Murine mammary tumor virus deficient in the major glycoprotein: Biochemical and biological studies on virions produced by a lymphoma cell line

Abstract

A cell line, designated MLA, was established from a spontaneous lymphoma of a DBA/2 mouse. These cells contained large amounts of intracytoplasmic type-A particles, the presumed precursors of mature MuMTV. Electron microscopy revealed that the MLA cells released MuMTV-like virions into the culture fluid, but these virions were devoid of the characteristic envelope “spikes” of MuMTV. Antigenic analysis of virions purified from the culture fluid of MLA cells showed that, by microimmunodiffusion, the glycoproteins of MuMTV, gp49 and gp36, were not detectable, nor was the glycoprotein gp70 of MuLV detectable. Internal structural proteins of MuMTV, p28 and p12, were however, readily detectable in these virions. By the more sensitive radioimmunoassay technique, it was found that the virions produced by MLA cells contained some gp49-related antigen, but the amount present was about 500-fold lower than in other tissue culture-derived MuMTV. MLA cells synthesized a 70,000-dalton precursor of MuMTV glycoproteins. Culture fluids from which the virions were removed by centrifugation did contain gp49, indicating processing of the precursor polypeptide, but this polypeptide was not incorporated into the virions. Molecular hybridization studies revealed that MLA cells contained a large amount of MuMTV-specific RNA; the amount of MuLV-related RNA was about 100-fold lower. Whereas 150–200 MuMTV particles were released into culture fluid per cell per day, few MuLV particles were released. Purified virions from the MLA cells were injected into BALB/c and C57BL weanling females. This resulted in mammary tumor development in BALB/c females, but not in C57BL. The tumors contained MuMTV with the normal complement of envelope spikes and gp49 was detectable in the virions.