Copurification of casein kinase II with 20 S proteasomes and phosphorylation of a 30-kDa proteasome subunit.

The 20 S proteasome is a multicatalytic protease that has been implicated in several processes including ATP/ubiquitin-dependent proteolysis. However, the ATP requirement(s) related to proteasome function is undefined. We demonstrate that a protein kinase activity copurifies through multiple steps utilized to isolate latent 20 S proteasomes from human erythrocytes. The kinase phosphorylates serine residues within a single 30-kDa proteasome subunit. The activity is not sensitive to cyclic AMP or protein kinase inhibitor, indicating that it is not a cyclic AMP-dependent kinase. It is sensitive to nanomolar levels of heparin and is able to utilize both ATP and GTP as phosphodonors, similar to casein kinase II activity. Moreover, a polyclonal antibody specific for casein kinase II recognizes the alpha' subunit of casein kinase II in the 20 S preparation and specifically immunoprecipitates the proteasome-phosphorylating activity. These characteristics suggest that the proteasome kinase is similar or identical to casein kinase II. It is suggested that phosphorylation of the 30-kDa proteasome subunit by casein kinase II may be involved in regulating the activity and/or assembly of proteasome complexes.