Specific binding of the type C viral core protein p12 with purified viral RNA.

The major viral phosphoproteins (p12) of the Rauscher murine leukemia virus (R-MuLV) and the simian sarcoma-associated virus (SSAV) bind in vitro to their homologous 70S and 35S viral RNAs. Using purified 32P-labeled RNA and 125I-labeled p12 protein, complexes that are stabilized by formaldehyde-cross-linking can be readily detected after velocity gradient centrifugation. The in vitro reconstructed ribonucleoprotein complexes are seen only with p12 proteins incubated with viral RNAs isolated from the same type C viruses; no such complexes form with heterologous protein-RNA mixtures. Homologous but not heterologous p12 molecules compete with radiolabeled p12 protein for the specific viral RNA binding sites. The competition assay permits the detection of 10 ng of viral p12 protein. The major internal protein of type C viruses (p30) does not bind to viral RNA using identical assay conditions. From the specific activities of the radiolabeled components and also by equilibrium sedimentation analysis, we estimate that fewer than 15 molecules of p12 protein bind to each molecule of viral RNA. Both the specificity and stoichiometry of the p12-RNA interactions suggest that these RNA tumor virus proteins have a regulatory role in cells.