The gag-mos hybrid protein of ts110 Moloney murine sarcoma virus: variation of gene expression with temperature

Membrane changes associated with the temperature-sensitive P85gag-mos-dependent transformation of rat kidney cells as determined by dielectrophoresis and electrorotation

Conventional dielectrophoresis (cDEP) and electrorotation (ROT) measurements have been used to determine the dielectric properties of a clone of normal rat kidney cells, designated 6m2, that exhibits a transformed phenotype at 33 degrees C and a non-transformed phenotype at 39 degrees C. cDEP measurements of the crossover frequencies at which individual 6m2 cells experienced zero cDEP force performed as a function of the conductivity of the suspension medium revealed that, in response to a temperature shift from 33 degrees C to 39 degrees C for 24 h, the mean specific cell membrane capacitance and conductance fell significantly (P < 0.01) from 42.3 (+/-1.3) to 30.3 (+/-2.9) mF/m2 and 743 (+/-422) to 567 (+/-326) S/m2, respectively. ROT analyses demonstrated a similar reduction for the membrane capacitance from 37.2 (+/-7.3) to 27.4 (+/-6.1) mF/m2, and also showed that accompanying changes in the mean internal electrical conductivity and dielectric permittivity of the cells were insignificant. Scanning electron microscopy was used to examine the surface morphology of the cells and, in agreement with our previous reports for leukemia cells, the observed membrane capacitance values correlated closely with the morphological complexity of the cell membrane surface. The observed changes in the membrane dielectric properties are discussed in terms of their biological significance and their relationship to previously-detected changes in cell surface charge.

Regulation of RNA splicing in gag-deficient mutants of Moloney murine sarcoma virus MuSVts110

We investigated whether the MuSVts110 gag gene product (P58gag) can regulate the novel growth temperature dependence of MuSVts110 RNA splicing. MuSVts110 mutants with either frameshifts or deletions in the gag gene were tested for their ability to maintain the MuSVts110 splicing phenotype. Only small decreases in splicing efficiency and no changes in the thermosensitivity of viral RNA splicing were observed in MuSVts110 gag gene frameshift mutants. Deletions within the gag gene, however, variably decreased MuSVts110 splicing efficiency but had no effect on its thermosensitivity. Another class of MuSVts110 splicing mutants generated by treatment of MuSVts110-infected cells with NiCl2 was also examined. In these "nickel revertants," P58gag is made, but splicing of the viral transcript is nearly complete at all growth temperatures. The splicing of "tagged" viral RNA transcribed from a modified MuSVts110 DNA introduced into nickel revertant cells remained thermosensitive, arguing against trans effects of viral gene products on splicing efficiency. These experiments indicated that neither the MuSVts110 P58gag protein nor any other viral gene product acts in trans to regulate MuSVts110 splicing.

Reversion of thermosensitive splicing defect of Moloney murine sarcoma virus ts110 by oversplicing of viral RNA

Moloney murine sarcoma virus ts110 possesses a thermosensitive splicing defect. By continuously growing nonproducer cells at the nonpermissive temperature, a new class of revertant cells, termed 6m3, that had lost the thermosensitive splicing defect was produced, and six distinct clones were selected. These cell clones were transformed at either permissive or restrictive temperatures. Unlike parental 6m2 cells, which contain two virus-specific RNA species of 4.0 and 3.5 kilobases (kb) at temperatures permissive for transformation, the 3.5-kb RNA was the only virus-specific RNA species detected in 6m3 clones. No new v-mos-containing DNA fragment was observed in Southern blot analysis of these cell clones compared with parental 6m2 cells, indicating that the 3.5-kb RNA was a splicing product rather than a direct transcript. Moreover, these cells expressed P85gag-mos but not P58gag at any temperature. The reversion of the phenotype in 6m3 cell clones appears to be the result of a selective loss of the temperature sensitivity of the splicing reaction, without affecting the thermosensitivity of the protein kinase activity. This change also appears to alter the mechanism regulating the efficiency of the genomic RNA-splicing reaction.

