Proteins of the murine C-type RNA tumour viruses: isolation of a group-specific antigen by isoelectric focusing

Stephan Oroszlan and the Proteolytic Processing of Retroviral Proteins: Following A Pro

Steve Oroszlan determined the sequences at the ends of virion proteins for a number of different retroviruses. This work led to the insight that the amino-terminal amino acid of the mature viral CA protein is always proline. In this remembrance, we review Steve’s work that led to this insight and show how that insight was a necessary precursor to the work we have done in the subsequent years exploring the cleavage rate determinants of viral protease processing sites and the multiple roles the amino-terminal proline of CA plays after protease cleavage liberates it from its position in a protease processing site.

Steve Oroszlan: A Personal Perspective

My memories of Steve go back over 50 years. While precise dates are no longer in my memory bank, circumstances and emotions remain alive and easy to recall. These memories tell the story of a remarkable human being, a true practitioner of his craft always, faithful to the basic principles of scientific pursuit, with integrity, honesty, and enthusiasm well beyond the norm. We had a professional symbiotic relationship that lasted over 20 years, resulting in over 50 publications in scientific journals and meeting abstracts. During that time, our fortunes rose in tandem, and when it was time to go our separate ways, he was more than ready to flourish on his own. Our personal friendship remained constant, and we enjoyed sharing meals and stories with family and friends over the years. In retrospect, I take pride in having played a role in a portion of his remarkable scientific journey. A few key anecdotes will illustrate some aspects of this summary. By way of a disclaimer, this is not a comprehensive review of the vast field of viral oncology and the selection of references is intentionally narrow. No slight is intended to the many outstanding investigators that were our contemporaries and at times collaborators during the period from the early 70s to the mid-80s.

The zinc finger of nucleocapsid protein of Friend murine leukemia virus is critical for proviral DNA synthesis in vivo

Nucleocapsid protein NCp10 of murine leukemia virus (MuLV) is encoded by the 3' domain of gag and contains a zinc finger of the form Cys-X2-Cys-X4-His-X4-Cys flanked by basic amino acids. In the course of virus assembly, NCp10 is necessary for core formation, and the zinc finger flanked by the basic residues is required for the packaging of the genomic RNA dimer. In vitro, NCp10 exhibits strong nucleic acid binding and annealing activities that appear to be critical for virus infectivity since NCp10 promotes dimerization of the viral RNA containing the E/DLS packaging-dimerization signal and annealing of replication primer tRNA(Pro) to the initiation site of reverse transcription (PBS). Recent in vitro studies have suggested that NCp10 may also play a role in proviral DNA synthesis. To investigate the function of NCp10 in proviral DNA synthesis in vivo, we developed a simple and convenient genetic packaging system consisting of two DNA constructs expressing the packaging components gag-pol and env of Friend MuLV and a Moloney MuLV-based lacZ vector with either the MuLV E+ or a rat VL30 E packaging signal. This system allowed us to examine the consequences of a set of mutations in NCp10 on a single round of recombinant virus replication. Most mutations in the N- or C-terminal domain of NCp10 do not significantly alter infectivity, while those in the zinc finger drastically impair infectivity. Analysis of the viral RNA content in virions showed that all mutations in the zinc finger decrease but do not abolish packaging of the recombinant genome. Interestingly enough, mutation of Y-28 to S (mutation Y28S) in the zinc finger results in RNA packaging at a level similar to that observed upon deletion of three prolines and three arginines in the C-terminal domain of NCp10 (mutant delta PR3). However, mutant Y28S is noninfectious while mutant delta PR3 is only threefold less infectious than the wild-type virus, which prompted us to examine the role of NCp10 protein in proviral DNA synthesis in vivo using these nucleocapsid mutants. PCR amplification was used to analyze viral DNA synthesized in newly infected cells, and results indicate that the Y28S zinc finger mutation impairs reverse transcription, thus suggesting that the nucleocapsid protein zinc finger plays a key role in proviral DNA synthesis in vivo.

