A cell line has been isolated that produces a defective pseudotype particle of Kirsten sarcoma virus without detectable infectious helper virus. Normal rat kidney cells were infected at low multiplicity with a virus stock obtained by the rescue of Kirsten sarcoma virus with woolly monkey leukemia virus. The individual transformed cell foci which resulted were propagated. One was found to produce noninfectious oncovirus particles containing high reverse transcriptase activity. These defective particles contained an active sarcoma genome but no detectable helper virus genome as cells fused to the particles became transformed but did not show evidence of helper woolly monkey leukemia virus expression. Nevertheless, the rat kidney cells producing the defective particles contained the complete woolly monkey leukemia virus genome; after prolonged culture, infectious woolly monkey leukemia viruses were released and rapidly spread through the culture. The defect of the noninfectious particle could be attributed to the absence of the viral envelope glycoprotein. Helper virus env gene products were not detected in cells producing the defective particle, nor did these cells exhibit interference to superinfection by woolly monkey leukemia virus. The protein composition of the defective particle was unusual. Major proteins were a woolly monkey virus p28 and a novel 55,000-dalton protein of rodent origin. The 55,000-dalton protein was not immunologically related to the known rodent type-C virus gag or env gene products; nevertheless, it was immunoprecipitated by several anti-murine leukemia virus sera, suggesting that a related protein is commonly present in preparations of purified mouse type-C viruses.
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