FUNCTIONAL-ANALYSIS OF THE CARBOXY-TERMINAL TRANSFORMING REGION OF V-MYC - BINDING TO MAX IS NECESSARY, BUT NOT SUFFICIENT, FOR CELLULAR-TRANSFORMATION

To understand the mechanism by which the Myc protein contributes to ce ll growth and development, our laboratory is studying functions of the avian myelocytomatosis virus 29 (MC29) Gag-Myc protein (v-Myc) in the mouse fibroblast cell line C3H10T1/2. Previously, we identified two s pecific regions in v-Myc which are required for co-transformation with activated H-ras. One maps to the amino-terminal portion of v-Myc (ami no acids 1-137) and has the potential to activate transcription of a b asal promoter. The second region spans the carboxy-terminal region of v-Myc (amino acids 244-410), contains a basic/helix-loop-helix/leucine zipper motif and specifies the nuclear location of the protein. In th is study, we have generated a series of deletion mutations within the MC29 gag-myc gene to define precisely the carboxy-terminal transformin g region using the co-transformation of C3H10T1/2 celts as an assay. v -Myc proteins encoded by selected deletion mutations were also examine d for their intracellular location, the ability to interact with the M ax protein and the potential to bind specifically to DNA. Our results demonstrate that integrity of both the basic/helix-loop-helix and the leucine zipper motifs of v-Myc is required for co-transforming activit y, but that the major nuclear localization signal sequence of v-Myc ca n be deleted without compromising the ability of v-Myc to cooperate wi th activated H-Ras p21 to transform C3H10T1/2 celts. In addition, whil e the binding of v-Myc to Max is necessary for ras/myc co-transformati on, it is not sufficient, and also requires the integrity of Myc seque nces specifying site-specific DNA binding.