Involvement of a high-molecular-weight polyprotein translational product of Snyder-Theilen Feline sarcoma virus in malignant transformation

The previously described high-molecular-weight polyprotein major translational product of the Snyder-Theilen strain of feline sarcoma virus (FeSV) was shown to possess protein kinase activity with specificity for tyrosine acceptor sites. Cells transformed by Snyder-Theilen FeSV exhibited constitutively elevated levels of phosphotyrosine and a concomitant reduction in epidermal growth factor (EGF) binding sites. By endpoint cloning in microtiter plates, a number of transformation-defective (tf) mutants of the Snyder-Theilen strain of FeSV were isolated. Mink cells nonproductively infected by such mutants were morphologically nontransformed, failed to grow in soft agar, bound EGF as efficiently as control mink cells, and lacked rescuable transforming virus. Although the level of expression of the major viral polyprotein translational product in td mutant-infected clones was comparable to that of wild-type (wt) transformants, the polyprotein in mutant clones lacked detectable protein kinase activity and total cellular phosphotyrosine levels were not elevated significantly above control values. Of a large number of wt Snyder-Theilen FeSV-transformed mink cell clones isolated, the majority were found to revert to a nontransformed morphology upon continuous passage in cell culture. Such nontransformed variants, as well as a Gardner FeSV-transformed mink cell revertant, lacked detectable polyprotein expression and exhibited levels of phosphotyrosine and EGF binding similar to those of control mink cells. These findings provide strong evidence favoring the involvement of the Snyder-Theilen FeSV-encoded high-molecular-weight polyprotein and its associated tyrosine-specific protein kinase activity in transformation.     

The nonstructural component of the Abelson murine leukemia virus polyprotein P120 is encoded by newly acquired genetic sequences.

The Abelson leukemia virus (AbLV) polyprotein P120 is compared to translational products representing the entire Moloney murine leukemia virus (MuLV) genome on the basis of [35S]methionine tryptic peptide composition. Three methionine-containing tryptic peptides present in Moloney Pr65gag are each shown to be present in both Pr75gag and in Pr180gag-pol. Of these, one peptide, corresponding to Moloney MuLV p12, but neither of two p30-specific peptides are present in AbLV P120. Among the 12 remaining methionine-containing peptides present in AbLV P120, many, if not all, are unique to AbLV P120 and not shared by either Moloney MuLV Pr180gag-pol or Pr82gag.     

Snyder-Theilen feline sarcoma virus P85 contains a single phosphotyrosine acceptor site recognized by its associated protein kinase.

Cells nonproductively transformed by a variant of the Snyder-Theilen strain of feline sarcoma virus (FeSV) expressed an 85,000-dalton polyprotein (P85) with associated tyrosine-specific protein kinase activity. We identified within this polyprotein a single tyrosine acceptor site for its enzyme activity. This acceptor site, as well as two serine phosphorylation sites localized with the p12 structural component of Snyder-Theilen FeSv P85, was phosphorylated in cells nonproductively transformed by Snyder-Theilen FeSv. In contrast, infection by Snyder-Theilen FeSV transformation-defective mutants resulted in phosphorylation only of the two serine acceptor sites, indicating phosphorylation of the tyrosine acceptor site to be transformation specific. In addition, we describe in vitro labeling conditions, using unfractionated cell extracts, which resulted in preferential phosphorylation of the single Snyder-Theilen FeSV tyrosine-specific acceptor site.     

