Frataxin Structure and Function

The FA gene (FRDA) (Chamberlain et al. 1988; Campuzano et al. 1996) encodes a small mitochondrial matrix protein, frataxin that is highly conserved in evolution. A single frataxin gene is found in all eukaryotes, including fungi and plants. A homologue, CyaY, is present in Gram-negative bacteria and in other prokaryotes like Rickettsia prowazeckii, thought to be related to the hypothetical mitochondrial precursor. FRDA is expressed in all cells, but at variable levels in different tissues and during development (Koutnikova et al. 1997; Jiralerspong et al. 1997). In adult humans, frataxin mRNA is most abundant in the heart, brain, and spinal cord, followed by liver, skeletal muscle, and pancreas.

FA patients have a profound but not complete frataxin deficiency, with a small residual amount of normal protein as a result of the d(GAA) triplet repeat expansion.

Structural studies have been carried out on frataxin (Dhe-Paganon et al. 2000; Musco et al. 2000) and its bacterial homologue, CyaY (Cho et al. 2000) by nuclear magnetic resonance and by crystallography. The structure is compact, overall globular, containing an N-terminal a-helix, a middle P-sheet region composed of seven P-strands, a second a-helix, and a C-terminal coil. On the outside, a ridge of negatively charged residues and a patch of hy-drophobic residues are highly conserved.

Knockout of the yeast frataxin homologous gene (YFH1) in yeast (Ayfh1) causes the loss of oxidative phosphorylation and of mitochondrial DNA (Bab-cock et al. 1997; Wilson and Roof 1997). Iron accumulates in mitochondria of Ayfh1 to more then tenfold its level in wild-type yeast. Loss of respiratory competence requires the presence of iron in the culture medium, and occurs more rapidly as the iron concentration in the medium is increased, suggesting that permanent mitochondrial damage is the consequence of iron toxicity (Radisky et al. 1999). Formation of the highly toxic hydroxyl radical through the Fenton reaction is suggested by the enhanced sensitivity of Ayfh1 to H2O2 (Babcock et al. 1997). In Ayfh1 yeast, there is a marked induction (tenfold to 50-fold) of the high-affinity iron trans port system on the cell membrane, normally not expressed in yeast cells that are iron-replete (Babcock et al. 1997). This induction has been recently related to a deficit in mitochondrial synthesis of iron-sulfur clusters (ISCs), rather than cytosolic iron depletion as previously thought (Chen et al. 2004). ISC-containing enzymes, such as respiratory chain complexes I, II, and III, and aconitase, are impaired in Ayfhl yeast (Rotig et al. 1997). Frataxin appears to be involved in an early step of ISC synthesis (Muh-lenhoff et al. 2003), through its interaction with the scaffold protein Isu1, where the first ISC assembly takes place, probably facilitating iron incorporation (Yoon and Cowan 2003). This finding suggests that frataxin may be a mitochondrial iron chaperone, protecting this metal from reactive oxygen species and making it bioavailable. Recent data support this view, suggesting that frataxin also acts as an iron chaperone in heme synthesis (Yoon and Cowan 2004), and in the modulation of aconitase activity (Bulteau et al. 2004). A much higher affinity of frataxin for the heme-synthesis enzyme ferrochelatase than for Isu1 (Yoon and Cowan 2004) would explain why heme synthesis is resistant to low frataxin levels and is essentially unaffected in FA patients.

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