Evaluation of the metal binding properties of a histidine-rich fusogenic peptide by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICRMS) was used to investigate metal ion interactions of the 18 amino acid peptide fragment B18 (LGLLLRHLRHHSNLLANI), derived from the membrane-associated protein bindin. The peptide sequence B18 represents the minimal membrane-binding motif of bindin and resembles a putative fusion peptide. The histidine-rich peptide has been shown to self-associate into distinct supramolecular structures, depending on the presence of Zn(2+) and Cu(2+). We examined the binding of B18 to the metal ions Cu(2+), Zn(2+), Mg(2+), Ca(2+), Mn(2+) and La(3+). For Cu(2+), we compared the metal binding affinities of the wild-type B18 peptide with those of its mutants in which one, two or three histidine residues have been replaced by serines. Upon titration of B18 with Cu(2+) ions, we found sequential binding of two Cu(2+) ions with dissociation constants of approximately 34 and approximately 725 micro M. Mutants of B18, in which one histidine residue is replaced by serine, still exhibit sequential binding of two copper ions with affinities for the first Cu(2+) ion comparable to that of wild-type B18 peptide, but with a greatly reduced affinity for the second Cu(2+) ion in mutants H112S and H113S. For mutants in which two histidines are replaced by serines, the affinity for the first Cu(2+) ion is reduced approximately 3-10 times in comparison with B18. The mutant in which all three histidine residues are replaced by serines exhibits an approximately 14-fold lower binding for the first Cu(2+) ion compared with B18. For the other metal ions under investigation (Zn(2+), Mg(2+), Ca(2+), Mn(2+) and La(3+)), a modest affinity to B18 was detected binding to the peptide in a 1 : 1 stoichiometry. Our results show a high affinity of the wild-type fusogenic peptide B18 for Cu(2+) ions whereas the Zn(2+) affinity was found to be comparable to that of other di- and trivalent metal ions.