P85gag-mos encoded by ts110 Moloney murine sarcoma virus has an associated protein kinase activity

Expression of mos in astrocytic tumors and its potential role in neoplastic progression

The c-mos gene and its protein product mos, components of the mitogen-activated protein kinase transduction pathway, are known to be involved in the control of meiosis and mitosis. Apart from a study on lung carcinomas, there is little information about its role in human neoplasia. The aim of this study was to investigate expression of mos in astrocytic tumors and to correlate it with accumulation of p53. We studied expression of mos in 62 cases of supratentorial astrocytic tumor. Intracytoplasmic immunostaining for mos was found in 28 (45%) cases: 3 of 20 (15%) grade 2 astrocytomas, 9 of 20 (45%) grade 3 anaplastic astrocytomas, and 16 of 22 (73%) glioblastomas. Immunopositivity for mos correlated significantly (P < 0.01) with tumor grade but not with p53 expression. In contrast to the findings in relation to lung tumors, immunopositivity for mos in astrocytic tumors did not predict recurrence-free or overall survival time. Cytoplasmic immunostaining was observed in scattered large cortical neurons adjacent to tumors, possibly due to stress-induced abortive entry into the cell cycle. The correlation of mos immunopositivity with tumor grade may reflect the expansion of more malignant mos-positive clones. This study provides evidence that mos may be involved in the neoplastic progression of a proportion of astrocytic tumors.

Mos and the cell cycle

The mos proto-oncogene-encoded serine/threonine protein kinase plays a key cell cycle-regulatory role during meiosis. The Mos protein is required for the activation and stabilisation of M phase-promoting factor MPF. As a component of a large multiprotein complex known as the cytostatic factor (CSF), Mos is involved in causing metaphase II arrest of eggs in vertebrates. Upon expression in somatic cells, Mos causes cell cycle perturbations resulting in cytotoxicity and neoplastic transformation. All the known biological activities of Mos are mediated through activation of the mitogen activated protein (MAP) kinase pathway. Here we discuss the interrelationship between Mos and other cell cycle regulators.

Moloney murine sarcoma virus MuSVts110 DNA: cloning, nucleotide sequence, and gene expression

We have cloned Moloney murine sarcoma virus (MuSV) MuSVts110 DNA by assembly of polymerase chain reaction (PCR)-amplified segments of integrated viral DNA from infected NRK cells (6m2 cells) and determined its complete sequence. Previously, by direct sequencing of MuSVts110 RNA transcribed in 6m2 cells, we established that the thermosensitive RNA splicing phenotype uniquely characteristic of MuSVts110 results from a deletion of 1,487 nucleotides of progenitor MuSV-124 sequences. As anticipated, the sequence obtained in this study contained precisely this same deletion. In addition, several other unexpected sequence differences were found between MuSVts110 and MuSV-124. For example, in the noncoding region upstream of the gag gene, MuSVts110 DNA contained a 52-nucleotide tract typical of murine leukemia virus rather than MuSV-124, suggesting that MuSVts110 originated as a MuSV-helper murine leukemia virus recombinant during reverse transcription rather than from a straightforward deletion within MuSV-124. In addition, both MuSVts110 long terminal repeats contained head-to-tail duplications of eight nucleotides in the U3 region. Finally, seven single-nucleotide substitutions were found scattered throughout MuSVts110 DNA. Three of the nucleotide substitutions were in the gag gene, resulting in one coding change in p15 and one in p30. All of the remaining nucleotide changes were found in the noncoding region between the 5' long terminal repeat and the gag gene. In NIH 3T3 cells transfected with the cloned MuSVts110 DNA, the pattern of viral RNA expression conformed with that observed in cells infected with authentic MuSVts110 virus in that viral RNA splicing was 30 to 40% efficient at growth temperatures between 28 and 33 degrees C but reduced to trace levels above 37 degrees C.

v-mos proteins encoded by myeloproliferative sarcoma virus and its ts159 mutant

The myeloproliferative sarcoma virus (MPSV) v-mos protein was predicted to be identical in size to p39c-mos because of an observed one-base deletion in the seventh codon of the env-mos open reading frame, which would allow translation to initiate at the methionine equivalent to codon 32 of the env-mos gene. On the basis of published results, p39c-mos is known to have greatly reduced in vitro protein kinase activity compared with p37env-mos encoded by Moloney murine sarcoma virus. Unexpectedly, the relative activity of the MPSV v-mos protein kinase was comparable to that of p37env-mos. Consistent with this finding, the size of MPSV v-mos protein was found to be similar to the size of p37env-mos. Moreover, the pattern and sizes of phosphorylated bands produced by autophosphorylation of the MPSV v-mos protein were similar to those of p37env-mos. These results were confirmed by in vitro transcription-translation of the MPSV v-mos gene. Resequencing portions of the MPSV mos gene failed to show the deletion within codon 7. Except for the codon 262 deletion, other mutations characteristic of MPSV and temperature-sensitive MPSV v-mos genes were confirmed. A glycine-to-arginine mutation at residue 338 of the MPSV env-mos sequence, previously shown to cause thermosensitivity of the mutant virus (termed ts159) transforming function, yielded a v-mos protein that had significantly reduced protein kinase activity in vitro. These findings indicate that MPSV, like other Moloney murine sarcoma virus strains, also encodes a functional env-mos protein.