Alterations in tubulin immunoreactivity; relation to secondary structure

Blood sinusoidal plasma membrane subfractions were isolated from normal mouse liver in the presence of the proteinase inhibitors PhMeSO2F and iodoacetamide. They were purified from smooth microsomal and Golgi vesicle contaminants. The phosphorylation reaction was studied at 33 degrees C, in the presence of 2 mM MnCl2. Addition of epidermal growth factor (EGF) to the preparations stimulated 32P incorporation from [gamma-32P]ATP or [gamma-32P]GTP essentially into one 170 000 Mr protein. Some incorporation was observed in a minor 120 000-Mr component which appears to be a degradation product of the 170 000-Mr component. No EGF-dependent phosphorylation of other membrane proteins or various exogenous proteins could be detected in vitro. The dephosphorylation of the 170 000-Mr component was observed after 4 min of incubation at 33 degrees C. This dephosphorylation reaction was inhibited by addition of 5 mM p-nitrophenyl phosphate but not by addition of micromolar Zn2+, Be2+ or orthovanadate. The 170 000-Mr protein specifically bound 125I-labeled EGF and thus appeared to be the hepatic EGF receptor. The EGF stimulatable kinase activity considerably enhances incorporation of 32P into tyrosine residues of the 170 000-Mr EGF receptor at 33 degrees C. Tryptic peptide maps of the 32P-labeled 170 000-Mr protein revealed a multiplicity of phosphorylated sites. Seven 32P-labeled phosphopeptides were observed after EGF stimulation, three of them being largely prominent. Tryptic peptide maps of the 170 000-Mr protein after it was covalently linked to 125I-labeled EGF showed only one 125I-labeled peptide, the migration of which appeared different from that of 32P-labeled phosphopeptides. These findings were confirmed by V8 protease unidimensional peptide mapping of the 170 000-Mr protein, labeled with 32P or 125I-EGF.     

Translation of type C viral RNAs in Xenopus laevis oocytes: evidence that the 120,000-molecular-weight polyprotein expressed in Abelson leukemia virus-transformed cells is virus coded.

The genomic RNA of Abelson leukemia virus (AbLV) has been purified and translated in Xenopus laevis oocytes. The primary AbLV-specific protein synthesized is a polyprotein corresponding in molecular weight and immunological properties to a previously described p15 and p12 containing 110,000- to 130,000-molecular-weight polyprotein expressed in AbLV-transformed cells. In contrast, translation of woolly monkey sarcoma virus genomic RNA resulted in symthesis of a 55,000-molecular-weight polyprotein consisting of woolly helper virus p30, p15, and p12. These findings demonstrate the value of the X. laevis oocyte in vitro system for studies of translational products of replication-defective transforming viruses and establish the virus-coded nature of the nonstructural component of the 110,000- to 130,000-molecular-weight polyprotein expressed in AbLV-transformed cells.     

Basal phosphorylation of cyclic AMP-regulated phosphoproteins in intact S49 mouse lymphoma cells.

Protein phosphorylation in intact S49 mouse lymphoma cells was studied by using high-resolution two-dimensional gel electrophoresis of proteins labelled with [35S]methionine or [32P]Pi. In wild-type cells substrates for cyclic AMP-stimulatable phosphorylation exhibited high basal phosphorylation; in mutant cells deficient in activities of either cyclic AMP-dependent protein kinase or adenylate cyclase, basal phosphorylation of most of these substrates was negligible. Analysis of tryptic phosphopeptides from proteins labelled with [32P]Pi in wild-type cells suggested that identical sites were phosphorylated under conditions of both basal and hormonally elevated concentrations of cyclic AMP. These results argue that most basal phosphorylation is a consequence of partial activation of cyclic AMP-dependent protein kinase and that this activation is attributable to basal concentrations of cyclic AMP. For the intermediate filament protein vimentin, basal phosphorylation was largely at a site distinct from that stimulated by increased cyclic AMP, and basal phosphorylation was not markedly different in mutant and wild-type cells. Vimentin phosphorylated at both sites was not observed. Cyclic AMP treatment resulted in enhanced phosphorylation at the cyclic AMP-specific site and decreased phosphorylation at the cyclic AMP-independent site.     

