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 implica ted in several processes including ATP/ubiquitin-dependent proteolysis . However, the ATP requirement(s) related to proteasome function is un defined. We demonstrate that a protein kinase activity copurifies thro ugh multiple steps utilized to isolate latent 20 S proteasomes from hu man erythrocytes. The kinase phosphorylates serine residues within a s ingle 30-kDa proteasome subunit. The activity is not sensitive to cycl ic 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 c asein 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 proteaso me-phosphorylating activity. These characteristics suggest that the pr oteasome kinase is similar or identical to casein kinase II. It is sug gested 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.