A second domain of simian virus 40 T antigen in which mutations can alter the cellular localization of the antigen

Development of quantitative and high-throughput assays of polyomavirus and papillomavirus DNA replication

Polyoma- and papillomaviruses genome replication is initiated by the binding of large T antigen (LT) and of E1 and E2, respectively, at the viral origin (ori). Replication of an ori-containing plasmid occurs in cells transiently expressing these viral proteins and is typically quantified by Southern blotting or PCR. To facilitate the study of SV40 and HPV31 DNA replication, we developed cellular assays in which transient replication of the ori-plasmid is quantified using a firefly luciferase gene located in cis to the ori. Under optimized conditions, replication of the SV40 and HPV31 ori-plasmids resulted in a 50- and 150-fold increase in firefly luciferase levels, respectively. These results were validated using replication-defective mutants of LT, E1 and E2 and with inhibitors of DNA replication and cell-cycle progression. These quantitative and high-throughput assays should greatly facilitate the study of SV40 and HPV31 DNA replication and the identification of small-molecule inhibitors of this process.

Putative nuclear localization signals (NLS) in protein transcription factors

We have recognized about ten distinct forms of strongly basic hexapeptides, containing at least four arginines and lysines, characteristic of nuclear proteins among all eukaryotic species, including yeast, plants, flies and mammals. These basic hexapeptides are considered to be different versions of a core nuclear localization signal, NLS. Core NLSs are present in nearly all nuclear proteins and absent from nearly all "nonassociated" cytoplasmic proteins that have been investigated. We suggest that the few (approximately 10%) protein factors lacking a typical NLS core peptide may enter the nucleus via their strong crosscomplexation with their protein factor partners that possess a core NLS. Those cytoplasmic proteins found to possess a NLS-like peptide are either tightly associated with cell membrane proteins or are integral components of large cytoplasmic protein complexes. On the other hand, some versions of core NLSs are found in many cell membrane proteins and secreted proteins. It is hypothesized that in these cases the N-terminal hydrophobic signal peptide of extracellular proteins and the internal hydrophobic domains of transmembrane proteins are stronger determinants for their subcellular localization. The position of core NLSs among homologous nuclear proteins may or may not be conserved; however, if lost from an homologous site it appears elsewhere in the protein. This search provides a set of rules to our understanding of the nature of core nuclear localization signals: (1) Core NLS are proposed to consist most frequently of an hexapeptide with 4 arginines and lysines; (2) aspartic and glutamic acid residues as well as bulky amino acids (F, Y, W) need not to be present in this hexapeptide; (3) acidic residues and proline or glycine that break the alpha-helix are frequently in the flanking region of this hexapeptide stretch; (4) hydrophobic residues ought not to be present in the core NLS flanking region allowing for the NLS to be exposed on the protein. In this study we attempt to classify putative core NLS from a wealth of nuclear protein transcription factors from diverse species into several categories, and we propose additional core NLS structures yet to be experimentally verified.

Spatial and temporal expression of the I factor during oogenesis in Drosophila melanogaster

The I factor is a functional non-viral retrotransposon, or LINE, from Drosophila melanogaster. Its mobility is associated with the I-R hybrid dysgenesis. In order to study the expression pattern of this LINE in vivo, a translational fusion between the first ORF of the I factor and the lacZ gene of Escherichia coli has been carried out and introduced in the genome of reactive (R) flies. Homozygous transgenic Drosophila lines have been established and analysed. ORF1 expression is limited to germ-line cells (nurse cells and oocyte) between stage 2 and 10 of oogenesis. No somatic expression is found. Position effects may limit the level of expression of a given transgene but do not modify its basic pattern of expression during the development of the fly. This reproducible control demonstrates both that I factor is driven by its own promoter, probably the internal one suggested by Mizrokhi et al. (Mizrokhi, L.J., Georgevia, S.G. and Ilying, Y.V. (1988). Cell 54, 685–691), and that tissue-specific regulatory sequences are present in the 5′ untranslated part of the I factor. The nuclear localization of the fusion protein reveals the presence of nuclear localization signals (NLS) in the ORF1-encoded protein correlating with the possible structural and/or regulatory role of this protein. This expression is restricted to dysgenic and reactive females, and is similar in the two conditions. All the results obtained in this work suggest that I factor transposition occurs as a meiotic event, between stage 2 and 10 of the oogenesis and is regulated at the transcriptional level. It also appears that our transgene is an efficient marker to follow I factor expression.

