Interactions between cell growth-regulating domains in the products of the adenovirus E1A oncogene.

Abstract

Among the various biological activities expressed by the products of the adenovirus EIA gene are the abilities to induce cellular DNA synthesis and proliferation in quiescent primary baby rat kidney cells. The functional sites for these activities lie principally within two regions of the ElA proteins: an N-terminal region and a small second region of approximately 20 amino acids further downstream. To study the biological functions of the first domain, we constructed an in-frame deletion of amino acid positions 23 through 107 of the ElA products. This deletion did not impede the ability of the ElA products to transactivate the adenovirus early region 3 promoter in a transient-expression assay in HeLa cells. The ability to induce DNA synthesis in quiescent baby rat kidney cells was, however, lost in the absence of these sequences. Deletion of the small second region induced a form of S phase in which DNA synthesis occurred in the apparent absence of controls required for the cessation of DNA synthesis and progression through the remainder of the cell cycle. These cells did not appear to accumulate in or before G2, and many appeared to have a DNA content greater than that in G2. The functions of both domains are required for production of transformed foci in a ras cooperation assay. Focus formation occurred, however, even when the two domains were introduced on two separate plasmids. This complementation effect appeared to require expression of both of the mutant proteins and did not appear to result merely from recombination at the DNA level.

Maintenance of cellular proliferation by adenovirus early region 1A in fibroblasts conditionally immortalized by using simian virus 40 large T antigen requires conserved region 1.

“…This was surprising as CR2 has been implicated as having a possible role in cellular proliferation on the basis of its ability to associate with the retinoblastoma susceptibility gene product RB1 (11,63) and its necessity for immortalization (10,34,39,41,52,65). Certainly the loss of the RB1 gene or an inability to synthesize its product has been correlated with cellular proliferation (14,23,32,33) and in human cells infected or transformed by adenovirus or SV40, the complexing of RB1 by the viral transforming proteins is postulated to deprive the cell of a negative governor of cellular proliferation (8,36,63,64 (9) and which may have a similar role to the CR1 of ElA.…”

“…Indeed it has been demonstrated that RB1 binding and transformation activity is maintained if the region homologous to CR2 of SV40 LT Ag replaces the CR2 of ElA (39). Thus these conserved regions are implicated in the transforming activity of the respective viral proteins (28,35,40,41,65).…”

Wild-type p53 mediates apoptosis by E1A, which is inhibited by E1B.

“…The amino terminus of E1A is required for induction of cellular DNA synthesis, enhancer repression, and transformation (for review, see Shenk and Flint 1991), but which of these activities is related to apoptosis remains to be determined. The PDdl mutation in E1A, however, can be distinguished from the other E 1A mutants examined because it produces a mutant protein that is completely defective for induction of cellular DNA synthesis (Moran and Zerler 1988;Stein et al 1991). The ability of E1A to release restrictions on growth control and induce S-phase, therefore, correlates with induction of apoptosis.…”

Inhibition of cell proliferation by p107, a relative of the retinoblastoma protein.

“…fragment and inserted into pJ31q to yield an SV40-based expression construct. The pCMVneoBam vector was also used to express CD20 (Stamenkovic and Seed 1988) and E1A12S (Moran and Zerler 1988) cDNAs. Expression constructs for human cyclins A, B1, B2, and E were kindly provided by P. Hinds (Hinds et al 1992).…”