Vimentin phosphorylation by p37mos protein kinase in vitro and generation of a 50-kDa cleavage product in v-mos-transformed cells

Previous studies have shown that vimentin, an intermediate filament protein, is reduced in amount in cells acutely infected with Moloney mouse sarcoma virus (Mo-MuSV). In this report, we provide evidence for specific alteration of vimentin in Mo-MuSV-transformed cells and demonstrate specific phosphorylation of vimentin by the p37mos protein kinase in vitro. Specificity of the phosphorylation reaction was demonstrated by using viral mos proteins encoded by various isolates of Mo-MuSV and p37mos produced in yeast. A phosphotransfer domain mutant lacking the ability to autophosphorylate p37mos failed to phosphorylate vimentin. Similarly, vimentin was not phosphorylated by the temperature-sensitive P85gag-mos kinase derived from infected cells maintained at the restrictive temperature. In ts110 MuSV-transformed NRK cells, vimentin was phosphorylated at both the permissive and nonpermissive temperatures for transformation. However, at the permissive temperature, an altered form of vimentin (about 50 kDa) with a more basic isoelectric point and lower apparent molecular weight was detected. This 50-kDa product was highly phosphorylated as compared to the bulk of the normal 55-kDa form of vimentin. On the basis of its mobility in two-dimensional gels, the 50-kDa form of vimentin should lack the carboxy terminus. This type of alteration could conceivably modulate the function of vimentin filaments in the transformed cell.

Activation of thermosensitive RNA splicing and production of a heat-labile P85gag-mos kinase by the introduction of a specific deletion in murine sarcoma virus-124 DNA

Murine sarcoma virus ts110 (MuSVts110) is a conditionally transformation-defective MuSV mutant lacking 1,487 bases found in its wild-type parent, MuSV-349 (MuSV-124). Expression of the MuSVts110 v-mos gene product, P85gag-mos, requires splicing of the viral transcript to align the gag and mos genes in frame. However, this splice event is restricted to growth temperatures of 33 degrees C or lower. No splicing of the viral RNA, no production of P85gag-mos, and, hence, no cell transformation is observed at growth temperatures above 33 degrees C. To determine whether thermosensitive splicing is an intrinsic property of To determine whether thermosensitive splicing is an intrinsic property of MuSVts110 RNA specified by the 1,487-base deletion or a result of a cellular defect, we examined an "equivalent" or MuSVts110 DNA (designated ts32 DNA) constructed by combining wild-type MuSV-124 DNA fragments with a synthetic oligonucleotide to yield an otherwise wild-type viral DNA containing the same 1,487-base deletion as authentic MuSVts110. As observed in control cells (6m2 cells) infected with the authentic MuSVts110 virus, NIH 3T3 cells transfected with ts32 DNA appeared morphologically transformed when grown at 33 degrees C, but were converted to a more normal, flattened shape within a few hours of a shift to 39 degrees C. In concert with these morphological changes, both the processing of the ts32 RNA transcripts and the production of ts32 p85gag-mos kinase were found to be optimal at growth temperatures from 28 to 33 degrees C, but dramatically reduced at 37 to 41 degrees C. Like authentic P85gag-mos, the ts32 P85gag-mos kinase activity was rapidly inactivated by brief exposure to 39 degrees C. These results suggested that the MuSVts110 equivalent is functionally indistinguishable from authentic MuSVts110 and that the novel temperature-sensitive splicing of MuSVts110 transcripts is specified by an intrinsic property of the viral RNA.

Analysis of metal-induced mutations altering the expression or structure of a retroviral gene in a mammalian cell line

Carcinogenic metal compounds, with the exception of chromium(VI), have been found to be poorly mutagenic in both prokaryotic and mammalian cell mutagenesis assays, yet they are clearly clastogenic (Hansen and Stern, 1984). Thus, the role of metals as initiators in carcinogenesis has been difficult to delineate. In an effort to develop a model system capable of assaying DNA damage caused by carcinogenic metals, we have investigated the role of NiCl2, CdCl2, Na2CrO4, and NMU in a murine sarcoma virus-infected mammalian cell line in which expression of the retroviral v-mos gene is growth-temperature regulated. This cell line, designated 6m2, contains a single-copy, stably integrated, mutant Moloney murine sarcoma virus DNA (designated MuSVts110) and is temperature sensitive for morphological transformation due to a conditionally defective viral RNA-splicing event that in turn regulates expression of the viral transforming gene. Mutations affecting the viral DNA in 6m2 cells can be detected if these alterations lead to changes in the structure or expression of the transforming protein encoded by the MuSVts110 v-mos gene. Analysis of the viral proteins from 6m2 'revertant' cell lines (as defined by reversion to the transformed phenotype at all growth temperatures) selected after treatment with the above agents showed that NiCl2, NMU, and Na2CrO4 each induced a different yet specific type of mutation. NiCl2 and NMU each altered the temperature sensitivity of viral RNA splicing, possibly due to base substitution mutations, but did so to distinctly different extents. Na2CrO4 affected the structure of the viral proteins by inducing what appear to be short frameshift mutations that resulted in the temperature-dependent translation of a novel virus-encoded transforming protein, P100gag-mos. CdCl2 also induced frameshift mutations but, in one case, induced a mutation which may result from a deletion of about 300 bases within the MuSVts110 DNA.

