The Zn2+ binding properties of the synthetic nucleocapsid protein (Ncp7) of HIV-1, containing two zinc-binding domains, have been studied using electrospray mass spectrometry (ES-MS). ES-MS measurements revealed strong binding of Zn2+ by Ncp7. Its shorter fragments, Ncp7-(1–35)- and (29–55)-peptides, each containing only one zinc-binding domain, bind one equivalent of Zn2+ ions tightly. ES-MS studies allows these fragments to be distinguished in terms of their binding affinity: they showed stronger binding of Zn2+ by Ncp7-(1–35)-peptide. Surprisingly, in addition to the expected two zinc-binding domains, a third metal binding site was detected in Ncp7. However, this site appears to bind different metal ions without selectivity and most probably reflects salt formation at the C-terminal acidic residues.
