Molecular Mechanisms Regulating The Response Of Budding Yeast To Cadmium Stress

Cadmium ions are not essential for eukarytic cells, but could induce cancers andare toxic to human health. Our lab previously identified112cadmium-sensitive genemutations. In this study, we find that81of these mutants are also sensitive tomanganese, zinc and iron divalent ions, but the rest31are not and only specificallysensitive to cadmium ions. Main functions of these genes are involved in protein fate,transcriptional regulation, cellular communication, signal transduction, and cellulartransport.Through serial dilution assay and Western blot analysis, we have identified twomitogen-activate protein （MAP） kinase signaling pathways, the cell wall integrity（CWI） pathway and high-osmolarity and glycerol （HOG） pathway, are involved in theresponse to cadmium stress. Cadmium stress activates Hog1through two independentupstream branches, Sho1and Sln1. Slt2phosphorylation dependes on the CWIpathway sensor Mid2, which activates the Rom1GEFs. The sensor stimulatednucleotide exchange on Rho1, which then activates the PKC pathway. In addition,phosphorylation of Slt2depends on Sit4, one of the elements in the target ofrapamycin （TOR） pathway, but does not require the HOG pathway. In response tocadmium stress, subcellular localization of Slt2and Hog1is not affected.Supplementation of1mM Mn²⁺or10mM Fe²⁺in media suppresses the cadmiumsensitivity of hog1Δ and slt2Δ cells, suggesting that manganese or iron deficiency isan important determinant of cadmium toxicity. In line with this, transporter smf1Δ,smf2Δ and fet4Δ mutants exhibit cadmium tolerance. Transcriptional expression ofSMF1, SMF2and PCA1are positively regulated by Hog1and Slt2. SMF1and SMF2promoter sequences have Swi6and Smp1transcription factor binding sites.VPS genes for nine cadmium-sensitive mutants participate in theubiquitin-dependent sorting of transmembrane proteins at the endosome. Zrt1is oneof the major pathways through which Cd²⁺enters yeast, but the zrt1Δ mutant is notsensitive to cadmium stress. Deletion of ZRT1or ZRT2does not affect the cadmiumsensitivity of VPS Class E gene mutations. Western blot analysis suggested that Zrt1degradation requires the VPS Class E family in response to cadmium stress.