Transformation by the vRel oncoprotein requires sequences carboxy-terminal to the Rel homology domain

The vRel oncoprotein of the avian Rev-T retrovirus is a member of the Rel/NF-kappa B family of transcription factors. The highly conserved amino-terminal Rel Homology (RH) domain in these proteins is required for DNA binding, protein-protein interactions and nuclear localization, and many mutations within this domain abolish transformation by vRel. We demonstrate here that overexpression of the vRel RH domain alone is insufficient to induce transformation of chicken spleen cells, indicating that sequences from the nonconserved carboxy terminus are necessary for the vRel transforming function. Therefore, we constructed and assayed several vRel mutants with deletions of carboxy-terminal sequences. These mutant vRel proteins did not transform spleen cells with equal efficiency, even though they were functionally similar by several other criteria. Our results demonstrate that there are two regions (aa 389 to 432 and aa 437 to 503) within the carboxy-terminal half of vRel that are important for transformation: mutant vRel proteins containing the RH domain and one or both of these carboxy-terminal regions can transform at roughly wild-type levels. Analysis of Gal4 fusion proteins containing carboxy-terminal sequences from the vRel mutants indicated that there is a correlation between the ability of these mutant proteins to transform avian spleen cells and their ability to activate transcription. These observations suggest that vRel induces malignant transformation by directly altering gene expression.