Apparent lack of trans-dominant negative effects of various vif mutants on the replication of HIV-1

Identification of dominant negative human immunodeficiency virus type 1 Vif mutants that interfere with the functional inactivation of APOBEC3G by virus-encoded Vif

APOBEC3G (A3G) is a host cytidine deaminase that serves as a potent intrinsic inhibitor of retroviral replication. A3G is packaged into human immunodeficiency virus type 1 virions and deaminates deoxycytidine to deoxyuridine on nascent minus-strand retroviral cDNA, leading to hyper-deoxyguanine-to-deoxyadenine mutations on positive-strand cDNA and inhibition of viral replication. The antiviral activity of A3G is suppressed by Vif, a lentiviral accessory protein that prevents encapsidation of A3G. In this study, we identified dominant negative mutants of Vif that interfered with the ability of wild-type Vif to inhibit the encapsidation and antiviral activity of A3G. These mutants were nonfunctional due to mutations in the highly conserved HCCH and/or SOCS box motifs, which are required for assembly of a functional Cul5-E3 ubiquitin ligase complex. Similarly, mutation or deletion of a PPLP motif, which was previously reported to be important for Vif dimerization, induced a dominant negative phenotype. Expression of dominant negative Vif counteracted the Vif-induced reduction of intracellular A3G levels, presumably by preventing Vif-induced A3G degradation. Consequently, dominant negative Vif interfered with wild-type Vif's ability to exclude A3G from viral particles and reduced viral infectivity despite the presence of wild-type Vif. The identification of dominant negative mutants of Vif presents exciting possibilities for the design of novel antiviral strategies.

Amino acid residues 88 and 89 in the central hydrophilic region of human immunodeficiency virus type 1 Vif are critical for viral infectivity by enhancing the steady-state expression of Vif

A hydrophilic region consisting of strikingly clustered charged amino acids is present at the center of human immunodeficiency virus type 1 (HIV-1) Vif. In this study, the role for this central hydrophilic region (E(88)WRKKR(93)) in the virus replication in nonpermissive H9 cells was investigated by extensive deletion and substitution analysis. A total of 31 mutants were constructed. Deletion of the E(88) or W(89) residue alone abolished viral infectivity in H9 cells and impaired virus replication in primary macrophage cultures. Substitution analysis indicated that the hydrophilicity and charge of the central region are insignificant for the function of Vif. Of the 16 substitution mutants, 3 mutants with substitution of E(88) and W(89) with an A residue did not grow in H9 cells. Upon transfection, four mutants (i.e., two mutants with deletion of E(88) or W(89); a mutant with substitution of E(88) and W(89) with A; and a mutant with substitution of E(88), W(89), and R(90) with A) were found to express Vif at a very reduced level relative to that by the wild-type clone. These results have thus demonstrated that amino acid residues 88 and 89 of Vif are critical for the replication of HIV-1 in target cells by enhancing the steady-state expression of Vif. In addition, E(88) and W(89) residues were found to be extremely conserved among the Vif proteins of naturally occurring HIV-1 field isolates as well as those of laboratory HIV-1 strains.