title: The asymmetric IscA homodimer with an exposed [2Fe-2S] cluster suggests the structural basis of the Fe-S cluster biosynthetic scaffold.
abstract: 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 alpha and beta, with asymmetric cysteinyl ligation by Cys37, Cys101, Cys103 from alpha and Cys103 from beta. In the crystal, two alphabeta dimers form an unusual domain-swapped tetramer via central domains of beta 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.
authors.affiliation: Institute for Protein Research, Osaka University, 3-2 Yamadaoka, Suita, Osaka 565-0871, Japan.
authors.forename: Kozo
authors.initials: K
authors.lastname: Morimoto
authors.forename: Eiki
authors.initials: E
authors.lastname: Yamashita
authors.forename: Youhei
authors.initials: Y
authors.lastname: Kondou
authors.forename: Soo Jae
authors.initials: SJ
authors.lastname: Lee
authors.forename: Fumio
authors.initials: F
authors.lastname: Arisaka
authors.forename: Tomitake
authors.initials: T
authors.lastname: Tsukihara
authors.forename: Masato
authors.initials: M
authors.lastname: Nakai
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry; D002352:Carrier Proteins / Q000737:chemistry*; D002850:Chromatography, Gel; D018360:Crystallography, X-Ray; D000458:Cyanobacteria / Q000378:metabolism*; D003545:Cysteine / Q000737:chemistry; D019281:Dimerization; D029968:Escherichia coli Proteins / Q000737:chemistry*; D007506:Iron-Sulfur Proteins / Q000737:chemistry*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011485:Protein Binding; D011487:Protein Conformation; D017386:Sequence Homology, Amino Acid
chemical_list: D001426:Bacterial Proteins; D002352:Carrier Proteins; D029968:Escherichia coli Proteins; D007506:Iron-Sulfur Proteins; C487901:IscA protein, E coli; D003545:Cysteine