The ubiquitin-conjugating enzymes, Ubc4 and Cdc34, mediate cadmium resistance in budding yeast through different mechanisms

[Cadmium induces p53-dependent apoptosis through the inhibition of Ube2d family gene expression]

Cadmium (Cd), a harmful metal, exerts severe toxic effects on various tissues such as those in the kidney, liver, lung, and bone. In particular, renal toxicity with damage to proximal tubule cells is caused by chronic exposure to Cd. However, the molecular mechanism underlying chronic Cd renal toxicity remains to be understood. In this review, we present our recent findings since we examined to search for the target molecules involved in the renal toxicity of Cd using toxicogenomics. In NRK-52E rat renal tubular epithelial cells, we found using DNA microarrays that Cd suppressed the expression of the gene encoding Ube2d4, a member of the Ube2d family. The Ube2d family consists of selective ubiquitin-conjugating enzymes associated with p53 degradation. Moreover, Cd suppressed the expressions of genes encoding all Ube2d family members (Ube2d1/2/3/4) prior to the appearance of cytotoxicity in NRK-52E cells. Cd markedly increased p53 protein level and induced p53 phosphorylation and apoptosis in the cells. In vivo studies showed that chronic Cd exposure also suppressed Ube2d family gene expression and induced p53 accumulation and apoptosis in the renal tubules of the mouse kidney. These findings suggest that Cd causes p53-dependent apoptosis due to the inhibition of p53 degradation through the down-regulation of Ube2d family genes in NRK-52E cells and mouse kidney. Thus, the Ube2d family genes may be one of the key targets of renal toxicity caused by Cd.

Global transcriptome profile of Cryptococcus neoformans during exposure to hydrogen peroxide induced oxidative stress

The ability of the opportunistic fungal pathogen Cryptococcus neoformans to resist oxidative stress is one of its most important virulence related traits. To cope with the deleterious effect of cellular damage caused by the oxidative burst inside the macrophages, C. neoformans has developed multilayered redundant molecular responses to neutralize the stress, to repair the damage and to eventually grow inside the hostile environment of the phagosome. We used microarray analysis of cells treated with hydrogen peroxide (H(2)O(2)) at multiple time points in a nutrient defined medium to identify a transcriptional signature associated with oxidative stress. We discovered that the composition of the medium in which fungal cells were grown and treated had a profound effect on their capacity to degrade exogenous H(2)O(2). We determined the kinetics of H(2)O(2) breakdown by growing yeast cells under different conditions and accordingly selected an appropriate media composition and range of time points for isolating RNA for hybridization. Microarray analysis revealed a robust transient transcriptional response and the intensity of the global response was consistent with the kinetics of H(2)O(2) breakdown by treated cells. Gene ontology analysis of differentially expressed genes related to oxidation-reduction, metabolic process and protein catabolic processes identified potential roles of mitochondrial function and protein ubiquitination in oxidative stress resistance. Interestingly, the metabolic pathway adaptation of C. neoformans to H(2)O(2) treatment was remarkably distinct from the response of other fungal organisms to oxidative stress. We also identified the induction of an antifungal drug resistance response upon the treatment of C. neoformans with H(2)O(2). These results highlight the complexity of the oxidative stress response and offer possible new avenues for improving our understanding of mechanisms of oxidative stress resistance in C. neoformans.

The essential Ubc4/Ubc5 function in yeast is HECT E3-dependent, and RING E3-dependent pathways require only monoubiquitin transfer by Ubc4

The ubiquitin (Ub)-conjugating enzymes Ubc4 and Ubc5 are involved in a variety of ubiquitination pathways in yeast, including Rsp5- and anaphase-promoting complex (APC)-mediated pathways. We have found the double deletion of UBC4 and UBC5 genes in yeast to be lethal. To investigate the essential pathway disrupted by the ubc4/ubc5 deletion, several point mutations were inserted in Ubc4. The Ubc4 active site mutation C86A and the E3-binding mutations A97D and F63A were both unable to rescue the lethal phenotype, indicating that an active E3/E2∼Ub complex is required for the essential function of Ubc4/Ubc5. A mutation that specifically eliminates RING E3-catalyzed isopeptide formation but not HECT E3 transthiolation (N78S-Ubc4) rescued the lethal phenotype. Thus, the essential redundant function performed by Ubc4 and Ubc5 in yeast is with a HECT-type E3, likely the only essential HECT in yeast, Rsp5. Our results also suggest that Ubc1 can weakly replace Ubc4 to transfer mono-Ub with APC, but Ubc4 cannot replace Ubc1 for poly-Ub chain extension on APC substrates. Finally, the backside Ub-binding mutant S23R-Ubc4 has no observable effect in yeast. Together, our results are consistent with a model in which Ubc4 and Ubc5 are 1) the primary E2s for Rsp5 in yeast and 2) act as monoubiquitinating E2s in RING E3-catalyzed pathways, in contrast to the processive human ortholog UbcH5.

Overexpression of the novel F-box protein Ymr258c confers resistance to methylmercury in Saccharomyces cerevisiae

We searched a yeast genomic library for genes that conferred methylmercury resistance to yeast. Ymr258c was identified as a factor that conferred strong methylmercury resistance when overexpressed in budding yeast. Ymr258c is regarded as one of the F-box proteins, i.e., component factors of the Skp1/Cullin/F-box protein (SCF) complex that functions as a ubiquitin ligase in the ubiquitin-proteasome system. In this study, Ymr258c was demonstrated to function as an F-box protein to reduce methylmercury toxicity. The overexpression of Ymr258c might promote the ubiquitination of proteins that are involved in the enhancement of methylmercury toxicity, and thereby promote their degradation by proteasomes to reduce methylmercury toxicity.