Casein kinase II induces c-fos expression via the serum response element pathway and p67SRF phosphorylation in living fibroblasts.

Elevation of intracellular casein kinase II (CKII) levels through microinjection of purified CKII results in the rapid and transient induction of c-fos in quiescent rat embryo fibroblasts, and activation of quiescent cells by serum is accompanied by the nuclear relocation of endogenous CKII. The induction of c-fos by CKII is inhibited by coinjection of oligonucleotides corresponding to the sequence of the serum response element (SRE) present in the c-fos promoter, indicating that competitive displacement of positive factors from the endogenous c-fos SRE prevents c-fos induction by CKII. Furthermore, the expression of c-fos induced by either CKII injection or serum activation is also inhibited by microinjection of antibodies against the 67 kDa serum response factor (p67SRF) indicating the absolute requirement of p67SRF in this process. Finally, we show the specific phosphorylation of p67SRF in vivo following microinjection of CKII into quiescent cells. Together, these data strongly support that CKII induces c-fos expression through binding/activation of the phosphorylated p67SRF at the SRE sequence.     

Purification of intercalator-released p67, a polypeptide that interacts specifically with the c-fos serum response element.

Incubation of intact nuclei in buffers containing the DNA intercalating drug chloroquine leads to release of proteins that interact with DNA. We demonstrate here that a protein which binds to a motif within the human c-fos promoter, identified as the serum response element (SRE), is quantitatively released from HeLa nuclei, whereas nuclear factor 1 (NF 1) is not. Purification of the SRE binding protein by affinity chromatography to greater than 95% homogeneity allowed us to identify it as a polypeptide of approximately 67,000 daltons. The DNA contacts made by p67, as identified by methylation interference experiments, are indistinguishable from those of the serum response factor described previously.