A neutralizable epitope common to the envelope glycoproteins of ecotropic, polytropic, xenotropic, and amphotropic murine leukemia viruses

An epitope common to all classes of murine leukemia viruses (MuLVs) was detected by reactivity of MuLVs with a rat monoclonal antibody (MAb) termed 83A25. The antibody is of the immunoglobulin G2a isotype and was derived after fusion of NS-1 myeloma cells with spleen cells from a Fischer rat immunized with a Friend polytropic MuLV. The antibody reacted with nearly all members of the ecotropic, polytropic, xenotropic, and amphotropic classes of MuLVs. Unreactive viruses were limited to the Friend ecotropic MuLV, Rauscher MuLV, and certain recombinant derivatives of Friend ecotropic MuLV. The presence of an epitope common to nearly all MuLVs facilitated a direct quantitative focal immunofluorescence assay for MuLVs, including the amphotropic MuLVs for which no direct assay has been previously available. Previously described MAbs which react with all classes of MuLVs have been limited to those which react with virion core or transmembrane proteins. In contrast, protein immunoblot and immunoprecipitation analyses established that the epitope reactive with MAb 83A25 resides in the envelope glycoproteins of the viruses. Structural comparisons of reactive and nonreactive Friend polytropic viruses localized the epitope near the carboxyl terminus of the glycoprotein. The epitope served as a target for neutralization of all classes of MuLV with MAb 83A25. The efficiency of neutralization varied with different MuLV isolates but did not correlate with MuLV interference groups.

Chemical and Immunological characterization of the major structural protein (p28) of MMC-1, a rhesus monkey endogenous type C virus: homology with the major structural protein of avian reticuloendotheliosis virus

The major core protein (p28) of MMC-1, an endogenous type C virus of the rhesus monkey (Macaca mulatta), was purified and subjected to structural and immunological analyses. The NH2-terminal amino acid sequence of MMC-1 p28 showed extensive homology to the sequences of the major structural proteins (p30) of known mammalian type C viruses. Similarly, interspecies antigenic determinants shared by all the above viral proteins were detected in MMC-1 p28. Competition radioimmunoassays together with the results of statistical analysis of the primary structure data provided evidence that MMC-1 p28 is not more closely related to primate type C viruses of the Papio genus than to those isolated from rodents, cats, or New World monkeys. MMC-1 p28 was found to be closely related structurally to the p30 protein of the avian reticuloendotheliosis virus (REV-A), a horizontally transmitted type C virus of putative mammalian origin. In addition, MMC-1 p28 and REV-A p30 shared a specific subset of antigenic determinants not present in any of the other avian or mammalian type C viruses studied. These findings suggest that MMC-1 and REV may have a common evolutionary origin.

Recombinants between temperature-sensitive mutants of rauscher murine leukemia virus and BALB:virus-2: genetic mapping of the Rauscher murine leukemia virus genome

Recombinant viruses were generated in tissue culture between Rauscher murine leukemia virus (MuLV) temperature-sensitive (ts) mutants restricted at different steps in virus replication and a mouse endogenous xenotropic virus, BALB:virus-2. Mutants used included ts 28, a late mutant which releases noninfectious viruses at 39 degrees C, and ts 29, a double mutant with a ts lesion in its reverse transcriptase and a late block affecting virus budding. Immunological typing of the translational products of clonal recombinant viruses made it possible to establish their partial genetic maps and localize regions of the viral genome affected by different ts lesions. Recombinants involving Rauscher MuLV ts 28 invariably contained BALB-virus-2 p15, p12, and p30 proteins, localizing the late defect in replication by this mutant to the 5' moiety of the viral gag gene. All ts 29-derived recombinants contained the entire BALB:virus-2 gag and pol genes. Substitution of the pol gene is in agreement with the reported thermolability of Rauscher MuLV ts 29 reverse transcriptase (Tronick et al., J. Virol. 16:1476-1482, 1975). Substitution of the gag gene suggests that internal structural proteins are actively involved in the virus budding processing. Rauscher MuLV recombinants were used to establish the genetic map of the Rauscher MuLV genome by T1 oligonucleotide fingerprinting analysis. Detection of Rauscher MuLV T1 oligonucleotides in representative recombinant viruses, whose protein phenotypes were established by immunological techniques, permitted their assignment to specific regions of the viral genome. The genetic map of Rauscher MuLV generated in these studies should be useful for identifying and characterizing the viral gene(s) involved in leukemogenesis.