Potassium inhibition of transforming protein P85gag-mos and reversal of the transformed phenotype in 6m2 cells

K+ at high concentrations (52-72 mM hypertonic KCl) has been reported to induce reverse transformation in the 6m2 cell, which is a clone of normal rat kidney cells (NRK) infected with a temperature-sensitive transformation virus. When exposed to high K+, 6m2 cells grown at the permissive temperature (33 degrees C) exhibit normal morphology and reduced soft agar growth, characteristics of cells grown at nonpermissive temperature (39 degrees C). In the current study, flattening of cells and rearrangement of surface microvilli were demonstrated by scanning electron microscopy to occur within 6 hr of exposure to high K+, similar to the effect of temperature shift to 39 degrees C. Exposure to K+ resulted in a 90% inhibition of P85gag-mos-associated serine kinase activity within 5 min, with a subsequent reduction of up to 75% of the synthesis of this protein. These alterations in the putative transforming protein were similar to those induced by temperature shift and were considered to be the basis for retrotransformation. The cell microtubular system and F-actin cables were affected more slowly by K+ than by a temperature shift to 39 degrees C. The former did not achieve the fine reticulum network seen in NRK cells until 72 hr later, but the latter remained aberrant. The effect on the enzyme might be mediated by alteration in phosphorylation, but the mechanism by which kinase inactivation induces retrotransformation is not yet known.

Monoclonal antibody study of the subcellular localization and DNA-stimulating activity of murine sarcoma virus-activated transformation-associated proteins

Previously, we reported the monoclonal antibody detection of transformation-associated proteins (TAP) in ts110 murine sarcoma virus-transformed normal rat kidney (6M2) cells (Chan et al., 1986). In this study, we used the same monoclonal antibody to investigate the subcellular localization, the fate and the mitogenic activity of TAP, as well as the correlationship between TAP synthesis and the expression of transformation properties of 6M2 cells. It was found that TAP were localized in the cytoplasm (probably the Golgi apparatus) of 6M2 cells. TAP were found as three intracellular polypeptides (mol wt of 66K, 63K, and 60K, respectively), and were rapidly released into extracellular medium. Upon release, TAP changed to two extracellular polypeptides (mol wt of 68K and 64K, respectively). Furthermore, the synthesis of TAP was temperature sensitive and correlated closely with the expression of transformation properties of the 6M2 cells. TAP have been purified by monoclonal antibody-affinity column chromatography and found to have a synergistic effect with insulin in stimulating the DNA synthesis of normal rat kidney cells.

Antibodies against small nuclear ribonucleoproteins immunoprecipitate complexes containing ts110 Moloney murine sarcoma virus genomic and messenger RNAs

Small nuclear ribonucleoproteins (snRNPs) are believed to play a role in processing premessenger RNAs. In this study, snRNPs were immunoprecipitated from extracts of cells infected with ts110 Moloney murine sarcoma virus (ts110 MoMuSV). Both the unspliced 4.0 kb and the spliced 3.5-kb ts110 MoMuSV specific RNA species were found in the immunoprecipitates obtained with monoclonal antibody anti-Sm and polyclonal anti-Sm, anti-(U1) RNP and anti-La sera. Although only a portion of the total ts110 RNAs was present in these immunoprecipitates, immune recognition by the anti-snRNPs was specific and not due to contaminating anti-RNA (at least for the anti-Sm sera) or, to anti-viral protein activities. Genomic 8.3-kb RNA and subgenomic 3.0-kb spliced env mRNA from Moloney murine leukemia virus (MoMuLV) infected cells as well as the cellular actin mRNA were also detected in immunoprecipitates obtained with the same antisera. The fact that pre-mRNAs and mature mRNAs of different origin can be recovered from immunoprecipitates formed with anti-snRNP sera establishes their tight association and confirms the role of snRNPs in mRNA processing.

