Decreased DNA-binding ability of purified transformation-specific proteins from deletion mutants of the acute avian leukemia virus MC29

The post-Loeffler-Frosch era: contribution of German virologists

This presentation dealt with the contributions of German virologists in the rapid development of virology following the Loeffler-Frosch era. Thereby, only research was included which was undertaken within German institutions, even though guest scientists from other countries or international cooperative efforts have in some cases contributed to the work. Contributions to the field of veterinary virology were not considered here, since this topic was treated separately during this centennial symposium. The overview includes contributions of the very early period when interest was focussed mainly on the determination of the physicochemical properties of the fast growing number of newly detected viruses, and of the pioneering period when fundamental discoveries of the nature of viruses were made. The concepts that derived from those studies made the development of modern virology possible. Some highlights of the present period were presented describing the findings of selected virus families. This part was followed by a description of the results which were relevant to problems of how viruses become pathogens, and the role of the immune response to virus infections. Finally, attention was drawn to the contributions of molecular studies which became important not only for the field of virology but also for life sciences in general.

Dissociation of cellular proliferation and c-myc expression by buttercup extract

Buttercup extract (BE), an extract of the buttercup plant (Zanthoriza simplicissima), inhibits RNA and DNA synthesis by HL-60 promyelocytic leukemia cells. Exposure of these cells to 3% BE for 48 hours results in dramatic inhibition of RNA synthesis without loss of cell viability. The effect of BE is partially reversible over 12-24 hours with the level of RNA synthesis returning nearly to control levels during this time period. DNA synthesis is also reversibly inhibited by exposure to BE. Despite the inhibition of RNA synthesis in HL-60 cells, there is no decrease in the level of c-myc mRNA, even at high BE concentrations. The level of gene-specific mRNA for the c-Ha-ras, c-fms, and c-mos genes in these cells also remained constant during exposure to BE. Ribosomal RNA is not degraded during 24 hours of BE treatment in vitro, suggesting that BE does not maintain the relative mRNA level for these genes by selective degradation of other RNA species. The inhibition of RNA and DNA synthesis by BE without a corresponding alteration in the level of expression of the c-myc gene suggests that this agent dissociates c-myc expression and cellular proliferation in these cells.

DNA-binding domain of human c-Myc produced in Escherichia coli

We have identified the domain of the human c-myc protein (c-Myc) produced in Escherichia coli that is responsible for the ability of the protein to bind sequence-nonspecific DNA. Using analysis of binding of DNA by proteins transferred to nitrocellulose, DNA-cellulose chromatography, and a nitrocellulose filter binding assay, we examined the binding properties of c-Myc peptides generated by cyanogen bromide cleavage, of mutant c-Myc, and of proteins that fuse portions of c-Myc to staphylococcal protein A. The results of these analyses indicated that c-Myc amino acids 265 to 318 were responsible for DNA binding and that other regions of the protein (including a highly conserved basic region and a region containing the leucine zipper motif) were not required. Some mutant c-Mycs that did not bind DNA maintained rat embryo cell-cotransforming activity, which indicated that the c-Myc property of in vitro DNA binding was not essential for this activity. These mutants, however, were unable to transform established rat fibroblasts (Rat-1a cells) that were susceptible to transformation by wild-type c-Myc, although this lack of activity may not have been due to their inability to bind DNA.

DNA-binding domain of human c-Myc produced in Escherichia coli

We have identified the domain of the human c-myc protein (c-Myc) produced in Escherichia coli that is responsible for the ability of the protein to bind sequence-nonspecific DNA. Using analysis of binding of DNA by proteins transferred to nitrocellulose, DNA-cellulose chromatography, and a nitrocellulose filter binding assay, we examined the binding properties of c-Myc peptides generated by cyanogen bromide cleavage, of mutant c-Myc, and of proteins that fuse portions of c-Myc to staphylococcal protein A. The results of these analyses indicated that c-Myc amino acids 265 to 318 were responsible for DNA binding and that other regions of the protein (including a highly conserved basic region and a region containing the leucine zipper motif) were not required. Some mutant c-Mycs that did not bind DNA maintained rat embryo cell-cotransforming activity, which indicated that the c-Myc property of in vitro DNA binding was not essential for this activity. These mutants, however, were unable to transform established rat fibroblasts (Rat-1a cells) that were susceptible to transformation by wild-type c-Myc, although this lack of activity may not have been due to their inability to bind DNA.

