Crystal Structure of IscA, an Iron-sulfur Cluster Assembly Protein from Escherichia coli

IscA, an 11 kDa member of the hesB family of proteins, binds iron and [2Fe-2S] clusters, and participates in the biosynthesis of iron-sulfur proteins. We report the crystal structure of the apo-protein form of IscA from Escherichia coli to a resolution of 2.3 Å. The crystals belong to the space group P3221 and have unit cell dimensions a=b=66.104 Å, c=150.167 Å (α=β=90°, γ=120°). The structure was solved using single-wavelength anomalous dispersion (SAD) phasing of a selenomethionyl derivative, and the IscA model was refined to R=21.4% (Rfree=25.4%). IscA exists as an (α1α2)2 homotetramer with the (α1α2) dimer comprising the asymmetric unit. Cys35, implicated in Fe-S cluster assembly, is located in a central cavity formed at the tetramer interface with the γ-sulfur atoms of residues from the α1 and α2′ monomers (and α1′α2) positioned close to one another (≅7 Å). C-terminal residues 99–107 are disordered, and the exact positions of Cys99 and Cys101 could not be determined. However, computer modeling of C-terminal residues in the tetramer suggests that Cys99 and Cys101 in the α1 monomer and those of the α1′ monomer (or α2 and α2′) are positioned sufficiently close to coordinate [2Fe-2S] clusters between the two dimers, whereas this is not possible within the (α1α2) or (α1′α2′) dimer. This symmetrical arrangement allows for binding of two [2Fe-2S] clusters on opposite sides of the tetramer. Modeling further reveals that Cys101 is positioned sufficiently close to Cys35 to allow Cys35 to participate in cluster assembly, formation, or transfer.