Phosphorylated serine residues and an arginine-rich domain of the moloney murine leukemia virus p12 protein are required for early events of viral infection

Mutational analyses of the p12 Gag phosphoprotein of Moloney murine leukemia virus have demonstrated its participation in both virus assembly and the early stages of infection. The molecular mechanisms by which p12 functions in these events are still poorly understood. We performed studies to examine the significance of p12 phosphorylation in the viral life cycle. Alanine substitutions were introduced at the potential phosphorylation sites in p12, and the resulting mutants were tested for replication. Mutant viruses with changes at S61 and S78 were severely impaired, whereas the other mutant viruses were viable. S61 was shown to be required for normal levels of phosphorylation of p12 in vivo. These defective mutant viruses showed no apparent alteration to Gag protein processing or reduction in the yield of virions after transient transfection, but the mutants failed to form circular viral DNAs in acutely infected cells. Sequence analysis of revertant clones derived from S(61,65)A mutant virus revealed two classes: one group with a single mutation at a residue adjacent to S61 and another group with mutations introducing new positive charges surrounding S61. In vivo [32P]orthophosphate labeling indicated that the rescue of the S(61,65)A mutant virus did not result in a significant increase in the phosphorylation level of p12. Alanine substitutions of an arginine-rich stretch near S61 (at R-66, -68, -70, and -71) resulted in the same phenotype as the S(61,65)A mutant virus. The restored function of S(61,65)A mutant virus by second or third site mutations may result from a structural change or the addition of positively charged residues in the arginine-rich region.     

Antibodies to the ATP-binding site of the human epidermal growth factor (EGF) receptor as specific inhibitors of EGF-stimulated protein-tyrosine kinase activity

A region of the primary amino acid sequence of the epidermal growth factor receptor (EGF) protein-tyrosine kinase, which is involved in ATP binding, was identified using chemical modification and immunological techniques. EGF receptor was 14C-labelled with the ATP analogue 5'-p-fluorosulphonylbenzoyladenosine and from a tryptic digest a single radiolabelled peptide was isolated. The amino acid sequence was determined to be residues 716-724 and hence lysine residue 721 is located within the ATP-binding site. Antisera were elicited in rabbits to a synthetic peptide identical to residues 716-727 of the EGF receptor and the homologous sequence in v-erb B transforming protein from avian erythroblastosis virus. The affinity-purified antibodies precipitated human ECF receptor from A431 cells and placenta, and the v-erb B protein from erythroblasts. The antibodies inhibited EGF-stimulated receptor protein-tyrosine kinase autophosphorylation and phosphorylation of an exogenous peptide substrate containing tyrosine. The antibodies did not immunoprecipitate the transforming proteins pp60v-src or P120gag-abl or cAMP-dependent protein kinase, proteins which have homologous but not identical sequences surrounding the lysine residue within the ATP-binding site, nor did they react with the platelet-derived growth factor receptor. The antibodies had no effect on the kinase activity of purified v-abl protein in solution. The antibodies may therefore be a specific inhibitor of the tyrosine kinase of the EGF receptor.     

Purification and characterization of the phosphorylated DNA-binding protein from adenovirus-type-2-infected cells.

The virus-coded 72000-Mr DNA-binding protein from adenovirus-type-2-infected cells has been purified to homogeneity by DEAE-cellulose chromatography, selective precipitation and gel filtration. The 72000-Mr DNA-binding protein is phosphorylated and the phosphate is covalently linked predominantly to serine. Analysis of tryptic digests of the 32P-labeled 72000-Mr protein showed that the phosphate residue(s) is present in only one peptide. The DNA-binding fraction contains an additional non-phosphorylated protein with an approximate molecular weight of 45000. Tryptic peptide maps of [35S]methionine-labeled 72000-Mr and 45000-Mr polypeptides are indistinguishable. The amino acid compositions of the 72000-Mr and 45000-Mr polypeptides show closely related distributions. An antiserum produced against the purified 72000-Mr DNA-binding protein precipitates both the 72000-Mr and the 45000-Mr protein from extracts of adenovirus-infected cells. Immunofluorescence studies revealed DNA-binding protein to be accumulated in characteristic structures in nuclei of the infected cells.     