Nickel mutagenesis: alteration of the MuSVts110 thermosensitive splicing phenotype by a nickel-induced duplication of the 3' splice site

We investigated DNA damage caused by carcinogenic metals in a murine sarcoma virus (MuSV)-based mutagenicity assay in which mutations targeted to v-mos expression can be selected. Nickel chloride treatment of NRK cells (termed 6m2 cells) infected with MuSVts110, a retrovirus conditionally defective in viral RNA splicing and cell transformation, caused the outgrowth of transformed "revertants" with changes in the MuSVts110 RNA splicing phenotype. Cadmium and chromium treatment of 6m2 cells resulted in the selection of a second class of revertants with what appeared to be frameshift mutations allowing the translation of a readthrough gag-mos protein. In both classes of metal-induced revertants, viral gene expression was distinct from that observed in revertants arising in untreated 6m2 cultures, arguing that metal treatment did not simply enhance the rate of spontaneous reversion. In one representative nickel revertant line the operative nickel-induced mutation affecting MuSVts110 RNA splicing was a duplication of 70 bases surrounding the 3' splice site. The effect of this mutation was to direct splicing to the most downstream of the duplicated 3' sites and concomitantly relax its characteristic thermosensitivity. These data establish the mutagenic potential of nickel and provide the first example of a defined nickel-induced mutation in a mammalian gene.

v-mos protein produced by in vitro translation has protein kinase activity

The v-mos protein, termed p37v-mos, has a closely associated serine/threonine protein kinase activity. To provide further information about its protein kinase activity, we tested the activity of p37v-mos produced in a cell-free translation system from transcripts generated from a cloned v-mos gene. Anti-mos(37-55) immunoprecipitates of in vitro-produced p37v-mos were found to possess serine/threonine protein kinase activity, whereas those obtained with anti-mos(260-271), known to block v-mos autophosphorylation, lacked kinase activity. The phosphorylated products were identical in size to p37v-mos and p43v-mos produced in protein kinase assays from Moloney murine sarcoma virus-infected cells expressing authentic p37v-mos. These results provide further proof that the protein kinase activity associated with p37v-mos is an intrinsic property of the v-mos gene product. This translation system also provides a useful experimental model to study the activation of the mos protein kinase. Thus, protein kinase assays performed on [35S]methionine-labeled p37v-mos produced p43v-mos at the expense of p37v-mos. Phosphatase treatment removed the p43v-mos species, resulting in increase of the p37v-mos-sized protein, confirming our previous interpretation that p43v-mos is a hyperphosphorylated form of p37v-mos.

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.

Association of protein phosphatase 2A with polyoma virus medium tumor antigen

The polyoma virus medium and small tumor antigens, as well as simian virus 40 small tumor antigen, form specific complexes with two cellular proteins designated 61- and 37-kDa proteins. In this report, we demonstrate that the 61- and 37-kDa proteins correspond to the A and C subunits, respectively, of the serine- and threonine-specific protein phosphatase 2A (PP2A). On the one hand, antibodies raised against the 61-kDa protein reacted specifically with the purified A subunit of PP2A. Furthermore, the amino acid sequences of seven tryptic peptides from the A subunit were almost identical to sequences of the 61-kDa protein as deduced from the corresponding cDNA sequence. On the other hand, antibodies against the purified C subunit (catalytic subunit) of PP2A reacted specifically with the medium tumor antigen-associated 37-kDa protein. These data suggest a role of PP2A in cell transformation by polyoma virus and simian virus 40.

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.

A cell mutant that exhibits temperature-dependent sensitivity to transformation by various oncogenes

We previously isolated a Fisher rat fibroblast mutant, B812, that has the unique property of temperature-dependent transformation by various oncogenic retroviruses. At the permissive temperature (35 degrees C), this mutant was sensitive to oncogenic transformation and formed foci on a dish at the same frequency as did the parental fibroblast cell line. When Kirsten murine sarcoma virus (Ki-MSV) was applied to the cells, the frequency of focus formation decreased more than 25-fold at the nonpermissive temperature (39 degrees C), whereas the cells expressed nearly the same level of the ras transcript as well as the ras protein. The temperature-restricted focus formation was fully reversible and was completely suppressed upon fusion with the wild-type parent cell. In addition to ras, the mos, fos, src, and erbB-2 oncogenes transformed this mutant with the same temperature dependence as described above; polyomavirus middle T antigen, adenovirus type 12, and human papillomavirus 16-E67 also transformed, but without temperature dependence. These results suggest that ras, fos, mos, src, and erbB-2 use a common cellular pathway for transforming cells.

A cell mutant that exhibits temperature-dependent sensitivity to transformation by various oncogenes

We previously isolated a Fisher rat fibroblast mutant, B812, that has the unique property of temperature-dependent transformation by various oncogenic retroviruses. At the permissive temperature (35 degrees C), this mutant was sensitive to oncogenic transformation and formed foci on a dish at the same frequency as did the parental fibroblast cell line. When Kirsten murine sarcoma virus (Ki-MSV) was applied to the cells, the frequency of focus formation decreased more than 25-fold at the nonpermissive temperature (39 degrees C), whereas the cells expressed nearly the same level of the ras transcript as well as the ras protein. The temperature-restricted focus formation was fully reversible and was completely suppressed upon fusion with the wild-type parent cell. In addition to ras, the mos, fos, src, and erbB-2 oncogenes transformed this mutant with the same temperature dependence as described above; polyomavirus middle T antigen, adenovirus type 12, and human papillomavirus 16-E67 also transformed, but without temperature dependence. These results suggest that ras, fos, mos, src, and erbB-2 use a common cellular pathway for transforming cells.

