Friedreich ataxia is an autosomal recessive neurodegenerative disease caused by defects in the FRDA gene, which encodes a mitochondrial protein called frataxin. Frataxin is evolutionarily conserved, with homologs identified in mammals, worms, yeast, and bacteria. The CyaY proteins of ␥-purple bacteria are believed to be closely related to the ancestor of frataxin. In this study, we have determined the crystal structure of the CyaY protein from Escherichia coli at 1.4-Å resolution. It reveals a protein fold consisting of a six-stranded antiparallel ␤-sheet flanked on one side by two ␣-helices. This fold is likely to be shared by all members of the conserved frataxin family. This study also provides a framework for the interpretation of disease-associated mutations in frataxin and for understanding the possible functions of this protein family.

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CyaY is a 106-residue protein from Escherichia coli. It shows amino-acid sequence similarity to human frataxin and a frataxin homologue in Saccharomyces cerevisiae, Yfh1p. The former is associated with the disease Friedreich ataxia and the latter plays a key role in iron homeostasis in mitochondria. CyaY has been overexpressed in soluble form in E. coli. The recombinant protein with a His(6) tag at its C-terminus has been crystallized at 296 K using polyethylene glycol (PEG) 4000 as a precipitant. Native diffraction data have been collected to 1.8 A using Cu Kalpha X-rays. The crystals belong to the trigonal space group P3(1)21 (or P3(2)21), with unit-cell parameters a = b = 44.66, c = 99.87 A, alpha = beta = 90.0, gamma = 120.0 degrees. The asymmetric unit contains one molecule of recombinant CyaY, with a corresponding V(m) of 2.13 A(3) Da(-1) and solvent content of 42.3%.

Ypd1p, a 167-residue protein from Saccharomyces cerevisiae, plays a key role in osmosensing phosphorelay signal transduction. It forms part of a multistep phosphorelay system which also includes Sln1p hybrid histidine kinase and two response regulators, Ssk1p and Skn7p. It has been overexpressed in soluble form in Escherichia coli with a His 6 -tag at its C-terminus. The recombinant protein has been crystallized at room temperature using ammonium sulfate and lithium sulfate as precipitants. Native diffraction data have been collected to 2.3 A Ê using synchrotron radiation. The crystals are triclinic, belonging to the space group P1, with unit-cell parameters a = 65. 78, b = 66.74, c = 65.75 A Ê , = 106.60, = 106.48, = 115.53 . The asymmetric unit contains four molecules of the monomeric recombinant Ypd1p, with a corresponding V m of 2.75 A Ê 3 Da À1 and a solvent content of 55.3%.