SELECTIVE RESPONSE OF TERNARY COMPLEX FACTOR SAP1A TO DIFFERENT MITOGEN-ACTIVATED PROTEIN-KINASE SUBGROUPS

Mitogenic and stress signals result in the activation of extracellular signal-regulated kinases (ERKs) and stress-activated protein kinase/c -Jun N-terminal kinases (SAPK/JNKs), respectively, which are two subgr oups of the mitogen-activated protein kinases. A nuclear target of mit ogen-activated protein (MAP) kinases is the ternary complex factor Elk -1, which underlies its involvement in the regulation of c-fos gene ex pression by mitogenic and stress signals. A second ternary complex fac tor, Sap1a, is coexpressed with Elk-1 in several cell types and shares attributes of Elk-1, the significance of which is not clear. Here we show that Sap1a is phosphorylated efficiently by ERKs but not by SAPK/ JNKs. Serum response factor-dependent ternary complex formation by Sap 1a is stimulated by ERK phosphorylation but not by SAPK/JNKs. Moreover , Sap1a-mediated transcription is activated by mitogenic signals but n ot by cell stress. These results suggest that Sap1a and Elk-1 have dis tinct physiological functions.