GAT1

GAT1 is a Skp1-specific glycosyltransferase in Toxoplasma gondii that governs the assembly of Skp1/F-box protein subcomplexes through both catalytic and non-enzymatic activities (PMID:32414843, PMID:42327091). Enzymatically, GAT1 caps the Skp1 pentasaccharide by adding a terminal fifth sugar via a Galα1,3Glcα linkage onto the hydroxyproline-linked glycan, a modification that imposes an ordered conformation on the glycan and thereby controls Skp1 structure through nonpolar packing of the terminal galactose (PMID:32414843); loss of gat1 produces a slow-growth phenotype, marking this terminal galactose as functionally important for glycan-mediated regulation of Skp1 (PMID:32414843). Beyond catalysis, GAT1 forms a homodimer that binds SKP1 monomers and competes with F-box proteins and the SKP1 homodimer for the same core hydrophobic interface, promoting SKP1 monomerization in a manner dependent on SKP1 glycostate and its intrinsically disordered C-terminal region, thereby biasing F-box protein binding preference (PMID:42327091).

1. 2020 High

   Establishing which enzyme completes the Skp1 glycan and whether that final sugar matters resolved the terminal step of a conserved cytoplasmic Skp1 glycosylation pathway and its physiological relevance.

   Evidence Glycosyltransferase identification with in vitro kinetic assays, X-ray crystallography, and gat1Δ knockout phenotyping in Toxoplasma gondii

   PMID:32414843

   Open questions at the time

   • The mechanism linking loss of the terminal galactose to the slow-growth phenotype at the cellular level was not resolved
   • The full set of downstream F-box proteins affected by the modification was not defined
2. 2020 Medium

   Modeling how the completed pentasaccharide behaves addressed why the terminal sugar matters, proposing that the glycan adopts an ordered conformation controlling Skp1 structure and subcomplex assembly.

   Evidence Computational structural analysis compared with the Dictyostelium glycan, supported by the same study's crystallographic and biochemical data

   PMID:32414843

   Open questions at the time

   • The conformational control model is computationally inferred rather than directly observed in the assembled Skp1/F-box complex
   • Direct measurement of how glycan conformation alters F-box protein affinity was not performed
3. 2026 Medium

   Testing whether GAT1 acts only as an enzyme revealed a distinct non-enzymatic scaffolding role, in which GAT1 competitively controls SKP1 monomer availability and F-box protein binding preference.

   Evidence Sedimentation velocity analytical ultracentrifugation, mutational probing, computational modeling, and gat1-knockout F-box protein interactome profiling in Toxoplasma gondii (preprint)

   PMID:42327091

   Open questions at the time

   • Not yet peer-reviewed
   • The in vivo consequences of GAT1-driven SKP1 monomerization for SCF ubiquitin ligase output were not established
   • How the enzymatic and scaffolding activities are coordinated on the same substrate is unresolved

• How GAT1's dual glycosyltransferase and scaffolding functions are integrated to regulate SCF complex assembly and parasite growth in vivo remains open.
• No in vivo demonstration linking GAT1-controlled SKP1 monomerization to ubiquitin ligase function
• The relationship between glycostate and scaffolding activity under physiological conditions is undefined