RETROVIRAL NUCLEOCAPSID DOMAINS MEDIATE THE SPECIFIC RECOGNITION OF GENOMIC VIRAL RNAS BY CHIMERIC GAG POLYPROTEINS DURING RNA PACKAGING IN-VIVO

The retroviral nucleocapsid (NC) protein is necessary for the specific encapsidation of the viral genomic RNA by the assembling virion. Howe ver, it is unclear whether NC contains the determinants for the specif ic recognition of the viral RNA or instead contributes nonspecific RNA contacts to strengthen a specific contact made elsewhere in the Gag p olyprotein. To discriminate between these two possibilities, we have s wapped the NC domains of the human immunodeficiency virus type 1 (HIV- 1) and Moloney murine leukemia virus (M-MuLV), generating an HIV-1 mut ant containing the M-MuLV NC domain and an M-MuLV mutant containing th e HIV-1 NC domain. These mutants, as well as several others, were char acterized for their abilities to encapsidate HIV-1, M-MuLV, and nonvir al RNAs and to preferentially package genomic viral RNAs over spliced viral RNAs. We found that the M-MuLV NC domain mediates the specific p ackaging of RNAs containing the M-MuLV Psi packaging element, while th e HIV-1 NC domain confers an ability to package the unspliced HIV-1 RN A over spliced HIV-1 RNAs. In addition, we found that the HIV-1 mutant containing the M-MuLV NC domain exhibited a 20-fold greater ability t han wild-type HIV-1 to package a nonviral RNA. These results help conf irm the notion that the NC domain specifically recognizes the retrovir al genomic RNA during RNA encapsidation.