Friend spleen focus-forming virus (SFFV) is a defective murine C-type retrovirus which causes a multi-stage erythroleukaemia in mice and erythroblastosis in bone marrow cultures. The SFFV env gene encodes a membrane glycoprotein, gp55, which is located on the cell surface and in the rough endoplasmic reticulum and is essential both for the induction of leukaemia in vivo and erythroblast proliferation in vitro. The mechanism by which gp55 causes increased erythroblastosis and ultimately leukaemia is unknown, but a reasonable suggestion is that gp55 can mimic the action of erythropoietin by binding to its receptor (Epo-R), thereby triggering prolonged proliferation of erythroid cells. To test this possibility, we have co-expressed gp55 and the murine Epo-R in a fibroblast cell line. We show here that in such cells, the SFFV glycoprotein binds directly to Epo-R. Furthermore, when an interleukin-3 (IL-3)-dependent lymphoid cell line was co-infected by SFFV and a virus that carries the Epo-R gene, it could grow without IL-3. We suggest that through direct binding to Epo-R, gp55 can stimulate the receptor and by-pass the normal requirement for Epo, causing prolonged proliferation of infected erythroid cells. This could be the first step of leukaemogenesis induced by Friend virus.

Activation of cell growth by binding of Friend spleen focus-forming virus gp55 glycoprotein to the erythropoietin receptor.

The envelope glycoprotein complex of replication competent retroviruses is comprised of two polypeptides, an external, glycosylated, hydrophilic polypeptide (SU) and a membrane-spanning protein (TM), that form a knob or knobbed spike on the surface of the virion Both polypeptides are encoded in the env gene and are synthesized in the form of a polyprotein precursor that is proteolytically cleaved during its transport to the surface of the cell While these proteins are not required for the assembly of enveloped virus particles, they do play a critical role in the virus replication cycle by recognizing and binding to specific receptors (SU) and by mediating the fusion of viral and cell membranes (TM): virus particles lacking envelope glycoproteins are thus noninfectious

Retrovirus envelope glycoproteins

Evidence for a glycoprotein 'signal' involved in transport between subcellular organelles. Two membrane glycoproteins encoded by murine leukemia virus reach the cell surface at different rates

Characterization of mouse monoclonal antibodies specific for Friend murine leukemia virus-induced erythroleukemia cells: friend-specific and FMR-specific antigens.

Friend virus induces rapid progressive erythroleukemia in susceptible mice. To oncologists and to cell biologists, this disease has provided a fascinating model for analyzing neoplastic progression, the role of host genes in controlling susceptibility to cancer, and the differentiation of erythroid cells in culture. However, to molecular biologists, Friend virus (and the closely related Rauscher and Cas virus complexes) has been generally viewed as a relatively complex anomaly. The virus lacks a classical oncogene of the sort typified by the src gene of Rous sarcoma virus. Such classical viral oncogenes (v-oncs) are modified versions of normal cellular genes (proto-oncogenes or c-oncs) and they are present in all of the other known retroviruses that cause rapidly developing neoplasms (Bishop 1983, 1985; Weinberg 1985). The c-oncs have been highly conserved throughout evolution, and there, is evidence that they perform important cellular functions. Because Friend virus lacks such a modified cellular gene and contains only retroviral-specific nucleic acid sequences, and because it induces a progressively developing neoplasm rather than an immediate cancer, it has been widely assumed that it is relatively “different” and “complex,” perhaps too different to provide general insights and too complex for molecular biological analysis.

Molecular Biology of Friend Viral Erythroleukemia

The Friend spleen focus-forming virus (F-SFFV) released from cultured erythroleukemia cells (cell line F4-6/K) was cloned free of its helper lymphatic leukemia virus (F-MuLV). After allowing adsorption to Sc-1 fibroblasts at a low multiplicity of infection, the cells were seeded individually into wells of a microtitier test plate and the resulting colonies were grown into large cultures. Among 14 of these cell cultures that have been analyzed thoroughly, 6 contained F-SFFV alone, 1 contained F-MuLV plus F-SFFV, and 7 were uninfected. Each of the Sc-1 cell lines which had been infected with cloned F-SFFV contained a glycoprotein with an apparent molecular weight of 55,000 (gp55) that was absent from the cell lines that lacked F-SFFV. gp55 was also present in Friend erythroleukemia cells and in fibroblasts infected with an F-SFFV that had been doubly cloned in another laboratory. These results indicate that gp55 is encoded by the F-SFFV genome. gp55 has the following additional properties. It can be immunoprecipitated with antiserum made to the F-MuLV virion envelope glycoprotein (gp75). Its unglycosylated polypeptide, formed in cells treated with 2-deoxy-D-glucose, has a molecular weight of approximately 45,000. Its tryptic peptide map contains peptides in common with F-MuLV gp75 but it also contains unique peptides. It appears to be absent or present in only low concentrations in erythroleukemia cell plasma membranes as determined by lactoperoxidase-catalyzed iodination, and it accumulates intracellularly in large amounts. In addition, it is absent from released virions. The majority of the cellular gp55 has an isoelectric point of 8.5 to 9.0. These results are consistent with the idea that an env gene recombination event was involved in the origin of F-SFFV.

S Dresler

Papers

Glycoprotein encoded by the Friend spleen focus-forming virus.