Understanding the binding properties of an unusual metal-binding protein − a study of bacterial frataxin (original) (raw)
Human Molecular Genetics, 2002
Friedreich's ataxia (FRDA), an autosomal recessive cardio-and neurodegenerative disease, is caused by low expression of frataxin, a small mitochondrial protein, encoded in the nucleus. At the biochemical level, the lack of frataxin leads to dysregulation of mitochondrial iron homeostasis and oxidative damage, which eventually causes neuronal death. It is, however, still unclear whether frataxin is directly involved in iron binding, since the yeast orthologue, but not the human protein, has been shown to form large aggregates in the presence of large iron excess. We have compared the properties of three proteins from the frataxin family-the bacterial CyaY from Escherichia coli, the yeast Yfh1 and human frataxin-as representative of organisms of increasing complexity. We show that the three proteins have the same fold but different thermal stabilities and iron-binding properties. While human frataxin has no tendency to bind iron, CyaY forms ironpromoted aggregates with a behaviour similar to that of yeast frataxin. However, aggregation can be competed by chelator agents or by ionic strength. At physiological salt conditions, almost no aggregation is observed. The design of mutants produced to identify the protein surface involved in iron-promoted aggregation allows us to demonstrate that the process is mediated by a negatively charged surface ridge. Mutation of three of these residues is sufficient to convert CyaY in a protein with properties similar to those of human frataxin. On the other hand, mutation of the exposed surface of the b sheet, which contains most of the conserved residues, does not affect aggregation, suggesting that iron binding is a non-conserved part of a more complex cellular function of frataxins.
Metal Ion Binding in Wild-Type and Mutated Frataxin: A Stability Study
Frontiers in Molecular Biosciences
This work studies the stability of wild-type frataxin and some of its variants found in cancer tissues upon Co2+ binding. Although the physiologically involved metal ion in the frataxin enzymatic activity is Fe2+, as it is customarily done, Co2+ is most often used in experiments because Fe2+ is extremely unstable owing to the fast oxidation reaction Fe2+ → Fe3+. Protein stability is monitored following the conformational changes induced by Co2+ binding as measured by circular dichroism, fluorescence spectroscopy, and melting temperature measurements. The stability ranking among the wild-type frataxin and its variants obtained in this way is confirmed by a detailed comparative analysis of the XAS spectra of the metal-protein complex at the Co K-edge. In particular, a fit to the EXAFS region of the spectrum allows positively identifying the frataxin acidic ridge as the most likely location of the metal-binding sites. Furthermore, we can explain the surprising feature emerging from a d...
Yeast frataxin solution structure, iron binding, and ferrochelatase interaction
Biochemistry, 2004
The mitochondrial protein frataxin is essential for cellular regulation of iron homeostasis. Although the exact function of frataxin is not yet clear, recent reports indicate the protein binds iron and can act as a mitochondrial iron chaperone to transport Fe(II) to ferrochelatase and ISU proteins within the heme and iron–sulfur cluster biosynthetic pathways, respectively. We have determined the solution structure of apo yeast frataxin to provide a structural basis of how frataxin binds and donates iron to the ferrochelatase. While the protein's α–β-sandwich structural motif is similar to that observed for human and bacterial frataxins, the yeast structure presented in this report includes the full N-terminus observed for the mature processed protein found within the mitochondrion. In addition, NMR spectroscopy was used to identify frataxin amino acids that are perturbed by the presence of iron. Conserved acidic residues in the helix 1–strand 1 protein region undergo amide chemical shift changes in the presence of Fe(II), indicating a possible iron-binding site on frataxin. NMR spectroscopy was further used to identify the intermolecular binding interface between ferrochelatase and frataxin. Ferrochelatase appears to bind to frataxin's helical plane in a manner that includes its iron-binding interface.
Journal of Biological Chemistry, 2013
Background: Iron-induced oligomerization of frataxin is still poorly understood. Results: The molecular basis of iron-induced oligomerization of yeast and bacterial frataxin is revealed. Catalyzed ferroxidation is required for correct oligomerization of Yfh1. Frataxin forms different oligomeric species at physiological conditions. Significance: Iron availability controls frataxin oligomerization, which in turn may control the processes that require iron delivery by frataxin. The role of the mitochondrial protein frataxin in iron storage and detoxification, iron delivery to iron-sulfur cluster biosynthesis, heme biosynthesis, and aconitase repair has been extensively studied during the last decade. However, still no general consensus exists on the details of the mechanism of frataxin function and oligomerization. Here, using small-angle x-ray scattering and x-ray crystallography, we describe the solution structure of the oligomers formed during the iron-dependent assembly of yeast (Yfh1) and Escherichia coli (CyaY) frataxin. At an iron-to-protein ratio of 2, the initially monomeric Yfh1 is converted to a trimeric form in solution. The trimer in turn serves as the assembly unit for higher order oligomers induced at higher iron-to-protein ratios. The x-ray crystallographic structure obtained from iron-soaked crystals demonstrates that iron binds at the trimer-trimer interaction sites, presumably contributing to oligomer stabilization. For the ferroxidationdeficient D79A/D82A variant of Yfh1, iron-dependent oligomerization may still take place, although >50% of the protein is found in the monomeric state at the highest iron-to-protein ratio used. This demonstrates that the ferroxidation reaction controls frataxin assembly and presumably the iron chaperone function of frataxin and its interactions with target proteins. For E. coli CyaY, the assembly unit of higher order oligomers is a tetramer, which could be an effect of the much shorter N-terminal region of this protein. The results show that understanding of the mechanistic features of frataxin function requires detailed knowledge of the interplay between the ferroxidation reaction, iron-induced oligomerization, and the structure of oligomers formed during assembly.
