Comparative tryptic peptide analysis of candidate P85gag-mos of ts110 Moloney murine sarcoma virus and P38-P23 mos gene-related proteins of wild-type virus

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.

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.

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.

Murine sarcoma virus ts110 RNA transcripts: origin from a single proviral DNA and sequence of the gag-mos junctions in both the precursor and spliced viral RNAs

Our previous studies have argued persuasively that in murine sarcoma virus ts110 (MuSVts110) the gag and mos genes are fused out of frame due to a approximately 1.5-kilobase (kb) deletion of wild-type murine sarcoma virus 349 (MuSV-349) viral information. As a consequence of this deletion, infected cells grown at 39 degrees C appear morphologically normal, producing a 4-kb viral RNA and a truncated gag gene product, P58gag. At 33 degrees C, however, MuSVts110-infected cells appear transformed, producing two viral RNAs, about 4 and 3.5 kb in length, and two viral proteins, P58gag and P85gag-mos. Recent S1 nuclease analyses (Nash et al., J. Virol. 50:478-488, 1984) suggested strongly that at 33 degrees C about 430 bases surrounding the out-of-frame gag-mos junction and bounded by consensus splice donor and acceptor sites are excised from the 4-kb RNA to form the 3.5-kb RNA. As a result of this apparent splicing event, the gag and mos genes seemed to be fused in frame and allowed the translation of P85gag-mos. In the present study, DNA primers hybridizing to the MuSVts110 4- and 3.5-kb RNAs just downstream of the gag-mos junction points were used to sequence these junctions by the primer extension method. We observed that, relative to wild-type MuSV-349 5.2-kb RNA, the MuSVts110 4-kb RNA had suffered a 1,488-base deletion as a result of the fusion of wild-type gag gene nucleotide 2404 to wild-type mos gene nucleotide 3892. This gag-mos junction is out of frame, containing both TAG and TGA termination codons in the reading frame 42 and 50 bases downstream of the gag-mos junction, respectively. Thus, the MuSVts110 4-kb RNA can only be translated into a truncated gag precursor containing an additional C-terminal 14 amino acid residues derived from an alternate mos gene reading frame. Similar analyses of the MuSVts110 3.5-kb RNA showed a further loss of both gag and mos sequences over those deleted in the original 1,488-base deletion. In the MuSVts110 3.5-kb RNA, we found that gag nucleotide 2017 was fused to mos nucleotide 3936 (nucleotide 2449 in the MuSVts110 4-kb genome). This 431-base excised fragment is bounded exactly by in-frame consensus splice donor and acceptor sequences. As a consequence of this splice event, the TAG codon is excised and the restoration of the original mos gene reading frame allows the TGA codon to be bypassed.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

S1 nuclease mapping of viral RNAs from a temperature-sensitive transformation mutant of murine sarcoma virus

The structures of murine sarcoma virus (MuSV) ts110 viral RNA and intracellular RNA present in MuSV ts110-infected cells (6m2 cells) have been examined by S1 nuclease analysis. A previous study involving heteroduplex analysis of MuSV ts110 viral RNAs hybridized to wild-type DNA revealed the presence of two MuSV ts110 RNAs, 4.0 and 3.5 kilobases (kb) in length, containing overlapping central deletions relative to wild-type MuSV 124 viral RNA (Junghans et al., J. Mol. Biol. 161:229-255, 1982). Here we show that the deletion (termed delta 1) in the 4.0-kb RNA has a 5' border located at about nucleotide 2409 (using the numbering system of Van Beveren et al., Cell 27:97-108, 1981), a position 63 bases upstream of the junction of the p30 and p10 coding sequences. The 3' border of the delta 1 deletion is found 1,473 bases downstream at approximately nucleotide 3883, 10 nucleotides downstream of the first mos gene initiation codon. In the 3.5-kb MuSV ts110 RNA, the 5' border of the deleted central region (termed delta 2) is located in a splice consensus donor site at approximately nucleotide 2017, 330 bases downstream from the junction of the p12 and p30 coding sequences, and extends about 1,915 bases in the downstream direction to nucleotide 3935, found in a splice consensus acceptor site about 55 nucleotides downstream of the first mos gene initiation codon and 30 bases upstream of the second initiation codon. No alteration of polyadenylate addition sites was observed in either MuSV ts110 RNA species, as compared with MuSV 349 RNA. The observation that the 5' and 3' borders of the deletion in the 3.5-kb RNA are within in-frame splice donor and acceptor sites suggests strongly that the 3.5-kb RNA is derived from the 4.0-kb RNA by a temperature-sensitive splice mechanism. Data presented here show unequivocally that formation of the 3.5-kb MuSV ts110 RNA from which the P85gag-mos polypeptide is translated is temperature sensitive. At 33 degrees C, with S1 analysis, the 3.5-kb RNA is found readily in 6m2 cells. Within 4 h of a shift to 39 degrees C, however, only trace amounts of this RNA can be found. Moreover, reshifting 6m2 cells to 33 degrees C permits the reappearance of the 3.5-kb RNA at its original level.

