How Escherichia coli and Saccharomyces cerevisiae build Fe/S proteins

Owing to the versatile electronic properties of iron and sulfur, iron sulfur (Fe/S) clusters are perfectly suited for sensing changes in environmental conditions and regulating protein properties accordingly. Fe/S proteins have been recruited in a wide array of diverse biological processes, including electron transfer chains, metabolic pathways and gene regulatory circuits. Chemistry has revealed the great diversity of Fe/S clusters occuring in proteins. The question now is to understand how iron and sulfur come together to form Fe/S clusters and how these clusters are subsequently inserted into apoproteins. Iron, sulfide and reducing conditions were found to be sufficient for successful maturation of many apoproteins in vitro, opening the possibility that insertion might be a spontaneous event. However, as in many other biological pathways such as protein folding, genetic analyses revealed that Fe/S cluster biogenesis and insertion depend in vivo upon auxiliary proteins. This was brought to light by studies on Azotobacter vinelandii nitrogenase, which, in particular, led to the concept of scaffold proteins, the role of which would be to allow transient assembly of Fe/S cluster. These studies paved the way toward the identification of the ISC and SUF systems, subjects of the present review that allow Fe/S cluster assembly into apoproteins of most organisms. Despite the recent discovery of the SUF and ISC systems, remarkable progress has been made in our understanding of their molecular composition and biochemical mechanisms. Such a rapid increase in our knowledge arose from a convergent interest from researchers engaged in unrelated fields and whose complementary expertise covered most experimental approaches used in biology. Also, the high conservation of ISC and SUF systems throughout a wide array of organisms helped cross-feeding between studies. The ISC system is conserved in eubacteria and most eukaryotes, while the SUF system arises in eubacteria, archaea, plants and parasites. ISC and SUF systems share a common core function made of a cysteine desulfurase, which acts as a sulfur donor, and scaffold proteins, which act as sulfur and iron acceptors. The ISC and SUF systems also exhibit important differences. In particular, the ISC system includes an Hsp70/Hsp40-like pair of chaperones, while the SUF system involves an unorthodox ATP-binding cassette (ABC)-like component. The role of these two sets of ATP-hydrolyzing proteins in Fe/S cluster biogenesis remains unclear. Both systems are likely to target overlapping sets of apoproteins. However, regulation and phenotypic studies in E coli, which synthesizes both types of systems, leads us to envisage ISC as the house-keeping one that functions under normal laboratory conditions, while the SUF system appears to be required in harsh environmental conditions such as oxidative stress and iron starvation. In Saccharomyces cerevisiae, the ISC system is located in the mitochondria and its function is necessary for maturation of both mitochondrial and cytosolic Fe/S proteins. Here, we attempt to provide the first comprehensive review of the ISC and SUF systems since their discovery in the mid and late 1990s. Most emphasis is put on E coli and S. cerevisiae models with reference to other organisms when their analysis provided us with information of particular significance. We aim at covering information made available on each Isc and Suf component by the different experimental approaches, including physiology, gene regulation, genetics, enzymology, biophysics and structural biology. It is ou hope that this parallel coverage will facilitate the identification of both similarities and specificities of ISC and SUF systems.