Expression of the v-mos gene alters a Mr 55,000 protein during acute infection by Moloney murine sarcoma virus

Infection of the rat myoblast cell line, L6E9, with Moloney murine sarcoma virus (Mo-MuSV) clone 124, altered a cellular protein of Mr 55,000 (P55) within 2 days of infection. The alteration of P55 was observed as a reduction in its steady-state level in cell extracts. The reduction of P55 correlated with the appearance of p37mos in infected cells. Except for P55 and one other protein, no change was detected in the total protein pattern of infected cells compared to uninfected cells, as judged by either immunoblots of one-dimensional NaDodSO4 gels or direct two-dimensional gel analysis. P55 levels were unchanged when L6E9 cells were infected with Moloney murine leukemia virus or several different transforming retroviruses. To determine the specificity of this v-mos-induced effect on P55, L6E9 cells were acutely infected with a temperature-sensitive variant (ts110) of Mo-MuSV. When these cells were shifted from 39 degrees C to 33 degrees C, which activates the gag-mos gene product, the P55 level dropped by greater than 50% within 2-3 hr. Conversely, with a shift in temperature from 33 degrees C to 39 degrees C, the cells' P55 level returned to normal within 5 hr, starting at 30 min after shift. These results clearly show that v-mos expression in acutely infected L6E9 cells alters the cellular protein, P55.

Temperature-sensitive splicing defect of ts110 Moloney murine sarcoma virus is virus encoded

ts110 Moloney murine sarcoma virus (Mo-MuSV)-nonproductively infected cells (6m2) have a transformed phenotype at 28 to 33 degrees C and a normal phenotype at 39 degrees C. At temperatures permissive for transformation, 6m2 cells contain P58gag produced from the 4.0-kilobase (kb) viral RNA genome and P85gag-mos translated from a 3.5-kb spliced mRNA. At 39 degrees C, only the 4.0-kb RNA and its product P58gag are detected. Two temperature-sensitive defects have been observed in ts110-infected 6m2 cells: (i) the splicing of the 4.0-kb RNA to the 3.5-kb RNA; and (ii) the thermolability of P85gag-mos and its kinase activity relative to the wild-type revertant protein, termed P100gag-mos (R.B. Arlinghaus, J. Gen. Virol. 66:1845-1853, 1985). In the present study, we examined the mos gene products of two cell lines (204-2F6 and 204-2F8) obtained by infection of normal rat kidney cells with ts110 Mo-MuSV as a simian sarcoma-associated virus pseudotype to see whether the temperature-sensitive splicing defect could be transferred by viral infection. Southern blot analysis of these two cell lines showed that viral DNAs containing restriction fragments from cellular DNA are different from those in 6m2 cells, indicating that 204-2F6 and 204-2F8 cells have different ts110 provirus integration sites from those of 6m2 cells. Northern blots, S1 mapping analyses, and immunoprecipitation experiments showed unequivocally that the splicing defect of ts110 Mo-MuSV is virus encoded and is independent of host cell factors.

Serine kinase activity associated with Maloney murine sarcoma virus-124-encoded p37mos

An antiserum directed against amino acid residues 37-55 [anti-mos (37-55) serum] of the predicted v-mos sequence was used to precipitate p37mos from Moloney murine sarcoma virus-124 (Mo-MuSV-124) acutely infected 3T3 cells. Proteins with sizes ranging from p37mos to 43 kDa (p43) were found to be phosphorylated when anti-mos (37-55) immune complexes containing p37mos were incubated with [gamma-32P]ATP and Mn2+. The phosphorylation of p37mos and p43 could be specifically blocked when the anti-mos (37-55) serum was incubated with 37-55 cyclic mos peptide prior to immunoprecipitation, but not if the serum was preincubated with an unrelated peptide representing amino acids of the myc protein sequence. Anti-mos (37-55) immune complexes from uninfected 3T3 cells did not produce any phosphorylated proteins the size of p37mos or p43. However, a 50-kDa protein (p50) was phosphorylated in both unblocked and mos peptide-blocked anti-mos (37-55) immune complexes from infected 3T3 cells, and in immune complexes from uninfected cells. Quercetin, an inhibitor of some protein kinases, inhibited the kinase phosphorylating p50 but not the kinase phosphorylating p37mos and p43. Preabsorption of the cell extract prior to immunoprecipitation with an excess of formalin-fixed Staphylococcus aureus, complexed with preimmune normal rabbit serum IgG, specifically removed the kinase phosphorylating p50. The amount of in vitro phosphorylated p37mos and p43 in the immune-complex kinase assay reached a maximum in extracts of 3T3 cells 2-3 days postinfection with Mo-MuSV 124 but decreased to trace levels after 5 days. Metabolically and in vitro phosphorylated p37mos generated an identical pattern of phosphopeptides upon partial V8 protease digestion. Based on peptide mapping and a kinetic analysis of the in vitro phosphorylation reaction, p37mos appears to be a precursor to the p43 phosphorylated species. Phosphoamino acid analyses revealed only phosphoserine in in vitro phosphorylated p37mos and p43mos. It was concluded that p37mos is closely associated with a serine kinase activity and that the in vitro phosphorylation of p37mos may lead to formation of a highly modified mos protein (p43) by way of superphosphorylation.

