Protein kinase CK1 is a p53-threonine 18 kinase which requires prior phosphorylation of serine 15

p53 is a potent transcription factor Which is regulated by sequential multi site phosphorylation and acetylation, in this paper, we identify threonine 18 of p53, a key site in regulating the interaction between p53 and its reg ulatory partner MDM2, as a novel site phosphorylated in vitro by purified r ecombinant casein kinase 1 (CK1) delta. Strikingly, phosphorylation of thre onine 18 is dependent upon prior phosphorylation of serine 15. These data h ighlight an additional and physiologically important target residue for CK1 in p53 and suggest a potential mechanism by which sequential modification of a pivotal N-terminal residue in p53 may occur following stress-activated modification of serine 15. (C) 1999 Federation of European Biochemical Soc ieties.