EFFECTS OF NUCLEOCAPSID MUTATIONS ON HUMAN-IMMUNODEFICIENCY-VIRUS ASSEMBLY AND RNA ENCAPSIDATION

The human immunodeficiency virus (HIV) pr55(Gag) precursor proteins di rect virus particle assembly. While Gag-Gag protein interactions which affect PW assembly occur in the capsid (CA) domain of pr55(Gag), the nueleocapsid (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 assemb ly and RNA encapsidation. In our experimental system, the p6 domain di d not appear to affect virus release efficiency but p6 deletions and t runcations reduced the specificity of genomic HIV-I RNA encapsidation. Mutations in the nucleocapsid region reduced particle release, especi ally when the p2 interdomain peptide or the amino-terminal portion of the NC region was mutated, and NC mutations also reduced both the spec ificity and the efficiency of HIV-1 RNA encapsidation. These results i mplicated a linkage between RNA encapsidation and virus particle assem bly or release. However, we found that the mutant ApoMTRB, in which th e nucleocapsid and p6 domains of HIV-1 pr55(Gag) were replaced with th e Bacillus subtilis MtrB protein domain, released particles efficientl y but packaged no detectable RNA. These results suggest that, for the purposes of virus-like particle assembly and release, NC can be replac ed by a protein that does not appear to encapsidate RNA.