4863lonate pathways in H . halobium are identical with those in other organisms.As discussed above, the cryptic stereochemistry involved in the incorporation of glycerol into the membrane lipid of H . halobium was firmly established. In contrast to eubacteria and eukaryotes, it is the sn-C-1 methylene group of 2,3-di-Gphytanyl glycerol that is derived from the sn-C-3 carbon of glycerol and the C-6 carbon of D-glucose. Although stereochemically feasible, a pathway including reduction of ~-glyceraldehyde-3-phosphate can be ruled out, since the present as well as previous studies clearly showed that no hydrogen was lost from either hydroxymethyl group of g l y~e r o l .~,~ Furthermore, the ether-forming reaction is also different from that of 0-alkyl dihydroxyacetone phosphate in mammalian cells?J0 in which a hydrogen of the sn-C-1 methylene group is stereospecifically replaced. Consequently, the most likely mechanism of formation of sn-2,3-0-diphytanyl glycerol in H . halobium apparently includes a unique stereochemical inversion a t the C-2 carbon of glycerol as shown in Scheme I.Acknowledgment. Proton and deuterium N M R spectra were recorded through the courtesy of Dr. M. Uramoto, Institute of Physical and Chemical Research, and Professor H. Set0 and Dr. K. Furihata, the University of Tokyo, to whom we are grateful. (9) Brown, A. The largest [S,M,(SR),]'-molecular aggregate structure reported to date is [E4Mlo(SPh)16]' (E = s, Se; M = Zn, Cd) (l).'

No abstract

Arsenic is omnipresent in soil, air, food and water. Chronic exposure to arsenic is a serious problem to human health. In-depth understanding of this metalloid's toxicity is a fundamental step towards development of arsenic-free foods and measures for bioremediation. By screening the complete set of gene deletion mutants (4873) of Saccharomyces cerevisiae, this study uncovered 75 sensitive and 39 resistant mutants against arsenite [As(III)]. Functional analysis of the corresponding genes revealed the molecular details for its uptake, toxicity and detoxification. On the basis of the hypersensitivity of yap3Δ, the transcription factor, Yap3p, is for the first time linked to the cell's detoxification against As(III). Apart from confirming the previously described role of the mitogen-activated protein kinase (MAPK) Hog1 pathway in combating arsenic toxicity, the results show that the regulatory subunits (Ckb1p and Ckb2p) of protein kinase CK2 are also involved in the process, suggesting possible crosstalk between the two key protein kinases. The sensitivity to As(III) conferred by deletion of the genes involved in protein degradation and chromatin remodelling demonstrates protein damage is the key mode of toxicity for the metalloid. Furthermore, the resistant phenotype of fps1Δ, snf3Δ and pho81Δ against As(III) links arsenic uptake with the corresponding plasma membrane-bound transporters-aquaglyceroporin (Fps1p), hexose (Snf3p) and phosphate transporters. The molecular details obtained in this screen for As(III) uptake, detoxification and toxicity provide the basis for future investigations into arsenic-related problems in the environment, agriculture and human health.

No abstract

Chromium toxicity is increasingly relevant to living organisms such as humans, due to the environmental contamination of chromium and the application of stainless steel-based medical devices like hip prostheses. Despite the investigations in past years, the molecular details for chromium toxicity remain to be delineated. In this study, we seek to gain insights into the molecular aspects of chromium toxicity by screening a genome-wide deletion set of individual genes in Saccharomyces cerevisiae against hexavalent chromium [Cr(vi)] using chromium trioxide. From the primary data collected in this study, two lists of deletion mutants in response to Cr(vi) exposure were obtained, one for the sensitive phenotype and the other for the resistant phenotype. The functional analysis of the genes corresponding to the sensitive mutants reveals the key features of Cr(vi) toxicity, which include genotoxicity, protein damage, disruption of energy and sulfur metabolisms. DNA repair, ubiquitination-mediated protein degradation, iron homeostasis and growth attenuation are the intrinsic facets of the cell's detoxification mechanisms. Protein kinase CK2 is, for the first time, found to be involved in regulating chromium toxicity by reducing the uptake of Cr(vi). Taken together, the findings provide meaningful details into the basic understanding of chromium toxicity in terms of its uptake, modes of action, cellular detoxification and molecular regulatory mechanisms.

Genome-wide screening using gene deletion mutants has been widely carried out with numerous toxicants including oxidants and metal ions. The focus of such studies usually centres on identifying sensitive phenotypes against a given toxicant. Here, we screened the complete collection of yeast gene deletion mutants (5047) with increasing concentrations of aluminium sulphate (0.4, 0.8, 1.6 and 3.2 mM) in order to discover aluminium (Al(3+)) tolerant phenotypes. Fifteen genes were found to be associated with Al(3+) transport because their deletion mutants exhibited Al(3+) tolerance, including lem3Δ, hal5Δ and cka2Δ. Deletion of CKA2, a catalytic subunit of tetrameric protein kinase CK2, gives rise to the most pronounced resistance to Al(3+) by showing significantly higher growth compared to the wild type. Functional analysis revealed that both molecular regulation and endocytosis are involved in Al(3+) transport for yeast. Further investigations were extended to all the four subunits of CK2 (CKA1, CKA2, CKB1 and CKB2) and the other 14 identified mutants under a spectrum of metal ions, including Al(3+), Zn(2+), Mn(2+), Fe(2+), Fe(3+), Co(3+), Ga(3+), Cd(2+), In(3+), Ni(2+) and Cu(2+), as well as hydrogen peroxide and diamide, in order to unravel cross-tolerance amongst metal ions and the effect of the oxidants. Finally, the implication of the findings in Al(3+) transport for the other species like plants and humans is discussed.

No abstract

Following our previous finding that the sulfhydryl-oxidising chemical diamide induced a marked elevation of cellular Al(3+) (Wu et al., Int J Mol Sci, 12:8119-8132, 2011), a further investigation into the underlying molecular mechanism was carried out, using the eukaryotic model organism Saccharomyces cerevisiae. The effects of non-toxic dose of diamide (0.8 mM) and a mild dose of aluminium sulphate (Al(3+)) (0.4 mM) were determined prior to the screening of gene deletion mutants. A total of 81 deletion mutants were selected for this study according to the available screening data against Al(3+) only (Kakimoto et al., BioMetals, 18: 467-474, 2005) and diamide only (Thorpe et al., Proc Natl Acad Sci USA, 101: 6564-6569, 2004). On the basis of our screening data and the cluster analysis, a cluster containing the gene deletions (rpe1∆, sec72∆, pdr5∆ and ric1∆) was found to be specifically sensitive to the mixture of diamide and Al(3+). However gnp1∆, mch5∆ and ccc1∆ mutants were resistant. Dithiothreitol (DTT) and ascorbate markedly reversed the diamide-induced Al(3+) toxicity. Inductively-coupled plasma optical emission spectrometry demonstrated that DTT reduced the intracellular Al(3+) content in diamide/Al(3+)-treated yeast cells six-fold compared to the non-DTT controls. These data together revealed that the pleiotropic drug resistance transporter (Pdr5p) and vacuolar/vesicular transport-related proteins (Ric1p and Sec72p) are the targets of diamide. A dysfunctional membrane-bound Pdr5p terminates the detoxification pathway for Al(3+) at the final step, leading to intracellular Al(3+) accumulation and hence toxicity. As Al(3+) toxicity has been a problem in agriculture and human health, this study has provided a significant step forward in understanding Al(3+) toxicity.