IDENTIFICATION OF A NEW VIRAL PROTEIN CONTAINING CAP30 AND NCP10 SEQUENCES IN MURINE AND FELINE LEUKEMIA RETROVIRUSES

Because Pr65(gag) is in part located in the nucleus and contains a put ative bipartite nuclear targeting signal, we investigated the cellular location and structure of P55(gag), a gag-encoded polyprotein known t o lack the nucleocapsid (NC) protein NCp10. P55(gag),vas found to be r estricted to the cytoplasm of Moloney murine leukemia virus-infected c ells. Of interest, P55(gag) was produced in cells infected by a viral protease deletion mutant and by a recombinant murine sarcoma virus kno ,un to lack the protease gene. Surprisingly, our structural and immuno logical studies indicated that P55(gag) also lacks carboxy-terminal re sidues of CAp30. During the course of studying P55(gag), we detected a new viral protein within purified virus particles that contained NCp1 0 tryptic peptide sequences and a CAp30 tryptic peptide lacking in P55 (gag). This viral protein, which we have named nucleocapsid-related pr otein (NCRP), also contained antigenic epitopes present in CAp30 and N Cp10. P55(gag)- and NCRP-like proteins were also observed in AKV murin e leukemia virus and feline leukemia virus systems. The precise site o f cleavage within Pr65(gag) that produces P55(gag) 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 fu nctional role in genomic RNA packaging and/or viral RNA dimerization.