Increased amount of a 25-kilodalton phosphoprotein after v-mos transfection of CHO cells

We transfected Chinese hamster ovary (CHO) cells with a cloned v-mos gene (pHT25). The mos family of oncogenes has previously been shown to have serine-threonine kinase activity. This kinase activity may be required for oncogenic transformation, although its exact biological role is unknown. We found that the transfected cells had an altered morphology, a slower doubling time, and an apparent increase in the amount of a 25-kilodalton (kDa) phosphoprotein that appeared to be of low abundance. Transfection of CHO cells with a cloned temperature-sensitive mos gene (ts159) led to isolation of a cell line that showed the presence of the 25-kDa phosphoprotein at the permissive but not at the nonpermissive temperature, suggesting a direct relationship between mos activity and the presence of this phosphoprotein. The characteristics of altered morphology and depressed growth rate were reminiscent of changes seen after the activation of the cyclic AMP-dependent protein kinase (PKA) in CHO cells. However, PKA activation did not stimulate phosphorylation of this 25-kDa protein, nor was there a change in total PKA activity in these cells. We suggest that the increased presence of the 25-kDa phosphoprotein is a consequence of the v-mos transfection and that it may be involved in the change of morphology and growth rate seen in the CHO cells. Phosphorylation of this protein may be a useful marker of mos and have some functional importance in the transformation of cells by the v-mos oncogene.

Oncogenes, protooncogenes, and signal transduction: toward a unified theory?

This paper has reviewed, in a broad sense, the potential involvement of the oncogenes and their progenitors, the protooncogenes, in signal transduction pathways. The membrane-associated oncogene products appear to be connected with the generation and/or regulation of secondary messengers, particularly those associated with Ca2+/phospholipid-dependent activation of the serine/threonine kinase protein kinase C. Activation of transmembrane receptors, either through binding their native ligand or through point mutations that lead to constitutive expression, results in the expression of their intrinsic tyrosine-specific protein kinases. In PDGF-stimulated cells, this results in the increased turnover of phosphatidylinositols and the subsequent release of IP3 (Habenicht et al., 1981; Berridge et al., 1984). This coincides with activation of a PI kinase activity (Kaplan et al., 1987). Likewise, the fms product, which is the receptor for CSF-1, induces a guanine nucleotide-dependent activation of phospholipase C (Jackowski et al., 1986). Receptor functions are potentially regulated through differential binding of ligands (as proposed with PDGF), through interactions with other receptors, and through the "feedback" regulation mediated by protein kinase C. PDGF stimulation leads to modulation of the EGF receptor through protein kinase C (Bowen-Pope et al., 1983; Collins et al., 1983; Davis and Czech, 1985). Similarly, the neu product becomes phosphorylated on tyrosine residues following treatment of cells with EGF, although the neu protein does not bind EGF itself (King et al., 1988; Stern and Kamps, 1988). The tyrosine kinases of the src family are not receptors themselves, although they may mediate specific receptor-generated signals. The clck product is physically and functionally associated with the T-cell receptors CD4 and CD8, and becomes active upon specific stimulation of cells expressing those markers (Veillette et al., 1988a,b). The precise physiological role of the src family products has not been established, but their kinase activity is intrinsic to that function. The v- and c-src products are hyperphosphorylated during mitosis (Chackalaparampil and Shalloway, 1988), which correlates with periods of reduced cell-to-cell adhesion and communication (Warren and Nelson, 1987; Azarnia et al., 1988). Furthermore, pp60c-src is associated with a PI kinase activity when complexed with MTAg of polyoma virus, suggesting a function in stimulating increased turnover of the phosphatidylinositols (Heber and Courtneidge, 1987; Kaplan et al., 1987).(ABSTRACT TRUNCATED AT 400 WORDS)

Increased amount of a 25-kilodalton phosphoprotein after v-mos transfection of CHO cells

We transfected Chinese hamster ovary (CHO) cells with a cloned v-mos gene (pHT25). The mos family of oncogenes has previously been shown to have serine-threonine kinase activity. This kinase activity may be required for oncogenic transformation, although its exact biological role is unknown. We found that the transfected cells had an altered morphology, a slower doubling time, and an apparent increase in the amount of a 25-kilodalton (kDa) phosphoprotein that appeared to be of low abundance. Transfection of CHO cells with a cloned temperature-sensitive mos gene (ts159) led to isolation of a cell line that showed the presence of the 25-kDa phosphoprotein at the permissive but not at the nonpermissive temperature, suggesting a direct relationship between mos activity and the presence of this phosphoprotein. The characteristics of altered morphology and depressed growth rate were reminiscent of changes seen after the activation of the cyclic AMP-dependent protein kinase (PKA) in CHO cells. However, PKA activation did not stimulate phosphorylation of this 25-kDa protein, nor was there a change in total PKA activity in these cells. We suggest that the increased presence of the 25-kDa phosphoprotein is a consequence of the v-mos transfection and that it may be involved in the change of morphology and growth rate seen in the CHO cells. Phosphorylation of this protein may be a useful marker of mos and have some functional importance in the transformation of cells by the v-mos oncogene.

