A structural protein with a molecular weight of approximately 12,000 has been purified from Gross murine oncornavirus by phosphocellulose column chromatography. The protein was distinct from all other of the principle Gross virus components and did not react with antisera prepared against other purified Gross virus proteins with molecular weights of about 30,000, 17,000, 14,000 and 10,000. The immunologic properties of the protein were analyzed by competition radioimmunoassay. The antigenic determinants were chiefly subgroup-specific with a close homology to the analogous protein of AKR, Kirsten, or BALB/c xenotropic viruses but not to proteins of Friend, Rauscher, Moloney, NIH Swiss xenotropic, or NZB xenotropic viruses. Group-specific determinants cross-reactive with the analogous protein of these latter viruses were present but were minor components of the total antigenic specificities of the protein. Weak interspecies cross-reactivity was demonstrated with the Theilen feline virus, but very little, if any, with different primate oncornaviruses. The identity of the related, cross-reactive proteins of other murine viruses was analyzed both by competition radioimmunoassay and immunoprecipitation. It was identified as the 15,000 MW component of Rauscher and Friend viruses, and a 12,000 MW component of Moloney and BALB xenotropic viruses. The close similarity in the properties of the p15 of Rauscher murine virus and this p12 of Gross murine virus establish these as the homologous subgroup-specific proteins of the FMR and Gross subgroups of viruses.
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