Nuclei of cells infected with Moloney murine leukemia virus (MoMuLV) were examined for the presence of gag proteins. This analysis was performed in conjunction with other studies suggesting a possible role for gag proteins in regulating nuclear events relating to processing and/or transport of viral genomic RNA. We detected Pr659'9 and a p30.related protein in a nuclear fraction of infected cells. We also found evidence that a highly conserved amino acid sequence, which is shared by p30 and Ul small nuclear ribonucleoprotein 70-kDa protein, is a component of the nuclear targeting sequence for Pr659'9. Immunoelectron microscopy studies with a monoclonal anti-p12 antibody established that approximately 18% ofgag-containing proteins of MoMuLV are located in the nucleus. Such gag-containing proteins from a mutant MoMuLV that lacks N-terminal myristic acid had greater affinity for the nucleus, suggesting that fatty acid acylation of Pr659ar plays a role in overcoming the proposed nuclear transport signal. The possible roles that nuclear gag proteins may play in retroviral replication are discussed.
Because Pr65 gag is in part located in the nucleus and contains a putative bipartite nuclear targeting signal, we investigated the cellular location and structure of P55 gag , a gag-encoded polyprotein known to lack the nucleocapsid (NC) protein NCp10. P55 gag was found to be restricted to the cytoplasm of Moloney murine leukemia virus-infected cells. Of interest, P55 gag was produced in cells infected by a viral protease deletion mutant and by a recombinant murine sarcoma virus known to lack the protease gene. Surprisingly, our structural and immunological studies indicated that P55 gag also lacks carboxy-terminal residues of CAp30. During the course of studying P55 gag , we detected a new viral protein within purified virus particles that contained NCp10 tryptic peptide sequences and a CAp30 tryptic peptide lacking in P55 gag. This viral protein, which we have named nucleocapsid-related protein (NCRP), also contained antigenic epitopes present in CAp30 and NCp10. P55 gag-and NCRP-like proteins were also observed in AKV murine leukemia virus and feline leukemia virus systems. The precise site of cleavage within Pr65 gag that produces P55 gag and NCRP is unknown but lies upstream of the CAp30-NCp10 junction within the carboxy-terminal domain of CAp30. The existence of a form of NCp10 containing carboxy-terminal CAp30 sequences raises interesting possibilities about its functional role in genomic RNA packaging and/or viral RNA dimerization.
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