Induction of a complex between rasGAP and a novel 110 kD protein is required for immortalization of primary epithelial cells by the E1A 12S oncoprotein of adenovirus.

Although established cell lines can be transformed with oncogenic ras, primary epithelial cells cannot, but require the coexpression of an immortalizing oncogene, such as the E1A region of adenovirus. We have previously shown that immortalization of primary epithelial cells by E1A 12S requires the expression of five regions encoded by both the first and second exons of the gene. However, only three of these regions, located in the first exon, are required for cotransformation of primary cells with an activated ras oncogene. Thus, the expression of oncogenic ras is able to abrogate the need for the E1A function(s) encoded by the second exon that are required for immortalization. This suggested the possibility that the functions encoded by the second exon of E1A may involve or interact with the normal ras signal transduction pathway. The results described herein demonstrate that immortalization-competent 12S gene products induce the expression of a novel 110 kD protein, p110, that forms a stable complex with rasGAP. Failure to induce the p110-rasGAP complex results in the concomitant loss of ability of 12S to immortalize primary epithelial cells. The appearance of this complex parallels the expression of the 12S protein and is sensitive to the levels of E1A 12S. p110 induction is independent of the ability of 12S to activate the cell cycle and of the presence of adenovirus E1B and is not observed in the presence of the large T antigen of SV40. Thus, it is not a general response to proliferation or tumorigenic transformation, but rather seems to be specific to the immortalization function(s) of E1A 12S.