Characterization of polyoma mutants with altered middle and large T-antigens.

The viable polyoma mutants dl1013, dl1014, and dl1015 produced shortened middle and large T-antigens. In mouse 3T3 cells, dl1013 and dl1014 grew at normal rates, and dl1015 grew at a reduced rate. dl1015 behaved phenotypically as a double mutant, with deficiencies both in the stimulation of the host cell and the replication of viral DNA. Only the former defect could be complemented by the ts-a mutant, which produced a normal middle T-antigen and a temperature-sensitive large T-antigen. This result suggests that middle T-antigen is involved in the induction of cellular DNA synthesis. Of the three mutants, dl1015 alone failed to transform rat fibroblasts to growth in semisolid medium. This defect could not be complemented by the ts-a mutant. Determination of the base sequences of the mutant DNAs showed that dl1013 and dl1014 had overlapping deletions of 21 and 9 base pairs, respectively, and that the dl1015 deletion of 30 base pairs was contiguous to the other mutations on their 3' sides. Analyses of the mutant t-antigens showed that all three mutants produced shortened middle T-antigens, whereas only dl1015 large T-antigen was detectably reduced in size.     

Extraction and fingerprint analysis of simian virus 40 large and small T-antigens.

A study of simian virus 40 (SV40) T-antigens isolated from productively infected CV1 cells using a variety of different extraction procedures showed that under some conditions the highest molecular weight form of T-Ag (large-T) isolated comigrated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with large-T from SV40-transformed H65-90B cells. Other faster-migrating forms of large-T are probably generated during the extraction procedure by a protease which is active at low pH, and such forms are probably experimental artifacts. After extraction under conditions which minimize proteolytic degradation of large-T, a further form of T-antigen was isolated; this has an apparent molecular weight in the range 15,000 to 20,000 and is referred to as small-t. Fingerprint analysis of [35S]methionine-labeled SV40 proteins showed that small-t has 10 to 12 methionine peptides whereas large-T has 15 to 18 methionine peptides. All but two of the methionine tryptic peptides present in small-t are also present in large-T. The fingerprint data also showed that T-antigens have no peptides in common with SV40 VP1. Experiments using reagents which inhibit posttranslational cleavage of encephalomyocarditis virus polyproteins showed that these reagents do not affect the synthesis of small-t and suggest that it is not made by proteolytic cleavage of large-T in vivo. An alternative model, which proposes that large-T and small-t are synthesized independently, is discussed in terms of the fingerprint data and the number of methionine tryptic peptides predicted from the primary sequence of SV40 DNA.     

Mechanisms of transformation by polyoma virus middle T antigen

This review addresses a fundamental question of polyoma virus biology: What is the molecular mechanism by which the polyoma virus middle T antigen (MTAg) transforms cells in culture? Since MTAg has no known intrinsic biochemical activity, it is believed to act by modulating the properties of the host cell's proteins (see review by Courtneidge [26]). Experiments to date have largely focused on the interaction of MTAg with the cellular tyrosine kinase, pp60c-src. However, recent data from a number of laboratories have demonstrated the importance of other MTAg-associating cellular proteins in MTAg-mediated transformation, including pp62c-yes and a phosphatidylinositol kinase. In this review, we will summarize what is presently known about the proteins interacting with MTAg. The extent to which the currently known details of the biochemistry of MTAg and its associated proteins can explain the transforming properties of the various mutant alleles of MTAg will be assessed.     

Polyoma virus. The early region and its T-antigens.

The DNA sequence of the early coding region of polyoma virus is presented. It consists of 2739 nucleotides. The sequence predicts that more than one reading frame can be used to code for the three known polyoma virus early proteins (designated small, middle and large T-antigens). From the DNA sequence, the 'splicing' signals used in the processing of viral RNA to functional messenger RNAs can be predicted, as well as the sizes and sequences of the three proteins. Other unusual aspects of the DNA sequence are noted. Comparisons are made between the DNA sequences and the predicted amino acid sequences of the respective large T-antigens of polyoma virus and the related virus Simian Virus (SV) 40.     

Studies of polyoma virus DNA: cleavage map of the polyoma virus genome.

A small-plaque polyoma virus, MPC-1, was isolated from a mouse plasmacytoma. The DNA of this polyoma virus was cleaved with a restriction enzyme from Haemophilus influenzae (Hin d), and the molecular weights of the limit products were analyzed by electrophoresis and electron microscopy. The fragments produced by this enzyme have been ordered by analysis of partial digest products. A physical map of the polyoma virus genome was then constructed.     

Salt-stable binding of the large T protein to DNA in polyoma virus chromatin.

