Waiting for frataxin

Sticky DNA, a self-associated complex formed at long GAA*TTC repeats in intron 1 of the frataxin gene, inhibits transcription

Friedreich's ataxia (FRDA) is an autosomal recessive neurodegenerative disease caused by the expansion of GAA.TTC repeats in the first intron of the frataxin (X25) gene. FRDA patients carrying two expanded GAA.TTC repeats show very low levels of mature frataxin mRNA and protein. A novel type of unusual DNA structure, sticky DNA, was previously found in the expanded GAA.TTC repeats from FRDA patients. To evaluate the effect of sticky DNA on transcription, in vitro transcription studies of (GAA.TTC)(n) repeats (where n = 9-150) were carried out using T7 or SP6 RNA polymerase. When a gel-isolated sticky DNA template was transcribed, the amount of full-length RNA synthesized was significantly reduced compared with the transcription of the linear template. Surprisingly, transcriptional inhibition was observed not only for the sticky DNA template but also another DNA molecule used as an internal control in an orientation-independent manner. The molecular mechanism of transcriptional inhibition by sticky DNA was a sequestration of the RNA polymerases by direct binding to the complex DNA structure. Moreover, plasmids containing the (GAAGGA.TCCTTC)(65) repeat, which does not form sticky DNA, did not inhibit in vitro transcription, as expected. These results suggest that the role of sticky DNA in FRDA may be the sequestration of transcription factors.

Frataxin activates mitochondrial energy conversion and oxidative phosphorylation

Friedreich's ataxia (FA) is an autosomal recessive disease caused by decreased expression of the mitochondrial protein frataxin. The biological function of frataxin is unclear. The homologue of frataxin in yeast, YFH1, is required for cellular respiration and was suggested to regulate mitochondrial iron homeostasis. Patients suffering from FA exhibit decreased ATP production in skeletal muscle. We now demonstrate that overexpression of frataxin in mammalian cells causes a Ca(2+)-induced up-regulation of tricarboxylic acid cycle flux and respiration, which, in turn, leads to an increased mitochondrial membrane potential (delta psi(m)) and results in an elevated cellular ATP content. Thus, frataxin appears to be a key activator of mitochondrial energy conversion and oxidative phosphorylation.

Crystal structure of human frataxin

Friedreich's ataxia, an autosomal recessive neurodegenerative disorder characterized by progressive gait and limb ataxia, cardiomyopathy, and diabetes mellitus, is caused by decreased frataxin production or function. The structure of human frataxin, which we have determined at 1.8-A resolution, reveals a novel protein fold. A five-stranded, antiparallel beta sheet provides a flat platform, which supports a pair of parallel alpha helices, to form a compact alphabeta sandwich. A cluster of 12 acidic residues from the first helix and the first strand of the large sheet form a contiguous anionic surface on the protein. The overall protein structure and the anionic patch are conserved in eukaryotes, including animals, plants, and yeast, and in prokaryotes. Additional conserved residues create an extended 1008-A(2) patch on a distinct surface of the protein. Side chains of disease-associated mutations either contribute to the anionic patch, help create the second conserved surface, or point toward frataxin's hydrophobic core. These structural findings predict potential modes of protein-protein and protein-iron binding.