Nuclear phosphoproteins. II. Metabolism of exogenous phosphoprotein by intact nuclei.

When a2p-labeled phosphoprotein is added to a suspension of calf thynms nuclei, the protein enters the nuclei and a rapid metabolic ' turnover ' of the previously incorporated phosphate groups occurs. The only a2P-labeled product which can be detected in these nuclei is inorganic phosphate. The results indicate that the phosphoprotein is probably not involved in the metabolic transfer of phosphate groups from one molecule to another, but favors the hypothesis that the phosphorylation and dephosphorylation of the nuclear phosphoproteins serve the purpose of modifying the structural and functional characteristics of the protein.