Structure and functional dynamics of the mitochondrial Fe/S cluster synthesis complex

Michal T Boniecki,Sven A Freibert,Miroslaw Cygler,Roland Lill,Ulrich Mühlenhoff

doi:10.1038/s41467-017-01497-1

Abstract

Iron–sulfur (Fe/S) clusters are essential protein cofactors crucial for many cellular functions including DNA maintenance, protein translation, and energy conversion. De novo Fe/S cluster synthesis occurs on the mitochondrial scaffold protein ISCU and requires cysteine desulfurase NFS1, ferredoxin, frataxin, and the small factors ISD11 and ACP (acyl carrier protein). Both the mechanism of Fe/S cluster synthesis and function of ISD11-ACP are poorly understood. Here, we present crystal structures of three different NFS1-ISD11-ACP complexes with and without ISCU, and we use SAXS analyses to define the 3D architecture of the complete mitochondrial Fe/S cluster biosynthetic complex. Our structural and biochemical studies provide mechanistic insights into Fe/S cluster synthesis at the catalytic center defined by the active-site Cys of NFS1 and conserved Cys, Asp, and His residues of ISCU. We assign specific regulatory rather than catalytic roles to ISD11-ACP that link Fe/S cluster synthesis with mitochondrial lipid synthesis and cellular energy status.

Highlights

Iron–sulfur (Fe/S) clusters are essential protein cofactors crucial for many cellular functions including DNA maintenance, protein translation, and energy conversion
De novo synthesis of a [2Fe–2S] cluster on the ISCU scaffold protein further involves the function of reduced ferredoxin (FDX2), presumably converting the persulfide sulfur (S0) to sulfide (S2−)[27, 28], and frataxin (FXN), which may fulfill a regulatory role in persulfide sulfur transfer from NFS1 to ISCU and/or provide iron[29, 30]
For crystallographic studies we have used the M107I mutant of ISCU in the expectation that frataxin would be dispensable as part of the core iron–sulfur cluster assembly (ISC) complex[32]

Summary

Introduction

Iron–sulfur (Fe/S) clusters are essential protein cofactors crucial for many cellular functions including DNA maintenance, protein translation, and energy conversion. De novo Fe/S cluster synthesis occurs on the mitochondrial scaffold protein ISCU and requires cysteine desulfurase NFS1, ferredoxin, frataxin, and the small factors ISD11 and ACP (acyl carrier protein). The biochemical mechanism of persulfide generation on a conserved cysteine residue was worked out with NifS, the bacterial founding member of this family[11], followed by determination of crystal structures of prokaryotic NifS/IscS12–14 or SufS desulfurases[15], and their complex with the scaffold protein IscU16, 17. One of the latter structures contains a bridging [2Fe–2S] cluster.

Methods

Results

Conclusion