Glycoregulation of E3(SCF) ubiquitin ligases in unicellular eukaryotes

Skp1 is an essential adaptor within the Skp1/Cul1/F-box (SCF) class of E3 polyubiquitin ligases that regulate protein degradation in all eukaryotes. Skp1 is also a target of a 5-enzyme glycosylation pathway in parasites and other unicellular eukaryotes. Glycosylation of Skp1 is contingent upon oxygen-dependent hydroxylation of a critical Pro residue by a homolog of the HIFα PHD2 oxygen sensor of animals. The resulting hydroxyproline is modified by a series of soluble, cytoplasmic, sugar nucleotide-dependent glycosyltransferases that vary among branches of protist evolution, and are evolutionarily related to counterparts in the Golgi and the cytoplasm of prokaryotes. Pair-wise gene fusions of the six enzymes occur in various protists, suggesting processing efficiency. The terminal glycosyltransferases exhibit a second site interaction with Skp1 that may modulate its function irrespective of glycosylation status. The pentasaccharide adopts a constrained fold that in turn promotes Skp1 conformations that inhibit sequestration by homodimerization and encourage binding to select F-box protein substrate receptors with varied effects on their expression levels. The occurrence of a second Skp1 copy in some protists that is resistant to modification indicates a mechanism to bypass glycoregulation. This review details evidence from the social amoeba Dictyostelium discoideum and the pathogens Toxoplasma gondii and Pythium ultimum for the specificity of the enzymes for Skp1 and their regulation, as support for a role in regulating protein turnover via E3(SCF) ubiquitin ligases, and in turn sensing oxygen at the cellular level.