We have described a heterogeneously processed glycoprotein with an apparent molecular weight of 55,000 (gp55) that is encoded by the Friend spleen focus-forming virus, an acute erythroleukemia virus [Dresler, S., Ruta, M., Murray, M.

The factors that control oncornavirus formation were analyzed in Friend leukemia cells that undergo hematopoiesis when treated with dimethyl sulfoxide. Suspension cultures of Ostertag FSD-1 cell line were found to enter a G,, or resting state at the end of their proliferative phase and to simultaneously cease producing helper and dependent components of Friend virus. Whereas the decline in virus production is at least 100-fold, rates of cellular RNA and protein synthesis are only slightly lower in resting than in growing cells. Both resting and growing cells contain similarly large concentrations of the viral proteins P(30) and P(12). Dimethyl sulfoxide induces hemoglobin synthesis in growing cells, but its effects on virus production appear to be indirect results of its action to inhibit cell growth and thus to delay entry of cells into the Gn resting state. Furthermore, variant cell lines were obtained with differing abilities to synthesize virus or hemoglobin. Some lines no longer produce infectious virus, although they all harbor murine leukemia virus genes which are expressed to varying extents. The major internal protein of these oncornaviruses, P(30), is

The subcellular localization of mouse mammary tumor virus (MMTV) glycoproteins was analyzed in infected and. cloned rat hepatocarcinoma cells cultured with the MMTV transcriptional inducer dexamethasone. When reacted with protein A~coated erythrocytes in the presence of antisera specific for viral glycoproteins or with fluorescent antisera, only some of the cells acquired surface label. This diversity was dependent on cell anchorage to the substratum. In general, the more rounded, less adherent cells contained the MMTV glycoproteins on their surfaces, whereas the flatter, more adherent cells did not. After a change in adherence, a delay preceded complete remodeling of the plasma membranes. Fluorescent antibody studies of fixed cells and analyses of viral glycoprotein synthesis and shedding using L-[35S]methionine indicated that the different expression of MMTV glycoproteins in round versus flat cells is caused by a switch in posttranslational processing. In round cells, the MMTVencoded precursor glycoprotein is proteolytically cleaved and then transported to plasma membranes as a complex of two subunits, the smaller being the membrane anchor. In flat adherent cells, the smaller subunit is rapidly degraded in an intracellular organelle and the larger is then secreted into the medium. As indicated by labeling of cells with 1251, the concentrations of several host-encoded plasma membrane components are also influenced by cell anchorage. We propose that this switch in cell surfaces and in secetions dependent upon cell-substratum attachments may be a common control mechanism important for embryogenesis, wound healing, and cancer.

Friend spleen focus-forming virus shuts down its gene expression frequently (ca. 10-3 per generation) in a cis-dominant hereditable fashion in various murine cells but much less frequently in rat cells (less than 10-6 per generation). Thus. nonexpresser variants were isolated at high frequency from murine cell lines by immunoselection directed against virus-encoded cell surface glycoproteins and also simply by subcloning cells from lines which had been cultured for many generations. Studies of independently infected cell clones indicate that shutdown is a property of the cell line rather than of the specific proviral site. Nucleic acid blot analyses suggest that shutdown correlates with decreased transcription. Moreover, preliminary evidence indicates that other murine retroviruses also shut down frequently in murine but not

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