The SufBCD Fe−S Scaffold Complex Interacts with SufA for Fe−S Cluster Transfer

Iron−sulfur clusters are key iron cofactors in biological pathways ranging from nitrogen fixation to respiration. Because of the toxicity of ferrous iron and sulfide to the cell, in vivo Fe−S cluster assembly transpires via multiprotein biosynthetic pathways. Fe−S cluster assembly proteins traffic iron and sulfide, assemble nascent Fe−S clusters, and correctly transfer Fe−S clusters to the appropriate target metalloproteins in vivo. The Gram-negative bacterium Escherichia coli contains a stress-responsive Fe−S cluster assembly system, the SufABCDSE pathway, that functions under iron starvation and oxidative stress conditions that compromise Fe−S homeostasis. Using a combination of protein−protein interaction and in vitro Fe−S cluster assembly assays, we have characterized the relative roles of the SufBCD complex and the SufA protein during Suf Fe−S cluster biosynthesis. These studies reveal that SufA interacts with SufBCD to accept Fe−S clusters formed de novo on the SufBCD complex. Our results represent the first biochemical evidence that the SufBCD complex within the Suf pathway functions as a novel Fe−S scaffold system to assemble nascent clusters and transfer them to the SufA Fe−S shuttle.