Phosphorylation of talin at tyrosine in Rous sarcoma virus-transformed cells.

The cytoskeletal protein talin was found to undergo enhanced phosphorylation at tyrosine residues in chicken embryo fibroblasts following transformation by Rous sarcoma virus. An increase in the tyrosine phosphorylation of talin was also observed within 6 h in cells infected by the temperature-sensitive mutant tsNY68 after a shift from the nonpermissive to the permissive temperature. The overall extent of phosphorylation was 0.07 mol of phosphate per mol of talin and was not appreciably altered by transformation. In uninfected cells talin was shown to be phosphorylated at multiple sites by tryptic peptide mapping. Following transformation most of these sites remained phosphorylated, to the same or to a lesser extent, while novel, phosphotyrosine-containing phosphopeptides appeared. Talin was phosphorylated at tyrosine in cells infected by Rous sarcoma virus mutants which induce altered or partial transformation morphologies; thus the increased phosphorylation of talin at tyrosine occurred irrespective of the morphology induced. Transformation by Y73 also induced elevated levels of phosphotyrosine in talin, whereas transformation by the avian erythroblastosis and Fujinami sarcoma viruses did not.     

Identification of serine 24 as the unique site on the transferrin receptor phosphorylated by protein kinase C

Addition of tumor-promoting phorbol diesters to [32P]phosphate-labeled A431 human epidermoid carcinoma cells caused an increase in the phosphorylation state of the transferrin receptor. The A431 cell transferrin receptor was also found to be a substrate for protein kinase C in vitro. Tryptic phosphopeptide mapping of the transferrin receptor resolved the same two phosphopeptides (X and Y) after either protein kinase C phosphorylation in vitro or treatment of labeled A431 cells with phorbol diesters. [32P]Phosphoserine was the only labeled phosphoamino acid detected. Phosphopeptide X was shown to be an incomplete tryptic digestion product which could be further digested with trypsin to generate the limit tryptic phosphopeptide (Y). Radiosequence analysis of [32P]phosphopeptide Y demonstrated that the [32P]phosphoserine was the second residue from amino terminus of the peptide. This receptor phosphopeptide was found to co-migrate with the synthetic peptide Phe-Ser(P)-Leu-Ala-Arg (where Ser(P) is phosphoserine) during reverse-phase high pressure liquid chromatography and two-dimensional thin layer electrophoresis and chromatography. The peptide Phe-Ser(P)-Leu-Ala-Arg is an expected tryptic fragment of the cytoplasmic domain of the transferrin receptor corresponding to residues 23-27. We conclude that the major site of protein kinase C phosphorylation of the transferrin receptor in vivo and in vitro is serine 24. This phosphorylation site is located within the intracellular domain of the transferrin receptor, 38 residues away from the predicted transmembrane domain.     

Translational products encoded by newly acquired sequences of independently derived feline sarcoma virus isolates are structurally related

Polyproteins encoded by several independent isolates of feline sarcoma virus (FeSV) were analyzed with respect to molecular weight, extent of phosphorylation, and tryptic peptide composition. As previously reported, cells nonproductively transformed by the Gardner strain of FeSV express a polyprotein which has a molecular weight of approximately 115,000 and contains feline leukemia virus p15, p12, and minor portion of p30. In addition, a major 72,000-dalton possible cleavage product can be identified. Snyder-Theilen FeSV-transformed cells express a major polyprotein of approximately 115,000 daltons and a second highly related 80,000-dalton protein. The p12 structural component of Gardner FeSV P115, but not Snyder-Theilen FeSV 115, corresponds to feline leukemia virus subgroup A with respect to immunological type specificity, a finding consistent with the independent origin of these viruses. Tryptic peptide analysis revealed five methionine-containing peptides specific to the nonstructural portion of Gardner FeSV 115, three of which were also represented in Snyder-Theilen FeSV P115, three of which were also represented in Snyder-Theilen FeSV P115. None of these [35S]methionine-labeled tryptic peptides were present in translational products representative of the complete feline leukemia virus subgroup A genome, including Pr180gag-pol, Pr65gag, and Pr82env. Similarly phosphorylated tryptic peptides within the structural (p12) and nonstructural components of Gardner FeSV P115 and Snyder-Theilen FeSV P115 Are highly related. These findings support the possibility that acquired sequences of two independently derived isolates of FeSV encode structurally related proteins.     