Altered junctional permeability between cells transformed by v-ras, v-mos, or v-src

Junctional permeability in normal and transformed NRK cells was quantitatively assessed by microinjecting fluorescent dye into one cell of a pair, digitizing the changes of fluorescence intensity using video analysis techniques, and applying the digital values to a solution of Fick's diffusion equation. We show that this approach reliably estimates the junctional permeance of a cell pair. Cells that are temperature sensitive for transformation were shown to also be temperature sensitive vis-ă-vis junctional permeance. Thus permeance values were reduced approximately 80-90% on transformation by either a mutant Rous sarcoma virus or a mutant Moloney murine sarcoma virus. Cells transformed by wild-type Kirsten sarcoma virus were also shown to possess levels of junctional permeance significantly lower than nontransformed controls. The transformed junctional phenotype could be observed as early as 15 min after shifting to transformation-permissive conditions. Our results suggest that the oncogenes src, ras, and mos exert their effects on NRK cell junctions via converging pathways, of which one may be phosphorylation of junctional proteins.

Exploitation of a thermosensitive splicing event to study pre-mRNA splicing in vivo

The spliced form of MuSVts110 viral RNA is approximately 20-fold more abundant at growth temperatures of 33 degrees C or lower than at 37 to 41 degrees C. This difference is due to changes in the efficiency of MuSVts110 RNA splicing rather than selective thermolability of the spliced species at 37 to 41 degrees C or general thermosensitivity of RNA splicing in MuSVts110-infected cells. Moreover, RNA transcribed from MuSVts110 DNA introduced into a variety of cell lines is spliced in a temperature-sensitive fashion, suggesting that the structure of the viral RNA controls the efficiency of the event. We exploited this novel splicing event to study the cleavage and ligation events during splicing in vivo. No spliced viral mRNA or splicing intermediates were observed in MuSVts110-infected cells (6m2 cells) at 39 degrees C. However, after a short (about 30-min) lag following a shift to 33 degrees C, viral pre-mRNA cleaved at the 5' splice site began to accumulate. Ligated exons were not detected until about 60 min following the initial detection of cleavage at the 5' splice site, suggesting that these two splicing reactions did not occur concurrently. Splicing of viral RNA in the MuSVts110 revertant 54-5A4, which lacks the sequence -AG/TGT- at the usual 3' splice site, was studied. Cleavage at the 5' splice site in the revertant viral RNA proceeded in a temperature-sensitive fashion. No novel cryptic 3' splice sites were activated; however, splicing at an alternate upstream 3' splice site used at low efficiency in normal MuSVts110 RNA was increased to a level close to that of 5'-splice-site cleavage in the revertant viral RNA. Increased splicing at this site in 54-5A4 viral RNA is probably driven by the unavailability of the usual 3' splice site for exon ligation. The thermosensitivity of this alternate splice event suggests that the sequences governing the thermodependence of MuSVts110 RNA splicing do not involve any particular 3' splice site or branch point sequence, but rather lie near the 5' end of the intron.

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.

Exploitation of a thermosensitive splicing event to study pre-mRNA splicing in vivo

The spliced form of MuSVts110 viral RNA is approximately 20-fold more abundant at growth temperatures of 33 degrees C or lower than at 37 to 41 degrees C. This difference is due to changes in the efficiency of MuSVts110 RNA splicing rather than selective thermolability of the spliced species at 37 to 41 degrees C or general thermosensitivity of RNA splicing in MuSVts110-infected cells. Moreover, RNA transcribed from MuSVts110 DNA introduced into a variety of cell lines is spliced in a temperature-sensitive fashion, suggesting that the structure of the viral RNA controls the efficiency of the event. We exploited this novel splicing event to study the cleavage and ligation events during splicing in vivo. No spliced viral mRNA or splicing intermediates were observed in MuSVts110-infected cells (6m2 cells) at 39 degrees C. However, after a short (about 30-min) lag following a shift to 33 degrees C, viral pre-mRNA cleaved at the 5' splice site began to accumulate. Ligated exons were not detected until about 60 min following the initial detection of cleavage at the 5' splice site, suggesting that these two splicing reactions did not occur concurrently. Splicing of viral RNA in the MuSVts110 revertant 54-5A4, which lacks the sequence -AG/TGT- at the usual 3' splice site, was studied. Cleavage at the 5' splice site in the revertant viral RNA proceeded in a temperature-sensitive fashion. No novel cryptic 3' splice sites were activated; however, splicing at an alternate upstream 3' splice site used at low efficiency in normal MuSVts110 RNA was increased to a level close to that of 5'-splice-site cleavage in the revertant viral RNA. Increased splicing at this site in 54-5A4 viral RNA is probably driven by the unavailability of the usual 3' splice site for exon ligation. The thermosensitivity of this alternate splice event suggests that the sequences governing the thermodependence of MuSVts110 RNA splicing do not involve any particular 3' splice site or branch point sequence, but rather lie near the 5' end of the intron.

The P55 protein affected by v-mos expression is vimentin

Rabbit antiserum prepared against a cyclic 19-amino-acid peptide predicted from the sequence of the viral mos gene (v-mos) of Moloney murine sarcoma virus not only recognized v-mos gene products but also specifically detected a 55,000-Mr polypeptide expressed in a variety of cells that grow on solid surfaces. This normal cellular protein, previously shown to be reduced in amount in cells expressing the v-mos gene, was found to be the intermediate filament structural protein, vimentin. This conclusion was reached by comparing relative mobilities in denaturing gels, isoelectric points, immunoreactivities, location in the cell, and peptide maps.

