Characterization of Moloney murine leukemia virus mutants with single-amino-acid substitutions in the Cys-His box of the nucleocapsid protein.

To study the function of the retroviral nucleocapsid protein (NC), we have constructed point mutations in the gag gene of Moloney murine leukemia virus (MuLV) that affect a conserved cysteine-histidine motif of NC. The mutants were characterized biologically and biochemically. Cell lines producing the mutant virions were constructed in NIH 3T3 and rat2 cells, and the viral particles released by these cells were characterized for protein and RNA content. The results indicated that most mutations block replication and specifically inhibit the packaging of the MuLV genomic RNA. In some of the mutants, the packaging of the endogenous rat VL30 RNA was not affected as profoundly as was MuLV RNA. NC also seems to have another function distinct from dimer formation and packaging: one mutation reduced viral RNA packaging by only fivefold but completely abolished viral cDNA synthesis, suggesting a defect in reverse transcription.