Crystal structure of Escherichia coli SufA involved in biosynthesis of iron-sulfur clusters:: Implications for a functional dimer

IscA and SufA are paralogous proteins that play crucial roles in the biosynthesis of Fe-S clusters, perhaps through a mechanism involving transient Fe-S cluster formation. We have determined the crystal structure of E. coli SufA at 2.7 angstrom resolution. SufA exists as a homodimer, in contrast to the tetrameric organization of IscA. Furthermore, a C-terminal segment containing two essential cysteine residues (Cys-Gly-Cys), which is disordered in the IscA structure, is clearly visible in one molecule (the a, subunit) of the SufA homodimer. Although this segment is disordered in the other molecule (the alpha(2) subunit), computer modeling of this segment based on the well-defined conformation of alpha(2) subunit suggests that the four cysteine residues (Cys114 and Cys116 in each subunit) in the Cys-Gly-Cys motif are positioned in close proximity at the dimer interface. The arrangement of these cysteines together with the nearby Glu118 in SufA dimer may allow coordination of an Fe-S cluster and/or an Fe atom. (c) 2005 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.