Nickel-induced heritable alterations in retroviral transforming gene expression

Determination of the mutagenic effects of carcinogenic nickel compounds has been difficult because, like many metals, nickel is poorly or nonmutagenic in procaryotic mutagenicity assays. We attempted to characterize nickel-induced genetic lesions by assessing the effect of nickel chloride on the conditionally defective expression of the v-mos transforming gene in normal rat kidney cells infected with the Murine sarcoma virus mutant ts110 (MuSVts110) retrovirus. MuSVts110 contains an out-of-frame gag gene-mos gene junction that prevents the expression of the v-mos gene at the nonpermissive temperature (39 degrees C). In MuSVts110-infected cells (6m2 cells) grown at 33 degrees C, however, this defect can be suppressed by a splicing event that restores the mos reading frame, allowing the expression of a gag-mos fusion protein which induces the transformed phenotype. The capacity to splice the viral transcript at 33 degrees C, but not at 39 degrees C, is an intrinsic property of the viral RNA. This property allowed us to target the MuSVts110 genome using a positive selection scheme whereby nickel was used to induce genetic changes which resulted in expression of the transformed phenotype at 39 degrees C. We treated 6m2 cells with NiCl2 and isolated foci consisting of cells which had reverted to the transformed phenotype at 39 degrees C. We found that brief nickel treatment increased the reversion frequency of 6m2 cells grown at 39 degrees C sevenfold over the spontaneous reversion frequency. The nickel-induced revertants displayed the following heritable characteristics: They stably maintained the transformed phenotype at 39 degrees C; unlike the MuSVts110 RNA in 6m2 cells, the nickel-induced revertant viral RNA could be spliced efficiently at 39 degrees C; as a consequence of the enhanced accumulation of spliced viral RNA, the nickel-induced revertants produced substantial amounts of the transforming v-mos protein P85gag-mos at 39 degrees C; the nickel-induced revertant P85gag-mos serine kinase, like the parental 6m2 P85gag-mos kinase, was found to be rapidly inactivated at 39 degrees C; however, in the nickel-induced revertants, overproduction of P85gag-mos allowed the transformed state to be maintained; and even though viral RNA processing was much changed, no rearrangements of the viral DNA in the nickel-induced revertant cells were detected by partial restriction analysis.

Nucleoside triphosphate-dependent DNA-binding properties of mos protein

We have previously shown that the mos gene product, p40mos, produced in Escherichia coli binds ATP and has ATPase activity. In the present study, we investigated the DNA-binding properties of p40mos and two mos deletion mutant proteins. Nitrocellulose blot protein-DNA binding assays showed that p40mos binds DNA in the presence of Mg2+-ATP and certain other nucleoside triphosphates. Ninety percent of the p40mos-bound DNA is dissociated if the complex is washed in the presence of 1 M NaCl or in the absence of ATP. p40mos-DNA binding is not observed in the presence of AMP or the nonhydrolyzable ATP analog adenosine 5'-[beta, gamma-methylene]-triphosphate; however, in the presence of ADP, p40mos binds DNA at 20% of the level that is observed with ATP. An N-terminal-deletion mutant protein, p19mos, has no DNA-binding activity, whereas a C-terminal-deletion mutant protein, p25mos, does. p25mos contains the ATP-binding domain, binds DNA in the presence of either ADP or ATP, and shows 5% and 45% binding (relative to that in the presence of ATP) in the presence of AMP and adenosine 5'-[beta, gamma-methylene]triphosphate, respectively. These results suggest that the N-terminal domain of p40mos is responsible for nucleoside triphosphate-mediated DNA binding. We also observed differential histone-DNA binding in the presence and absence of ATP.

A temperature-sensitive mutant of the myeloproliferative sarcoma virus, altered by a point mutation in the mos oncogene, has been modified as a selectable retroviral vector

The myeloproliferative sarcoma virus (MPSV) is a mos-oncogenic retrovirus which induces an acute myeloproliferative disease in adult mice. The isolation and molecular cloning of two mutants of MPSV temperature sensitive (ts) for mos transformation (Kollek et al., J. Virol. 50:717-724, 1984) have been described previously. In this report, we describe the biological activity of these clones, the molecular basis of the ts lesion of one clone, and the construction of a selectable vector based on the MPSV ts genome. Both molecular clones, ts159 and ts124, proved to have retained the ts phenotype, the former being tighter for the induction and maintenance of the transformed phenotype. A single transition (G----A) at position 1888 in the mos coding region, resulting in the change of Gly to Arg at position 307, was responsible for the ts phenotype of clone ts159. Substitution of sequences carrying this mutation with the corresponding sequences of the wild-type virus generated a virus that was ts for transformation. Insertion of the dominant selectable marker gene for geneticin resistance (neor) into ts159 did not disrupt mos expression or its ts phenotype. neor-ts159 facilitates the study of mos action by allowing the selection of infected cells at the nonpermissive temperature before mos transformation has been induced. Furthermore, infected cells which show no obvious phenotype alteration due to mos expression can be identified by their Neor phenotype.

p37mos-associated serine/threonine protein kinase activity correlates with the cellular transformation function of v-mos

