Abstract

Cap43 protein has been tested for metal binding domains. The protein, specifically induced by nickel compounds in cultured human cells, had a new mono-histidinic motif consisting of 10 amino acids repeated three times in the C-terminus.
The 20-Ac-TRSRSHTSEG–TRSRSHTSEG (Thr³⁴¹–Arg–Ser–Arg–Ser–His ³⁴⁶–Thr–Ser–Glu–Gly–Thr–Arg–Ser–Arg–Ser–His³⁵⁶ –Thr–Ser–Glu–Gly³⁶⁰ – peptide 1) and the 30–Ac-TRSRSHTSEG–TRSRSHTSEG–TRSRSHTSEG (Thr³⁴¹ –Arg–Ser–Arg–Ser–His³⁴⁶ –Thr–Ser–Glu–Gly–Thr–Arg–Ser–Arg–Ser–His³⁵⁶ –Thr–Ser–Glu–Gly–Thr–Arg–Ser–Arg–Ser–His³⁶⁶ –Thr–Ser–Glu–Gly ³⁷⁰ – peptide 2) amino acids sequence has been analyzed as a site for Ni(II) binding.
A combined pH-metric and spectroscopic (UV–visible, CD, NMR) studies of Ni(II) binding to both fragments were performed. The 20-amino acid peptide can bind one and two metal ions while the 30-amino acid fragment one, two and three metal ions. At physiological pH, depending on the metal to ligand molar ratio, peptide 1 forms the Ni ₂L species while peptide 2 the NiL, Ni₂L and Ni₃ L complexes where each metal ion is coordinated to the imidazole nitrogen atom of the histidine residue of the 10-amino acid fragment. Octahedral complexes at pH 8–9 and planar 4N complexes with (N _Im , 3N ⁻ ) bonding mode at pH above 9, are formed.
This work supports the existence of an interesting binding site at the COOH-terminal domain of the Cap43 protein.

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