Iron-sulfur Cluster Assembly in Plants

Plant cells use the ISC pathway for assembling iron-sulfur (FeS) clusters in the mitochondria and the SUF pathway for FeS clusters in the plastids. The ISC and SUF pathways are similar to those found in bacteria, although there are interesting differences in the green lineage (Balk & Pilon, 2011; Balk & Schaedler 2014). Each pathway consists of 10+ proteins, including a cysteine desulfurase that generates persulfide, and scaffold proteins.

The assembly of FeS clusters in the cytosol and nucleus is dependent on the mitochondria. This was first shown in yeast by Lill and coworkers, and the Balk lab confirmed that Arabidopsis NFS1, the mitochondrial cysteine desulfurase, is also required for extra-mitochondrial FeS enzymes (Bernard et al., 2013). Based on in-vitro studies they propose that the persulfide formed by NFS1 is exported to the cytosol in the form of glutathione trisulfide by the ATP-Binding Cassette transporter ATM3 in plants and Atm1 in yeast (Schaedler et al., 2014).

Interestingly, a recent study by Debkumar Pain and coworkers showed that Atm1 exports a sulfur compound for tRNA thiolation (Pandey et al., 2018, Cell Chem. Biol.). The plant ATM3 transporter is also involved in the biosynthesis of molybdenum cofactor assembly, for more details see the Balk lab’s recent paper (Kruse et al., 2018).

The cytosolic FeS assembly pathway (CIA) consists of 8 proteins of which 7 are conserved in plants. All except MET18 are essential for embryo development in Arabidopsis (Bych et al., 2008a; Bernard et al., 2013; Luo et al., 2012). Viable mutants in NBP35 have serious developmental defects (Bastow et al., 2017). The CIA proteins are functionally associated with DNA polymerases and DNA integrity, but their precise molecular functions in eukaryotes remain unknown.