Figure 1.

Cartoon model of the mitochondrial Fe/S protein assembly process. Figure was produced based on Braymer and Lill (2017). A cascade of ISC proteins is required for the de novo synthesis of [2Fe-2S] and [4Fe-4S] clusters and their proper trafficking to target apoproteins in mitochondria. Initially, a [2Fe-2S] cluster is synthesized by the early ISC machinery, composed of the Isu1 scaffold protein requiring sulfide from the cysteine desulfurase complex Nfs1-Isd11-Acp1, electrons from the transfer chain NADPH-Arh1 and the ferredoxin Yah1, and the regulator and/or iron donor Yfh1. The Isu1-bound [2Fe-2S] cluster is then delivered to the monothiol glutaredoxin Grx5, a reaction accomplished by the Hsp70 chaperone Ssq1 with the help of the J-type co-chaperone Jac1. This reaction is dependent on ATP hydrolysis by Ssq1. The exchange factor Mge1 facilitates the exchange of ADP for ATP. The resulting bridging [2Fe-2S] cluster on a Grx5 dimer is inserted directly into [2Fe-2S] recipient apoproteins or trafficked to the late ISC machinery for [4Fe-4S] cluster biogenesis. The early ISC machinery, including the chaperones and Grx5, is also responsible for generating the component X-S for transport of sulfur out of the mitochondria to the CIA machinery for cytosolic-nuclear Fe/S protein biogenesis. The late ISC machinery consists of the yet structurally and functionally uncharacterized Isa1-Isa2-Iba57 complex and is needed for the generation of [4Fe-4S] clusters. Trafficking and insertion of the [4Fe-4S] clusters into target Fe/S proteins are facilitated by specific ISC targeting factors, such as Nfu1, the complex I-specific Ind1, and the Bol proteins. Dashed arrows indicate steps that remain poorly elucidated on the biochemical level.