A NOVEL SIMIAN-VIRUS-40 EARLY-REGION DOMAIN MEDIATES TRANSACTIVATION OF THE CYCLIN A PROMOTER BY SMALL-T ANTIGEN AND IS REQUIRED FOR TRANSFORMATION IN SMALL-T ANTIGEN-DEPENDENT ASSAYS

At least three regions of the simian virus 40 small-t antigen (small-t ) contribute to the protein's ability to enhance cellular transformati on. As we showed previously for rat F111 cells, one region includes se quences from residues 97 to 103 that are involved in the binding and i nhibition of protein phosphatase 2A. In the present study, the role of the protein phosphatase 2A binding region was confirmed in two additi onal small-t-dependent transformation systems. Second, small-t was fou nd to provide a function previously identified as a large-T transforma tion domain. Mutations in residues 19 to 28 of large-T affected its tr ansforming ability, but these mutations were complemented by a wild-ty pe small-t. A third region of small-t was also required for efficient transformation. This region, the 42-47 region, is shared by large-T an d small-t and contains a conserved HPDKGG hexapeptide. The 42-47 regio n function could be provided by either small-t or large-T in small-t-d ependent systems. Mutations in the 42-47 region reduced the ability of small-t to transactivate the cyclin A promoter, of interest because s mall-t increased endogenous cyclin A mRNA levels in both human and mon key cells, as well as transactivating the promoter in transient assays .