ERK PHOSPHORYLATION POTENTIATES ELK-1-MEDIATED TERNARY COMPLEX-FORMATION AND TRANSACTIVATION

Induction of the human c-fos proto-oncogene by mitogens depends on the formation of a ternary complex by p62(TCF) with the serum response fa ctor (SRF) and the serum response element (SRE). We demonstrate that E lk-1, a protein closely related to p62(TCF) in function, is a nuclear target of two members of the MAP kinase family, ERK1 and ERK2. Phospho rylation of Elk-1 increases the yield of ternary complex in vitro. At least five residues in the C-terminal domain of Elk-1 are phosphorylat ed upon growth factor stimulation of NIH3T3 cells. These residues are also phosphorylated by purified ERK1 in vitro, as determined by a comb ination of phosphopeptide sequencing and 2-D peptide mapping. Conversi on of two of these phospho-acceptor sites to alanine impairs the forma tion of ternary complexes by the resulting Elk-1 proteins. Removal of these serine residues also drastically diminishes activation of the c- fos promoter in epidermal growth factor-treated cells. Analogous mutat ions at other sites impair activation to a lesser extent without affec ting ternary complex formation in vitro. Our results indicate that pho sphorylation regulates ternary complex formation by Elk-1, which is a prerequisite for the manifestation of its transactivation potential at the c-fos SRE.