Overexpression of Bop3 confers resistance to methylmercury in Saccharomyces cerevisiae through interaction with other proteins such as Fkh1, Rts1, and Msn2

An account of fungal 14-3-3 proteins

14-3-3s are a group of relatively low molecular weight, acidic, dimeric, protein(s) conserved from single-celled yeast to multicellular vertebrates including humans. Despite lacking catalytic activity, these proteins have been shown to be involved in multiple cellular processes. Apart from their role in normal cellular physiology, recently these proteins have been implicated in various medical consequences. In this present review, fungal 14-3-3 protein localization, interactions, transcription, regulation, their role in the diverse cellular process including DNA duplication, cell cycle, protein trafficking or secretion, apoptosis, autophagy, cell viability under stress, gene expression, spindle positioning, role in carbon metabolism have been discussed. In the end, I also highlighted various roles of yeasts 14-3-3 proteins in tabular form. Thus this review with primary emphasis on yeast will help in appreciating the significance of 14-3-3 proteins in cell physiology.

The protein transportation pathway from Golgi to vacuoles via endosomes plays a role in enhancement of methylmercury toxicity

Methylmercury causes serious damage to the central nervous system, but the molecular mechanisms of methylmercury toxicity are only marginally understood. In this study, we used a gene-deletion mutant library of budding yeast to conduct genome-wide screening for gene knockouts affecting the sensitivity of methylmercury toxicity. We successfully identified 31 genes whose deletions confer resistance to methylmercury in yeast, and 18 genes whose deletions confer hypersensitivity to methylmercury. Yeast genes whose deletions conferred resistance to methylmercury included many gene encoding factors involved in protein transport to vacuoles. Detailed examination of the relationship between the factors involved in this transport system and methylmercury toxicity revealed that mutants with loss of the factors involved in the transportation pathway from the trans-Golgi network (TGN) to the endosome, protein uptake into the endosome, and endosome-vacuole fusion showed higher methylmercury resistance than did wild-type yeast. The results of our genetic engineering study suggest that this vesicle transport system (proteins moving from the TGN to vacuole via endosome) is responsible for enhancing methylmercury toxicity due to the interrelationship between the pathways. There is a possibility that there may be proteins in the cell that enhance methylmercury toxicity through the protein transport system.

How Saccharomyces cerevisiae copes with toxic metals and metalloids

Toxic metals and metalloids are widespread in nature and can locally reach fairly high concentrations. To ensure cellular protection and survival in such environments, all organisms possess systems to evade toxicity and acquire tolerance. This review provides an overview of the molecular mechanisms that contribute to metal toxicity, detoxification and tolerance acquisition in budding yeast Saccharomyces cerevisiae. We mainly focus on the metals/metalloids arsenic, cadmium, antimony, mercury, chromium and selenium, and emphasize recent findings on sensing and signalling mechanisms and on the regulation of tolerance and detoxification systems that safeguard cellular and genetic integrity.

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.

A Ubiquitin-proteasome System as a Factor that Determine the Sensitivity to Methylmercury

To elucidate the mechanism of toxicity of methylmercury (MeHg), we searched for factors that determine the sensitivity of yeast cells to MeHg and found that overexpression of Cdc34 or Rad23, both proteins related to the ubiquitin-proteasome (UP) system, induces resistance to MeHg toxicity. The acquisition of resistance to MeHg in Cdc34-overexpressing yeast cells requires the ubiquitin-conjugating activity of Cdc34 and the proteolytic activity of proteasomes. Therefore, it seems likely that certain as-yet-unidentified proteins that increase MeHg toxicity might exist in cells and that the toxicity of MeHg might be reduced by the enhanced degradation of such proteins through the UP system when Cdc34 is overexpressed. Unlike Cdc34, Rad23 suppresses the degradation of ubiquitinated proteins by proteasomes. This activity of Rad23 might be involved in the acquisition of resistance to MeHg toxicity when Rad23 is overexpressed. Overexpression of Rad23 might induce resistance to MeHg by suppressing the degradation of proteins that reduce the MeHg toxicity. Moreover, when we overexpressed Cdc34 in normal and Rad23-defective yeasts, resistance to MeHg was enhanced to almost the same extent in both lines of yeast cells. Thus it is possible that the binding of Rad23 to ubiquitinated proteins might be regulated by a mechanism that involves the recognition of substrate proteins and that the functions of Rad23 might not affect the protein-degradation system in which Cdc34 is involved. Many proteins that reduce or enhance MeHg toxicity and are ubiquitinated might exist in cells. The UP system and related proteins might determine the extent of MeHg toxicity by regulating the cellular concentrations of these various proteins.

Identification of F-box proteins that are involved in resistance to methylmercury inSaccharomyces cerevisiae

We searched for F-box proteins that might be related to the mechanism that protects Saccharomyces cerevisiae against the toxic effects of methylmercury. We found that overexpression of Hrt3 and of Ylr224w rendered yeast cells resistant to methylmercury. Yeast cells that overexpressed Hrt3 and Ylr224w were barely resistant to methylmercury in the presence of a proteasome inhibitor. Our results suggest the existence of some protein(s) that enhances the toxicity of methylmercury in yeast cells and, also, that overexpression of Hrt3 or Ylr224w can confer resistance to methylmercury by enhancing the polyubiquitination of this protein(s) and its degradation in proteasomes.