A serine/threonine-specific protein kinase activity is closely associated with v-mos-encoded proteins. Experiments were conducted with several mutant forms of p37mos to determine whether or not the kinase function correlates with the biological activity of the mutant v-mos genes. Two mutants lacking cell transformation activity, one an arginine substitution for lysine-121 in the putative ATP-binding site and the other a 23-amino acid deletion from the C-terminal end of p37mos, had no kinase activity associated with their mutant proteins. However, a third mutant with reduced biological activity had drastically less kinase activity than the wild-type protein. The latter mutant was able to phosphorylate the kinase-inactive p37mos(Arg-121) protein in vitro. These results indicate that even though p37mos(Arg-121) can be phosphorylated in trans by other kinase molecules, it lacks the ability to phosphorylate itself in vitro. This provides a compelling argument that the protein kinase function of p37mos is an intrinsic property of the protein. Moreover, since the kinase function correlates with the cellular transformation activity of the v-mos gene, we predict that it is required for the biological activity of the v-mos gene.

Identification of the protein encoded by the human diffuse B-cell lymphoma (dbl) oncogene

The dbl oncogene was initially isolated from a human diffuse B-cell lymphoma. Antisera from mice bearing tumors induced by this oncogene specifically detected a protein of about 66 kDa (p66) in dbl transformants. dbl cDNA-selected poly(A)+ RNA isolated from a transfectant clone expressing p66 directed the in vitro synthesis of this protein, establishing that it is encoded by dbl. Subcellular localization studies revealed that p66 is a cytoplasmic protein distributed between cytosol and crude membrane fractions. Moreover, p66 was shown to be a phosphoprotein, with phosphorylation specific to serine residues. Our characterization of the dbl-encoded protein appears to distinguish this transforming gene product from those of other known oncogenes.

Members of the src and ras oncogene families supplant the epidermal growth factor requirement of BALB/MK-2 keratinocytes and induce distinct alterations in their terminal differentiation program

BALB-/MK-2 mouse epidermal keratinocytes required epidermal growth factor for proliferation and terminally differentiated in response to high Ca2+ concentration. Infection with retroviruses containing transforming genes of the src and ras oncogene families led to rapid loss of epidermal growth factor dependence, in some cases, accompanied by alterations in cellular morphology. The virus-altered cells continued to proliferate in the presence of high levels of extracellular calcium but exhibited alterations in normal keratinocyte terminal differentiation that appear to be specific to the particular oncogene. These alterations bore similarities to abnormalities in differentiation observed in naturally occurring squamous epithelial malignancies.

Detection of c-abl tyrosine kinase activity in vitro permits direct comparison of normal and altered abl gene products

The v-abl transforming protein P160v-abl and the P210c-abl gene product of the translocated c-abl gene in Philadelphia chromosome-positive chronic myelogenous leukemia cells have tyrosine-specific protein kinase activity. Under similar assay conditions the normal c-abl gene products, murine P150c-abl and human P145c-abl, lacked detectable kinase activity. Reaction conditions were modified to identify conditions which would permit the detection of c-abl tyrosine kinase activity. It was found that the Formalin-fixed Staphylococcus aureus formerly used for immunoprecipitation inhibits in vitro abl kinase activity. In addition, the sodium dodecyl sulfate and deoxycholate detergents formerly used in the cell lysis buffer were found to decrease recovered abl kinase activity. The discovery of assay conditions for c-abl kinase activity now makes it possible to compare P150c-abl and P145c-abl kinase activity with the altered abl proteins P160v-abl and P210c-abl. Although all of the abl proteins have in vitro tyrosine kinase activity, they differ in the way they utilize themselves as substrates in vitro. Comparison of in vitro and in vivo tyrosine phosphorylation sites of the abl proteins suggests that they function differently in vivo. The development of c-abl kinase assay conditions should be useful in elucidating c-abl function.

Biologically active mutants with deletions in the v-mos oncogene assayed with retroviral vectors

We have constructed retroviral expression vectors by manipulation of the Moloney murine leukemia virus genome such that an exogenous DNA sequence may be inserted and subsequently expressed when introduced into mammalian cells. A series of N-terminal deletions of the v-mos oncogene was constructed and assayed for biological activity with these retroviral expression vectors. The results of the deletion analysis demonstrate that the region of p37mos coding region upstream of the third methionine codon is dispensable with respect to transformation. However, deletion mutants of v-mos which allow initiation of translation at the fourth methionine codon have lost the biological activity of the parental v-mos gene. Furthermore, experiments were also carried out to define the C-terminal limit of the active region of p37mos by the construction of premature termination mutants by the insertion of a termination oligonucleotide. Insertion of the oligonucleotide just 69 base pairs upstream from the wild-type termination site abolished the focus-forming ability of v-mos. Thus, we have shown the N-terminal limit of the active region of p37mos to be between the third and fourth methionines, while the C-terminal limit is within the last 23 amino acids of the protein.

Pks, a raf-related sequence in humans

A human fetal liver cDNA library was screened at reduced hybridization stringency for v-raf-related sequences. In addition to the expected c-raf-1 cDNA, a second sequence was isolated. Comparison of the second gene (pks) to the other raf-related sequences revealed nucleotide homologies of 71%. The predicted amino acid sequence of the kinase domain is sufficiently similar to that of v-raf to suggest that pks may encode a polypeptide that exhibits serine/threonine kinase activity. The expression of pks mRNA (2.7 kilobases long) is elevated in peripheral blood mononuclear cells isolated from two patients with angioimmunoblastic lymphadenopathy with dysproteinemia, a disease in which autoantibodies are produced following the lymphoproliferative activation of B cells. Analysis of somatic cell hybrids for segregation of the pks locus revealed the presence of an additional locus closely related to the pks sequence.