Structure of glycosylated and unglycosylated gag polyproteins of Rauscher murine leukemia virus: carbohydrate attachment sites

The structural relationships among the gag polyproteins Pr65gag, Pr75gag, and gPr80gag of Rauscher murine leukemia virus were studied by endoglycosidase H digestion and formic acid cleavage. Fragments were identified by precipitation with specific antisera to constituent virion structural proteins followed by one-dimensional mapping. Endoglycosidase H reduced the size of gPr80gag to that of Pr75gag. By comparing fragments of gPr80gag and the apoprotein Pr75gag, the former was shown to contain two mannose-rich oligosaccharide units. By comparing fragments of Pr65gag and Pr75gag, the latter was shown to differ from Pr65gag at the amino terminus by the presence of a leader peptide approximately 7,000 daltons in size. The internal and carboxyl-terminal peptides of the two unglycosylated polyproteins were not detectably different. The location of the two N-linked carbohydrate chains in gPr80gag has been specified. One occurs in the carboxyl-terminal half of the polyprotein at asparagine177 of the p30 sequence and the other is found in a 23,000-dalton fragment located in the amino-terminal region of gPr80gag and containing the additional amino acid sequences not found in Pr65gag plus a substantial portion of p15.

Effect of chemical modification and fragmentation on antigenic determinants of internal protein p30 and surface glycoprotein gp70 of type C retroviruses

The effects of protein modification on the antigenic determinants of p30 and gp70 of type C retroviruses were investigated by using solid-phase competition radioimmunoassays. Proteins were modified by reduction with 2-mercaptoethanol and subsequent carboxymethylation of SH groups with iodoacetamide or by amidination of alpha and epsilon amino groups with methylacetimidate. The type-specific determinants of gp70 were found to be conformational in nature, as they were destroyed by these chemical modifications. Group- and interspecies-specific determinants of gp70 antigens, however, appear to be sequential and do not involve residues susceptible to these chemical reagents. Conformation-dependent type-specific determinants of p30 were affected only by methylacetimidate. Group- and interspecies-specific determinants of p30 are similar to those of gp70 in that they also appear to be sequential antigenic sites. Therefore, the broadly reactive group- and interspecies-specific determinants of gp70 and p30 can be followed into small peptides. Accordingly, a cyanogen bromide cleavage fragment derived from the carboxyl-terminal one-third of Rauscher leukemia virus p30 was found to carry group-specific determinants but no detectable interspecies-specific determinants. In contrast, a peptide obtained by limited trypsin cleavage of p30, which was derived from the NH(2)-terminal region of the protein, contained at least one of the interspecies determinants shared with feline leukemia virus p27.

Major structural proteins of type B, type C, and type D oncoviruses share interspecies antigenic determinants

Interspecies antigenic determinants have been used as markers to study the evolution of oncoviruses. By radioimmunologic techniques, it has been possible to detect crossreactivities among the major structural proteins of prototype viruses representing each of the three major Oncovirinae genera. Two different subsets of interspecies antigenic determinants were demonstrated. One encompasses known type B and type D viruses; the other is shared by type D and mammalian type C viruses. Such determinants were also demonstrated in certain as yet unclassified oncoviruses. These findings provide experimental evidence supporting the concept of an evolutionary relationship among the three major oncovirus genera.

Immunological relationships of an endogenous guinea pig retrovirus with prototype mammalian type B and type D retroviruses

A retrovirus endogenous to guinea pig cells was earlier shown to be morphologically similar to type B and type D prototype retroviruses. Molecular hybridization techniques were used to show that guinea pig virus nucleotide sequences are endogenous to both domestic (Cavia porcellus) and indigenous (Cavia aperea) guinea pigs, but cannot be detected in the DNA of either other hystricomorph rodents or other mammals tested. Using radioimmunological techniques designed to detect interspecies relationships, the major internal polypeptide of guinea pig virus (p26) was shown to share three different sets of interspecies antigenic determinants with squirrel monkey retrovirus, viper retrovirus, and mouse mammary tumor virus. Thus, guinea pig virus appears to provide an evolutionary link between type B and D retroviruses.

