Protein composition of a defective murine sarcoma virus particle possessing the enveloped type-A morphology

The protein composition of a noninfectious murine sarcoma virus (Gazdar MSV) isolated from the HTG2 cell line was characterized. Electron microscopy of thin sections of the infected cells demonstrated that the virus particles were assembled via the budding mechanism at the plasma membrane, and the released virions possessed almost exclusively the enveloped type-A morphology characteristic of the immature form of retroviral particle. The protein composition of the virions was shown by SDS-PAGE analysis under reducing conditions to consist of a nonglycosylated protein of approximately 65,000 daltons, p65. When analyzed under nonreducing conditions this band was resolved into at least two components, and higher molecular weight forms of p65 were observed as well. p65 could be immunoprecipitated with monospecific antisera against the individual "gag" gene-coded MuLV proteins p30, p15, and p12 but not with antisera against the MuLV envelope proteins, reverse transcriptase, or p10, indicating that the MSV p65 is related to but not identical with the Pr65^gag of MuLV. The processed MuLV core proteins were not observed in purified HTG2 virions or in HTG2 cell extracts, indicating that proteolytic cleavage of the precursor is not required for the assembly or release of the MSV particles. Immunoprecipitation of lysed virions and HTG2 cell lysates with antisera to the MuLV envelope proteins gp70 and p15(E) demonstrated that detectable amounts of these and immunologically related components are not present in the virions and are not expressed in the infected cells. This suggests that the envelope proteins are not essential for the assembly and release of budding retroviral particles.