In budding yeast, uptake of iron is largely controlled by the
transcription factor Aft1. cDNA microarrays were used to identify new
iron- and AFT1-regulated genes. Four genes regulated as part of
the AFT1-regulon (ARN1-4) encoded members of a subfamily
of the major facilitator superfamily of transporters. These proteins
have 14 predicted transmembrane domains and were from 26% - 53%
identical at the amino acid level. ARN1-4 were found to
facilitate the transport of siderophores: low molecular weight compounds
that specifically bind ferric iron. Our data indicate that S.
cerevisiae maintains two systems of siderophore-mediated iron
uptake, one occurring at the plasma membrane and the other occurring in
an intracellular compartment. The plasma membrane-based system required
the reduction of the siderophore-bound iron by the surface reductases
FRE1, FRE2, and FRE3. Uptake of reduced
siderophore-iron required the high-affinity, ferrous iron transport
complex encoded by FET3 and FTR1. A second system of
uptake required the Arn transporters. We determined the specificity of
different siderophores for individual Arn transporters. Arn1p and Arn3p
were localized to intracellular vesicles that co-sediment with the late
endosomal protein Pep12p. Siderophore uptake was deficient in an
endocytosis mutant, end4. The intracellular distribution of Arn1p
did not change in the end4 mutant, however, suggesting that
siderophores are delivered to the Arn transporters through an endocytic
pathway.
cerevisiae can be exploited to clone nucleoside transporters from
other species or from tissues of pharmacological relevance.
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