New insight in cadmium detoxification system in Saccharomyces Cerevisiae

ATP-binding cassette (ABC) proteins constitute one of the largest protein superfamilies, with more than 3000 members operating from bacteria to man. These proteins catalyze the ATP-dependent transport of different molecules across cellular membranes (1). Generally, ABCC subfamily members transport most substrates in the form of GSH conjugates and, in some cases, glucuronide or sulfate conjugates, rather than transporting the unmodified substrates themselves (2). Mutations in several members of the ABC transporter genes cause human diseases, including cystic fibrosis, pseudoxanthoma elasticum, and Dubin-Johnson syndrome (3). Because yeast harbors several homologues of mammalian ABC proteins with medical importance, understanding their molecular mechanisms and substrate interaction might help identifying new approaches aimed to overcome drug resistance or other ABC-mediated diseases. The yeast genome contains 30 ABC proteins. Phylogenetic analyses established the existence of six ABC subfamilies (ABCB-ABCG using the mammalian nomenclature). Five members of ABCC subfamily (Ycf1p, Bpt1p, Ybt1, Nft1p and Vmr1p) are “full length” and one (Yor1p) is “short” . While Yor1p localizes to the plasma membrane the others localized to the vacuolar membrane (4). Vmr1 and Nft1p are the least characterized of the yeast ABCC transporters. Recent studies have shown that Vmr1p is specifically involved in multidrug resistance and contributes to cadmium resistance on ethanol/glycerol medium, while Ycf1p (Yeast Cadmium Factor) is the main transporter of GSH - cadmium on glucose medium (5). In this study, experiments carried out on Saccharomyces Cerevisiae wild type and Vmr1p- deleted strain in presence of L-Buthionine-sulfoximine (BSO), a specific inhibitor of γglutamylcysteine synthetase, suggested that on ethanol/glycerol medium the Vmr1p mediates cadmium detoxification but not through formation of Cd[GS]2 complexes. Also, fluorescence microscopy experiments of wild type, YCF1 and VMR1 cells, grown on respiratory medium containing monochlorobimane, suggest that, compared to Ycf1p, Vmr1p does not contribute to the vacuolar accumulation of monochlorobimane-GS by intact cells. This result leaves assume that Vmr1p does not transport glutathione S-coniugate. References 1. Holland I., et al. (2003) ABC proteins from bacteria to men. Academic Press-Elsevier Science. 2. Cole S. P. and Deeley R. G. (2006) Trends Pharmacol. Sci. 27:438–446. 3. Dean M. et al. (2005) Methods Enzymol. 400:409–429. 4. Paumi C, et al. (2009) Microbiol.Mol.Biol.Rev 73, 577-593. 5. Wawrzycka D. et al. (2010) FEMS Yeast Res.;10(7):828-38