Mutant cells that abnormally process plasma membrane glycoproteins encoded by murine leukemia virus

Interaction between the dominant negative mutant and the wild-type envelope proteins of Friend murine leukemia virus

Interaction between the previously obtained dominant negative mutant, referred to as fcr (T. Matano, T. Odawara, M. Ohshima, H. Yoshikura, and A. Iwamoto, J. Virol. 67:2026-2033, 1993), and the wild-type envelope proteins (Env) of Friend murine leukemia virus was examined. The wild-type Env was bound to the fcr mutant Env and trapped in the endoplasmic reticulum. The virus receptor was not involved in this interaction.

trans-dominant interference with virus infection at two different stages by a mutant envelope protein of Friend murine leukemia virus

A dominant negative mutant Friend murine leukemia virus (FMLV) env gene was cloned from an immunoselected Friend erythroleukemia cell. The mutant env had a point mutation which resulted in a Cys-to-Arg substitution at the 361st amino acid in the FMLV envelope protein (Env). The mutant Env was retained in the endoplasmic reticulum (ER) and accumulated because of its slow degradation. The NIH 3T3 cells expressing the mutant env were resistant to ecotropic Moloney MLV (MoMLV) penetration, suggesting that the mutant Env traps the ecotropic MLV receptors in the ER. When the mutant env gene was transfected into and expressed in the cells persistently infected with MoMLV, the wild-type Env was trapped in the ER, and the MoMLV production was suppressed. Thus, the mutant Env accumulating in the ER trans-dominantly and efficiently interfered with the ecotropic MLV infection at both the early and the late stages.

Syngeneic monoclonal antibodies directed against Rauscher virus-induced myeloid leukemic cells: isolation and characterization

Hybridomas producing syngeneic monoclonal antibodies (MAbs) were prepared by fusion of spleen cells of BALB/c mice, which were immunized with sublethal doses of RMB-I cells. This cell line originates from a Rauscher virus (R-MuLV)-induced myeloid leukemia and forms tumors when re-inoculated into mice. MAbs were characterized as regards their reactivity against virally and non-virally induced cell lines. Two selected MAbs, IC5F5 and 4D2B4, were analyzed further. Their binding to subcellular structures was determined, and so were the properties of the antigens to which they are directed. MAb IC5F5 is of the IgG2A and 4D2B4 of the IgG2b subclass. Both bind to R-MuLV-infected or -transformed cell lines and are not mutually competitive. The antibodies do not react with other murine and human myeloid leukemic cells. As shown by immuno-electron microscopy, these MAbs have affinity to the cell membrane of non-virus producing RMB-I cells. When lysates of purified virus were analyzed, the MAbs were found to be directed to the gag precursor protein Pr65, and one of them (IC5F5) also to be directed to the core protein p12. In RMB-I cells, binding occurs to a 50-kDa glycoprotein and 2 proteins of 26 and 29 kDa. Since RMB-I cells do not produce virus, but express aberrant viral proteins, these MAbs are tumor-specific and useful for immunotherapy.

Glucocorticoid-regulated glycoprotein maturation in wild-type and mutant rat cell lines

Glucocorticoid hormones can regulate the posttranslational maturation of mouse mammary tumor virus (MTV) precursor polyproteins in M1.54, a stably infected rat hepatoma cell line. We have used complement-mediated cytolysis to recover variants of M1.54 that fail to express MTV cell surface glycoproteins in a hormone-regulated manner (Firestone, G.L., and K.R. Yamamoto, 1983, Mol. Cell. Biol., 3:149-160). One such clonal isolate, CR4, is similar to wild-type with respect to synthesis of MTV mRNAs, production of the MTV glycoprotein precursor (gPr74env) and a glycosylated maturation product (gp51), and hormone-induced processing of two MTV phosphoproteins. In contrast, three viral cell surface glycoproteins (gp78, gp70, and gp32) and one extracellular species (gp70s), which derive from gPr74env in glucocorticoid-treated wild-type cells, fail to appear in CR4. CR4 showed no apparent alterations in proliferation rate, cell shape, or expression of total functional mRNA and bulk glycoproteins. We conclude that the genetic lesion in CR4 defines a highly selective hormone-regulated glycoprotein maturation pathway that alters the fate of a restricted subset of precursor species.

Monoclonal antibody to the amino-terminal L sequence of murine leukemia virus glycosylated gag polyproteins demonstrates their unusual orientation in the cell membrane

To analyze cell surface murine leukemia virus gag protein expression, we have prepared monoclonal antibodies against the spontaneous AKR T lymphoma KKT-2. One of these antibodies, 43-13, detects an AKR-specific viral p12 determinant. A second monoclonal antibody, 43-17, detects a novel murine leukemia virus-related antigen found on glycosylated gag polyproteins (gp95gag, gp85gag, and gp55gag) on the surface of cells infected with and producing ecotropic endogenous viruses, but does not detect antigens within these virions. The 43-17 antibody immunoprecipitates the precursor of the cell surface gag protein whether in its glycosylated or unglycosylated state, but does not detect the cytoplasmic precursor of the virion gag proteins (Pr65gag). Based on these findings, we have localized the 43-17 determinant to the unique amino-terminal part of the glycosylated gag polyprotein (the L domain). We have determined that gp95gag contains L-p15-p12-p30-p10 determinants, whereas gp85gag lacks the carboxyterminal p10 determinant, and gp55gag lacks both p30 and p10 carboxy terminal determinants. Analysis of cell surface gag expression with the 43-17 antibody leads us to propose that the L domain plays a crucial role in (i) the insertion and orientation of murine leukemia virus gag polyproteins in the cell membrane and (ii) the relative abundance of expression of AKR leukemia virus versus Moloney murine leukemia virus glycosylated gag polyproteins in infected cells.