Evolutionary relatedness of viper and primate endogenous retroviruses

A retrovirus previously isolated from a tumored Russell's viper is shown by molecular hybridization to be an endogenous virus of this reptilian species. Radio-immunologic techniques revealed that the viper retrovirus is immunologically and, hence, evolutionarily related to endogenous type D retorviruses of Old World primates. These findings extend the number of vertebrate classes possessing endogenous retroviruses and suggest that type D retroviruses may even be more widely distributed in nature than type C retroviruses.

Conditions required for induction of murine p30 by herpes simplex virus

Mouse cells (line N cIA cl10) contain 1.2-2.5 ng murine leukaemia virus (MuLV) p30 antigen/mg of protein; this amount of antigen is measurable by competition radioimmunoassay (RIA) but is not detectable by indirect immunofluorescence (IF). Infection of N cIA cl10 cells with herpes simplex virus type 2 (HSV-2) induces expression of MuLV p30. Induction by HSV-2 does not require either cell or virus DNA synthesis and is optimal 8 h post infection when cells at 50-70% confluence are infected at a multiplicity of infection (MOI) of 5-8 PFU/cell. At an MOI of 2.5, 70-80% of the cells express HSV antigens while none of the cells express p30; at an MOI of 5.0, 70-80% of the cells express HSV antigens but 55% of the cells express p30. Using the conditions reported in this paper for preparation of competing antigen, induction of p30 by HSV-2 (strain 333) infection is not measurable by competition RIA.

Quantitative immunoelectrophoretic assay for murine oncornavirus p30: noncovalent facilitation by sodium dodecyl sulfate

Treatment of Rauscher murine leukemia virus lysates with the anionic detergent sodium dodecyl sulfate (SDS) at concentrations between 0.2 to 2.0% SDS per mg of viral protein greatly increased the anodal electrophoretic mobility of p30, the major internal polypeptide. SDS treatment did not reduce p30 antigenicity or cause nonspecific precipitation of normal serum proteins during subsequent immunoanalysis. The increased anodal electrophoretic mobility allowed assay of Rauscher murine leukemia virus p30 by Laurell rocket immunoelectrophoresis. An SDS-facilitated rocket immunoelectrophoresis assay is described that was highly reproducible (coefficient of variability, less than 3.0%) and capable of detecting 125 ng of viral protein. To our knowledge, this is the first report of a quantitative immunoelectrophoretic assay for an oncornavirus antigen. Since SDS binding is a general property of proteins, this method of noncovalently altering electrophoretic mobility appears to be applicable to other antigen-antibody systems.

Immunologic studies of the low molecular weight DNA binding protein of murine oncornaviruses

A low molecular weight, highly basic DNA-binding protein was purified from several oncornaviruses by the sequential procedures of gel filtration in guanidine-hydrochloride, DEAE-cellulose chromatography and affinity chromatography on single-stranded DNA sepharose. The binding protein from Rauscher and woolly monkey type-C viruses was the fastest migrating of the virion proteins in SDS-polyacrylamide gels and thus is designated p10 according to previous convention although our estimates of molecular weight were 8-9,000 daltons. The binding protein from these two viruses was resolved into two bands by acid-urea electrophoresis although only a single NH2 terminal amino acid was detected (S. Oroszlan, personal communication), thus indicating charge heterogeneity. Antibody to Rauscher virus p10 species-specific in gel diffusion and complement-fixation tests and did not exhibit cross-reactivity with other virion proteins. A DNA-binding protein was also detected in preparations of mouse mammary tumor virus. This purified protein had an apparent molecular weight of 12,500, was the second fastest migrating component in the virus preparations, and was antigenically unrelated to the mouse type-C virus p10.

