate(s) of the TgFBXL2 SCF complex, and how substrate turnover mechanistically links to sexual-stage gene repression and centrosome replication, remain unknown.
• No substrate identified
• No mechanistic link established between ubiquitination activity and the transcriptional or division phenotypes