Human frataxin: a model to understand sequence-structure-function relationships

ASTOR MIGNONE; SANTIAGO FARAJ; LEANDRO VENTURUTTI; ERNESTO A ROMÁN; MARIANO CAMILO GONZÁLEZ LEBRERO; JOSÉ MARÍA DELFINO; JAVIER SANTOS

Ciudad de Salta, Salta, Argentina

Congreso; 3rd Latin American Protein Society Meeting & XXXIX Annual Meeting of the Argentinean Biophysical Society; 2010

Latin American Protein Society & Sociedad Argentina de Biofísica

Friedreich ataxia (FA) is caused by the deficient expression of frataxin (FXN), an α+β protein of 120 amino acid residues. FXN is capable of complexing significant amounts of Fe metal ion. The 3D structure of human FXN (hFXN) in its monomeric form has been thoroughly studied by NMR and X-ray crystallography. Variations in the C-terminal region correlate with the conformational stability of different homologues. A detailed study of this part of the molecule is necessary in order to understand its role in the consolidation of the tertiary and quaternary structure of the protein. The quaternary structure of yeast FXN (yFXN) is induced by iron. On the contrary, this phenomenon does not occur in hFXN, and it is not clear whether multimerization of hFXN occurs in-vivo. The N-terminal region of the molecule, which is not conserved along the evolution of FXNs, has a role in the oligomerization process. However, we do not know the molecular mechanism that defines the in-vitro multimerization of hFXN. For this project we decided to explore the role of N- and C-terminal regions in FXN stability. We studied three FXN variants: (a) FXN56-210, that aggregates during in-vitro expression and can be unfolded and refolded into the monomeric form; (b) FXN90-210, a truncated and monomeric version that is very stable, folds cooperatively and retains biological function; and (c) FXN90-180, a partially folded fragment that conserves iron binding sites, exhibits secondary structure –but only residual tertiary structure–  and experiences non-cooperative temperature unfolding. In addition, we ran molecular dynamics simulations of fragments FXN90-195 and FXN90-210 and applied bioinformatic tools to study sequence 56-90. Our results confirm the role of C-terminal moiety on the stabilization of the cooperative α‑unit of hFXN. We hypothesize that the N-terminal may have a role in the multimerization through the stabilization of a partially folded state.