Phosphorylation of polyoma T antigens.

The T antigens of polyoma virus have been examined for phosphorylation in vivo and associated protein kinase activities in vitro. The 100K "large" T antigen is the major phosphoprotein among the T antigen species in vivo as determined by labeling virus-infected cells with 32P-orthophosphate. Hr-t mutants show normal phosphorylation of their 100K T antigens. The wild-type 56K plasma membrane-associated "middle" T antigen is also phosphorylated in the cell, but to a lesser extent than the 100K; this low level phosphorylation is also observed in the presumably altered 56K protein induced by hr-t mutant NG59 and in the 50K truncated "middle" T of hr-t mutant SD15. Addition of dibutyryl cyclic AMP to the medium does not affect labeling of either large or middle T antigens in wild-type- or mutant-infected cells. Thus no differences are observed in T antigen phosphorylation in vivo between wild-type virus and hr-t mutants. Hr-t mutants are defective in a protein kinase activity assayed in vitro by adding gamma-32P-ATP to T antigen immunoprecipitates. In the case of wild-type virus, the 56K protein is the major phosphate acceptor in the in vitro kinase reaction, with a somewhat lower level of phosphorylation observed in the 100K band. Hr-t mutants NG59 and SD15 show no labeling of the altered 56K or 50K, respectively, but do show detectable levels of 32P in the 100K bands. A wild-type virus carrying a small deletion affecting the 100K and 56k bands shows a normal level of kinase activity associated with the truncated T antigens. Ts-a mutants appear to be normal with respect to the middle T antigen-associated kinase. Photoaffinity labeling of infected cell extracts with 8-azido cyclic AMP shows that the two major classes of regulatory subunits of cyclic AMP-dependent protein kinases are present in the immunoprecipitates. Phosphorylation of histone H1 occurs when this substrate is added to immunoprecipitates of either mock-infected or virus-infected cells, again demonstrating the presence of cellular kinases. Further experiments will be required to determine whether the middle T antigen of polyoma virus is itself a protein kinase or simply a substrate for one or more cellular kinases.     

Structural and functional modification of pp60c-src associated with polyoma middle tumor antigen from infected or transformed cells.

The polyoma middle tumor antigen (MTAg) associates with the src proto-oncogene product pp60c-src in infected or transformed rodent cells. The tyrosine protein kinase activity of pp60c-src, as measured by in vitro phosphorylation of pp60c-src itself or the exogenous substrate enolase, was increased 10- to 20-fold in cells transformed or infected with transformation-competent polyoma virus compared with controls. pp60c-src associated with MTAg and precipitated with polyoma antitumor serum had a novel site(s) of in vitro tyrosine phosphorylation within its amino-terminal domain. These observations suggest that association of MTAg with pp60c-src alters the accessibility of pp60c-src tyrosine residues for phosphorylation in vitro and increases pp60c-src protein kinase activity. Several transformation-defective mutants of MTAg did not cause amino-terminal tyrosine phosphorylation of pp60c-src in vitro or enhance its protein kinase activity, suggesting that these properties correlate with the transforming ability of MTAg. However, one transformation-defective MTAg mutant, dl1015, did cause amino-terminal tyrosine phosphorylation of pp60c-src in vitro and did enhance its protein kinase activity. This suggests that properties of MTAg, in addition to modifying the structure and function of pp60c-src, may be important for transformation.     

