The Asymmetric IscA Homodimer with an Exposed [2Fe-2S] Cluster Suggests the Structural Basis of the Fe-S Cluster Biosynthetic Scaffold

It has been shown that the so-called scaffold proteins are vital in Fe-S cluster biosynthesis by providing an intermediate site for the assembly of Fe-S clusters. However, since no structural information on such scaffold proteins with bound Fe-S cluster intermediates is available, the structural basis of the core of Fe-S cluster biosynthesis remains poorly understood. Here we report the first Fe-S cluster-bound crystal structure of a scaffold protein, IscA, from Thermosynechococcus elongatus, which carries three strictly conserved cysteine residues. Surprisingly, one partially exposed [2Fe-2S] cluster is coordinated by two conformationally distinct IscA protomers, termed α and β, with asymmetric cysteinyl ligation by Cys37, Cys101, Cys103 from α and Cys103 from β. In the crystal, two αβ dimers form an unusual domain-swapped tetramer via central domains of β protomers. Together with additional biochemical data supporting its physiologically relevant configuration, we propose that the unique asymmetric Fe-S cluster coordination and the resulting distinct conformational stabilities of the two IscA protomers are central to the function of IscA-type Fe-S cluster biosynthetic scaffold.