Nucleoprotein complexes extracted from the nuclei of mouse cells lytically infected with polyoma virus contain an ATPase activity which appears to correspond to that of the viral large T protein, as it exhibits the same characteristic properties; in particular, the activity is extensively inhibited by polyclonal antibodies from animals bearing polyoma tumors (anti-T antigen antibodies) and by monoclonal antibodies against large T. Significant amounts of DNA were immunoprecipitated by adding these antibodies to the nucleoprotein complex, suggesting that the protein is tightly bound to DNA in the viral chromatin. Since one of the monoclonal antibodies quantitatively immunoprecipitated the pulse-labeled replicative intermediates, we conclude that some large T protein remains physically associated with the DNA throughout its replication cycle. After exposure to salt concentrations higher than 1 M KCl, about half of the large T-specific ATPase activity was still observed to co-sediment with 21S form I viral DNA. The observations that the sedimentation coefficient of the salt-stable complexes was shifted to 16S after a limited endonucleolytic digestion, and that both the viral DNA and the ATPase activity were co-precipitated in the presence of polyethylene glycol at high ionic strength, further demonstrated that the protein is engaged in an unusually stable complex with DNA in the viral chromatin.     

Viable deletion mutant in the medium and large T-antigen-coding sequences of the polyoma virus genome.

A polyoma virus mutant that maps in the early region between the known hr-t and ts-a mutants has been isolated. Its 66-base-pair deletion results in structural changes in both medium and large T-antigens but causes no substantial alterations in viral replication or cell transformation.     

Protein kinase activity associated with polyoma virus middle T antigen in vitro.

A protein kinase activity can be detected in immunoprecipitates of extracts from polyoma virus (Py)-infected cells using antiserum raised against Py-transformed cells (anti-T serum). The activity is not detected in uninfected cells or when using control serum. Using rat anti-T serum both Py middle T and the heavy chain of rat IgG are phosphorylated, whereas using hamster anti-T serum only Py middle T is phosphorylated. Experiments using a number of different mutants of Py indicate that the kinase activity detected is under viral control and is associated with Py middle T. Consistent with this the kinase, like middle T, can be detected in purified preparations of plasma membranes. The kinase can also be detected in a large number of Py-transformed cells, but not in untransformed cells or in cells transformed by other viruses. Some of the Pytransformed cells which contain kinase activity lack full sized Py large T but all contain middle T. Kinase activity is not detected in a cell line (18.37) which contains integrated viral DNA of a nontransforming hr-t deletion mutant and which contains Py large T but not middle T or small t. These results show that Py middle T or a protein which specifically binds to it has protein kinase activity in vitro. Although these results raise the possibility that protein kinases play an essential role in Py-induced transformation, however, thus far we have no data which show unequivocally that the results are physiologically significant.     

Expression of biologically active middle T antigen of polyoma virus from recombinant baculoviruses.

Two different recombinant baculoviruses have been generated for expressing the middle T antigen (MT) of polyoma virus in insect (Sf9) cells. One (pAcI-PyMT) produces moderate levels of MT and the other (pVL-PyMT) high levels. Indirect immunofluorescence and cellular fractionation studies with pAcI-PyMT infected Sf9 cells give results similar to those observed with wild type polyoma virus infected mouse cells, and show MT to be mainly associated with cytoplasmic membranes in the insect cell. In the latter, a sub-population of MT is phosphorylated in in vitro protein kinase assays. The yields of MT from pVL-PyMT infected cells are high enough to suggest that this protein can now be produced by this method in sufficient amounts for definitive biochemical and crystallographic analyses.     

Subcellular localisation of guinea pig hepatic molybdenum hydroxylases.

Molybdenum hydroxylase activity in guinea pig liver has been compared with that of marker enzymes in mitochondria (succinate dehydrogenase), microsomes (glucose-6-phosphatase) and cytosol (lactate dehydrogenase). Aldehyde oxidase activity was highest in the cytosol, with about 10-fold activity of xanthine oxidase. Significant molybdenum hydroxylase activity was found in mitochondria with minimal levels in microsomes. Mitochondrial and cytosolic aldehyde oxidase varied in substrate specificity and electrophoretic mobility with two major bands in each fraction, one of which was common to cytosol and mitochondria.     

Comparison of phosphorylation of two polyoma virus middle T antigens in vivo and in vitro.

Two species of polyoma virus middle T antigen were detected in both lytically infected and transformed cells by in vitro kinase assay of immunoprecipitates. A minor species with an apparent molecular weight of 58,000 (58K) represented less than 10% of the total middle T protein. This species was roughly 10 times more active as a phosphate acceptor than was the predominant 56K form. Partial proteolytic mapping experiments showed that the same site was phosphorylated in both species. Mapping of the middle T antigens from a series of deletion mutants suggested that the major site of phosphorylation is tyrosine residue 315. Phosphorylation occurred on both middle T species in vivo, involving sites predominantly other than the tyrosine labeled in vitro. The 56K and 58K middle T forms differed from each other in their in vivo phosphorylation patterns. Some phosphate was incorporated into the 58K species in a region of the molecule to which at least part of the apparent molecular weight different could be mapped. hr-t mutant NG-59, which codes for a slightly altered middle T, produced only a single species (56K) which was inactive in the in vitro kinase reaction. Moreover, no 58K species appeared in vivo with this mutant. hr-t mutants are therefore defective in both aspects of phosphorylation. Phenotypically normal revertant cells of a polyoma transformed line failed to express any middle T antigens or associated kinase activity.     

