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Zhang D.,BRC 417 | Zhang D.,Fujian Agriculture and forestry University | Van Der Wel H.,BRC 417 | Johnson J.M.,The University of Oklahoma Health Sciences Center | And 2 more authors.
Journal of Biological Chemistry | Year: 2012

Cytoplasmic prolyl 4-hydroxylases (PHDs) have a primary role in O 2 sensing in animals via modification of the transcriptional factor subunit HIFα, resulting in its polyubiquitination by the E3VHLubiquitin (Ub) ligase and degradation in the 26 S proteasome. Previously thought to be restricted to animals, a homolog (P4H1) of HIFα-type PHDs is expressed in the social amoeba Dictyostelium where it also exhibits characteristics of an O 2 sensor for development. Dictyostelium lacks HIFα, and P4H1 modifies a different protein, Skp1, an adaptor of the SCF class of E3-Ub ligases related to the E3 VHLUb ligase that targets animal HIFα. Normally, the HO-Skp1 product of the P4H1 reaction is capped by a GlcNAc sugar that can be subsequently extended to a pentasaccharide by novel glycosyltransferases. To analyze the role of glycosylation, the Skp1 GlcNAc-transferase locus gnt1 was modified with a missense mutation to block catalysis or a stop codon to truncate the protein. Despite the accumulation of the hydroxylated form of Skp1, Skp1 was not destabilized based on metabolic labeling. However, hydroxylation alone allowed for partial correction of the high O 2 requirement of P4H1-null cells, therefore revealing both glycosylation-independent and glycosylation-dependent roles for hydroxylation. Genetic complementation of the latter function required an enzymatically active form of Gnt1. Because the effect of the gnt1 deficiency depended on P4H1, and Skp1 was the only protein labeled when the GlcNAc-transferase was restored to mutant extracts, Skp1 apparently mediates the cellular functions of both P4H1 and Gnt1. Although Skp1 stability itself is not affected by hydroxylation, its modification may affect the stability of targets of Skp1-dependent Ub ligases. © 2012 by The American Society for Biochemistry and Molecular Biology, Inc. Source

Xu Y.,BRC 417 | Brown K.M.,The University of Oklahoma Health Sciences Center | Wang Z.A.,BRC 417 | Van Der Wel H.,BRC 417 | And 4 more authors.
Journal of Biological Chemistry | Year: 2012

In diverse types of organisms, cellular hypoxic responses are mediated by prolyl 4-hydroxylases that use O2 and α-ketoglutarate as substrates to hydroxylate conserved proline residues in target proteins. Whereas in metazoans these enzymes control the stability of the HIFα family of transcription factor subunits, the Dictyostelium enzyme (DdPhyA) contributes to O2 regulation of development by a divergent mechanism involving hydroxylation and subsequent glycosylation of DdSkp1, an adaptor subunit in E3SCF ubiquitin ligases. Sequences related to DdPhyA, DdSkp1, and the glycosyltransferases that cap Skp1 hydroxyproline occur also in the genomes of Toxoplasma and other protists, suggesting that this O2 sensing mechanism may be widespread. Here we show by disruption of the TgphyA locus that this enzyme is required for Skp1 glycosylation in Toxoplasma and that disrupted parasites grow slowly at physiological O2 levels. Conservation of cellular function was tested by expression of TgPhyA in DdphyA-null cells. Simple gene replacement did not rescue Skp1 glycosylation, whereas overexpression not only corrected Skp1 modification but also restored the O 2 requirement to a level comparable to that of overexpressed DdPhyA. Bacterially expressed TgPhyA protein can prolyl hydroxylate both Toxoplasma and Dictyostelium Skp1s. Kinetic analyses showed that TgPhyA has similar properties to DdPhyA, including a superimposable dependence on the concentration of its co-substrate α-ketoglutarate. Remarkably, however, TgPhyA had a significantly higher apparent affinity for O2. The findings suggest that Skp1 hydroxylation by PhyA is a conserved process among protists and that this biochemical pathway may indirectly sense O2 by detecting the levels of O2-regulated metabolites such as α-ketoglutarate. © 2012 by The American Society for Biochemistry and Molecular Biology, Inc. Source

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