The nuclear protein v-Myb, encoded by the avian myeloblastosis virus (AMV), can induce acute monoblastic leukemia in vivo and transform chicken myelomonocytic cells in culture. The N terminus of v-Myb functions as the DNA-binding domain, and multiple central and C-terminal regions of this protein have been reported to function in transcriptional activation of model reporter genes. We showed previously that a C-terminal domain (amino acids 296 to 371) is required for transcriptional activation 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 observed between transcriptional activation and leukemic transformation by this series of mutants. Furthermore, deletion analyses demonstrate that the C-terminal 41 amino acids of v=MybAMV (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. Hence, we defined a 10-amino-acid subregion (the "FAETL" motif) required 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 mutants with substitutions of the zipper leucines or multiple adjacent phosphorylation sites demonstrates that the function of the FAETL motif is not dependent on an intact leucine zipper structure or adjacent phosphorylation sites. The study of GAL4-Myb fusions suggests that this region is important in maintaining a fully functional conformation of v-Myb. The putative leucine zipper structure has previously been proposed to exert inhibitory effects on c-Myb because its mutation caused increased transcriptional transactivation and transformation. Interestingly, our results show that this region is essential for the functions of v-Myb without requiring a heptad leucine repeat
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