The nucleocapsid protein NCp7 of the human immunodeficiency virus type 1 contains two zinc fingers of the CX 2 CX 4 HX 4 C type, flanked by several basic residues, and plays a major role in viral infectivity. Thus, NCp7 was shown to promote annealing of the tRNA 3 Lys to the primer binding site, a key step in reverse transcription. However, previous in vitro experiments were unable to clarify the role of the zinc fingers in this process, due to nucleic acid aggregation induced by the basic N-and C-terminal domains of NCp7. We show here that deletion of these sequences in (12-53)NCp7 strongly reduces the formation of aggregates and allows a direct visualization of the binary or ternary complexes between NCp7 and nucleic acids by gel electrophoresis. (12-53)NCp7 is able to induce hybridization of the 33 P tRNA 3 Lys and the human immunodeficiency virus type 1 viral RNA-(77-257), which contains the primer binding site. Modification of the proximal zinc finger conformation in Cys 23 (12-53)NCp7 led to a large reduction in this hybridization process, while replacement of Trp 37 by Leu in the distal zinc fingers resulted in a complete absence of annealing activity. These data account for the in vivo loss of viral infectivity following these mutations and emphasize the critical role of the structure of the zinc finger domain of NCp7. This could facilitate a rational approach to new antiviral agents directed toward NCp7.
The 96-amino acid protein Vpr functions as a regulator of cellular processes involved in human immunodeficiency virus, type 1 (HIV-1) life cycle, in particular by interrupting cells division in the G 2 phase. Incorporation of Vpr in the virion was reported to be mediated by the C-terminal domain of the Pr55Gag polyprotein precursor, which includes NCp7, a protein involved in the genomic RNA encapsidation and p6, a protein required for particle budding. To precisely define the Gag and Vpr sequences involved in this protein-protein interaction, NCp7, p6, and Vpr as well as a series of derived peptides were synthesized using Fmoc (N-(9-fluorenyl)-methoxycarbonyl) chemistry. Binding assays were carried out by Far Western experiments and by competition studies using (52-96)Vpr immobilized onto agarose beads. The results show that interaction between NCp7 and Vpr occurs in vitro by a recognition mechanism requiring the zinc fingers of NCp7 and the last 16 amino acids of Vpr. Moreover, NCp10, the equivalent of NCp7 in Moloney murine leukemia virus but not polysine inhibits Vpr-NCp7 complexation. Interestingly enough, Vpr was found to interact with Gag, NCp15, and NCp7 but not with mature p6 in vitro. In vivo mutations in NCp7 zinc fingers in an HIV-1 molecular clone led to viruses with important defects in Vpr encapsidation. Together, these results suggest that NCp7 cooperates with p6 to induce Vpr encapsidation in HIV-1 mature particles. The NCp7-Vpr complex could also be important for interaction of Vpr with cellular proteins involved in cell division.
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