Two regions of the adenovirus early region 1A proteins are required for transformation.

Regions of the adenovirus type 5 early region 1A (E1A) proteins that are required for transformation were defined by using a series of deletion mutants. Deletion mutations collectively spanning the entire protein-coding region of E1A were constructed and assayed for their ability to cooperate with an activated ras oncogene to induce transformation in primary baby rat kidney cells. Two regions of E1A (amino acids 1 to 85 and 121 to 127) were found to be essential for transformation. Deletion of all or part of the region from amino acids 121 to 127 resulted in a total loss of transforming ability. An adjacent stretch of amino acids (residues 128 to 139), largely consisting of acidic residues, was found to be dispensable for transformation but appeared to influence the efficiency of transformation. Amino acids 1 to 85 made up a second region of the E1A protein that was essential for transformation. Deletion of all or part of this region resulted in a loss of the transforming activity. Even a mutation resulting in a single amino acid change at position 2 of the polypeptide chain was sufficient to eliminate transformation. Deletion of amino acids 86 to 120 or 128 to 289 did not eliminate transformation, although some mutations in these regions had lowered efficiencies of transformation. Foci induced by transformation-competent mutants could be expanded into cell lines that retained their transformed morphology and constitutively expressed the mutant E1A proteins.     

Mutant adenovirus type 9 E4 ORF1 genes define three protein regions required for transformation of CREF cells.

Human adenovirus type 9 (Ad9) elicits exclusively estrogen-dependent mammary tumors in rats, and an essential oncogenic determinant for this virus is Ad9 E4 open reading frame 1 (9ORF1), which encodes a 125-residue cytoplasmic protein with cellular growth-transforming activity in vitro. In this study, we engineered 48 different mutant 9ORF1 genes in an attempt to identify regions of this viral protein essential for transformation of the established rat embryo fibroblast cell line CREF. In initial assays with CREF cells, 17 of the 48 mutant 9ORF1 genes proved to be severely defective for generating transformed foci but only 7 of these defective genes expressed detectable amounts of protein. To further examine the defects of the seven mutant proteins, we selected individual cell pools of stable CREF transformants for the wild-type and mutant 9ORF1 genes. Compared to cell pools expressing the wild-type 9ORF1 protein, most cell pools expressing mutant proteins displayed decreased growth in soft agar, and all generated significantly smaller tumors in syngeneic animals. The altered amino acid residues of the seven mutant 9ORF1 polypeptides clustered within three separate regions referred to as region I (residues 34 to 41), region II (residues 89 to 91), and C-terminal region III (residues 122 to 125). By using indirect immunofluorescence, we also assessed whether the mutant proteins localized properly to the cytoplasm of cells. The region I and region II mutants displayed approximately wild-type subcellular localizations, whereas most region III mutants aberrantly accumulated within the nucleus of cells. In summary, we have identified three 9ORF1 protein regions necessary for cellular transformation and have demonstrated that C-terminal region III sequences significantly influence the proper localization of the 9ORF1 polypeptide in cells.     

An adenovirus early region 1A protein is required for maximal viral DNA replication in growth-arrested human cells.

Two closely related adenovirus early region 1A proteins are expressed in transformed cells. The smaller of these, which is 243 amino acids in length, is required for the transformation of primary rat cells and for the transformation of immortalized rat cells to anchorage-independent growth. This protein is not required for productive infection of exponentially growing HeLa cells but is required for maximal replication in growth (G0)-arrested human lung fibroblasts (WI-38 cells). To determine the function of this protein in viral replication in these G0-arrested cells, we compared viral early mRNA, early protein, and late protein synthesis after infection with wild type or a mutant which does not express the protein. No differences were found. However, viral DNA synthesis by the mutant was delayed and decreased to 20 to 30% that of wild type in these cells. Viral DNA synthesis was much less defective in growing WI-38 cells, and in the transformed human HeLa cell line it occurred at wild-type levels. Furthermore, the mutant which can express only the 243-amino-acid early region 1A protein induced cellular DNA synthesis in G0-arrested rat cells to the same level as wild-type virus. A mutant which can express only the 289-amino-acid early region 1A protein induced less cellular DNA synthesis in G0-arrested rat cells. We propose that the early region 1A 243-amino-acid protein alters the physiology of arrested permissive cells to allow maximal viral DNA replication. In nonpermissive rodent cells, the 243-amino-acid protein drives G0-arrested cells into S phase. This activity is probably important for the immortalization of primary cells.     

A carboxy-terminal region required by the adenovirus type 9 E4 ORF1 oncoprotein for transformation mediates direct binding to cellular polypeptides.

Human adenovirus type 9 (Ad9) is unique among oncogenic adenoviruses in that it elicits exclusively mammary tumors in rats and requires the viral E4 region open reading frame 1 (9ORF1) gene for tumorigenicity. The 9ORF1 oncogenic determinant codes for a 14-kDa transforming protein, and three separate regions of this polypeptide, including one at the extreme C terminus, are necessary for transforming activity. In this study, we investigated whether the 9ORF1 transforming protein interacts with cellular factors. Following incubation with cell extracts, a glutathione S-transferase (GST)-9ORF1 fusion protein associated with several cellular phosphoproteins (p220, p180, p160, p155), whereas GST fusion proteins of transformation-defective 9ORF1 C-terminal mutants did not. Similar interactions requiring the 9ORF1 C terminus were revealed with protein-blotting assays, in which a GST-9ORF1 protein probe reacted specifically with cellular polypeptides having gel mobilities resembling those of the 9ORF1-associated cellular phosphoproteins, as well as with additional cellular polypeptides designated p140/p130. In addition, GST fusion proteins containing 9ORF1 C-terminal fragments associated with some of the 9ORF1-associated cellular polypeptides, as did GST fusion proteins of full-length wild-type Ad5 and Ad12 E4 ORF1 transforming proteins. Significantly, the results of coimmunoprecipitation analyses suggested that the same cellular polypeptides also associate with wild-type but not C-terminal-mutant 9ORF1 proteins in vivo. Together, these findings suggest that the 9ORF1 C terminus, which is essential for transformation, participates in specific and direct binding of the 9ORF1 oncoprotein to multiple cellular polypeptides. We propose that interactions with these cellular factors may be responsible, at least in part, for the transforming activity of the 9ORF1 viral oncoprotein.     

Cellular targets for transformation by the adenovirus E1A proteins

Three cellular proteins, including species of 300,000 daltons and 107,000 daltons as well as p105-RB, the product of the retinoblastoma susceptibility gene, stably interact with the adenovirus E1A proteins. To help determine the functional basis of these interactions, the regions of E1A that participate in these interactions were mapped using a series of deletion mutants. The 300,000 dalton and the 107,000 dalton proteins interacted with sequences within amino acids 1 to 76 and 121 to 127, respectively. Interaction with the third cellular protein, p105-RB, required the presence of sequences from two noncontiguous regions of the E1A polypeptide chain, amino acids 30 to 60 and 121 to 127. The regions of E1A that are required for these interactions coincided precisely with the regions of E1A that are required for its transforming function. These results suggest that the interactions with these cellular proteins are fundamental to the transforming activity of E1A.