Temperature-sensitive viral RNA expression in Moloney murine sarcoma virus ts110-infected cells

We examined the mos-specific intracellular RNA species in 6m2 cells, an NRK cell line nonproductively infected with the ts110 mutant of Moloney murine sarcoma virus. These cells present a normal phenotype at 39 degrees C and a transformed phenotype at 28 or 33 degrees C, expressing two viral proteins, termed P85gag-mos and P58gag, at 28 to 33 degrees C, whereas only P58gag is expressed at 39 degrees C. It has been previously shown that 6m2 cells contain two virus-specific RNA species, a 4.0-kilobase (kb) RNA coding for P58gag and a 3.5-kb RNA coding for P85gag-mos. Using both Northern blot and S1 nuclease analyses, we show here that the 3.5-kb RNA is the predominant viral RNA species in 6m2 cells grown at 28 degrees C, whereas only the 4.0-kb RNA is detected at 39 degrees C. During temperature shift experiments, the 3.5-kb RNA species disappears after a shift from 28 to 39 degrees C and is detected again after a shift back from 39 to 28 degrees C. By Southern blot analysis, we have detected only one ts110 proviral DNA in the 6m2 genome. This observation, as well as previously published heteroduplex and S1 nuclease analyses which showed that the 3.5-kb RNA species lacks about 430 bases found at the gag gene-mos gene junction in the 4.0-kb RNA, suggests that the 3.5-kb RNA is a splicing product of the 4.0-kb RNA. The absence of the 3.5-kb RNA when 6m2 cells are grown at 39 degrees C indicates that the splicing reaction is thermosensitive. The splicing defect of the ts110 Moloney murine sarcoma virus viral RNA in 6m2 cells cannot be complemented by acute Moloney murine leukemia virus superinfection, since no 3.5-kb ts110 RNA was detected in acutely superinfected 6m2 cells maintained at 39 degrees C. The spliced Moloney murine leukemia virus env mRNA, however, is found in acutely infected cells maintained at 39 degrees C, suggesting that the lack of ts110 viral RNA splicing at 39 degrees C is not due to an obvious host defect. In sharp contrast, however, 6m2 cells chronically superinfected with Moloney murine leukemia virus produce a 3.5-kb RNA species at 39 degrees C as well as at 28 degrees C and contain proviral DNAs corresponding to the two viral RNA species.(ABSTRACT TRUNCATED AT 400 WORDS)

Further characterization of the P85gag-mos -associated protein kinase activity

The gag-mos hybrid protein encoded by ts110 MoMuSV was shown to have an associated protein kinase activity which phosphorylated both P85gag-mos and P58gag when [gamma-32P]ATP and a manganese cofactor were added to an immune complex containing P85gag-mos. Immunoprecipitation and removal of P85gag-mos from the reaction mixture by either an anti-mos or anti-gag serum resulted in a subsequent elimination of in vitro P85gag-mos and P58gag phosphorylation. This kinase activity was shown to be either an intrinsic property of P85gag-mos or else a tightly bound cellular enzyme activity resistant to elution with 2.0 M NaCl, 0.5% deoxycholate, and 0.1% SDS. A correlation was made between the amount of kinase activity and the concentration of P85gag-mos. Viral gag antisera were also used to show immune complex phosphorylation of another gag-mos hybrid protein termed P100gag-mos, derived from a revertant of ts110. In vitro phosphorylation experiments derived from v-mos transformed MuSV 124 cells using viral gag antisera were completely negative which shows that the gag-mos kinase in 6m2 cells is not merely a gag-associated kinase that phosphorylates MuSV coded gag gene products. When shifting 6m2 cells from a permissive temperature to the nonpermissive temperature of 39 degrees for 2-4 hr, a noticeable change toward a more normal morphology occurs. NRK 54-5A4 cells infected with a revertant of ts110 with wild-type phenotype, showed little change in morphology between permissive and nonpermissive temperatures. In addition to the ts defect affecting P85gag-mos production previously reported, a second ts defect in ts110 is reported here which is functional in nature; it can be detected within 5 min after shift to 39 degrees by the heat lability of the P85-associated kinase activity. The P100gag-mos protein kinase from the wild-type revertant cells did not exhibit this heat sensitivity under similar conditions. The thermal inactivation of the P85 kinase was shown to precede events that occur as cells are shifted to the restricted temperature including morphological reversion to the normal phenotype, and the decrease in P85gag-mos concentration. Based on all of these observations, it is suggested that the P85-associated kinase activity is not merely an adherent cellular kinase, but actually a function of the gag-mos gene product.