| Budget Amount *help |
¥3,600,000 (Direct Cost: ¥3,600,000)
Fiscal Year 2005: ¥1,500,000 (Direct Cost: ¥1,500,000)
Fiscal Year 2004: ¥2,100,000 (Direct Cost: ¥2,100,000)
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| Research Abstract |
IscA has been proposed to be involved in the iron-sulfur cluster biosynthesis as a so-called scaffold protein, on which a transient iron-sulfur cluster is assembled and from with the pre-assembled cluster is delivered to various substrate proteins. We have identified the IscA homolog to be localized to plastids, termed AtIscA-I, in Arabidopsis thaliana. The AtIscA-I protein was apparently constitutively expressed in all tissues analyzed in Arabidopsis. The AtIscA-I protein exists in the stroma as a soluble protein which tends to form a homo-dimer and can host a [2Fe-2S]-like cluster. Complete loss of the protein from plastids did not cause any significant defect either in normal plant growth or in biogenesis of major iron-sulfur proteins, indicating this protein is not essential or redundant for these functions. In contrast, loss of one of the three plastid-localized CnfU scaffold proteins, CnfU-V, caused significant reduction in the level of AtIscA-I. Hence, efficient biogenesis of At
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IscA-I scaffold requires function of another essential scaffold protein CnfU.
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. 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 a and b, with asymmetric cysteinyl ligation by Cys37,Cys101l,Cys103 from a and Cys103 from b. In the crystal, two ab dimers form an unusual domain-swapped tetramer via central domains of b protomers. Together with additional biochemical data supporting its physiologically dimeric configuration, we propose that the unique asymmetric Fe-S cluster coordination and resulting distinct conformational stabilities between the two IscA protomers is central to the function of IscA-type Fe-S cluster biosynthetic scaffold. Less
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