CASEIN KINASE-2 AND PROTEIN SUBSTRATES ARE RELEASED FROM RAT-LIVER CELLS NUCLEI BY DNASE OR RNASE DIGESTION

Casein kinase 2 was released from rat liver cells nuclei by digestion with DNase I plus RNase A. This treatment also released three major su sbstrates of 50, 40-42, and 37 kDa. Casein kinase 2 and substrates wer e also extracted by DNase or RNase separately. However, in DNase extra cts only the 37 kDa protein was phosphorylated by casein kinase 2, whe reas in RNase extracts all three substrates were phosphorylated. When the DNase extracts were subsequently treated with RNase the 40-42 subs trates were then phosphorylated, indicating that their interaction wit h RNA prevents their phosphorylation by casein kinase 2. The ratio of beta:alpha subunits of casein kinase 2 present in the nuclease extract s was higher than that of the purified enzyme, which is assumed to be 1:1. A further analysis by sucrose gradient centrifugation revealed th at under physiological salt conditions casein kinase 2 from nuclease e xtracts formed large aggregates (higher than 300 kDa) which were disru pted at 400 mM KCl. At the latter KCl concentration CK-2 activity was localized at a position corresponding to a Mr of 230-250 kDa, which is still higher than the typical tetrameric form Of the enzyme. (C) 1994 Academic Press, Inc.