Evolutionary relationships between gag gene-coded proteins of murine and primate endogenous type C RNA viruses

Several low molecular weight proteins of endogenous type C viruses of the RD114/baboon group are compared with the gag gene translational products of endogenous type C viruses of murine origin. The p10 proteins of each virus group are shown to be immunologically and biochemically related, while the p12 proteins of RD114/baboon viruses are demonstrated to share antigenic determinants with murine viral p15. Moreover, highly type-specific phosphoproteins, p15 of RD114/baboon viruses and p12 of murine viruses, are shown to possess very similar biochemical properties. These findings, along with previous studies indicating immunologic cross-reactivity between their major internal antigens, p30, demonstrate that each of the gag gene-coded proteins of murine type C viruses has a analogue in viruses of the RD114/baboon group. The immunologic and biochemical relatedness of their gag gene translational products supports the concept of a common progenitor in the evolution of these endogenous viruses.

Correlation of apparent molecular weight and antigenicity of viral proteins: an SDS-page separation followed by acrylamide-agarose electrophoresis and immunoprecipitation

A simple method is described which combines a sodium dodecyl sulfate polyacrylamide gel electrophoresis (SOS-PAGE) in the first demension with a second electrophoresis, at right angles to the first, into an agarose matrix. The proteins, separated by SDS-PAGE, are exposed to appropriate antisera after the second stage electrophoresis and immunoprecipitates form in the agarose corresponding to the relative electrophoretic mobilities of proteins in the first stage SDS-PAGE separation. The method thus provides a simple, reproducible means for correlating antigenicity with apparent molecular weight of proteins. The technique is qualtitative, but requires smaller quantities of antisera than more conventional immunoelectrophoretic methods such as rocket electrophoresis.

Characterization of a splenic factor inhibiting DNA-synthesis in lymphocytes in vitro

A factor inhibiting DNA synthesis in lymphocytes from lymph nodes and thymus of guinea pigs and rats has been isolated from the calf spleen. A crude extract was obtained by homogenization of the spleen in saline, centrifugation at 1,200 and 46,000 g, heat precipitation at 60 degrees C, acetone precipitation, followed by lyophilization at the acetone precipitate. The active material was partly purified and characterized by gel chromatography, ion exchange chromatography, ultrafiltration and preparative electrofocusing. The active factor was found to be acidic and heat stable (60 degrees C, 10 min) with a molecular weight between 75,000 and 17,000 daltons. At preparative electrofocusing inhibiting material focused at pH 4.0. The splenic factor inhibited the DNA synthesis in lymphocytes from guinea pigs and rats, but not the DNA synthesis in HeLa cells. During the purification, material which enhanced the uptake of 3H-thymidine in DNA of lymphocytes was separated from the inhibitor. The stimulating factor had a molecular weight below 10,000 daltons as judged by ultrafiltration and gel chromatography. At preparative electrofocusing stimulating material focused around pH 8.5.

Leukemia following Reye's syndrome treated with exchange transfusions: search for type-C viral infection

A 5-year-old child developed acute lymphoblastic leukemia during convalescence from an episode of Reye's syndrome that was treated with multiple exchange transfusions. Routine laboratory, histology, and viral serology were unable to establish a common etiology for the 2 illnesses. Cultural and immunologic methods to search for evidence of infection with type-C viruses or viral genes in lymphoblasts from the buffy coat and bone marrow failed to reveal these agents. Although no common infectious etiology was defined for the close temporal occurrence of 2 rare diseases, the possibility of an iatrogenically induced malignancy was considered.

Humoral immune responses of cats to mammalian type-C virus p30s

Natural and experimental cat sera were tested in radioimmune precipitation assays vs purified p30s from FeLV, RD114 and MuLV. Antibodies with specificity for FeLV p30 comparable to hyperimmune sera from heterologous species but of low titer were found in a high percentage of normal cats from households with a high incidence of FeLV and neoplasia. Sera from cats with neoplasms were generally negative. Cats immunized with FeLV gave low-level immune response, also of the same general specificity as heterologous hyperimmune sera. Cat sera do not normally show antibody to RD114 p30 although two immunized weanling cats produced low titered but highly specific p30 antibody. Thus, for both classes of feline type-C virus p30s, there is an evident capability of the cat to mount an immune response to natural or experimental exposure to the respective proteins. The magnitude of the response is between 100 and 1,000 fold below that seen in heterologous species. In contrast, cats immunized with MuLV p30 gave immune responses comparable to those seen in guinea-pigs, rabbits and goats. Several very old cats with carcinoma had antibody which preferentially precipitated MuVL p30. A competition assay using one such serum and labelled MuVL p30 was inhibited by FeVL, RD114, and MuLV p30s. This indicates that the assay is "interspecies" in nature. Among the possible explanations of this reaction category is that it represents antibody to the p30 of an as yet undefined class of feline type-C virus.

