FAETL MOTIF REQUIRED FOR LEUKEMIC TRANSFORMATION BY V-MYB

The nuclear protein v-Myb, encoded by the avian myeloblastosis virus ( AMV), can induce acute monoblastic leukemia in vivo and transform chic ken myelomonocytic cells in culture, The N terminus of v-Myb functions as the DNA-binding domain, and multiple central and C-terminal region s of this protein have been reported to function in transcriptional ac tivation of model reporter genes, We showed previously that a C-termin al domain (amino acids 296 to 371) is required for transcriptional act ivation and transformation of primary chicken myelomonocytic cells, In this study, we have now analyzed a series of C-terminal mutants of v- Myb to further investigate this domain, A strong correlation was obser ved between transcriptional activation and leukemic transformation by this series of mutants. Furthermore, deletion analyses demonstrate tha t the C-terminal 41 amino acids of v-Myb(AMV) (amino acids 331 to 371 of the Myb portion) are nonessential whereas further deletion of amino acids 321 to 330 (EFAETLQLID) results in a nonfunctional protein, Hen ce, we defined a 10-amino-acid subregion (the ''FAETL'' motif) require d for transcriptional activation and oncogenic transformation by v-Myb (AMV). The FAETL region is part of a putative leucine zipper structure and lies near a cluster of phosphorylation sites. Our analysis of mut ants with substitutions of the zipper leucines or multiple adjacent ph osphorylation sites demonstrates that the function of the FAETL motif is not dependent on an intact leucine zipper structure or adjacent pho sphorylation sites, The study of GAL4-Myb fusions suggests that this r egion is important in maintaining a fully functional conformation of v -Myb, The putative leucine zipper structure has previously been propos ed to exert inhibitory effects on c-Myb because its mutation caused in creased transcriptional transactivation and transformation, Interestin gly, our results show that this region is essential for the functions of v-Myb without requiring a heptad leucine repeat.