Incorporation of lipids into variants of Moloney sarcoma virus which produce gag-mos fusion proteins

Several transforming gene products of mammalian retroviruses undergo post-translational addition of fatty acids. This report demonstrates that gag-mos fusion proteins from variant strains of murine sarcoma viruses incorporate both myristate and palmitate. However, the inefficiency of palmitate incorporation, as well as incorporation of that fatty acid into gag proteins, suggest that palmitate is metabolized to myristate before becoming bound to virus proteins. Furthermore, no detectable post-translational processing event is apparently required for addition of myristate, suggesting that such an event is unnecessary for association of gag-mos proteins with cell membranes.

Restriction of Moloney murine leukemia virus replication in Moloney murine sarcoma virus-infected cells

MuSV349 is a TB-cell line which produces infectious MuSV with little or no MuLV detectable by the XC assay. The apparent restriction of MuLV replication in MuSV349 cells was investigated. A replication-competent helper virus, with protein composition nearly identical to that of Mo-MuLV was isolated from the viruses produced by MuSV349 cells. This helper MuLV after it was separated from MuSV and upon infection of TB cells produced viral titer similar to that of Mo-MuLV-infected TB cells indicating that its replication might have been restricted in the MuSV349 cells. To find out whether the suppression of the helper virus replication is due to the genetic peculiarities of the virus or MuSV349 cells, the relative amounts of MuLV and MuSV produced by several distinct clonal MuSV isolates (derived from a common progenitor) upon superinfection with Mo-MuLV were determined. The results of these experiments showed that while both SV7 and SV15F on coinfection with Mo-MuLV produced MuSV in excess over MuLV; and ts110 and tsSV13 on coinfection with Mo-MuLV produced MuLV in excess over MuSV. Since the same Mo-MuLV is used in these experiments and since upon transfer to a different cell, SV7, SV15F, and ts110 retain the property to restrict or not restrict MuLV replication it appears that the above property is determined by the genetics of the MuSV.

The role of calmodulin in cell transformation

Two cell lines transformed with temperature sensitive retroviruses were examined for: their ability to grow in low Ca2+ medium, their calmodulin levels and changes in calmodulin acceptor proteins. Both cell lines grow in low Ca2+ medium at the permissive temperature 34 degrees C while both lines did not replicate at the non-permissive temperature 39 degrees C. The NRKLA23 cells have nearly twice as much calmodulin at the permissive temperature than they do at the non-permissive temperature while the 6M2 cells have an equal amount of calmodulin at both temperatures. Both cell lines exhibit changes in the calmodulin acceptor proteins going from the permissive to the non-permissive temperature. We suspect that the changes in the calmodulin acceptor proteins may be involved in the altered Ca2+-sensitivity of growth in the cells going from the permissive to non-permissive temperature.

Gag-mos Polyproteins encoded by variants of the Moloney strain of mouse sarcoma virus

Two revertants of ts110 Moloney murine sarcoma virus (MuSV) with wild-type MuSV phenotype were examined for the presence of mos gene products, ts110 MuSV has a temperature-sensitive defect in a function required to maintain the transformed phenotype. The nonproducer 6m2 cell clone transformed by ts110 produces an 85,000-Da gag-mos protein (P85gag-mos) and a 58,000-Da gag protein (P58gag). A spontaneous revertant (clone 54-5A4) of the 6m2 cell clone produces a 100,000-Da protein (P100) recognized by antisera raised against murine leukemia virus p15, p12, and p30 but lacks determinants of p10, reverse transcriptase, and gp70. P100 was specifically recognized by antisera (anti-C3) prepared against a synthetic peptide representing the predicted C-terminal 12 amino acids of Moloney MuSV v-mos gene. Normal sera or anti-C3 blocked with excess synthetic peptide did not recognize P100. Thus, P100 is a product of the gag and mos genes. P100 was found to be phosphorylated. A second wild-type revertant (clone 204-3) was obtained by superinfection of ts110 nonproducer cells with Simian sarcoma associated virus (SSAV); it was also found to contain a phosphorylated P100gag-mos protein. The 204-3 cell clone also contained two gag polyproteins (Pr60gag and Pr55gag) of the size and antigenic properties of those found in SSAV-infected cells. These results provide two examples of P100 gag-mos proteins both derived from the P85gag-mos producing 6m2 cell clone. The P100 gag-mos polyproteins are made in amounts that are easily detected by radiolabeling experiments using [3H]leucine. The intracellular viral RNAs present in 6m2 cells and the two revertant clones were also examined. All three cell clones contained a 4.0 kb RNA hybridizing to v-mos sequences but only the 6m2 clone contained a 3.5 kb mos-containing RNA. Our findings indicate that the 3.5 kb RNA codes for P85gag-mos in cell-free translation experiments (Junghans et al., 1982, J. Mol. Biol. 161, 229). These findings as they relate to the mechanism that produces P100gag-mos instead of P85gag-mos are discussed.

