Review
Ancient and essential: the assembly of iron–sulfur clusters in plants

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In plants iron–sulfur (Fe–S) proteins are found in the plastids, mitochondria, cytosol and nucleus, where they are essential for numerous physiological and developmental processes. Recent mutant studies, mostly in Arabidopsis thaliana, have identified three pathways for the assembly of Fe–S clusters. The plastids harbor the SUF (sulfur mobilization) pathway and operate independently, whereas cluster assembly in the cytosol depends on the emerging CIA (cytosolic iron–sulfur cluster assembly) pathway and mitochondria. The latter organelles use the ISC (iron–sulfur cluster) assembly pathway. In all three pathways the assembly process can be divided into a first stage where S and Fe are combined on a scaffold protein, and a second stage in which the Fe–S cluster is transferred to a target protein. The second stage might involve different carrier proteins with specialized functions.

Section snippets

The basics of Fe–S cluster assembly

Iron–sulfur (Fe–S) clusters are cofactors of proteins that function in vital processes such as respiration, photosynthesis, sulfur and nitrogen assimilation, amino acid and purine metabolism, plant hormone and coenzyme synthesis, DNA repair and translation (see Box 1 and Supplementary Table S1). Plants and algae are particularly rich in Fe–S proteins, both in diversity and abundance, owing to their photo-autotrophic life style. Fe–S proteins are found in the plastids, mitochondria, cytosol and

The sulfur mobilization (SUF)-like machinery in the plastids

Chloroplasts harbor their own Fe–S cluster assembly system (Table 1) which was largely inherited from their cyanobacterial evolutionary ancestor. The source of sulfur is cysteine but the source of Fe is not known. Although it has been assumed that the iron-sequestering protein ferritin could be a source of Fe it is now clear that this is not the case from the analysis of Arabidopsis triple ferritin mutants [14]. By contrast, it seems that ferritin serves to sequester iron in order to prevent

The plastid contains several scaffolds and carrier proteins

A number of plastid proteins have been identified that bind and transfer Fe–S clusters in vitro, and could function either as a scaffold for cluster assembly or as carrier/transfer proteins. These are the SUFBCD complex, NFU1-3, HCF101, SUFA (CpISCA), GRXS14 or GRXS16 (Table 1 and Figure 1a).

The SUFBCD proteins are best characterized in E. coli. They form a BC2D complex with structural homology to non-integral ABC proteins. SufB can assemble a labile 4Fe–4S cluster, and binds FADH2 [26]. The BC2

The iron–sulfur cluster (ISC) pathway in the mitochondria

Plant mitochondria operate the so-called ISC pathway for the assembly of Fe–S clusters (Table 2 and Figure 1b). Key features of this pathway are a group-I cysteine desulfurase [18], and a single-domain scaffold protein (IscU/ISU). The isc gene cluster in E. coli also encodes an IscA protein, chaperones and ferredoxin [48], whereas additional proteins assisting the core machinery have been identified in yeast mitochondria 1, 10.

In the first stage of cluster assembly, persulfide is delivered by

Mitochondrial dependence and the emerging cytosolic iron–sulfur cluster assembly (CIA) pathway in the cytosol

Direct functional evidence for the cytosolic Fe–S cluster assembly pathway in plants is fragmentary, but a number of recent reports indicate that the situation is similar to that in Baker's yeast: (i) Cytosolic Fe–S proteins depend on ISC components and a mitochondrial ABC transporter; (ii) localization results show that most ISC proteins are exclusively mitochondrial (except HSCB and HSCA1 [56]); and (iii) highly conserved CIA proteins are present in the cytosol (Table 2 and Figure 1c).

The

Conclusions and future prospects

A fairly complete draft of the pathways of Fe–S cluster assembly in the plastids, mitochondria and cytosol is now available. The plastids generate Fe–S clusters for their own requirements, whereas the mitochondria provide both their resident clusters and play a role in the assembly of cytosolic/nuclear Fe–S proteins. The general localization of the SUF proteins to the plastids and the ISC proteins to the mitochondria reflects their inheritance through their cyanobacterial and α-proteobacterial

Acknowledgements

Janneke Balk is supported by a University Research Fellowship from the Royal Society. Her research on Fe–S protein biogenesis is supported by grants from the EU FP7, BBSRC and the Wellcome Trust. Research in the laboratory of Marinus Pilon is supported by a grant from the US National Science Foundation (Grant # NSF-MCB 0950726). We would like to thank Delphine Bernard and the reviewers for helpful comments to the manuscript and figures.

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