Cell anchorage determines whether mammary tumor virus glycoproteins are processed for plasma membranes or secretion

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 MMTV-encoded 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 125I, 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 secretions dependent upon cell-substratum attachments may be a common control mechanism important for embryogenesis, wound healing, and cancer.

A temperature-sensitive Chinese hamster ovary cell mutant pleiotropically defective in protein export

We have developed a new selection procedure for mammalian cell mutants defective in protein export by the use of diphtheria toxin, and devised a new screening method for defective protein secretion using nitrocellulose membranes. By the combination of these procedures, we have isolated a temperature-sensitive mutant clone of Chinese hamster ovary cells which shows a pleiotropic defect in protein export. This mutant, designated DS28-6, is temperature-sensitive for growth. Secretion of a series of proteins is markedly inhibited at the non-permissive temperature. These proteins seem to be normally synthesized and accumulated within the cell at the non-permissive temperature and secreted upon shift down to the permissive temperature. When this mutant is infected with vesicular stomatitis virus, oligosaccharide processing of G-protein is arrested at an endoglycosidase-H-sensitive stage at the non-permissive temperature. The lesion of this mutant appears to be in the endoplasmic reticulum or the cis Golgi or both.

Identification and characterization of a mouse mammary tumor virus protein uniquely expressed on the surface of BALB/cV mammary tumor cells

A unique subline of BALB/c mice, designated BALB/cV, exhibits an intermediate mammary tumor incidence (47%) and harbors a distinct milk-transmitted mouse mammary tumor virus (MMTV). The BALB/cV subline was used to study the molecular basis of potential virus-host interactions involving cell surface-expressed MMTV proteins. Cell surface iodination identified virus-specific proteins expressed on BALB/cV primary mammary tumor cells grown in culture. In contrast to (C3H)MMTV-producing cell lines which expressed MMTV gp52, BALB/cV tumor cells lacked gp52 and expressed instead a 68K, env-related protein. The 68Kenv protein was also detected on the surface of metabolically labeled BALB/cV tumor cells by an external immunoprecipitation technique. The expression of 68Kenv was restricted to mammary tissues of BALB/cV mice that also expressed other MMTV proteins. Biochemical analysis established that 68Kenv was not modified by N-linked glycosylation. 125I-labeled 68Kenv was rapidly released into the media of tumor cell cultures and was recovered both in the form of a soluble protein and in a 100,000 g pellet. The biologic function of this cell surface-expressed viral protein remains unknown.

Frequent hereditable shutdown of murine retrovirus gene expression in murine cell lines

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 in rat cells and that shutdown of replication-competent murine leukemia viruses with accompanying loss in interference to superinfection may be the rate-limiting reaction enabling cells to acquire multiple proviruses in their chromosomes. High-frequency shutdown in vivo would have important pathological consequences.

Frequent hereditable shutdown of murine retrovirus gene expression in murine cell lines

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 in rat cells and that shutdown of replication-competent murine leukemia viruses with accompanying loss in interference to superinfection may be the rate-limiting reaction enabling cells to acquire multiple proviruses in their chromosomes. High-frequency shutdown in vivo would have important pathological consequences.

Deletion mutants of Moloney murine leukemia virus which lack glycosylated gag protein are replication competent

A series of deletion mutations localized near the 5' end of the Moloney murine leukemia virus genome was generated by site-specific mutagenesis of cloned viral DNA. The mutants recovered from such deleted DNAs failed to synthesize the normal glycosylated gag protein gPr80gag. Two of the mutants made no detectable protein, and a third mutant, containing a 66-base pair deletion, synthesized an altered gag protein which was not glycosylated. All the mutants made normal amounts of the internal Pr65gag protein. The viruses were XC positive and replicated normally in NIH/3T3 cells as well as in lymphoid cell lines. These results indicate that the additional peptides of the glycosylated gag protein are encoded near the 5' end, that the glycosylated and internal gag proteins are synthesized independently, and that the glycosylated gag protein is not required during the normal replication cycle. In addition, the region deleted in these mutants apparently encodes no cis-acting function needed for replication. Thus, all essential sequences, including those for packaging viral RNA, must lie outside this area.

