NUCLEOCAPSID AND MATRIX PROTEIN CONTRIBUTIONS TO SELECTIVE HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 GENOMIC RNA PACKAGING

The nucleocapsid protein (NC) of retroviruses plays a major role in ge nomic RNA packaging, and some evidence has implicated the matrix: prot ein (MA) of certain retroviruses in viral RNA binding. To further inve stigate the role of NC in the selective recognition of genomic viral R NA and to address the potential contribution of MA in this process, we constructed chimeric and deletion human immunodeficiency virus type 1 (HIV-1) mutants that alter the NC or MA protein. Both HIV and mouse m ammary tumor virus (MMTV) NC proteins have two zinc-binding domains an d similar basic amino acid compositions but differ substantially in to tal length, amino acid sequence, and spacing of the zinc-binding motif s. When the entire NC coding sequence of HIV was replaced with the MMT V NC coding sequence, we found that the HIV genome was incorporated in to virions at 50% of wild-type levels, Viruses produced from chimeric HIV genomes with complete NC replacements, or with the two NC zinc-bin ding domains replaced with MMTV sequences, preferentially incorporated HIV genomes when both HN and MMTV genomes were simultaneously present in the cell. Viruses produced from chimeric MMTV genomes in which the MMTV NC had been replaced with HIV NC preferentially incorporated MMT V genomes when both HIV and MMTV genomes were simultaneously present i n the cell. In contrast, viruses produced from chimeric HIV genomes co ntaining the Moloney NC, which contains a single zinc-binding motif, w ere previously shown to preferentially incorporate Moloney genomic RNA Taken together, these results indicate that an NC protein with two zi nc-binding motifs is required for specific HIV RNA packaging and that the amino acid contest of these motifs, while contributing to the proc ess, is less crucial for specificity. The data also suggest that HIV N C may not be the exclusive determinant of RNA selectivity. Analysis of an HIV MA mutant revealed that specific RNA packaging does not requir e MA protein.