Design of a Protein Motif Responsive to Tyrosine Nitration and an Encoded Turn-Off Sensor of Tyrosine Nitration

Tyrosine nitration is a protein post-translational modification that is predominantly non-enzymatic and is observed to be increased under conditions of nitrosative stress and in numerous disease states. A small protein motif (14-18 amino acids) responsive to tyrosine nitration has been developed. In this design, nitrotyrosine replaced the conserved Glu12 of an EF-hand metal-binding motif. Thus, the non-nitrated peptide bound terbium weakly. In contrast, tyrosine nitration resulted in a 45-fold increase in terbium affinity. Nuclear magnetic resonance spectroscopy indicated direct binding of nitrotyrosine to the metal and EF-hand-like metal contacts in this designed peptide. Nitrotyrosine is an efficient quencher of fluorescence. To develop a sensor of tyrosine nitration, the initial design was modified to incorporate Glu residues at EF-hand positions 9 and 16 as additional metal-binding residues, to increase the terbium affinity of the peptide with unmodified tyrosine. This peptide with a tyrosine at residue 12 bound terbium and effectively sensitized terbium luminescence. Tyrosine nitration resulted in a 180-fold increase in terbium affinity (K-d = 1.6 mu M) and quenching of terbium luminescence. This sequence was incorporated as an encoded protein tag and applied as a turn-off fluorescent protein sensor of tyrosine nitration. The sensor was responsive to nitration by peroxynitrite, with fluorescence quenched upon nitration. The greater terbium affinity upon tyrosine nitration resulted in a large dynamic range and sensitivity to substoichiometric nitration. An improved approach for the synthesis of peptides containing nitrotyrosine was also developed, via the in situ silyl protection of nitrotyrosine. This work represents the first designed, encodable protein motif that is responsive to tyrosine nitration.