Impaired Maturation of Mouse Mammary Tumor Virus Precursor Polypeptides in Lymphoid Leukemia Cells, Producing Intracytoplasmic A Particles and No Extracellular B-Type Virions

Abstract

Processing of polypeptides of the mouse mammary tumor virus, a type B retrovirus, was investigated in a transplanted thymic lymphoma cell line of the GR strain (GRSL). This cell line was maintained in vivo in ascites form and in vitro as a suspension culture. GRSL cells produce clusters of intracytoplasmic A particles and are virtually deficient in the production of mature extracellular Btype particles. As control, a mammary tumor cell line of the same mouse strain capable of complete virion synthesis was used. The kinetics of viral polypeptide synthesis were studied by pulse labeling with various isotopes (including 35S and 32p), followed by immunoprecipitation of cell lysates with monospecific antisera to the major mouse mammary tumor virus gag and env proteins, p27 and gp52, respectively. Both the primary gag and env precursor polypeptides were synthesized in the GRSL cells, but their conversion into viral proteins was impaired. The major gag precursor, Pr739G', was stable over a period of 8 h, and mature viral core polypeptides could not be detected. Also, the highly phosphorylated intermediates in the proteolytic processing of Pr739a9 in virus-producing cells were absent in GRSL cells. By immunoprecipitation, Pr73-"9 was detected in a GRSL particle fraction with the density of intracytoplasmic A particles. The precursor for envelope proteins, Pr73en', was turned over without the generation of mature viral envelope components gp52 and gp36. The in vivo-transplanted ascites GRSL cells, however, were shown to express gp52 on the cell surface together with a 73,000-dalton polypeptide, as indicated by cell surface iodination and immunoprecipitation.

Both T and B cells shed infectious mouse mammary tumor virus

“…It had previously been reported that a T-cell lymphoma line was not able to process virus proteins and therefore could not produce infectious virus particles (16). To determine if all of the cells were able to produce MMTV proteins, Western blot analysis was performed on total cell extracts with monospecific anti-SU ( Fig.…”

“…4). Thus, there is no general block to the production of infectious MMTV virions in B-or T-cell tumor lines, although some cell lines may not process the viral proteins (16).…”

Altered mouse mammary tumor virus transcript synthesis in T-cell lymphoma cells

“…Mouse mammary tumor virus (MMTV), the causative agent of mammary tumors in susceptible mice, is also known to be expressed in tissues and tumors other than those of the mammary gland. The most extensively documented association of MMTV with nonmammary tumors is that with T-cell lymphomas (1,8,22,27,32). A characteristic common to all the MMTV-expressing T-cell lymphomas is the presence of numerous acquired (amplified) MMTV proviral genomes (8,22), all containing long deletions in their long terminal repeat (LTR) regions (1,14,17,18,21,23).…”

“…Transcription assay constructs containing MMTV LTRs bearing deletions appear to be more efficient in lymphoid cells than constructs with full-length LTRs (35). The T-cell lymphoma cells do not produce mature viral particles (27,32), although they do transcribe high levels of MMTV RNA (9,17,23,30). The mRNAs are translated into gag and env polyprotein precursors but are then not processed further into mature viral structural proteins because of an apparent block in the maturation pathway (27,32).…”

“…The T-cell lymphoma cells do not produce mature viral particles (27,32), although they do transcribe high levels of MMTV RNA (9,17,23,30). The mRNAs are translated into gag and env polyprotein precursors but are then not processed further into mature viral structural proteins because of an apparent block in the maturation pathway (27,32). A C57BL/6 mouse-derived T-cell lymphoma line, EL-4, has also been reported to synthesize an unusual 1.0-kilobase (kb) LTR transcript (9,30) which initiates within the envelope gene in response to phorbol ester treatment (9).…”