Oxygen-dependent regulation of F-box proteins in Toxoplasma gondii is mediated by Skp1 glycosylation

A dynamic proteome is required for cellular adaption to changing environments including levels of O₂, and the SKP1/CULLIN-1/F-box protein/RBX1 (SCF) family of E3 ubiquitin ligases contributes importantly to proteasome-mediated degradation. We examine, in the apicomplexan parasite Toxoplasma gondii, the influence on the interactome of SKP1 by its novel glycan attached to hydroxyproline generated by PHYa, the likely ortholog of the HIFα PHD2 oxygen-sensor of human host cells. Strikingly, the representation of several putative F-box proteins (FBPs) is substantially reduced in PHYaΔ parasites grown in fibroblasts. One, FBXO13, is a predicted lysyl hydroxylase related to the human JmjD6 oncogene except for its F-box domain. The abundance of FBXO13, epitope-tagged at its genetic locus, was reduced in PHYaΔ parasites thus explaining its diminished presence in the SKP1 interactome. A similar effect was observed for FBXO14, a cytoplasmic protein of unknown function that may have co-evolved with PHYa in apicomplexans. Similar findings in glycosylation-mutant cells, rescue by proteasomal inhibitors, and unchanged transcript levels suggested the involvement of the SCF in their degradation. The effect was selective because FBXO1 was not affected by loss of PHYa. These findings are physiologically significant because the effects were phenocopied in parasites reared at 0.5% O₂. Modest impact on steady-state SKP1 modification levels suggests that effects are mediated during a lag phase in hydroxylation of nascent SKP1. The dependence of FBP abundance on O₂-dependent SKP1 modification likely contributes to the reduced virulence of PHYaΔ parasites owing to impaired ability to sense O₂ as an environmental signal.