Effects of nucleocapsid mutations on human immunodeficiency virus assembly and RNA encapsidation.

The human immunodeficiency virus (HIV) Pr55Gag precursor proteins direct virus particle assembly. While Gag-Gag protein interactions which affect HIV assembly occur in the capsid (CA) domain of Pr55Gag, the nucleocapsid (NC) domain, which functions in viral RNA encapsidation, also appears to participate in virus assembly. In order to dissect the roles of the NC domain and the p6 domain, the C-terminal Gag protein domain, we examined the effects of NC and p6 mutations on virus assembly and RNA encapsidation. In our experimental system, the p6 domain did not appear to affect virus release efficiency but p6 deletions and truncations reduced the specificity of genomic HIV-1 RNA encapsidation. Mutations in the nucleocapsid region reduced particle release, especially when the p2 interdomain peptide or the amino-terminal portion of the NC region was mutated, and NC mutations also reduced both the specificity and the efficiency of HIV-1 RNA encapsidation. These results implicated a linkage between RNA encapsidation and virus particle assembly or release. However, we found that the mutant ApoMTRB, in which the nucleocapsid and p6 domains of HIV-1 Pr55Gag were replaced with the Bacillus subtilis MtrB protein domain, released particles efficiently but packaged no detectable RNA. These results suggest that, for the purposes of virus-like particle assembly and release, NC can be replaced by a protein that does not appear to encapsidate RNA.