The leucine zipper domain of avian cMyc is required for transformation and autoregulation

Small deletions of 7 to 48 amino acids have been generated in the leucine zipper domain of the avian cMyc protein and the mutant cMyc proteins expressed using an avian retroviral vector. Retrovirally encoded cMyc protein transforms primary chick embryo fibroblasts and leads to abnormal regulation of the endogenous c-myc gene. Deletion of the most C-terminal leucine of the zipper motif confers a partial phenotype affecting some but not all parameters of transformation. Complete loss of transforming activity results from deletion of further leucine residues, including one which is not part of the heptad repeat. In cMyc transformed cells endogenous c-myc mRNA is expressed at a low level and is abnormally refractory to induction by serum stimulation. In contrast, a non-transforming cMyc protein which lacks the zipper does not affect normal c-myc expression. These results demonstrate that the leucine zipper domain of avian cMyc is required for both transformation and autoregulation, and suggests that essential leucine residues within the motif may be spaced differently from those in the zippers of Fos and Jun.