Mutations altering the Moloney murine leukemia virus p12 Gag protein affect virion production and early events of the virus life cycle

The p12 Gag protein of Moloney murine leukemia virus is a small polypeptide of unknown function, containing two proline‐rich motifs. To determine its role in replication, we introduced a series of deletion and alanine‐scanning substitution mutations throughout the p12 coding region of a proviral DNA, and characterized the phenotypes of the resulting mutant viruses. Complete deletion of p12 and mutations affecting the PPPY motif caused substantial reduction in the yield of virions and a modest reduction in Gag processing. Proteolytic cleavage of the R‐peptide from the cytoplasmic tail of the envelope protein TM was abolished in these mutants, suggesting that the PPPY motif is crucial for the viral protease to access the TM tail. The resulting virions were non‐infectious, and unable to initiate DNA synthesis in infected cells. Mutants with alterations in both the N‐ and C‐terminal portions of p12 exhibited a distinct phenotype. The production of virions and processing of Gag, Pol and Env precursors were normal. The viruses were able to direct synthesis of linear viral DNA, but there was almost no detectable circular DNAs or LTR–LTR junction. These data suggest that p12 plays a critical role in the early events of the virus life cycle.