Yct1p, a novel, high-affinity, cysteine specific transporter from the yeast Saccharomyces cerevisiae

Cysteine transport in the yeast Saccharomyces cerevisiae is mediated by at least eight different permeases, none of which are specific for cysteine. We describe a novel, high affinity, (Km = 55 µM), cysteine-specific transporter encoded by the ORF YLL055w that was initially identified by a combined strategy of data mining, bioinformatics and genetic analysis. Null mutants of YLL055w, but not of the other genes encoding for transporters that mediate cysteine uptake such as GAP1, GNP1, MUP1 or AGP1, in a met15 background, resulted in a growth defect when cysteine, at low concentrations, was provided as the sole sulphur source. Transport experiments further revealed that Yll055wp was the major contributor to cysteine transport under these conditions. The contributions of the other transporters became relevant only at higher concentrations of cysteine, or when YLL055w was either deleted or repressed. YLL055w expression was repressed by organic sulphur sources, and was mediated by the Met4p-dependent sulphur regulatory network. The results reveal that YLL055w encodes the principal cysteine transporter in S. cerevisiae, which we have named as YCT1 (Yeast Cysteine Transporter). Interestingly, Yct1p belongs to the Dal5p family of transporters rather than the amino acid permease family to which all the known amino acid transporters belong.

Key Words: Cysteine, DAL5, Sulphur, Transporter, Yeast

Online ISSN: 1943-2361  Print ISSN: 0016-6731
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