Identification of the human c-myc protein nuclear translocation signal

We identified and characterized two regions of the human c-myc protein that target proteins into the nucleus. Using mutant c-myc proteins and proteins that fuse portions of c-myc to chicken muscle pyruvate kinase, we found that residues 320 to 328 (PAAKRVKLD; peptide M1) induced complete nuclear localization, and their removal from c-myc resulted in mutant proteins that distributed in both the nucleus and cytoplasm but retained rat embryo cell cotransforming activity. Residues 364 to 374 (RQRRNELKRSP; peptide M2) induced only partial nuclear targeting, and their removal from c-myc resulted in mutant proteins that remained nuclear but were cotransformationally inactive. We conjugated synthetic peptides containing M1 or M2 to human serum albumin and microinjected the conjugate into the cytoplasm of Vero cells. The peptide containing M1 caused rapid and complete nuclear accumulation, whereas that containing M2 caused slower and only partial nuclear localization. Thus, M1 functions as the nuclear localization signal of c-myc, and M2 serves some other and essential function.

Identification of the human c-myc protein nuclear translocation signal

We identified and characterized two regions of the human c-myc protein that target proteins into the nucleus. Using mutant c-myc proteins and proteins that fuse portions of c-myc to chicken muscle pyruvate kinase, we found that residues 320 to 328 (PAAKRVKLD; peptide M1) induced complete nuclear localization, and their removal from c-myc resulted in mutant proteins that distributed in both the nucleus and cytoplasm but retained rat embryo cell cotransforming activity. Residues 364 to 374 (RQRRNELKRSP; peptide M2) induced only partial nuclear targeting, and their removal from c-myc resulted in mutant proteins that remained nuclear but were cotransformationally inactive. We conjugated synthetic peptides containing M1 or M2 to human serum albumin and microinjected the conjugate into the cytoplasm of Vero cells. The peptide containing M1 caused rapid and complete nuclear accumulation, whereas that containing M2 caused slower and only partial nuclear localization. Thus, M1 functions as the nuclear localization signal of c-myc, and M2 serves some other and essential function.

Protein that induces cell differentiation causes nicks in double-stranded DNA

The growth and differentiation of myeloid hematopoietic cells are regulated by different macrophage and granulocyte inducing proteins, those that induce growth and others that induce differentiation. The proteins that induce differentiation but not those that induce growth bind to double-stranded DNA. We now report that purified myeloid cell differentiation-inducing protein causes single strand breaks (nicks) in double-stranded DNA. This DNA nicking may initiate the changes in gene expression that are required for differentiation.

Isolation of an MH2 retrovirus mutant temperature sensitive for macrophage but not fibroblast transformation

A conditional mutant of the MH2 avian retrovirus, termed ts41MH2, was isolated. Unlike wtMH2, ts41MH2 permitted transformed macrophages to differentiate during a 5- to 7-day temperature shift from 37 to 42 degrees C. Mutant-infected cells incubated at 42 degrees C exhibited a flattened morphology and then fused to form giant multinucleated cells that closely resembled normal macrophage maturation in vitro. These differentiated cells reacted strongly with a myeloid-macrophage-specific monoclonal antibody. The process of differentiation was inhibited when ts41MH2-transformed nonproducer clones were superinfected before the temperature shift with the myc gene-containing MC29 or OK10 viruses. By contrast, no inhibition was observed in clones superinfected with the MH2-PA200 virus that contains only the mil gene. The mutant also demonstrated a reduced oncogenic potential relative to that of wtMH2 when it was inoculated intravenously into young birds. However, in contrast to the results obtained with hematopoietic cells, none of the five fibroblast transformation parameters tested for ts41MH2 were altered from those induced by wtMH2. These results suggest that the mutation in ts41MH2 is located in a region of myc required for macrophage transformation, but not required for fibroblast transformation.

Molecular analysis of myc gene mutants

We describe the generation and characterization of a series of deletion mutants of the avian acute leukaemia virus MC29 which allow the study of the function of the myc in transformation of quail embryo fibroblasts in vitro and tumour induction in vivo. These mutants, which are deleted in the 3' portion of the myc gene, fail to transform macrophages in vitro or induce tumours in vivo but are still able to transform morphologically fibroblasts. From one of these mutants a 'recovered' MC29 virus was generated which, like wild type MC29, transformed fibroblasts and macrophages in vitro. When tested in vivo this virus induced lymphomas of T and B cells rather that the endotheliomas induced by wild type MC29. This system allows us to investigate another question which is the mechanism by which the virus (or oncogene it contains) preferentially transforms one cell type.

Proteins expressed by the c-myc oncogene in lymphomas of human and avian origin

We have prepared antisera against synthetic peptides corresponding to the C-terminal region of the avian and human myc oncogene coding sequences. Immunoprecipitates from avian and human cells show two major proteins which, by the criteria of hybrid-selected translation, transfection, and peptide-blocking assays, are the c-myc protein products. These proteins are phosphorylated nuclear proteins which are tightly bound to the nuclear matrix-lamin and which have a short half-life. Analysis of avian and human lymphoma cell lines containing rearranged c-myc alleles show significant changes in the ratio of the two proteins although only the avian lymphomas have increased quantities of c-myc protein.

