Intracellular Iron Transport

Inside the cells Fe is translocated to target compartments or target components. It is likely that intracellular organelles express on their membranes their own specific transporters for import or export of Fe. Only few Fe transport proteins have been localised to organellar membranes. AtNRAMP3 protein was found in the vacuolar membrane, where it may serve the export of Fe from vacuoles upon Fe deficiency (Thomine et al., 2003). IDE7, an Fe-regulated ABC-type transporter of unknown function, was also described to be present in the tonoplast (Yamaguchi et al., 2002). Chloroplasts are a major site of Fe function in electron transport chains and storage of excess Fe bound to ferritin. Iron import into chloroplasts is apparently carried out by uniport-mechanism in a non-specific mechanism, as suggested by successful competition of Fe transport by other micronutrient ions (Shingles et al., 2002). The nicotianamine-free chloronerva mutant accumulates Fe precipitates in the chloroplast, but contains no detectable ferritin (Liu et al., 1998; Becker et al., 1995), suggesting that nicotianamine is a prerequisite for transport between chelation in the cytosol and proper storage in target compartments. Mitochondria are another target for Fe, mostly in Fe-sulfar clusters. An Arabidopsis mutant lacking a mitochondrial ATP binding cassette transporter shows increased contents of non-heme and non-protein bound Fe, together with increased expression of genes encoding oxygen radical detoxifying enzymes (Kushnir et al., 2001). Frataxin is important for maintenance of mitochondrial Fe levels. Loss-of-function frataxin mutants show altered mitochondrial Fe accumulation and oxidative damage in yeast and humans (known as Friedreich's ataxia) (Babcock et al., 1997; Campuzano et al., 1996). It was proposed that Frataxin binds Fe and might function in mitochondrial Fe-sulfar cluster assembly (Aloria et al., 2004; Bulteau et al., 2004). Frataxin is well conserved in sequence in diverse organisms including plants. AtFH (Arabidopsis frataxin) is a functional protein in yeast complementation assays, but its precise function in plants has not been analysed yet (Busi et al., 2004).

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