Down-regulation of cytokeratin 14 mRNA in polyoma virus middle T-transformed rat liver epithelial cells.

We have recently shown that rat liver nonparenchymal epithelial cells, such as T51B cells, selectively express cytokeratin (CK) 14 as a partner of CK8 in their intermediate filaments, and we proposed CK14 as a unique cell lineage marker of the liver epithelial cell population (R. Blouin, M-J. Blouin, I. Royal, A. Grenier, A. Loranger, D. R. Roop, and N. Marceau, Differentiation, submitted for publication, 1992). In the present study, T51B-261A (spontaneously transformed) and T51B-261B (aflatoxin B1-treated) clones and clones derived from T51B cells transfected with SV40 large T (LT) and polyoma virus middle T (MT) were used to investigate CK gene expression in nontransformed and transformed liver epithelial cells. T51B-261A, T51B-261B, MT-T51B, and LT/MT-T51B clones all grew in calcium-deficient medium and formed colonies in soft agar, whereas LT-T51B clones did not grow at all in either one of these assays. T51B-261A and T51B-261B clones formed small, slow growing tumors when injected into newborn syngenic rats, whereas the MT-T51B and LT/MT-T51B clones produced rapidly forming, large tumors. There was no effect of cell transformation on CK expression, except in the clones expressing MT, where the CK intermediate filaments were completely lost. Analyses of [35S]methionine incorporation into the Triton-resistant cytoskeleton and of total proteins confirmed that CKs were absent. In contrast, vimentin intermediate filaments remained unaffected in all of the clones.(ABSTRACT TRUNCATED AT 250 WORDS)     

Deletion mapping of a short polyoma virus middle T antigen segment important for transformation.

Viable polyoma virus mutants were constructed that had small deletions in the early region of the genome. The deletions together removed most of the segment missing from the genome of the nontransforming mutant dl23 (N. Smolar and B. E. Griffin, J. Virol. 38:958-967, 1981). The transformation properties, as measured by colony formation in soft agar, of mutants with overlapping or contiguous deletions showed that part or all of the middle T antigen segment, consisting of the short amino acid sequence Glu4-Tyr-Met-Pro-Met, was essential for the activity of the protein in transformation. However, the segment could be deleted without significant effect on the in vitro protein kinase activity associated with the middle T antigen.     

Inhibition of polyoma virus middle T antigen-associated tyrosyl kinase activity by N-ethylmaleimide

The middle T antigen (MT Ag) encoded by polyoma virus has an associated protein kinase activity which transfers a phosphoryl group from ATP or GTP to a tyrosine residue on MT Ag in immunoprecipitates formed between polyoma virus-infected or transformed cell extracts and serum from animals bearing polyoma-induced tumors. Incubation of such immunoprecipitates or polyoma-transformed cell extracts prior to immunoprecipitation with the sulfhydryl reagent, N-ethylmaleimide (NEM), resulted in a significant inhibition of MT Ag-associated kinase activity. Inactivation of this enzyme activity by NEM was found to be dependent upon the incubation pH, time of incubation, and NEM concentration. ATP, GTP, and ADP in the presence of Mg2+ were found to decrease the rate of NEM-mediated inactivation of MT Ag-associated kinase activity, while CTP and UTP did not detectably alter the rate of enzyme inhibition by NEM. These results suggest that the MT Ag-associated kinase possesses at least one NEM-sensitive sulfhydryl group essential for phosphotransferase activity which may be present at or near the enzyme catalytic site.     

Requirement for the C-terminal region of middle T-antigen in cellular transformation by polyoma virus

Deletions of polyoma virus DNA around the region that codes for the C-terminus of the viral middle T-antigen were created using a transforming fragment (BamH I/EcoR I) of viral DNA cloned in the plasmid vector pAT153. These species were recloned and assayed for their ability to transform Rat-1 cells in culture. Our results showed that whereas the DNA sequence between the presumed translational termination codon for the viral middle T-antigen and the single viral EcoR I site could be removed with no apparent effect on transformation, the removal of the termination codon itself or any amino acid coding sequences of this protein caused a drastic decrease in the transforming ability of the DNA. Transfection of Rat-1 cells with plasmids that contained viral DNA with deletions which corresponded to the last fourteen or more amino acids of the middle T-antigen never gave rise to cellular transformation.     

Sequence from early region of polyoma virus DNA containing viral replication origin and encoding small, middle and (part of) large T antigens.

The sequence of about one third of the polyoma virus genome is presented. This sequence covers the origin of replication of two large plaque strains (A2 and A3) of polyoma virus. The two strains differ by 11 bp in the origin region. A model for replication is suggested. The sequence probably also covers the entire coding region of two of the polyoma virus early proteins--small and middle T antigens--as well as part of the coding region for large T antigen. Over a small region of the DNA, all three coding frames contain termination codons, which argues a need for spliced early messenger RNAs. In another region of the DNA, two coding frames can be used. Correlation with protein data suggests that one frame codes for part of middle T antigen and the other for part of large T antigen.