Journal of Molecular Biology, 2005
The mitochondrial protein frataxin is emerging as a novel mechanism to promote iron metabolism while also providing anti-oxidant protection. Recombinant frataxin proteins from different species are able to form large molecular assemblies that store Fe(III) as a stable mineral in vitro. Furthermore, monomeric and assembled forms of frataxin donate Fe(II) to the Fe-S cluster scaffold protein IscU, [3Fe-4S] 1C aconitase, and ferrochelatase in vitro. However, little is known about the speciation of frataxin in vivo, and the physiologically relevant form(s) of the protein remains undefined. Here, we report that human heart mitochondria contain frataxin species of increasing negative surface charge and molecular mass, ranging from monomer to polymers of O1 MDa. Moreover, we show that the main partner protein of frataxin, IscU, binds in a stable manner to frataxin oligomers. These results suggest that assembly is a physiologic property of frataxin. Biochemical analyses further reveal that, unlike the prokaryotic and yeast frataxin homologues, which require ironprotein interactions for assembly, human frataxin uses stable subunitsubunit interactions involving a non-conserved amino-terminal region. We propose that human frataxin is a modular protein that depends on selfassembly to accomplish its diverse functions.
Dynamics, stability and iron-binding activity of frataxin clinical mutants
FEBS Journal, 2008
Human frataxin is a mitochondrial protein whose deficiency is associated with the neurodegenerative disorder Friedreich ataxia (FRDA; OMIM 229300), a pathology characterized by neuronal death, cardiomyopathy and diabetes . At the molecular level, the disease involves iron homeostasis deregulation and an impairment of the biosynthesis of iron-sulfur proteins . The majority of FRDA patients (> 95%) are homozygous for a GAA repeat expansion within the first intron of the frataxin gene . The expansion affects frataxin transcription, which results in a reduction of protein levels by 5-35%, depending on the insertion length. A small but significant number of FRDA patients are compound heterozygotes,
Nature Communications, 2010
Reduced levels of frataxin, an essential protein of as yet unknown function, are responsible for causing the neurodegenerative pathology Friedreich's ataxia. Independent reports have linked frataxin to iron-sulphur cluster assembly through interactions with the two central components of this machinery: desulphurase nfs1/Iscs and the scaffold protein Isu/Iscu. In this study, we use a combination of biophysical methods to define the structural bases of the interaction of CyaY (the bacterial orthologue of frataxin) with the Iscs/Iscu complex. We show that CyaY binds Iscs as a monomer in a pocket between the active site and the Iscs dimer interface. Recognition does not require iron and occurs through electrostatic interactions of complementary charged residues. mutations at the complex interface affect the rates of enzymatic cluster formation. CyaY binding strengthens the affinity of the Iscs/Iscu complex. our data suggest a new paradigm for understanding the role of frataxin as a regulator of Iscs functions.
Structure, 2006
Defects in the mitochondrial protein frataxin are responsible for Friedreich ataxia, a neurodegenerative and cardiac disease that affects 1:40,000 children. Here, we present the crystal structures of the ironfree and iron-loaded frataxin trimers, and a singleparticle electron microscopy reconstruction of a 24 subunit oligomer. The structures reveal fundamental aspects of the frataxin mechanism. The trimer has a central channel in which one atom of iron binds. Two conformations of the channel with different metalbinding affinities suggest that a gating mechanism controls whether the bound iron is delivered to other proteins or transferred to detoxification sites. The trimer constitutes the basic structural unit of the 24 subunit oligomer. The architecture of this oligomer and several features of the trimer structure demonstrate striking similarities to the iron-storage protein ferritin. The data reveal how stepwise assembly provides frataxin with the structural flexibility to perform two functions: metal delivery and detoxification.
Monomeric Yeast Frataxin Is an Iron-Binding Protein †
Biochemistry, 2006
Friedreich's ataxia, an autosomal cardio-and neurodegenerative disorder that affects 1 in 50,000 humans, is caused by decreased levels of the protein frataxin. Although nuclear encoded, frataxin is targeted to the mitochondrial matrix and necessary for proper regulation of cellular iron homeostasis. Frataxin is required for the cellular production of both heme and iron-sulfur clusters. Monomeric frataxin binds with high affinity to ferrochelatase, the enzyme involved in iron insertion into porphyrin during heme production. Monomeric frataxin also binds to Isu, the scaffold protein required for assembly of Fe-S cluster intermediates. These processes (heme and Fe-S cluster assembly) share requirements for iron, suggesting monomeric frataxin might function as the common iron donor.