Dissociation of Rb-binding and anchorage-independent growth from immortalization and tumorigenicity using SV40 mutants producing N-terminally truncated large T antigens

The large T antigen of SV40 is both necessary and sufficient for conversion of primary mouse cells to cells with fully transformed phenotype. In this investigation, the influence of the N-terminal portion of T antigen on individual transformed cell characteristics was probed by using mutants bearing deletions in the 5'T antigen coding sequence. Specifically, DNA constructs expected to produce T antigens missing the first 109, 127, 150, or 176 amino acids or internal amino acid segments between 117 and 250 were tested for the ability to immortalize C57Bl/6 mouse embryo fibroblasts. The transformed cell properties displayed by clonally derived cell lines were then examined. The results indicated that neither the first 127 amino acids nor amino acids 127-250 of T antigen were necessary for efficient immortalization of primary cells or for their tumorigenicity. Functions mapped within these regions, including binding of the retinoblastoma susceptibility gene product (Rb) and transactivation of heterologous promoters, therefore, were not required to confer either of these growth properties. In addition the results showed that anchorage-independent growth was separable genetically from tumorigenicity and that removal of amino acids within the first 250 residues of T antigen compromised other transformed cell growth properties.

A mutant SV40 large T antigen interferes with nuclear localization of a heterologous protein

A mutant SV40 genome carrying a frameshift at the carboxyl terminus of the large T antigen failed to replicate SV40 DNA and to transform rat2 cells, although the altered region is known to be dispensable for these functions. The mutant T antigen also failed to localize normally in the nucleus and interfered with nuclear localization of at least one other nuclear protein, adenovirus fiber. A double mutant carrying an additional lesion in the nuclear localization signal was also localized in the cytoplasm, but regained the ability to transform rat2 cells and no longer affected the nuclear localization of fiber protein. We suggest that the frameshift T antigen may disrupt a mechanism required for nuclear localization of proteins.

Requirements for immortalization of primary mouse embryo fibroblasts probed with mutants bearing deletions in the 3' end of SV40 gene A

The influence of specific contiguous stretches of amino acids predominantly in the carboxy terminal third of the SV40 large T antigen on the immortalization of cells in culture was investigated. Mutants that bear either small in-phase or frameshift deletions in the large T antigen coding sequence were transfected into primary mouse embryo fibroblasts of C57Bl/6 origin (B6/MEF). The frequency of immortalization was determined as the number of colonies that developed from cells escaping senescence. The results indicated that the terminal 81 amino acids of large T antigen are not needed for efficient immortalization or tumorigenicity. In contrast removal of as few as three amino acids encoded in the vicinity of the Dde-1 site at 0.234 map units (m.u.) severely restricted immortalization, suggesting that this region of the coding sequence either structurally or functionally is essential to at least one parameter of the transformed cell phenotype. The T antigen produced by dlA2433 which bears a deletion of nine nucleotides at 0.234 m.u. fails to associate stably with the cellular protein p53. The results showed that the addition of long stretches of amino acids (96 or 97 residues) from the open reading frame at the 3' end of the early region inactivated immortalizing functions, although the addition of as many as 18 amino acids from other reading frames was not detrimental. The evidence presented also confirmed that wild-type levels of ATPase activity are not necessary for immortalization or tumorigenicity of B6/MEF. Finally, we show that one of the mutants that immortalized primary cells did not produce dense foci on a cell monolayer. This last result indicated that independent functions are required for these two parameters of the transformed cell phenotype.

Cellular mutation mediates T-antigen-positive revertant cells resistant to simian virus 40 transformation but not to retransformation by polyomavirus and adenovirus type 2

T-antigen-positive transformation revertant cell lines were isolated from fully simian virus 40 (SV40)-transformed Fisher rat embryo fibroblast cells (REF 52 cells) by methionine starvation. Reversion of the transformed cells (SV-52 cells) was caused by a mutation within the cellular genome. To demonstrate this, we isolated SV40 DNA from the host genome, inserted it into plasmid pSPT18 DNA, cloned it in Escherichia coli, and microinjected it into the nuclei of the REF 52 cells. Fully transformed cells were obtained with the same efficiency (20 to 25%) as after microinjection of wild-type SV40 DNA I. Furthermore, the revertant cells were resistant to retransformation by SV40. Following microinjection of wild-type SV40 DNA I, 42 independent cell lines were isolated. Cells of all analyzed lines acquired additional SV40 DNA copies, but changes in the cell morphology or growth characteristic were not demonstrable. However, the revertants were retransformable with a high efficiency after polyomavirus and adenovirus type 2 infections or microinjection. Also, fusion of the revertant cells with the grandparental REF 52 cells led to restoration of the transformed state.

trans-activation of cellular and viral promoters by a transforming nonkaryophilic simian virus 40 large T antigen

We used the chloramphenicol acetyl transferase (CAT) transient expression system to study the transactivating ability of a simian virus 40 (SV40) mutant that was unable to transport and localize large T antigen into the nucleus and which retained the competence to transform established cell lines. The effect of the SV40 wild type and the SV40 mutant for the large T antigen was tested in both mouse and simian cells on a series of plasmids in which the CAT gene was regulated by one of the following promoters: SV40 early and late, herpes simplex virus thymidine kinase, chicken alpha 2(I) collagen, mouse H-2Kk. Our results indicated that both the SV40 wild type and the cytoplasmic mutant for the large T antigen regulated transcription from any promoter tested, suggesting that the trans-activation by SV40 does not require the nuclear localization of the 100,000-molecular-weight large T-antigen protein.