PHOSPHORYLATION SITES MAPPING IN THE N-TERMINAL DOMAIN OF C-MYC MODULATE ITS TRANSFORMING POTENTIAL

The nuclear proto-oncoprotein c-Myc is involved in the regulation of c ell growth and differentiation. c-Myc is phosphorylated at multiple si tes in vivo, two of which we have identified near the amino terminus. In chicken Thr-61/Ser-65 are phosphorylated, as are the comparable pos itions, Thr-58/Ser-62 in human c-Myc. These residues are located withi n a domain that is implicated in transactivation and is important for the transforming potential of the protein. Furthermore, these phosphor ylation sites or nearby amino acids are frequently mutated in v-myc an d in several c-myc genes from Burkitt's lymphoma cells. In vitro these two phosphorylation sites can be modified by glycogen synthase kinase 3 and mitogen activated protein kinase. To address their biological i mportance we mutated these amino terminal phosphorylation sites separa tely and together. Stably transfected Rat1A cells expressing the mutat ed proteins have an increased growth potential in soft agar Compared t o wt-c-myc transfectants. These altered transformation characteristics indicate that Myc function may be negatively regulated by the amino t erminal phosphorylation.