Immunopurified mutant mouse p53 from transformed cells is known to be tightly associated with a protein kinase which phosphorylates p53 in an in vitro kinase reaction. Wild-type p53 from a non-transformed cell line was not associated with a protein kinase whereas immunopurified wild-type p53 from a transformed cell line was tightly associated with a protein kinase which phosphorylated p53. In order to compare wild-type and mutant p53 in the same cellular environment both forms were cloned in a baculovirus expression system and in in vitro transcription/translation vectors and both p53 proteins were expressed in the two systems. Wild-type and mutant p53 from baculovirus infected insect cells were tightly associated with a protein kinase which phosphorylates p53. In contrast, immunopurified wild-type and mutant p53 from an in vitro transcription/translation reaction were not associated with a protein kinase but could be phosphorylated by added casein kinase II. Thus, in the present paper we demonstrate that the association of p53 with a protein kinase and the in vitro phosphorylation of p53 seems to depend on the surrounding cellular environment.

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