Iron-sulfur cluster assembly proteins are NifU-like proteins that help to assemble or repair iron-sulfur (Fe-S) clusters by acting as a scaffold while these clusters are built - Fe-S clusters are integral parts of the important Fe-S proteins, which are found in respiratory chain complexes and numerous enzymes. In fact, the proteins involved in forming these clusters are among the most highly conserved and prevalent in nature. In eukaryotes, Fe-S assembly usually occurs in the mitochondria, though there can also be activity in the cytoplasm and nucleus. This protein, from Bacillus subtilis, is part of the Suf FE-S assembly family, which is functionally and structurally similar to the IscU family. SufU acts as an enhancer of the cysteine desulfurase activity of SufS as well as acting as a scaffold for Fe-S cluster assembly. Its structure consists of five alpha helices (in blue) and three antiparellel beta sheets (in green). Its active site is made up of four side chains, Cys41, Cys66, Cys128, and Asp43 (in red). It is pictured here bonded to a zinc atom (in silver).