Functional complementation of nucleocapsid and late domain PTAP mutants of human immunodeficiency virus type 1 during replication.

During human immunodeficiency virus type 1 (HIV-1) assembly, the nucleocapsid (NC) and the PTAP motif in p6 of Gag play important roles in RNA encapsidation and virus release, respectively. We have previously demonstrated that functional complementation occurs between an NC mutant and a PTAP mutant to rescue viral replication. In this report, we examined the amounts of functional NC and PTAP motif that are required during virus replication. When NC and PTAP mutants were coexpressed at 5:1, 5:5, and 1:5 ratios, virus titers were rescued at 5%, 51%, and 86% of the wild-type level, respectively. These results indicate that HIV-1 requires a small amount of functional PTAP motif but far more functional NC to complete efficient replication. Further analyses reveal that RNA packaging can be significantly rescued in viruses containing a small amount of functional NC. However, most of the NC proteins must be functional to generate the wild-type level of R-U5 DNA product. Once the R-U5 product is generated, viruses containing half of the functional NC can complete reverse transcription and DNA integration at near-wild-type efficiency. These results define the quantitative requirements of NC and p6 during HIV-1 replication and provide insights into the requirement for the development of anti-HIV strategies using NC and p6 as targets.