3Y1 rat cells are defective in processing of the envelope precursor protein of AKR virus

Rat 3Y1 cells were infected by AKR virus through microinjection of molecularly cloned proviral DNA. Based on a strong immunofluorescence using anti-p30 as an antiserum a cell clone (RESA-2) was selected that had a high expression of viral antigens. Subsequent restriction analysis of its DNA revealed that the RESA-2 clone contained at least 30 apparently intact integrated proviruses per genome. There was an apparently normal synthesis and processing of gag and pol gene products. The viral envelope precursor polyprotein gPr82env, however, did not yield the major envelope glycoprotein gp70. The gag precursor polyprotein, Pr65gag, as well as the gPr82env from RESA-2 cells were identified as AKR viral proteins by gel electrophoresis of hydroxylamine cleavage fragments. The virions formed by RESA-2 cells lacked gp70 and were noninfectious. After fusion of RESA-2 cells and mouse cells an infectious N-tropic virus was produced. The results indicate that rat 3Y1 cells lack (a) factor(s) necessary for the correct processing of gPr82env. The high incidence of abortive infections of murine leukemia virus (MLV) in susceptible rat cells reported by others is therefore probably due to defective particles in the virus stock and/or to the lack of (a) cellular factor(s) necessary for reverse transcription and subsequent integration of the viral genome.

Effects of monensin on morphogenesis and infectivity of Friend murine leukemia virus

The transport of the gp70 glycoprotein to the cell surface and concomitant release of infectious virus was inhibited by treatment of Friend murine leukemia virus-infected Eveline cells with the sodium ionophore monensin. Virus yields were reduced more than 50-fold by 10(-5) M monensin, whereas particle production was reduced by 50% in monensin-treated cells. The resulting particles failed to incorporate newly synthesized gp70 and p15(E), whereas the other structural proteins, p30, p15, p12, and p10, were incorporated into virions. However, monensin did not inhibit the incorporation into virions of preformed gp70. A reduction in the efficiency of cleavage of the PrENV glycoprotein precursor and a defect in the processing of simple endo-H-sensitive to complex endo-H-resistant oligosaccharides suggest that intracellular transport of gp70 may be blocked before its entry into the Golgi apparatus. Fewer particles were found to bud from the cell surface, but intracellular vacuoles with budding virions were detected. Ferritin labeling and pulse-chase studies suggested a cell surface origin for these vacuoles. These experiments indicate that monensin inhibits the transport of Friend murine leukemia virus glycoproteins at an early stage, with a resultant block in the assembly and release of infectious virus.

Immunogenic variants obtained by mutagenesis of mouse mastocytoma P815. IV. Analysis of variant-specific antigens by selection of antigen-loss variants with cytolytic T cell clones

Mouse mastocytoma P815 tumor cell variants that express new individual antigens were subjected to immunoselection in vitro with specific cytolytic T lymphocyte (CTL) clones. From one variant (P35) a number of resistant clones were isolated after a single-step selection. These immunoselected clones (P35.iscA-) proved to have acquired a stable and complete resistance to lysis by the selecting CTL, but remained sensitive to lysis by a CTL clone that recognizes a common P815 antigen. However, when these P35.iscA- clones were used to stimulate in vitro spleen cells from mice immunized with variant P35, the cytolytic activity was significantly higher on P35 and P35.iscA- targets than on the original P815 clone (P1). This indicated that P35.iscA- cells, which had lost a P35-specific determinant (P35A), had nevertheless retained another P35-specific determinant (P35B). CTL clones directed against the residual P35B determinant were then isolated. When P35 cells were submitted to selection with anti-P35B CTL, resistant clones (P35.iscB-) were obtained that were still lysed by anti-P35A and anti-P815 CTL clones. Tum- variant P35 therefore carries at least two specific determinants that can be lost independently. These results show the possibility of using selection with CTL clones to separate different components of complex cell surface antigens. The correlation between antigen expression and tumorigenicity was also examined. Antigen-loss P35.iscA- clones were found to be more tumorigenic than variant P35 which has a greatly reduced tumorigenic capacity compared to the original P815 tumor. This correlation suggests that the P35 determinant that was no longer expressed by the P35.iscA- clones is recognized on the original P35 tum- variant as a tumor rejection antigen.

Two distinct intracellular pathways transport secretory and membrane glycoproteins to the surface of pituitary tumor cells

The pituitary cell line, AtT-20, synthesizes adrenocorticotropic hormone (ACTH) as a glycoprotein precursor that is cleaved into mature hormones during packaging into secretory granules. The cells also produce an endogenous leukemia virus (MuLV) that is glycosylated after translation similar to the glycosylation of the ACTH precursor. Our evidence suggests that the envelope glycoprotein and some precursor ACTH get to the cell surface in a vesicle different from the mature ACTH secretory granule. Viral glycoproteins and ACTH precursor are released from the cells much sooner after synthesis than mature ACTH. Isolated secretory granules do not contain significant amounts of the envelope glycoprotein or ACTH precursor. Exposing cells to 8Br-cAMP stimulates release of mature ACTH four to five fold, but has little effect on the release of the ACTH precursor or the viral glycoproteins. We propose that the viral glycoproteins and some of the ACTH precursor are transported by a constitutive pathway, while mature ACTH is stored in secretory granules where its release is enhanced by stimulation.