Characterization of non-producer human cells induced by Kirsten sarcoma virus

Non-producer (NP) human cells induced by the Kirsten sarcoma virus were characterized. These morphologically altered NP cells produced neither infectious virus nor complement-fixing antigens of the murine sarcoma-leukemia virus complex. The NP cells did not release RNA-dependent DNA polymerase and type-C virus particles with a density of approximately 1.15 g/ml in sucrose gradients by 3H-uridine labelling. The NP cells produced tumors when transplanted subcutaneously into athymic nude mice. The tumor cells re-established in culture resembled the orginal NP cells, were confirmed as human cells by karyological analysis and were also found to be "non-producer". The sarcoma virus genome in NP cells could be rescued not only by co-cultivation with "helper virus"-releasing cells but also by superinfection with helper type-C viruses. Murine (Rauscher, Ki-MuLV, AT-124 and two other xenotropic viruses), feline, RD-114 and Simian (woolly monkey and baboon) type-C viruses possessed the ability to rescue the sarcoma genome from NP cells but not AKR leukemia virus. In addition, the feline leukemia virus titer obtained by the rescuing technique in NP cells was the same as those obtained in feline embryo and NP cells by CF induction assay.

Distinction between tumor-specific transplantation antigen and virion antigens in solubilized products from membranes of virus-induced leukemic cells

A membrane antigen from RBL-5 leukemic cells that was solubilized and partially purified is further characterized in this study. This soluble antigen is capable of immunizating syngeneic hosts to reject neoplastic cells and thus resembles TSTA. It also induces cytotoxic antibody in syngeneic hosts capable of specifically lysing RBL-5 cells in vitro. RBL-5, however, releases infectious virus (RLV); it was necessary therefore to rule out virus or structural virion antigens as the effective immunogen. Infectious virus was not detectable in our initial crude membrane (CM) material, nor in the papain-solubilized CS or the G-150 Sephadex-chromatographed fraction. Virus-neutralizing antibody was not detected, under stringent assay conditions, in the syngeneic anti-CM sera. Antigen preparations CM, CS and the chromatographeal fractions F1, F2 and F3 were assayed in a complement-fixation test against brood-reacting antisera capable of detecting virus envelope antigen and gs antigen and against syngeneic antisera. Although our antigen preparations were positive for virion antigens, CS and F2 contained an antigen that reacted only with syngeneic antiserum. These same fractions were those reactive as immunogens. On the basis of these data, it is postulated that a cellular membrane component, other than viral, functions as TSTA.

Cell-mediated immunity to Moloney sarcoma virus in mice. II. Analysis of antigenic specificities involved in T lymphocyte-mediated in vivo rejection of murine sarcoma virus-induced tumors

Sublethally irradiated BALB/c mice innoculated with Moloney sarcoma virus (MSVm) develop progressively growing tumors and die within 30 days of virus innoculation. These animals can be protected from tumor progression (and death) by innoculation of small numbers of MSV-immune T lymphocytes from MSV-M innoculated (but unirradiated) animals. T lymphocytes in these donor animals have been shown to express immunity to a variety of viral and virally-induced antigens. We have investigated whether immunity to any one of these antigens was critically important in leading to protection of the irradiated animals by sensitizing normal T lymphocytes in vitro to different viral antigens and examining the ability of these sensitized cells to protect the irradiated recipients. Data is presented to show that cells sensitized in vitro to MSV-transformed fibroblasts, and to purified antigens with group specificity, but not to viral envelope antigens, or whole virus, are capable of protecting the irradiated MS innoculated animals.