V-myc- and c-myc-encoded proteins are associated with the nuclear matrix

A series of extraction procedures were applied to avian nuclei which allowed us to define three types of association of v-myc- and c-myc-encoded proteins with nuclei: (i) a major fraction (60 to 90%) which is retained in DNA- and RNA-depleted nuclei after low- and high-salt extraction, (ii) a small fraction (1%) released during nuclease digestion of DNA in intact nuclei in the presence of low-salt buffer, and (iii) a fraction of myc protein (less than 10%) extractable with salt or detergents and found to have affinity for both single- and double-stranded DNA. Immunofluorescence analysis with anti-myc peptide sera on cells extracted sequentially with nucleases and salts confirmed the idea that myc proteins were associated with a complex residual nuclear structure (matrix-lamin fraction) which also contained avian nuclear lamin protein. Dispersal of myc proteins into the cytoplasm was found to occur during mitosis. Both c-myc and v-myc proteins were associated with the matrix-lamin, suggesting that the function of myc may relate to nuclear structural organization.

MC29 virus-coded protein occurs as monomers and dimers in transformed cells

The MC29 virus-coded protein p110gag-myc was found exclusively in the nucleus of transformed Japanese quail (Q8) cells, and time course experiments indicated that the protein had a half-life of about 30 min. When extracts of either Q8 or chicken embryo cells infected with MC29 virus were prepared with nondenaturing detergents and then sedimented in sucrose gradients, p110 was found in the fractions expected to contain monomers (5.9S), dimers (9.3S), or mixtures of the two. The same extracts treated with denaturing detergent (0.2% sodium dodecyl sulfate) exhibited p110 only in fractions expected for the monomeric protein, but beta-mercaptoethanol had no effect on the original distribution. Gradients prepared with 0.5 or 1.0 M NaCl failed to dissociate the faster-sedimenting form. No other protein or polyribonucleotide which could increase the sedimentation rate of p110 was found, and neither RNase nor DNase altered the sedimentation pattern of p110 in nondenatured extracts. A reassociation of monomeric p110 into dimers discernible by gel electrophoresis was demonstrated.

V-myc- and c-myc-encoded proteins are associated with the nuclear matrix

A series of extraction procedures were applied to avian nuclei which allowed us to define three types of association of v-myc- and c-myc-encoded proteins with nuclei: (i) a major fraction (60 to 90%) which is retained in DNA- and RNA-depleted nuclei after low- and high-salt extraction, (ii) a small fraction (1%) released during nuclease digestion of DNA in intact nuclei in the presence of low-salt buffer, and (iii) a fraction of myc protein (less than 10%) extractable with salt or detergents and found to have affinity for both single- and double-stranded DNA. Immunofluorescence analysis with anti-myc peptide sera on cells extracted sequentially with nucleases and salts confirmed the idea that myc proteins were associated with a complex residual nuclear structure (matrix-lamin fraction) which also contained avian nuclear lamin protein. Dispersal of myc proteins into the cytoplasm was found to occur during mitosis. Both c-myc and v-myc proteins were associated with the matrix-lamin, suggesting that the function of myc may relate to nuclear structural organization.

Chromosomal translocations activating myc sequences and transduction of v-abl are critical events in the rapid induction of plasmacytomas by pristane and abelson virus

Plasmacytomas with short latent periods can be induced in BALB/c mice by a single intraperitoneal (i.p.) injection of 0.5 ml pristane followed 20-40 d later by an injection of Abelson virus. The karyotypes of 18 such tumors were determined; 10 of these had rcpt 12;15, 5 had rcpt 6;15 and 3 had no translocations, but two of these have been shown to have interstitial deletions of chromosome 15. The specific breakpoints were the same as described in pristane-induced plasmacytomas, i.e., at 15D2 /3, 6C2 , and 12F2 . Near diploid karyotypes and trisomy of chromosome 11 were frequently seen. All of the Abelson-plus-pristane-induced plasmacytomas (ABPC) were studied as transplanted tumors, contained integrated v- abl sequences, and actively transcribed v- abl mRNA. All but one of these tumors contained abundant myc RNA transcripts. The shortness of the latent periods of the ABPC suggests that the rcpt 12;15 and rcpt 6;15 occur soon after pristane administration and are present at the time Abelson virus is introduced. In this form of plasmacytomagenesis , activated v- abl genes appear to bypass other genetic changes that require a much longer period of time in pristane plasmacytomagenesis . Nonetheless, the consistent finding of chromosome-15 alterations and abundant myc expression in these plasmacytomas emphasize the apparent need for multiple events even in the genesis of some tumors induced by rapid transforming viruses.