Phosphorylation of the DNA repair protein APE/REF-1 by CKII affects redox regulation of AP-1

The DNA repair protein apurinic endonuclease (APE/Ref-1) exerts several phy siological functions such as cleavage of apurinic/apyrimidinic sites and re dox regulation of the transcription factor AP-1, whose activation is part o f the cellular response to DNA damaging treatments. Here we demonstrate tha t APE/Ref-1 is phosphorylated by casein kinase II (CKII), This was shown fo r both the recombinant APE/Ref-1 protein (Km = 0.55 mM) and for APE/Ref-1 e xpressed in COS cells. Phosphorylation of APE/Ref-1 did not alter the repai r activity of the enzyme, whereas it stimulated its redox capability toward s AP-1, thus promoting DNA binding activity of AP-1, Inhibition of CKII med iated phosphorylation of APE/Ref-1 blocked mutagen-stimulated increase in A P-1 binding. It also abrogated the induction of c-Jun protein and rendered cells more sensitive to induced DNA damage. Thus, phosphorylation of APE/Re f-1 appears to be involved in regulating the different physiological activi ties of the enzyme, CKII mediated phosphorylation of APE/Ref-1 and concomit ant increase in AP-1 binding activity appears to be a novel mechanism of ce llular stress response, forcing transcription of AP-1 target gene(s) the pr oduct(s) of which may exert protective function.