Iron-Sulfur Cluster Biosynthesis: Characterization of a Molten Globule Domain in Human NFU

Human NFU (also known as HIRIP5) has been implicated in cellular iron-sulfur cluster biosynthesis. Bacterial and yeast forms arc smaller than the human protein and arc homologous to the C-terminal domain of human NFU. This C-terminal domain contains a pair of redox active cysteines and demonstrates thioiredoxin-like activity by both binding to and mediating persulfide bond cleavage of sulfurloaded IscS, the sulfide donor for [2Fe-2S] cluster assembly oil ISU-type scaffold proteins. Herein, human NFU is shown to possess a novel combination of a molten globule-type C-terminal domain and an N-terminal domain with a fully folded regular tertiary structure. The molten globule characteristics of the C-terminal domain have been evaluated by 1-anilino-8-naphthalenesulfonic acid binding, the kinetics of trypsin digestion, and heteronuclear single-quantum coherence nuclear magnetic resonance Studies. Human NFU is a functionally competent reducing agent for cysteinyl persulfide bond cleavage, releasing inorganic sulfide for incorporation into the ISU-bound [2Fe-2S] cluster, a reactivity that might be facilitated by the flexibility of the C-terminal domain.