Cysteine-Persulfide Sulfane Sulfur-Ligated Zn Complex of Sulfur-Carrying SufU in the SufCDSUB System for Fe–S Cluster Biosynthesis

SufU, a component of the SufCDSUB Fe-S cluster biosynthetic system, serves as a Zn-dependent sulfur-carrying protein that delivers inorganic sulfur in the form of cysteine persulfide from SufS to SufBCD. To understand this sulfur delivery mechanism, we studied the X-ray crystal structure of SufU and its sulfur-carrying state (persulfurated SufU) and performed functional analysis of the conserved amino acid residues around the Zn sites. Interestingly, sulfur-carrying SufU with Cys41-persulfide (Cys41-S_γ-S_δ^-) exhibited a unique Zn coordination structure, in which electrophilic S_γ is ligated to Zn and nucleophilic/anionic S_δ is bound to distally conserved Arg125. This structure is distinct from those of other Cys-persulfide-S_δ-ligated metals of metalloproteins, such as hybrid cluster proteins and SoxAX. Functional analysis of SufU variants with Zn-ligand and Arg125 substitutions revealed that both Zn and Arg125 are critical for the function of SufU with SufS. The Zn-persulfide structure of SufU provides insight into the sulfur-transfer process, suggesting that persulfide-S_δ^- is stabilized via bridging by Zn and Arg125 of SufU.