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.

Molecular basis underlying phenotypic revertants of Moloney murine sarcoma virus MuSVts110

We examined the molecular basis for phenotypic reversion in cells infected with a transformation mutant of murine sarcoma virus, MuSVts110. In MuSVts110-infected NRK cells (6m2 cells), the manifestation of the transformed phenotype at 33 degrees C and the normal phenotype at 39 degrees C is governed by thermosensitive splicing of the MuSVts110 primary transcript, a 4.0-kilobase (kb) RNA which contains the gag and mos genes joined out of frame. At 33 degrees C, selectively, the 4.0-kb RNA is processed to a spliced 3.5-kb RNA in which the gag and mos genes are rejoined in a continuous open reading frame, thus allowing synthesis of the P85gag-mos-transforming protein. In contrast, the MuSVts110 revertant cell lines (designated 54-5A4 and 204-3) appear transformed at all growth temperatures from 33 to 39 degrees C and express a P100gag-mos-transforming protein from an apparently unprocessed 4.0-kb viral RNA. In the current study we established both by S1 nuclease analysis and primer extension sequencing that the revertant 54-5A4 and 204-3 4.0-kb viral RNAs suffered a 5-base deletion at the intron-exon border of the 3' splice site. The effect of this deletion is twofold. First, because of the damage to the 3' splice site, the revertant viral 4.0-kb RNAs cannot be processed to the spliced 3.5-kb RNA and, consequently, cannot be translated to P85gag-mos. Second, the 5-base deletion excises an in-frame stop codon positioned at the intron-exon border in the parental RNA and restores the original mos gene reading frame. The net effect is to produce a continuous open reading frame from the gag, alternate mos, and authentic mos gene reading frames which are fused together in the revertant 4.0-kb RNA. This continuous open reading frame can be translated into the P100gag-mos-transforming protein at any growth temperature.

Members of the src and ras oncogene families supplant the epidermal growth factor requirement of BALB/MK-2 keratinocytes and induce distinct alterations in their terminal differentiation program

BALB-/MK-2 mouse epidermal keratinocytes required epidermal growth factor for proliferation and terminally differentiated in response to high Ca2+ concentration. Infection with retroviruses containing transforming genes of the src and ras oncogene families led to rapid loss of epidermal growth factor dependence, in some cases, accompanied by alterations in cellular morphology. The virus-altered cells continued to proliferate in the presence of high levels of extracellular calcium but exhibited alterations in normal keratinocyte terminal differentiation that appear to be specific to the particular oncogene. These alterations bore similarities to abnormalities in differentiation observed in naturally occurring squamous epithelial malignancies.

Biologically active mutants with deletions in the v-mos oncogene assayed with retroviral vectors

We have constructed retroviral expression vectors by manipulation of the Moloney murine leukemia virus genome such that an exogenous DNA sequence may be inserted and subsequently expressed when introduced into mammalian cells. A series of N-terminal deletions of the v-mos oncogene was constructed and assayed for biological activity with these retroviral expression vectors. The results of the deletion analysis demonstrate that the region of p37mos coding region upstream of the third methionine codon is dispensable with respect to transformation. However, deletion mutants of v-mos which allow initiation of translation at the fourth methionine codon have lost the biological activity of the parental v-mos gene. Furthermore, experiments were also carried out to define the C-terminal limit of the active region of p37mos by the construction of premature termination mutants by the insertion of a termination oligonucleotide. Insertion of the oligonucleotide just 69 base pairs upstream from the wild-type termination site abolished the focus-forming ability of v-mos. Thus, we have shown the N-terminal limit of the active region of p37mos to be between the third and fourth methionines, while the C-terminal limit is within the last 23 amino acids of the protein.

Detection of c-abl tyrosine kinase activity in vitro permits direct comparison of normal and altered abl gene products

The v-abl transforming protein P160v-abl and the P210c-abl gene product of the translocated c-abl gene in Philadelphia chromosome-positive chronic myelogenous leukemia cells have tyrosine-specific protein kinase activity. Under similar assay conditions the normal c-abl gene products, murine P150c-abl and human P145c-abl, lacked detectable kinase activity. Reaction conditions were modified to identify conditions which would permit the detection of c-abl tyrosine kinase activity. It was found that the Formalin-fixed Staphylococcus aureus formerly used for immunoprecipitation inhibits in vitro abl kinase activity. In addition, the sodium dodecyl sulfate and deoxycholate detergents formerly used in the cell lysis buffer were found to decrease recovered abl kinase activity. The discovery of assay conditions for c-abl kinase activity now makes it possible to compare P150c-abl and P145c-abl kinase activity with the altered abl proteins P160v-abl and P210c-abl. Although all of the abl proteins have in vitro tyrosine kinase activity, they differ in the way they utilize themselves as substrates in vitro. Comparison of in vitro and in vivo tyrosine phosphorylation sites of the abl proteins suggests that they function differently in vivo. The development of c-abl kinase assay conditions should be useful in elucidating c-abl function.

