ABSTRACT All retroviral nucleocapsid (NC) protein contain one or two copies of the invariant Cys-X2-Cys-X4-His- X4-Cys sequence that chelates zinc through cysteine thiolate and histidine imidazole coordination. The CCHC residues form a zinc finger motif which plays a major role in virion morphogenesis, genomic RNA packaging and viral infectivity. To elucidate the importance of this particular motif, well conserved in retroviruses and retroelements, we closely investigated the zinc binding properties of NC corresponding to the human immunodeficiency virus type 2 (HIV-2). To this end we synthesized a 39-residue peptide encompassing the two zinc fingers (NC39), using Fmoc (N-(9-fluorenyl) methoxycarbonyl) chemistry. Electrospray ionization mass spectrometry (ES-MS) was used to control the purety of the corresponding synthetic peptide and to determine the stoichiometry of our complex, while complete sequence specific 1H NMR assignment of this complex was achieved by two-dimensional NMR experiments in aqueous solution. Circular dichroism study and qualitative analysis of NOE data reveal that this zinc complex adopts a compact, folded conformation characterized by successive loops and turns containing a tetrahedrally coordinated zinc atom, with the two fingers independently folded. At neutral pH the N-terminal motif was found to bind zinc more strongly than does the C-terminal motif indicating that the two Zn2+ fingers of HIV-2 are not functionally equivalent.
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