Enterococcus faecalis SufU scaffold protein enhances SufS desulfurase activity by acquiring sulfur from its cysteine-153

Abstract

Iron–sulfur [Fe–S] clusters are inorganic prosthetic groups that play essential roles in all living organisms. In vivo [Fe–S] cluster biogenesis requires enzymes involved in iron and sulfur mobilization, assembly of clusters, and delivery to their final acceptor. In these systems, a cysteine desulfurase is responsible for the release of sulfide ions, which are incorporated into a scaffold protein for subsequent [Fe–S] cluster assembly. Although three machineries have been shown to be present in Proteobacteria for [Fe–S] cluster biogenesis (NIF, ISC, and SUF), only the SUF machinery has been found in Firmicutes. We have recently described the structural similarities and differences between Enterococcus faecalis and Escherichia coli SufU proteins, which prompted the proposal that SufU is the scaffold protein of the E. faecalis sufCDSUB system. The present work aims at elucidating the biological roles of E. faecalis SufS and SufU proteins in [Fe–S] cluster assembly. We show that SufS has cysteine desulfurase activity and cysteine-365 plays an essential role in catalysis. SufS requires SufU as activator to [4Fe–4S] cluster assembly, as its ortholog, IscU, in which the conserved cysteine-153 acts as a proximal sulfur acceptor for transpersulfurization reaction.