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Protein Science (2005), 14:1863-1869. Published by Cold Spring Harbor Laboratory Press. Copyright © 2005 The Protein Society
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The crystal structure of TrxA(CACA): Insights into the formation of a [2Fe-2S] iron–sulfur cluster in an Escherichia coli thioredoxin mutant

Jean-Francois Collet1,3, Daniel Peisach2, James C.A. Bardwell1 and Zhaohui Xu2

1 Department of Molecular, Cellular, and Developmental Biology and 2 Life Sciences Institute, University of Michigan, Ann Arbor, Michigan 48109-1048, USA

(RECEIVED March 16, 2005; FINAL REVISION April 25, 2005; ACCEPTED April 25, 2005)

Escherichia coli thioredoxin is a small monomeric protein that reduces disulfide bonds in cytoplasmic proteins. Two cysteine residues present in a conserved CGPC motif are essential for this activity. Recently, we identified mutations of this motif that changed thioredoxin into a homodimer bridged by a [2Fe-2S] iron–sulfur cluster. When exported to the periplasm, these thioredoxin mutants could restore disulfide bond formation in strains lacking the entire periplasmic oxidative pathway. Essential for the assembly of the iron–sulfur was an additional cysteine that replaced the proline at position three of the CGPC motif. We solved the crystalline structure at 2.3 Å for one of these variants, TrxA(CACA). The mutant protein crystallized as a dimer in which the iron–sulfur cluster is replaced by two intermolecular disulfide bonds. The catalytic site, which forms the dimer interface, crystallized in two different conformations. In one of them, the replacement of the CGPC motif by CACA has a dramatic effect on the structure and causes the unraveling of an extended {alpha}-helix. In both conformations, the second cysteine residue of the CACA motif is surface-exposed, which contrasts with wildtype thioredoxin where the second cysteine of the CXXC motif is buried. This exposure of a pair of vicinal cysteine residues apparently allows thioredoxin to acquire an iron–sulfur cofactor at its active site, and thus a new activity and mechanism of action.

Keywords: thioredoxin; iron–sulfur cluster; crystal structure; disulfide bond; periplasm

Article and publication are at http://www.proteinscience.org/cgi/doi/10.1110/ps.051464705.

Reprint requests to: Zhaohui Xu, Life Sciences Institute, University of Michigan, Ann Arbor, MI 48109, USA; e-mail: zhaohui{at}umich.edu; fax: (734) 763-6492.


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