Cell-mediated immunity to moloney sarcoma virus in mice. I. Analysis of antigens responsible for lymphocyte stimulation in regressor mice

Purified viruses, viral antigens, and cell extracts have been tested for their ability to stimulate protein synthesis by MSV-M-1 regressor spleen cells from BALB/c mice. Immune, but not normal cells, responded to MSV-M, but not to MSV-G virus-1, and to the type-specific viral envelope glycoprotein from MSV-M virus. Extracts of mouse embryo fibroblasts transformed by either MSV-M or MSV-G, however, specifically stimulated MSV-M regressor spleen cells. Cells stimulated by different antigens, and by phytohaemagglutinin, had the same sedimentation profile and were identified as T-lymphocytes.

Murine type-C virus group-specific antigens: interstrain immunochemical, biophysical, and amino acid sequence differences

The 30,000-molecular-weight internal protein, p30, was purified from seven strains of mouse type-C viruses. The individual p30's showed variation in isoelectric points and also intrastrain heterogeneity. The individual p30's could be distinguished by peptide map and quantitative complement fixation techniques with relatedness estimates of >95%. Amino terminal sequence analysis showed variability at position 4 for several p30's with complete homology otherwise through 24 residues. The intrastrain heterogeneity in p30 isoelectric points could not be explained by common contaminants, as shown by peptide mapping, and is more likely based on post-transcriptional modifications. These data provide a chemical basis for the recently described type-specific immunological properties of individual p30's.

Biological expression of antigenic determinants of murine leukemia virus proteins gp69-71 and p30

Antisera to purified structural proteins of Rauscher murine leukemia virus, the major envelope glycoprotein, gp69/71, and the major internal protein, p30, were studied by immunofluorescence of viable and fixed virus-infected cells and by virus neutralization. Group-specific and type-specific determinants of gp69/71 were demonstrated by immunofluorescence and virus neutralization tests, indicating that these determinants are located in the cytoplasm and probably on the cell surface as well as on virus envelope. Antisera against p30 showed anti-group and anti-interspecies activities by immunofluorescence with no virus-neutralizing activity. Both antigenic determinants of gp69/71 were sensitive to guanidine-hydrochloride and to a lesser degree to ether treatment, whereas the group-specific determinants of p30 were relatively stable to these treatments.

Expression of the major internal viral polypeptide in cells transformed by wild-type and temperature-sensitive murine sarcoma virus

Phenotypic expression of the murine intraspecies and interspecies antigenic determinants of the major type C viral structural 30,000-dalton polypeptide, p30, was measured by radioimmunoassay inhibition in cell lines from different species. Uninfected normal rat kidney (NRK) cells did not contain detectable levels of murine intraspecies and interspecies p30 antigen, whereas rat cells transformed by and producing murine sarcoma virus (MSV)-Moloney leukemia virus (M-MSV-MuLV) contained high levels of both murine intraspecies and interspecies p30 antigen. Significant amounts of murine intraspecies and interspecies p30 antigen were detected in wild-type MSV-transformed nonproducer NRK cells. The control of p30 antigen expression was examined in temperature-sensitive MSV-transformed nonproducer cells [NRK(MSV-1b)] which are cold sensitive for maintenance of the transformed phenotype. Both murine intraspecies and interspecies p30 antigens were detected in NRK(MSV-1b) cells when grown at the permissive (39 C) or nonpermissive (33 C) temperature, suggesting that p30 antigen expression is not correlated with maintenance of the transformed phenotype. The results demonstrate that previously undetectable p30 antigens are expressed in MSV-transformed nonproducer NRK cells, and suggest that the expression of p30 antigen may be a useful marker for viral gene expression in mammalian cells.

Radioimmunoassay for the major structural protein of hamster type C viruses

A radioimmunoassay for the major, group-specific antigen (p30) of hamster type C viruses was developed. The test detected approximately 5 ng of viral protein per ml and was highly specific for hamster viruses when used with homologous antibody. Comparison of three hamster viruses, two being mouse-hamster pseudotypes, in homologous and heterologous intraspecies assays, showed no evidence of type specificity for these proteins. The pseudotype viruses showed no evidence of mouse virus p30 antigenic determinants. An interspecies antigen assay employing (125)I-labeled hamster p30 and anti-feline p30 was completely inhibited by cat (feline leukemia virus), hamster, and rat viruses, to a slightly lesser degree by mouse viruses, and only poorly by RD 114 and Gibbon ape viruses. The Mason-Pfizer virus did not inhibit this assay. Hamster p30 was detected by radioimmunoassay in individual embryos from two LSH hamsters and in several adult tissues, excluding muscle at levels below that required for detection in complement-fixation tests.