P85: a gag-mos polyprotein encoded by ts110 Moloney murine sarcoma virus

Antibody to a synthetic peptide (anti-C3 serum) with the predicted sequence of the C terminus of the Moloney murine sarcoma virus (strain 124) v-mos gene was used in immunoprecipitation experiments with cytoplasmic extracts of a clone of NRK cells infected with ts110 Moloney murine sarcoma virus, termed 6m2 cells. ts110 Moloney murine sarcoma virus codes for two viral proteins of 85,000 and 58,000 M(r), termed P85 and P58, respectively, in nonproducer 6m2 cells maintained at 33 degrees C. Anti-C3 serum specifically recognized [(3)H]leucine-labeled P85, but not P58, from infected cells maintained at 33 degrees C, whereas antiserum prepared against murine leukemia virus p12 recognized both proteins. Normal serum and anti-C3 serum pretreated with excess C3 peptide did not precipitate P85. Immunoprecipitation experiments after metabolic labeling of 6m2 cells with (32)P(i) showed that P85 is phosphorylated. Both anti-C3 and anti-p12 sera specifically detected (32)P-labeled P85. Cell-free translation of ts110 murine sarcoma virus/murine lukemia virus RNA produces P85, P58, and helper virus protein Pr63(gag). Anti-C3 serum specifically precipitated P85 but neither P58 nor Pr63(gag). We conclude from these studies that P85 is a product of both the gag and mos genes of ts110 murine sarcoma virus, and, therefore, it is referred to as P85(gag-mos). We have not detected any other v-mos gene product in ts110-infected cells.

Detection of an 85,000-dalton phosphoprotein in ts110 murine sarcoma virus-infected cells with antiserum against a v-mos peptide

We have used an antiserum directed against a synthetic v-mos peptide (anti-C3 serum) to screen ts110 murine sarcoma virus (MuSV)-infected cells for the presence of v-mos-encoded proteins. Anti-C3 serum specifically recognized an 85,000-dalton protein doublet (P85) from [35S]methionine-labeled ts110 MuSV-infected producer cells grown at 32 degrees C, the permissive temperature for transformation. The P85 doublet was also recognized by an antiserum directed against the viral gag protein p15. P85 was present but at 2- to 10-fold-lower levels in ts110 MuSV-infected producer cells grown at 39 degrees C, the restrictive temperature for transformation. The P85gag-mos fusion product was the only v-mos protein reproducibly detected in this ts110 MuSV-transformed cell line. Immunoprecipitation of 32P-labeled cells with anti-C3 serum revealed that the upper band of the P85 doublet is phosphorylated, containing mostly phosphoserine and some phosphothreonine. Cells acutely infected with ts110 MuSV contained slightly higher levels of P85 than did the ts110 MuSV-infected producer cell line. Anti-C3 serum specifically recognized a 33,000-dalton protein (p33) in the acutely infected cells labeled with [35S]methionine. p33 was present in trace amounts and may represent a previously unidentified ts110 MuSV-encoded v-mos protein.

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

A protein identified as P85(gag-mos) was shown to be phosphorylated when immunoprecipitates from ts110 Moloney murine sarcoma virus transformed nonproducer cells (clone 6m2) were incubated with [gamma-(32)P]ATP. The in vitro-labeled 85,000-dalton phosphoprotein comigrated on NaDodSO(4)/polyacrylamide gels with authentic phosphorylated P85(gag-mos). Immunoprecipitates obtained with antisera prepared against Rauscher murine leukemia virus core protein p30 were active in the immune complex kinase assay but anti-murine leukemia virus p10 precipitates were not. Previous studies have shown that anti-p30 but not anti-p10 antisera recognize P85(gag-mos). The 6m2 clone has been shown to express P85(gag-mos) at 33 degrees C but not at 39 degrees C. Anti-p30 immune complexes from 6m2 cells maintained at 39 degrees C failed to phosphorylate the 85,000-dalton protein. Furthermore, the in vitro phosphorylated 85,000-dalton protein gave the same pattern of V8 protease-generated cleavage products as in vivo(32)P-labeled P85(gag-mos). We conclude from these results that P85(gag-mos) is phosphorylated in anti-p30 immune complex kinase reactions. Phosphoamino acid analyses indicated that the in vitro phosphorylated P85(gag-mos) contained phosphoserine and phosphothreonine. Our findings indicate that incubation of anti-p30 immunoprecipitates at 39 degrees C drastically reduced, in a specific way, the kinase activity associated with P85(gag-mos). This result and other data suggest that the kinase is virus-encoded. Because P85(gag-mos), but not Pr65(gag) is phosphorylated in anti-p30 immunoprecipitates from MuLV-MuSV ts110 producer cells, the kinase enzyme is associated with P85(gag-mos) and not gag gene products. A second major polypeptide of the size of P58(gag) was also phosphorylated in anti-p30 immunoprecipitates from cells maintained at 33 degrees C but not at 39 degrees C. Since 6m2 cells at 39 degrees C contain P58(gag), this is also consistent with the kinase activity being associated with P85(gag-mos).