Structural Insights into the N-Terminal Domain of Human Frataxin Provided by the Stable Intermediate FXN45-210

FEDERICO IACOVELLI; MARIANA GALLO; MARCO FRANCONERI; JAVIER SANTOS; SANTIAGO FARAJ; MATTIA FALCONI; ALESSANDRO DESIDERI; DANIEL O. CICERO

Frauenwörth

Congreso; 35th FGMR Discussion Meeting and Joint Conference; 2013

German, Italian and Slovenian Magnetic Resonance Societies

Frataxin (FXN) is a mitochondrial protein highly conserved across evolution, with homologues in almost all known organisms. FXN is an iron-binding protein critical for mitochondrial iron metabolism, global cellular iron homeostasis, and antioxidant protection. In humans, deficiency of FXN is associated with Friedreich ataxia, a neurological and autosomal recessive disease. The FXN mRNA is translated to a precursor polypeptide that is transported to the mitochondrial matrix and processed to at least two isoforms, FXN42?210 and FXN81?210. Human cells normally contain both isoforms and it was found that they have different biochemical properties and functional roles [1]. Structural studies have shown that the frataxin fold comprises a flexible N-terminal region present only in eukaryotes and a highly conserved C-terminal globular domain that in humans spans residues 90- 210. The structure of the C-terminal domain of FXN was solved both by NMR and X-ray crystallography consisting of a slightly elongated domain in which two helices pack against an antiparallel β-sheet of six strands. Complications from proteolysis and degradation of the protein?s N-terminus hindered the characterization beyond residues 88?210, though studies on FXN61-210 and FXN81-210 indicate that the N-terminus is unfolded and highly dynamic. However, recently FXN45-210 was reported to be stable and nearly complete resonance assignments of these residues were published [2]. With the aim of providing a structural basis for understanding the biological role of the human FXN N-terminus we carried out an NMR characterization of FXN45-210, complemented with molecular dynamics calculations. We discuss the results in the context of the biochemical effects that may contribute to the pathophysiology of Friedreich ataxia. [1] Gakh et al. (2010) J. Biol. Chem. 285, 38486-501. [2] Kondapalli et al. (2010). Biomol. NMR Assign. 4, 61-4.