The phosphorylation of nucleoplasmin by casein kinase‐2 is resistant to heparin inhibition

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Abstract
Highly purified preparations of casein kinase‐2 from the nuclei of Xenopus laevis oocytes and from calf thymus can phosphorylate in vitro purified nucleoplasmin from X. laevis oocytes and eggs. The phosphorylation of nucleoplasmin by both kinase preparations is quite insensitive to heparin in contrast with casein phosphorylation which is completely abolished by heparin concentrations above 10 μg/ml. However, the phosphorylation of nucleoplasmin and casein are inhibited in a very similar fashion by 5,6‐dichloro‐1‐β‐D‐ribofuranosylbenzimidazole (DRB), a well characterized specific inhibitor of casein kinase‐2. Similarly, nucleoplasmin phosphorylation by the oocyte enzyme can be stimulated several‐fold by spermine, another characteristic of this enzyme. These findings indicate that the phosphorylation of nucleoplasmin by purified casein kinase‐2, while showing typical response to DRB and spermine, exhibits anomalous behavior in its resistance to heparin inhibition. It is possible that the large clusters of acidic amino acids in nucleoplasmin permit this substrate to interact with the enzyme more efficiently than other protein substrates. Heparin is generally considered a potent and specific inhibitor of casein kinase‐2. This study, however, questions the validity of utilizing heparin inhibition as a criterion for casein kinase‐2 involvement.