Nucleotide sequence and biochemical activities of the Moloney murine sarcoma virus strain HT-1 mos gene

The nucleotide sequence of the Moloney murine sarcoma virus strain HT-1 (HT1MSV) mos gene differs from that of the cellular mos gene in three positions, but these are silent changes, and the amino acid sequence of the v-mos and c-mos open reading frames are identical. We have overproduced the mos HT1MSV (equivalent to c-mos) in Escherichia coli under the control of phage lambda promoter (pL). The E. coli p40mos protein thus obtained was partially purified and examined for several biochemical activities. We show that the p40mos binds ATP analog p-fluorosulfonylbenzoyladenosine and exhibits ATPase activity.

Use of site-specific antipeptide antibodies to perturb the serine kinase catalytic activity of p37mos

The mos oncogene of Moloney murine sarcoma virus encodes a protein of approximately 37,000 daltons (designated p37mos). We have detected a serine protein kinase activity which is closely associated with p37mos in immune complexes obtained with antibodies [anti-mos(37-55) serum] that were generated with a peptide containing amino acids 37 through 55 of the v-mos protein (S. A. Maxwell and R. B. Arlinghaus, Virology 143:321-333, 1985). Immune complexes that were derived with antibodies generated against peptides representing the C-terminal 8 or 12 amino acids of v-mos (anti-C2 and anti-C3 serum, respectively) exhibited very little kinase activity capable of phosphorylating p37mos. Treatment of anti-mos(37-55) complexes containing active v-mos kinase with anti-C3 or anti-C2 serum resulted in a dramatic reduction of the in vitro phosphorylation of p37mos. Antiserum blocked with the appropriate C-terminal peptide had no inhibitory effect on the phosphorylation of p37mos in anti-mos(37-55) complexes which indicated that the inhibition of v-mos kinase activity was a specific effect of these antibodies. The specific inhibition of the in vitro phosphorylation of p37mos by antibodies directed against the C terminus of the v-mos protein provides strong evidence that the v-mos gene encodes a serine protein kinase. In addition, the extreme C terminus of p37mos may be critical for an active v-mos kinase.

Protein kinase activity associated with the product of the yeast cell division cycle gene CDC28

Antibodies raised against the protein encoded by a lacZ-CDC28 in-frame fusion were shown to immunoprecipitate the CDC28 product from yeast cell lysates. The polypeptide p36CDC28 is a phosphoprotein of apparent Mr 36,000. Immune complexes prepared from yeast cell lysates by using anti-CDC28 antibody were found to possess a protein kinase activity, as determined by the transfer of label from [gamma-32P]ATP to a coprecipitated Mr 40,000 protein of unknown identity or function (p40). This activity was absent or thermolabile when extracts were prepared from several different cdc28 temperature-sensitive strains. The protein kinase activity was dependent on Zn2+ and transferred phosphate specifically to serine and threonine residues.

Activation of the c-abl oncogene by viral transduction or chromosomal translocation generates altered c-abl proteins with similar in vitro kinase properties

The v-abl protein of Abelson murine leukemia virus is a tyrosine-specific kinase. Its normal cellular homolog, murine c-abl, does not possess detectable tyrosine kinase activity in vitro. Previously, we have detected tyrosine kinase activity in vitro for an altered c-abl gene product (c-abl P210) in the K562 human chronic myelogenous leukemia cell line. The expression of this variant c-abl gene product correlates with chromosomal translocation and amplification of the c-abl gene in K562 cells. Like v-abl, c-abl P210 is a fusion protein containing non-abl sequences near the amino terminus of c-abl. We compared the in vitro tyrosine kinase activity of c-abl P210 with that of wild-type murine v-abl. The remarkable similarities of these two proteins with respect to cis-acting autophosphorylation, trans-acting phosphorylation of exogenous substrates, and kinase inhibition, using site-directed abl-specific antisera, suggested that c-abl P210 could function similarly to v-abl in vivo. In addition, c-abl P210 possessed an associated serine kinase activity in immunoprecipitates. The serine kinase activity was not inhibited by site-directed, abl-specific antisera that inhibit the tyrosine kinase activity, suggesting that the serine kinase activity is not an intrinsic property of c-abl P210. Thus, the activation of the c-abl gene in a human leukemia cell line may have functional consequences analogous to activation of the c-abl gene in Abelson murine leukemia virus.

Epidermal growth factor precursor is related to the translation product of the Moloney sarcoma virus oncogene mos

Murine epidermal growth factor (EGF) is synthesized as part of a large precursor (pro-EGF), which is thought to span the cell membrane. Comparison of the published pro-EGF sequence with the sequences of the translation products of viral oncogenes reveals that pro-EGF is related to the translation product of mos, the oncogene of Moloney murine sarcoma virus. Similarity is greatest between the COOH-terminal region of v-mos (residues 317-360) and part of the cytoplasmic domain of pro-EGF (residues 1127-1174). Statistical comparison of these sequences indicates that the probability of the similarity arising by chance is less than 2 X 10(-8). This similarity extends to the corresponding regions of the translation products of the cellular homologues (c-mos) of the v-mos gene present in normal murine and human DNA. Similarities are also observed between two other regions of the murine c-mos sequence (residues 48-134 and 196-275) and parts of the extracellular domain of pro-EGF (residues 565-651 and 741-817, respectively). All three mos genes are members of the tyrosine kinase family of oncogenes, as is erbB, the oncogene of avian erythroblastosis virus. Since the sequences of the erbB translation product and the EGF receptor are closely related, the relationship between mos and pro-EGF suggests that pro-EGF and the EGF receptor have evolved from a common ancestor.