Batoul Srour, Sylvain Gervason, Maren Hellen Hoock, Beata Monfort, Kristian Want, Djabir Larkem, Nadine Trabelsi, Gautier Landrot, Andrea Zitolo, Emiliano Fonda, Emilien Etienne, Guillaume Gerbaud, Christina Sophia Müller, Jonathan Oltmanns, Jesse B. Gordon, Vishal Yadav, Malgorzata Kleczewska, Marcin Jelen, Michel B. Toledano, Rafal Dutkiewicz, David P. Goldberg, Volker Schünemann, Bruno Guigliarelli, Bénédicte Burlat, Christina Sizun, Benoit D’Autréaux, Institut de Biologie Intégrative de la Cellule (I2BC), Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Université Paris-Saclay-Centre National de la Recherche Scientifique (CNRS), Technische Universität Kaiserslautern (TU Kaiserslautern), Synchrotron SOLEIL (SSOLEIL), Centre National de la Recherche Scientifique (CNRS), Bioénergétique et Ingénierie des Protéines (BIP ), Aix Marseille Université (AMU)-Centre National de la Recherche Scientifique (CNRS), Johns Hopkins University (JHU), Medical University of Gdańsk, University of Gdańsk (UG), Institut de Chimie des Substances Naturelles (ICSN), Institut de Chimie du CNRS (INC)-Université Paris-Saclay-Centre National de la Recherche Scientifique (CNRS), and ANR-17-CE11-0021,FRATAXUR,Bases moléculaires et structurales de la biogenèse des centres fer-soufre permettant d'élucider la fonction moléculaire de la frataxine(2017)
International audience; Iron−sulfur (Fe−S) clusters are prosthetic groups of proteins biosynthesized on scaffold proteins by highly conserved multi-protein machineries. Biosynthesis of Fe−S clusters into the ISCU scaffold protein is initiated by ferrous iron insertion, followed by sulfur acquisition, via a still elusive mechanism. Notably, whether iron initially binds to the ISCU cysteine-rich assembly site or to a cysteine-less auxiliary site via N/O ligands remains unclear. We show here by SEC, circular dichroism (CD), and Mossbauer spectroscopies that iron binds to the assembly site of the monomeric form of prokaryotic and eukaryotic ISCU proteins via either one or two cysteines, referred to the 1-Cys and 2-Cys forms, respectively. The latter predominated at pH 8.0 and correlated with the Fe−S cluster assembly activity, whereas the former increased at a more acidic pH, together with free iron, suggesting that it constitutes an intermediate of the iron insertion process. Iron not binding to the assembly site was non-specifically bound to the aggregated ISCU, ruling out the existence of a structurally defined auxiliary site in ISCU. Characterization of the 2-Cys form by site-directed mutagenesis, CD, NMR, X-ray absorption, Mossbauer, and electron paramagnetic resonance spectroscopies showed that the iron center is coordinated by four strictly conserved amino acids of the assembly site, Cys35, Asp37, Cys61, and His103, in a tetrahedral geometry. The sulfur receptor Cys104 was at a very close distance and apparently bound to the iron center when His103 was missing, which may enable iron-dependent sulfur acquisition. Altogether, these data provide the structural basis to elucidate the Fe−S cluster assembly process and establish that the initiation of Fe−S cluster biosynthesis by insertion of a ferrous iron in the assembly site of ISCU is a conserved mechanism.