The v-Rel oncoprotein blocks apoptosis and proteolysis of I kappa B-alpha in transformed chicken spleen cells.

The v-Rel oncoprotein of the avian Rev-T retrovirus malignantly transforms chicken spleen cells in vivo and in vitro. We previously described two temperature-sensitive (ts) mutants of v-Rel (v-G37E and v-R273H) that show a ts ability to transform chicken spleen cells and to bind to DNA in vitro. We now show that spleen cell lines transformed by ts v-Rel proteins at the permissive temperature undergo apoptosis when cells are shifted to the nonpermissive temperature. The levels of most proteins (including v-Rel, p53, c-Myc, Rb and Bcl-2) do not change in these cells even at advanced stages of apoptosis. However, the chicken I kappa B-alpha protein (also called p40), which is in a complex with v-Rel in transformed cells, is degraded when ts v-Rel-transformed cells are shifted to the nonpermissive temperature. In v-R273H-transformed cells, p40 is degraded without the appearance of proteolytic intermediates. In contrast, in v-G37E-transformed cells, p40 is cleaved to an intermediate species that is missing approximately 3-4 kDa from its amino terminus. This truncated form of p40 is found in a detergent-insoluble fraction and can also be detected in wild-type v-Rel-transformed cells that are induced to undergo apoptosis by treatment with cycloheximide. Both ts v-Rel proteins are ts for interaction with p40 in vitro. The results reported here indicate that v-Rel blocks a normal pathway of programmed cell death and that I kappa B-alpha can undergo multiple degradative pathways, which can be induced by alterations in the structure of the Rel protein to which it is bound.