Autogenous immunity to endogenous RNA tumor virus: differential reactivities of immunoglobulins M and G to virus envelope antigens

The autogenous humoral immune response of mice to their endogenous leukemia virus has been examined in terms of the reactivities of individual classes of antibody present in normal B6C3F(1) serum. Whole serum and the immunoglobulin (Ig) M and IgG fractions of serum from animals of different age groups were compared by radioimmune precipitation assays and viral infectivity neutralization assays. Both IgM and IgG fractions were able to precipitate virus, although not as effectively as whole serum. Virus-specific antibody levels, as well as total antibody concentrations in whole serum, appeared to increase with age. Sodium dodecyl sulfate gel electrophoresis analysis was performed with immune precipitates obtained when whole serum or 19 or 7S fractions from animals of different age groups were reacted with disrupted virus. The 19S antibody fraction reacted with three antigenic determinants on the viral envelope. These antigens have apparent molecular weights of 17,000, 43,000, and 68,000. The last two appear to be glycoproteins and may correspond to the M(2) and M(1) antigens. In contrast, the 7S component reacted only with the 17,000-molecular-weight protein. Neutralization assays against BALB:virus-2, a xenotropic endogenous mouse type C virus, revealed that 19S and whole serum but not the 7S fraction possessed neutralizing activity. These findings indicate that there are differential reactivities of IgM and IgG antibodies in normal serum of B6C3F(1) mice, with respect to both recognition of viral envelope antigens and neutralization of endogenous MuLV. These results are consistent with the hypothesis that the autogenous humoral immune response is a systemic host function that may be important in the regulation of endogenous type C virus expression in vivo.

Properties of feline leukemia virus. I. Chromatographic separation and analysis of the polypeptides

Rickard's strain of feline leukemia virus (FeLV) contains two large glycoproteins and five smaller polypeptides of molecular weights 100,000 (gp >/= 100), 70,000 (gp70), 30,000 (p30), 21,000 (p21), 15,000 (p15), 11,200 (p11), and 10,000 (p10) when chromatographed on 6% agarose in the presence of 6 M guanidine hydrochloride (GuHCl). P21 is a minor component which was not previously described for mammalian leukemia-sarcoma viruses and may be analogous to the seventh protein found in avian viruses. Analysis on 4% agarose and by sodium dodecyl sulfate polyacrylamide gel electrophoresis revealed that gp >/= 100 is actually >/= 200,000 daltons and dissociates to polypeptides of approximately 100,000 to 115,000 daltons, whereas gp70 can be resolved into six stained bands ranging from 40,000 to 80,000 daltons despite being rechromatographed as a single symmetrical peak on 6% agarose. Rechromatography on 8% agarose was found to be more effective than on 6% agarose or sodium dodecyl sulfate polyacrylamide gel electrophoresis for obtaining the five small polypeptides, especially p11 and p10, in a highly purified form suitable for further analysis and for obtaining more precise estimates of their molecular weights, especially when done by co-chromatography with iodinated standard proteins markers. Rechromatographed p30, p21, p15, p11, and p10 had molecular weights of 27,000, 18,000, 15,000, 12,000, and 12,000 respectively, by co-electrophoresis with the marker proteins on sodium dodecyl sulfate polyacrylamide gel electrophoresis, clearly establishing that the latter two FeLV polypeptides comigrate to form one less band when compared to elution from agarose. The isoelectric points of p30 and p15 were 5.5 and 8.9, respectively, after renaturation from GuHCl and 5.6 and 8.7, respectively, when isolated from Tween-ether treated virus. Rechromatographed p30, p15, and p11, renatured by removing GuHCl with dialysis, reacted only with their homologous antisera in immunodiffusion analysis, indicating that they are immunologically unrelated. Also the interspecies gs-3 determinant associated with p30 could be regained by removal of GuHCl.