Nerve growth factor action is mediated by cyclic AMP- and Ca+2/phospholipid-dependent protein kinases

Nerve growth factor (NGF) mediates the phosphorylation of tyrosine hydroxylase in PC12 cells on two distinct peptide fragments, separable by two-dimensional tryptic phosphopeptide mapping (phosphopeptides T1 and T3). Phorbol diester derivatives capable of activating Ca+2/phospholipid-dependent protein kinase (C-kinase) cause a specific phosphorylation of peptide T3 in a dose-dependent, saturable manner. Derivatives of the endogenous C-kinase activator diacylglycerol, also cause the phosphorylation of tyrosine hydroxylase on peptide T3. The C-kinase inhibitors chlorpromazine and trifluoperazine inhibit the phorbol diester stimulated phosphorylation of site T3 in a dose-dependent manner. These agents inhibit the phosphorylation of T3 in response to NGF, but have no effect on NGF's ability to cause T1 phosphorylation. In a PC12 mutant deficient in cAMP-dependent protein kinase activity, NGF mediates the phosphorylation of tyrosine hydroxylase on peptide T3 but not on T1. We conclude that NGF mediates the activation of both the cAMP-dependent protein kinase and the C-kinase to phosphorylate substrate proteins. These kinases can act independently to phosphorylate tyrosine hydroxylase, each at a different site, and each of which results in the enzyme activation. A molecular framework is thus provided for events underlying NGF action.     

Ribosomal proteins P0, P1, and P2 are phosphorylated by casein kinase II at their conserved carboxyl termini

A potential casein kinase II (CK II) recognition site is located within the conserved carboxyl (COOH) terminus of the ribosomal P (phospho) proteins P0, P1, and P2. To determine whether the COOH termini of the P proteins are physiological substrates for CK II, we studied the phosphorylation of the P proteins in vitro and in intact cells. The results show that the addition of exogenous purified CK II and ATP to intact ribosomes in vitro resulted in the relatively selective phosphorylation of all three P proteins. A synthetic peptide corresponding to the COOH-terminal 22 amino acids of P2 (C-22) was also phosphorylated by CK II with a Km of 13.4 microM. An endogenous ribosome-associated, CK II-like enzyme also phosphorylated the P proteins relatively selectively in the presence of 10 mM Mg2+ and ATP. The endogenous kinase was inhibited by heparin, utilized either ATP or GTP as a phosphate donor, and phosphorylated casein. A CK II-specific peptide (Arg-Arg-Arg-Glu-Glu-Glu-Thr-Glu-Glu-Glu) and the C-22 peptide inhibited the phosphorylation of the P proteins by the endogenous kinase, providing further evidence for its CK II-like properties and for localization of the CK II phosphorylation site to the COOH termini of the P proteins. Tryptic phosphopeptide maps of P1 and P2 phosphorylated by exogenous CK II and the endogenous ribosome-bound kinase were virtually identical. These phosphopeptides comigrated with the tryptic digest of C-22 and with the tryptic phosphopeptides derived from P1 and P2 isolated from intact cells metabolically labeled with [32P]orthophosphate in vivo. These studies demonstrate that exogenous CK II and a ribosome-bound, CK II-like enzyme phosphorylate the ribosomal P proteins in vitro and localize the target site for phosphorylation to the COOH terminus. The incorporation of phosphate into the same target site in intact cells indicates that the P proteins are in vivo substrates of CK II.     

The substrate specificity of the protein kinase induced in cells infected with herpesviruses: studies with synthetic substrates [corrected] indicate structural requirements distinct from other protein kinases

Synthetic peptides have been used to investigate the site specificity of highly purified virus induced protein kinase, a recently discovered protein kinase isolated from cells infected with alpha-herpesviruses. The enzyme from cells infected with pseudorabies virus can catalyse the phosphorylation of both seryl and threonyl residues in peptides that contain several arginyl residues on the amino-terminal side of the target residue. At least two arginyl residues are required, and the best substrates examined contain four to six such residues. Virus induced protein kinase differs in site specificity from protein kinase C in being unable to phosphorylate peptides in which multiple arginyl residues are on the carboxyl-terminal side of the target residue, or to phosphorylate peptides in which the arginyl residues are replaced by ornithyl residues. Virus induced protein kinase from cells infected with herpes simplex virus type I had similar substrate preferences to virus induced protein kinase from cells infected with pseudorabies virus. Although virus induced protein kinase and the cyclic AMP-dependent protein kinase have several peptide substrates in common, their relative preferences for these (as indicated by Km values) were found to be very different.     

