SKP1

Toxoplasma SKP1 is the adaptor subunit of SCF-class E3 ubiquitin ligases whose engagement of F-box protein substrate receptors is gated by an O2-dependent post-translational modification cascade unique to this lineage (PMID:39307307). A prolyl 4-hydroxylase (TgPhyA/PHYa) hydroxylates a specific SKP1 proline in an O2-dependent reaction, and TgPhyA is tuned for activity at low O2, allowing SKP1 modification to report on ambient oxygen (PMID:22648409, PMID:32934009). The resulting hydroxyproline is elaborated into a pentasaccharide by sequential cytoplasmic glycosyltransferases — Gnt1 and PgtA add the first three sugars, Glt1 (GT32) adds a fourth glucose in a Glcα1,3Fuc linkage, and the glycogenin-related Gat1 caps the chain with a Galα1,3Glcα linkage — each step being functionally important for normal growth (PMID:26719340, PMID:28928220, PMID:32414843). This glycan controls F-box binding through two coupled conformational effects: it remodels the loop between the two α-helices of the F-box-combining site to open a more accessible interface (PMID:28928219), and it weakens SKP1 homodimerization mediated by electrostatic charge clusters in the disordered C-terminal region, freeing SKP1 monomers for F-box protein capture (PMID:40296701). The terminal enzyme GAT1 additionally acts non-enzymatically as a homodimer that competes for the same hydrophobic interface used by F-box proteins and the SKP1 homodimer, further shaping F-box-binding preference in a glycosylation- and CTR-dependent manner (PMID:42327091). Functionally, loss of PHYa-dependent glycosylation selectively depletes the F-box receptors FBXO13 and FBXO14 from the SKP1 interactome via proteasomal degradation while sparing FBXO1, linking O2 sensing to selective stabilization of distinct SCF substrate receptors (PMID:39307307). SKP1-associated SCF complexes carry out defined cellular roles: TgFBXO1 is an Inner Membrane Complex/daughter-cell scaffold effector required for replication (PMID:31348812), and TgFBXL2 forms an SCF complex associated with the COP9 signalosome that controls centrosome replication, daughter-cell development, and repression of sexual-commitment genes (PMID:38814984). In vivo, PHYa-dependent SKP1 modification is required for virulence and brain cyst formation and enables the parasite to survive IFNγ-mediated nutritional immunity through tryptophan scavenging under IDO induction (PMID:38857298).

1. 2012 High

   Establishing that an O2-dependent prolyl hydroxylase initiates SKP1 modification connected SKP1 processing to oxygen sensing and to a measurable growth requirement.

   Evidence Genetic disruption of TgphyA with growth phenotype, recombinant TgPhyA prolyl-hydroxylase assay, and cross-species complementation in Dictyostelium

   PMID:22648409

   Open questions at the time

   • Downstream glycan structure not yet defined at this stage
   • Consequence for SCF F-box binding not addressed
2. 2015 High

   Identifying the first three sugars and their transferases (Gnt1, PgtA) defined the pentasaccharide assembly pathway and showed the glycan is needed for growth.

   Evidence MS of glycoforms, glycosyltransferase assays in extracts, and knockout/complementation of gnt1 and pgtA

   PMID:26719340

   Open questions at the time

   • Terminal sugars and their enzymes not yet identified
   • Structural basis for growth requirement unresolved
3. 2017 High

   Structural and biophysical analysis revealed how the glycan opens the F-box-combining site, providing the mechanistic link between glycosylation and enhanced F-box protein binding.

   Evidence NMR spectroscopy and MD simulations validated by relaxation kinetics of the glycosylated F-box interface

   PMID:28928219

   Open questions at the time

   • Effect on specific endogenous F-box proteins not tested here
   • Role of homodimerization not yet integrated
4. 2017 High

   Characterizing Glt1 as the fourth-sugar transferase extended the assembly pathway and, via catalytic-dead rescue failure, showed the enzymatic product itself is required for normal growth.

   Evidence Recombinant Glt1 assay with [3H]UDP-glucose, NMR linkage determination, glt1 knockout, and catalytic-dead rescue

   PMID:28928220

   Open questions at the time

   • Terminal fifth sugar still uncharacterized
   • Substrate-receptor consequences not addressed
5. 2019 High

   Confirming TgFBXO1 as a SKP1-binding F-box protein assigned an SCF complex a concrete cellular role in daughter-cell scaffolding during replication.