Threonine 1336 of the human insulin receptor is a major target for phosphorylation by protein kinase C

The ability of tumor-promoting phorbol diesters to inhibit both insulin receptor tyrosine kinase activity and its intracellular signaling correlates with the phosphorylation of the insulin receptor beta subunit on serine and threonine residues. In the present studies, mouse 3T3 fibroblasts transfected with a human insulin receptor cDNA and expressing greater than one million of these receptors per cell were labeled with [32P]phosphate and treated with or without 100 nM 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA). Phosphorylated insulin receptors were immunoprecipitated and digested with trypsin. Alternatively, insulin receptors affinity purified from human term placenta were phosphorylated by protein kinase C prior to trypsin digestion of the 32P-labeled beta subunit. Analysis of the tryptic phosphopeptides from both the in vivo and in vitro labeled receptors by reversed-phase HPLC and two-dimensional thin-layer separation revealed that PMA and protein kinase C enhanced the phosphorylation of a peptide with identical chromatographic properties. Partial hydrolysis and radiosequence analysis of the phosphopeptide derived from insulin receptor phosphorylated by protein kinase C indicated that the phosphorylation of this tryptic peptide occurred specifically on a threonine, three amino acids from the amino terminus of the tryptic fragment. Comparison of these data with the known, deduced receptor sequence suggested that the receptor-derived tryptic phosphopeptide might be Ile-Leu-Thr(P)-Leu-Pro-Arg. Comigration of a phosphorylated synthetic peptide containing this sequence with the receptor-derived phosphopeptide confirmed the identity of the tryptic fragment. The phosphorylation site corresponds to threonine 1336 in the human insulin receptor beta subunit.(ABSTRACT TRUNCATED AT 250 WORDS)     

12-O-tetradecanoylphorbol-13-acetate stimulates phosphorylation of the 58,000-Mr form of polyomavirus middle T antigen in vivo: implications for a possible role of protein kinase C in middle T function.

The 58,000-Mr form (58K form) of the polyomavirus middle T antigen (mT) is a minor species distinguished by its phosphorylation in vivo on serine and by its efficient phosphorylation on tyrosine in immune complexes (B.S. Schaffhausen and T.L. Benjamin, J. Virol. 40:184-196, 1981). Here we report that the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), an activator of protein kinase C, rapidly stimulates phosphorylation of this mT species when added to cultures of wild-type polyomavirus-infected or polyomavirus-transformed 3T3 cells. Incubation with TPA leads to an accumulation of the 58K mT species to levels 1.5- to 5-fold higher than that in untreated cells within 15 min. TPA specifically stimulates phosphorylation of the 58K mT species without affecting that of the 56K species. Mapping by partial proteolysis shows that TPA-stimulated phosphorylation occurs at or near the site in 58K mT that is normally phosphorylated in the absence of TPA. A synthetic diacyl glycerol, 1-oleoyl-2-acetyl-glycerol, also specifically stimulates phosphorylation of 58K mT in vivo, while an inactive phorbol analog does not. TPA fails to induce phosphorylation of a 58K mT species encoded by certain nontransforming virus mutants with altered mT proteins that normally fail to undergo phosphorylation at the 58K site. These results indicate that the 58K form of mT is phosphorylated by or through the action of protein kinase C. TPA treatment of infected cells also leads to increased levels of 58K mT as measured in the immune complex kinase reaction, in which mT becomes phosphorylated on tyrosine by pp60c-src. These results are discussed in terms of a possible role for protein kinase C in activating mT function(s), including the formation of stable complexes with pp60c-src.