   Evidence SKP1–FBXO1 Co-IP, live imaging/IFA localization, myristoylation and TgRab11b-dependence tests, and conditional knockdown growth phenotype

   PMID:31348812

   Open questions at the time

   • Ubiquitination substrates of the FBXO1 SCF not identified
   • Whether FBXO1 binding depends on SKP1 glycosylation not tested here
6. 2020 High

   Solving Gat1 as the glycogenin-related terminal transferase with a distinct linkage completed the pentasaccharide pathway and showed the capping sugar is functionally important.

   Evidence Glycosyltransferase assay, X-ray crystallography, kinetics, and gat1 knockout growth phenotype

   PMID:32414843

   Open questions at the time

   • Non-catalytic roles of Gat1 not yet examined at this stage
   • Link to specific F-box selection undefined
7. 2020 High

   Comparative enzymology established that TgPhyA is tuned to low O2 and is functionally distinct from animal PHDs, positioning the SKP1 pathway as a bespoke oxygen sensor.

   Evidence In vitro prolyl hydroxylase kinetics with full-length Skp1 substrates, cross-species substrate tests, and DdPhyA crystallography

   PMID:32934009

   Open questions at the time

   • In vivo O2 thresholds in the parasite not defined here
   • Downstream interactome consequences of O2 changes not yet measured
8. 2024 High

   Interactome profiling in PHYa-null parasites showed O2-dependent glycosylation selectively stabilizes specific F-box receptors, linking oxygen sensing to differential SCF assembly.

   Evidence Reciprocal interactome MS in PHYaΔ vs wild-type, proteasome inhibitor rescue, low-O2 phenocopy, and transcript controls

   PMID:39307307

   Open questions at the time

   • Why FBXO13/FBXO14 but not FBXO1 are affected not fully resolved
   • Substrates ubiquitinated by the stabilized SCFs not identified
9. 2024 High

   Genetic and infection studies placed the PHYa/SKP1 O2-sensing pathway in host immune evasion, showing it is required for virulence, cyst formation, and survival of IFNγ-induced nutritional immunity via tryptophan scavenging.

   Evidence TgPHYa knockout in ME49, mouse infection with IFNγ-KO and IDO-KO backgrounds, and intracellular survival assays in IFNγ-treated cells

   PMID:38857298

   Open questions at the time

   • The specific SCF substrates mediating tryptophan scavenging not identified
   • Whether other strains behave identically not tested
10. 2024 High

    Characterizing the TgFBXL2 SCF complex, its COP9-signalosome association, and depletion phenotypes assigned SKP1-based SCF ligases roles in centrosome/daughter-cell cycle control and repression of sexual commitment.

    Evidence Co-IP complex characterization, conditional depletion with centrosome/daughter-cell imaging, and RNAseq

    PMID:38814984

    Open questions at the time

    • Ubiquitination substrates of FBXL2 SCF not identified
    • Whether FBXL2 engagement depends on SKP1 glycosylation not tested
11. 2025 High

    Biophysical dissection of the SKP1 C-terminal region revealed that glycosylation frees SKP1 for F-box binding by weakening electrostatically driven homodimerization, adding a second conformational control mechanism.

    Evidence Sedimentation velocity AUC, CTR deletion/poly-serine substitution mutants, high-salt tests, and MD simulations

    PMID:40296701

    Open questions at the time

    • Direct competition with individual F-box proteins not measured
    • In vivo contribution of dimerization to F-box selection not established
12. 2026 Medium

    AUC and interface mutagenesis proposed a non-enzymatic role for GAT1 as a glycosylation-sensitive competitor at the F-box interface, refining how SKP1's binding preference is set.

    Evidence Sedimentation velocity AUC, computational modeling, hydrophobic-interface mutants, and gat1-knockout interactome (preprint)

    PMID:42327091

    Open questions at the time

    • Preprint, single lab, not yet peer-reviewed
    • In vivo relevance of GAT1-driven monomerization to specific F-box selection not demonstrated

• The direct ubiquitination substrates of the SKP1-based SCF complexes and how O2-tuned F-box selection reshapes the parasite proteome during replication and stage conversion remain unresolved.
• No SCF substrate targets identified for any F-box receptor in the corpus
• Molecular basis for differential FBXO13/14 vs FBXO1 sensitivity to glycosylation state incomplete